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From t..@apache.org
Subject [33/46] incubator-apex-core git commit: SPOI-6737 #resolve Moving operator guides under shared space.
Date Mon, 29 Feb 2016 07:04:08 GMT
http://git-wip-us.apache.org/repos/asf/incubator-apex-core/blob/217c0651/application_development.md
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+Application Developer Guide
+===========================
+
+Real-time big data processing is not only important but has become
+critical for businesses which depend on accurate and timely analysis of
+their business data. A few businesses have yielded to very expensive
+solutions like building an in-house, real-time analytics infrastructure
+supported by an internal development team, or buying expensive
+proprietary software. A large number of businesses are dealing with the
+requirement just by trying to make Hadoop do their batch jobs in smaller
+iterations. Over the last few years, Hadoop has become ubiquitous in the
+big data processing space, replacing expensive proprietary hardware and
+software solutions for massive data processing with very cost-effective,
+fault-tolerant, open-sourced, and commodity-hardware-based solutions.
+While Hadoop has been a game changer for companies, it is primarily a
+batch-oriented system, and does not yet have a viable option for
+real-time data processing.  Most companies with real-time data
+processing end up having to build customized solutions in addition to
+their Hadoop infrastructure.
+
+ 
+
+The DataTorrent platform is designed to process massive amounts of
+real-time events natively in Hadoop. This can be event ingestion,
+processing, and aggregation for real-time data analytics, or can be
+real-time business logic decisioning such as cell tower load balancing,
+real-time ads bidding, or fraud detection.  The platform has the ability
+to repair itself in real-time (without data loss) if hardware fails, and
+adapt to changes in load by adding and removing computing resources
+automatically.
+
+
+
+DataTorrent is a native Hadoop application. It runs as a YARN
+(Hadoop 2.x) application and leverages Hadoop as a distributed operating
+system. All the basic distributed operating system capabilities of
+Hadoop like resource allocation ( [Resource Manager](#h.1ksv4uv) [)](#h.1ksv4uv),
+distributed file system ([HDFS](#h.3j2qqm3)[)](#h.3j2qqm3), [multi-tenancy](#h.3q5sasy)[,](#h.3q5sasy) 
+[security](#h.3q5sasy) [,](#h.3q5sasy) [fault-tolerance](#h.2nusc19)[,](#h.2nusc19) [scalability](#h.34g0dwd)[,](#h.34g0dwd) etc.
+are supported natively in all streaming applications.  Just as Hadoop
+for map-reduce handles all the details of the application allowing you
+to only focus on writing the application (the mapper and reducer
+functions), the platform handles all the details of streaming execution,
+allowing you to only focus on your business logic. Using the platform
+removes the need to maintain separate clusters for real-time
+applications.
+
+
+
+In the platform, building a streaming application can be extremely
+easy and intuitive.  The application is represented as a Directed
+Acyclic Graph (DAG) of computation units called [operators](#h.3o7alnk)[ ](#h.3o7alnk)interconnected
+by the data-flow edges called  [streams](#h.nmf14n)
+[.](#h.nmf14n) The operators process input
+streams and produce output streams. A library of common operators is
+provided to enable quick application development.  In case the desired
+processing is not available in the Operator Library, one can easily
+write a custom operator. We refer those interested in creating their own
+operators to the [Operator Development Guide](operator_development.md).
+
+Running A Test Application
+=======================================
+
+This chapter will help you with a quick start on running an
+application. If you are starting with the platform for the first time,
+it would be informative to open an existing application and see it run.
+Do the following steps to run the PI demo, which computes the value of
+PI  in a simple
+manner:
+
+1.  Open up platform files in your IDE (for example NetBeans, or Eclipse)
+2.  Open Demos project
+3.  Open Test Packages and run ApplicationTest.java in pi package
+4.  See the results in your system console
+
+
+
+Congratulations, you just ran your first real-time streaming demo
+:) This demo is very simple and has four operators. The first operator
+emits random integers between 0 to 30, 000. The second operator receives
+these coefficients and emits a hashmap with x and y values each time it
+receives two values. The third operator takes these values and computes
+x\*\*2+y\*\*2. The last operator counts how many computed values from
+the previous operator were less than or equal to 30, 000\*\*2. Assuming
+this count is N, then PI is computed as N/number of values received.
+Here is the code snippet for the PI application. This code populates the
+DAG. Do not worry about what each line does, we will cover these
+concepts later in this document.
+
+
+```java
+// Generates random numbers
+RandomEventGenerator rand = dag.addOperator("rand", new RandomEventGenerator());
+rand.setMinvalue(0);
+rand.setMaxvalue(30000);
+
+// Generates a round robin HashMap of "x" and "y"
+RoundRobinHashMap<String,Object> rrhm = dag.addOperator("rrhm", new RoundRobinHashMap<String, Object>());
+rrhm.setKeys(new String[] { "x", "y" });
+
+// Calculates pi from x and y
+JavaScriptOperator calc = dag.addOperator("picalc", new Script());
+calc.setPassThru(false);
+calc.put("i",0);
+calc.put("count",0);
+calc.addSetupScript("function pi() { if (x*x+y*y <= "+maxValue*maxValue+") { i++; } count++; return i / count * 4; }");
+calc.setInvoke("pi");
+dag.addStream("rand_rrhm", rand.integer_data, rrhm.data);
+dag.addStream("rrhm_calc", rrhm.map, calc.inBindings);
+
+// puts results on system console
+ConsoleOutputOperator console = dag.addOperator("console", new ConsoleOutputOperator());
+dag.addStream("rand_console",calc.result, console.input);
+```
+
+
+You can review the other demos and see what they do. The examples
+given in the Demos project cover various features of the platform and we
+strongly encourage you to read these to familiarize yourself with the
+platform. In the remaining part of this document we will go through
+details needed for you to develop and run streaming applications in
+Malhar.
+
+Test Application: Yahoo! Finance Quotes
+----------------------------------------------------
+
+The PI application was to
+get you started. It is a basic application and does not fully illustrate
+the features of the platform. For the purpose of describing concepts, we
+will consider the test application shown in Figure 1. The application
+downloads tick data from  [Yahoo! Finance](http://finance.yahoo.com)  and computes the
+following for four tickers, namely [IBM](http://finance.yahoo.com/q?s=IBM),
+[GOOG](http://finance.yahoo.com/q?s=GOOG), [YHOO](http://finance.yahoo.com/q?s=YHOO).
+
+1.  Quote: Consisting of last trade price, last trade time, and
+    total volume for the day
+2.  Per-minute chart data: Highest trade price, lowest trade
+    price, and volume during that minute
+3.  Simple Moving Average: trade price over 5 minutes
+
+
+Total volume must ensure that all trade volume for that day is
+added, i.e. data loss would result in wrong results. Charting data needs
+all the trades in the same minute to go to the same slot, and then on it
+starts afresh, so again data loss would result in wrong results. The
+aggregation for charting data is done over 1 minute. Simple moving
+average computes the average price over a 5 minute sliding window; it
+too would produce wrong results if there is data loss. Figure 1 shows
+the application with no partitioning.
+
+
+
+![](images/application_development/ApplicationDeveloperGuide.html-image00.png)
+
+
+
+The operator StockTickerInput: StockTickerInput[ ](http://docs.google.com/../apidocs/com/datatorrent/demos/yahoofinance/StockTickInput.html)is
+the input operator that reads live data from Yahoo! Finance once per
+interval (user configurable in milliseconds), and emits the price, the
+incremental volume, and the last trade time of each stock symbol, thus
+emulating real ticks from the exchange.  We utilize the Yahoo! Finance
+CSV web service interface.  For example:
+
+
+```
+$ GET 'http://download.finance.yahoo.com/d/quotes.csv?s=IBM,GOOG,AAPL,YHOO&f=sl1vt1'
+"IBM",203.966,1513041,"1:43pm"
+"GOOG",762.68,1879741,"1:43pm"
+"AAPL",444.3385,11738366,"1:43pm"
+"YHOO",19.3681,14707163,"1:43pm"
+```
+
+
+Among all the operators in Figure 1, StockTickerInput is the only
+operator that requires extra code because it contains a custom mechanism
+to get the input data.  Other operators are used unchanged from the
+Malhar library.
+
+
+Here is the class implementation for StockTickInput:
+
+
+```java
+package com.datatorrent.demos.yahoofinance;
+
+import au.com.bytecode.opencsv.CSVReader;
+import com.datatorrent.annotation.OutputPortFieldAnnotation;
+import com.datatorrent.api.Context.OperatorContext;
+import com.datatorrent.api.DefaultOutputPort;
+import com.datatorrent.api.InputOperator;
+import com.datatorrent.lib.util.KeyValPair;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.util.*;
+import org.apache.commons.httpclient.HttpClient;
+import org.apache.commons.httpclient.HttpStatus;
+import org.apache.commons.httpclient.cookie.CookiePolicy;
+import org.apache.commons.httpclient.methods.GetMethod;
+import org.apache.commons.httpclient.params.DefaultHttpParams;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This operator sends price, volume and time into separate ports and calculates incremental volume.
+ */
+public class StockTickInput implements InputOperator
+{
+  private static final Logger logger = LoggerFactory.getLogger(StockTickInput.class);
+  /**
+   * Timeout interval for reading from server. 0 or negative indicates no timeout.
+   */
+  public int readIntervalMillis = 500;
+  /**
+   * The URL of the web service resource for the POST request.
+   */
+  private String url;
+  public String[] symbols;
+  private transient HttpClient client;
+  private transient GetMethod method;
+  private HashMap<String, Long> lastVolume = new HashMap<String, Long>();
+  private boolean outputEvenIfZeroVolume = false;
+  /**
+   * The output port to emit price.
+   */
+  @OutputPortFieldAnnotation(optional = true)
+  public final transient DefaultOutputPort<KeyValPair<String, Double>> price = new DefaultOutputPort<KeyValPair<String, Double>>();
+  /**
+   * The output port to emit incremental volume.
+   */
+  @OutputPortFieldAnnotation(optional = true)
+  public final transient DefaultOutputPort<KeyValPair<String, Long>> volume = new DefaultOutputPort<KeyValPair<String, Long>>();
+  /**
+   * The output port to emit last traded time.
+   */
+  @OutputPortFieldAnnotation(optional = true)
+  public final transient DefaultOutputPort<KeyValPair<String, String>> time = new DefaultOutputPort<KeyValPair<String, String>>();
+
+  /**
+   * Prepare URL from symbols and parameters. URL will be something like: http://download.finance.yahoo.com/d/quotes.csv?s=IBM,GOOG,AAPL,YHOO&f=sl1vt1
+   *
+   * @return the URL
+   */
+  private String prepareURL()
+  {
+    String str = "http://download.finance.yahoo.com/d/quotes.csv?s=";
+    for (int i = 0; i < symbols.length; i++) {
+      if (i != 0) {
+        str += ",";
+      }
+      str += symbols[i];
+    }
+    str += "&f=sl1vt1&e=.csv";
+    return str;
+  }
+
+  @Override
+  public void setup(OperatorContext context)
+  {
+    url = prepareURL();
+    client = new HttpClient();
+    method = new GetMethod(url);
+    DefaultHttpParams.getDefaultParams().setParameter("http.protocol.cookie-policy", CookiePolicy.BROWSER_COMPATIBILITY);
+  }
+
+  @Override
+  public void teardown()
+  {
+  }
+
+  @Override
+  public void emitTuples()
+  {
+
+    try {
+      int statusCode = client.executeMethod(method);
+      if (statusCode != HttpStatus.SC_OK) {
+        System.err.println("Method failed: " + method.getStatusLine());
+      }
+      else {
+        InputStream istream = method.getResponseBodyAsStream();
+        // Process response
+        InputStreamReader isr = new InputStreamReader(istream);
+        CSVReader reader = new CSVReader(isr);
+        List<String[]> myEntries = reader.readAll();
+        for (String[] stringArr: myEntries) {
+          ArrayList<String> tuple = new ArrayList<String>(Arrays.asList(stringArr));
+          if (tuple.size() != 4) {
+            return;
+          }
+          // input csv is <Symbol>,<Price>,<Volume>,<Time>
+          String symbol = tuple.get(0);
+          double currentPrice = Double.valueOf(tuple.get(1));
+          long currentVolume = Long.valueOf(tuple.get(2));
+          String timeStamp = tuple.get(3);
+          long vol = currentVolume;
+          // Sends total volume in first tick, and incremental volume afterwards.
+          if (lastVolume.containsKey(symbol)) {
+            vol -= lastVolume.get(symbol);
+          }
+
+          if (vol > 0 || outputEvenIfZeroVolume) {
+            price.emit(new KeyValPair<String, Double>(symbol, currentPrice));
+            volume.emit(new KeyValPair<String, Long>(symbol, vol));
+            time.emit(new KeyValPair<String, String>(symbol, timeStamp));
+            lastVolume.put(symbol, currentVolume);
+          }
+        }
+      }
+      Thread.sleep(readIntervalMillis);
+    }
+    catch (InterruptedException ex) {
+      logger.debug(ex.toString());
+    }
+    catch (IOException ex) {
+      logger.debug(ex.toString());
+    }
+  }
+
+  @Override
+  public void beginWindow(long windowId)
+  {
+  }
+
+  @Override
+  public void endWindow()
+  {
+  }
+
+  public void setOutputEvenIfZeroVolume(boolean outputEvenIfZeroVolume)
+  {
+	   this.outputEvenIfZeroVolume = outputEvenIfZeroVolume;
+  }
+
+}
+```
+
+
+
+The operator has three output ports that emit the price of the
+stock, the volume of the stock and the last trade time of the stock,
+declared as public member variables price, volume and  time of the class.  The tuple of the
+price output port is a key-value
+pair with the stock symbol being the key, and the price being the value.
+ The tuple of the volume output
+port is a key value pair with the stock symbol being the key, and the
+incremental volume being the value.  The tuple of the  time output port is a key value pair with the
+stock symbol being the key, and the last trade time being the
+value.
+
+
+
+Important: Since operators will be
+serialized, all input and output ports need to be declared transient
+because they are stateless and should not be serialized.
+
+
+
+The method setup(OperatorContext)
+contains the code that is necessary for setting up the HTTP
+client for querying Yahoo! Finance.
+
+
+
+Method emitTuples() contains
+the code that reads from Yahoo! Finance, and emits the data to the
+output ports of the operator.  emitTuples() will be called one or more times
+within one application window as long as time is allowed within the
+window.
+
+
+
+Note that we want to emulate the tick input stream by having
+incremental volume data with Yahoo! Finance data.  We therefore subtract
+the previous volume from the current volume to emulate incremental
+volume for each tick.
+
+
+
+The operator
+DailyVolume: This operator
+reads from the input port, which contains the incremental volume tuples
+from StockTickInput, and
+aggregates the data to provide the cumulative volume.  It uses the
+library class  SumKeyVal&lt;K,V&gt; provided in math package.  In this case,
+SumKeyVal&lt;String,Long&gt;, where K is the stock symbol, V is the
+aggregated volume, with cumulative
+set to true. (Otherwise if  cumulativewas set to false, SumKeyVal would
+provide the sum for the application window.)  Malhar provides a number
+of built-in operators for simple operations like this so that
+application developers do not have to write them.  More examples to
+follow. This operator assumes that the application restarts before the
+market opens every day.
+
+
+
+The operator Quote:
+This operator has three input ports, which are price (from
+StockTickInput), daily\_vol (from
+Daily Volume), and time (from
+ StockTickInput).  This operator
+just consolidates the three data items and and emits the consolidated
+data.  It utilizes the class ConsolidatorKeyVal&lt;K&gt; from the
+stream package.
+
+
+
+The operator HighLow: This operator reads from the input port,
+which contains the price tuples from StockTickInput, and provides the high and the
+low price within the application window.  It utilizes the library class
+ RangeKeyVal&lt;K,V&gt; provided
+in the math package. In this case,
+RangeKeyVal&lt;String,Double&gt;.
+
+
+
+The operator MinuteVolume:
+This operator reads from the input port, which contains the
+volume tuples from StockTickInput,
+and aggregates the data to provide the sum of the volume within one
+minute.  Like the operator  DailyVolume, this operator also uses
+SumKeyVal&lt;String,Long&gt;, but
+with cumulative set to false.  The
+Application Window is set to one minute. We will explain how to set this
+later.
+
+
+
+The operator Chart:
+This operator is very similar to the operator Quote, except that it takes inputs from
+High Low and  Minute Vol and outputs the consolidated tuples
+to the output port.
+
+
+
+The operator PriceSMA:
+SMA stands for - Simple Moving Average. It reads from the
+input port, which contains the price tuples from StockTickInput, and
+provides the moving average price of the stock.  It utilizes
+SimpleMovingAverage&lt;String,Double&gt;, which is provided in the
+ multiwindow package.
+SimpleMovingAverage keeps track of the data of the previous N
+application windows in a sliding manner.  For each end window event, it
+provides the average of the data in those application windows.
+
+
+
+The operator Console:
+This operator just outputs the input tuples to the console
+(or stdout).  In this example, there are four console operators, which connect to the output
+of  Quote, Chart, PriceSMA and VolumeSMA.  In
+practice, they should be replaced by operators that use the data to
+produce visualization artifacts like charts.
+
+
+
+Connecting the operators together and constructing the
+DAG: Now that we know the
+operators used, we will create the DAG, set the streaming window size,
+instantiate the operators, and connect the operators together by adding
+streams that connect the output ports with the input ports among those
+operators.  This code is in the file  YahooFinanceApplication.java. Refer to Figure 1
+again for the graphical representation of the DAG.  The last method in
+the code, namely getApplication(),
+does all that.  The rest of the methods are just for setting up the
+operators.
+
+
+
+```java
+package com.datatorrent.demos.yahoofinance;
+
+import com.datatorrent.api.ApplicationFactory;
+import com.datatorrent.api.Context.OperatorContext;
+import com.datatorrent.api.DAG;
+import com.datatorrent.api.Operator.InputPort;
+import com.datatorrent.lib.io.ConsoleOutputOperator;
+import com.datatorrent.lib.math.RangeKeyVal;
+import com.datatorrent.lib.math.SumKeyVal;
+import com.datatorrent.lib.multiwindow.SimpleMovingAverage;
+import com.datatorrent.lib.stream.ConsolidatorKeyVal;
+import com.datatorrent.lib.util.HighLow;
+import org.apache.hadoop.conf.Configuration;
+
+/**
+ * Yahoo! Finance application demo. <p>
+ *
+ * Get Yahoo finance feed and calculate minute price range, minute volume, simple moving average of 5 minutes.
+ */
+public class Application implements StreamingApplication
+{
+  private int streamingWindowSizeMilliSeconds = 1000; // 1 second (default is 500ms)
+  private int appWindowCountMinute = 60;   // 1 minute
+  private int appWindowCountSMA = 5 * 60;  // 5 minute
+
+  /**
+   * Get actual Yahoo finance ticks of symbol, last price, total daily volume, and last traded price.
+   */
+  public StockTickInput getStockTickInputOperator(String name, DAG dag)
+  {
+    StockTickInput oper = dag.addOperator(name, StockTickInput.class);
+    oper.readIntervalMillis = 200;
+    return oper;
+  }
+
+  /**
+   * This sends total daily volume by adding volumes from each ticks.
+   */
+  public SumKeyVal<String, Long> getDailyVolumeOperator(String name, DAG dag)
+  {
+    SumKeyVal<String, Long> oper = dag.addOperator(name, new SumKeyVal<String, Long>());
+    oper.setType(Long.class);
+    oper.setCumulative(true);
+    return oper;
+  }
+
+  /**
+   * Get aggregated volume of 1 minute and send at the end window of 1 minute.
+   */
+  public SumKeyVal<String, Long> getMinuteVolumeOperator(String name, DAG dag, int appWindowCount)
+  {
+    SumKeyVal<String, Long> oper = dag.addOperator(name, new SumKeyVal<String, Long>());
+    oper.setType(Long.class);
+    oper.setEmitOnlyWhenChanged(true);
+dag.getOperatorMeta(name).getAttributes().put(OperatorContext.APPLICATION_WINDOW_COUNT,appWindowCount);
+    return oper;
+  }
+
+  /**
+   * Get High-low range for 1 minute.
+   */
+  public RangeKeyVal<String, Double> getHighLowOperator(String name, DAG dag, int appWindowCount)
+  {
+    RangeKeyVal<String, Double> oper = dag.addOperator(name, new RangeKeyVal<String, Double>());
+    dag.getOperatorMeta(name).getAttributes().put(OperatorContext.APPLICATION_WINDOW_COUNT,appWindowCount);
+    oper.setType(Double.class);
+    return oper;
+  }
+
+  /**
+   * Quote (Merge price, daily volume, time)
+   */
+  public ConsolidatorKeyVal<String,Double,Long,String,?,?> getQuoteOperator(String name, DAG dag)
+  {
+    ConsolidatorKeyVal<String,Double,Long,String,?,?> oper = dag.addOperator(name, new ConsolidatorKeyVal<String,Double,Long,String,Object,Object>());
+    return oper;
+  }
+
+  /**
+   * Chart (Merge minute volume and minute high-low)
+   */
+  public ConsolidatorKeyVal<String,HighLow,Long,?,?,?> getChartOperator(String name, DAG dag)
+  {
+    ConsolidatorKeyVal<String,HighLow,Long,?,?,?> oper = dag.addOperator(name, new ConsolidatorKeyVal<String,HighLow,Long,Object,Object,Object>());
+    return oper;
+  }
+
+  /**
+   * Get simple moving average of price.
+   */
+  public SimpleMovingAverage<String, Double> getPriceSimpleMovingAverageOperator(String name, DAG dag, int appWindowCount)
+  {
+    SimpleMovingAverage<String, Double> oper = dag.addOperator(name, new SimpleMovingAverage<String, Double>());
+    oper.setWindowSize(appWindowCount);
+    oper.setType(Double.class);
+    return oper;
+  }
+
+  /**
+   * Get console for output.
+   */
+  public InputPort<Object> getConsole(String name, /*String nodeName,*/ DAG dag, String prefix)
+  {
+    ConsoleOutputOperator oper = dag.addOperator(name, ConsoleOutputOperator.class);
+    oper.setStringFormat(prefix + ": %s");
+    return oper.input;
+  }
+
+  /**
+   * Create Yahoo Finance Application DAG.
+   */
+  @Override
+  public void populateDAG(DAG dag, Configuration conf)
+  {
+    dag.getAttributes().put(DAG.STRAM_WINDOW_SIZE_MILLIS,streamingWindowSizeMilliSeconds);
+
+    StockTickInput tick = getStockTickInputOperator("StockTickInput", dag);
+    SumKeyVal<String, Long> dailyVolume = getDailyVolumeOperator("DailyVolume", dag);
+    ConsolidatorKeyVal<String,Double,Long,String,?,?> quoteOperator = getQuoteOperator("Quote", dag);
+
+    RangeKeyVal<String, Double> highlow = getHighLowOperator("HighLow", dag, appWindowCountMinute);
+    SumKeyVal<String, Long> minuteVolume = getMinuteVolumeOperator("MinuteVolume", dag, appWindowCountMinute);
+    ConsolidatorKeyVal<String,HighLow,Long,?,?,?> chartOperator = getChartOperator("Chart", dag);
+
+    SimpleMovingAverage<String, Double> priceSMA = getPriceSimpleMovingAverageOperator("PriceSMA", dag, appWindowCountSMA);
+       DefaultPartitionCodec<String, Double> codec = new DefaultPartitionCodec<String, Double>();
+    dag.setInputPortAttribute(highlow.data, PortContext.STREAM_CODEC, codec);
+    dag.setInputPortAttribute(priceSMA.data, PortContext.STREAM_CODEC, codec);
+    dag.addStream("price", tick.price, quoteOperator.in1, highlow.data, priceSMA.data);
+    dag.addStream("vol", tick.volume, dailyVolume.data, minuteVolume.data);
+    dag.addStream("time", tick.time, quoteOperator.in3);
+    dag.addStream("daily_vol", dailyVolume.sum, quoteOperator.in2);
+
+    dag.addStream("quote_data", quoteOperator.out, getConsole("quoteConsole", dag, "QUOTE"));
+
+    dag.addStream("high_low", highlow.range, chartOperator.in1);
+    dag.addStream("vol_1min", minuteVolume.sum, chartOperator.in2);
+    dag.addStream("chart_data", chartOperator.out, getConsole("chartConsole", dag, "CHART"));
+
+    dag.addStream("sma_price", priceSMA.doubleSMA, getConsole("priceSMAConsole", dag, "Price SMA"));
+
+    return dag;
+  }
+
+}
+```
+
+
+
+Note that we also set a user-specific sliding window for SMA that
+keeps track of the previous N data points.  Do not confuse this with the
+attribute APPLICATION\_WINDOW\_COUNT.
+
+In the rest of this chapter we will run through the process of
+running this application. We assume that  you are familiar with details
+of your Hadoop infrastructure. For installation
+details please refer to the [Installation Guide](installation.md).
+
+
+Running a Test Application
+-----------------------------------------
+
+We will now describe how to run the yahoo
+finance application described above in different modes
+(local mode, single node on Hadoop, and multi-nodes on Hadoop).
+
+
+The platform runs streaming applications under the control of a
+light-weight Streaming Application Manager (STRAM). Each application has
+its own instance of STRAM. STRAM launches the application and
+continually provides run time monitoring, analysis, and takes action
+such as load scaling or outage recovery as needed.  We will discuss
+STRAM in more detail in the next chapter.
+
+
+
+The instructions below assume that the platform was installed in a
+directory &lt;INSTALL\_DIR&gt; and the command line interface (CLI) will
+be used to launch the demo application. An application can be run in
+[local mode](#h.3dy6vkm)[ ](#h.3dy6vkm)(in IDE or from command line) or on a  [Hadoop cluster](#h.1t3h5sf) [.](#h.1t3h5sf)
+
+
+
+To start the dtCli run
+
+    <INSTALL_DIR>/bin/dtcli
+
+The command line prompt appears.  To start the application in local mode (the actual version number in the file name may differ)
+
+    dt> launch -local <INSTALL_DIR>/yahoo-finance-demo-3.2.0-SNAPSHOT.apa
+
+To terminate the application in local mode, enter Ctrl-C
+
+Tu run the application on the Hadoop cluster (the actual version
+number in the file name may differ)
+
+    dt> launch <INSTALL_DIR>/yahoo-finance-demo-3.2.0-SNAPSHOT.apa
+
+
+To stop the application running in Hadoop, terminate it in the dtCli:
+
+    dt> kill-app
+
+
+
+Executing the application in either mode includes the following
+steps. At a top level, STRAM (Streaming Application Manager) validates
+the application (DAG), translates the logical plan to the physical plan
+and then launches the execution engine. The mode determines the
+resources needed and how how they are used.
+
+Local Mode
+-----------------------
+
+In local mode, the application is run as a single-process with multiple threads. Although a
+few Hadoop classes are needed, there is no dependency on a Hadoop
+cluster or Hadoop services. The local file system is used in place of
+HDFS. This mode allows a quick run of an application in a single process
+sandbox, and hence is the most suitable to debug and analyze the
+application logic. This mode is recommended for developing the
+application and can be used for running applications within the IDE for
+functional testing purposes. Due to limited resources and lack  of
+scalability an application running in this single process mode is more
+likely to encounter throughput bottlenecks. A distributed cluster is
+recommended for benchmarking and production testing.
+
+Hadoop Cluster
+---------------------------
+
+In this section we discuss various Hadoop cluster setups.
+
+### Single Node Cluster
+
+In a single node Hadoop cluster all services are deployed on a
+single server (a developer can use his/her development machine as a
+single node cluster). The platform does not distinguish between a single
+or multi-node setup and behaves exactly the same in both cases.
+
+
+
+In this mode, the resource manager, name node, data node, and node
+manager occupy one process each. This is an example of running a
+streaming application as a multi-process application on the same server.
+With prevalence of fast, multi-core systems, this mode is effective for
+debugging, fine tuning, and generic analysis before submitting the job
+to a larger Hadoop cluster. In this mode, execution uses the Hadoop
+services and hence is likely to identify issues that are related to the
+Hadoop environment (such issues will not be uncovered in local mode).
+The throughput will obviously not be as high as on a multi-node Hadoop
+cluster. Additionally, since each container (i.e. Java process) requires
+a significant amount of memory, you will be able to run a much smaller
+number of containers than on a multi-node cluster.
+
+### Multi-Node Cluster
+
+In a multi-node Hadoop cluster all the services of Hadoop are
+typically distributed across multiple nodes in a production or
+production-level test environment. Upon launch the application is
+submitted to the Hadoop cluster and executes as a  multi-processapplication on multiple nodes.
+
+
+
+Before you start deploying, testing and troubleshooting your
+application on a cluster, you should ensure that Hadoop (version 2.2.0
+or later) is properly installed and
+you have basic skills for working with it.
+
+------------------------------------------------------------------------
+
+
+
+
+
+Apache Apex Platform Overview
+========================================
+
+Streaming Computational Model
+------------------------------------------
+
+In this chapter, we describe the the basics of the real-time streaming platform and its computational model.
+
+
+The platform is designed to enable completely asynchronous real time computations done in as unblocked a way as possible with
+minimal overhead .
+
+
+
+Applications running in the platform are represented by a Directed
+Acyclic Graph (DAG) made up of  operators and streams. All computations
+are done in memory on arrival of
+the input data, with an option to save the output to disk (HDFS) in a
+non-blocking way. The data that flows between operators consists of
+atomic data elements. Each data element along with its type definition
+(henceforth called  schema) is
+called a tuple. An application is a
+design of the flow of these tuples to and from
+the appropriate compute units to enable the computation of the final
+desired results. A message queue (henceforth called
+ buffer server) manages tuples streaming
+between compute units in different processes.This server keeps track of
+all consumers, publishers, partitions, and enables replay. More
+information is given in later section.
+
+
+
+The streaming application is monitored by a decision making entity
+called STRAM (streaming application
+manager). STRAM is designed to be a light weight
+controller that has minimal but sufficient interaction with the
+application. This is done via periodic heartbeats. The
+STRAM does the initial launch and periodically analyzes the system
+metrics to decide if any run time action needs to be taken.
+
+
+
+A fundamental building block for the streaming platform
+is the concept of breaking up a stream into equal finite time slices
+called streaming windows. Each window contains the ordered
+set of tuples in that time slice. A typical duration of a window is 500
+ms, but can be configured per application (the Yahoo! Finance
+application configures this value in the  properties.xml file to be 1000ms = 1s). Each
+window is preceded by a begin\_window event and is terminated by an
+end\_window event, and is assigned
+a unique window ID. Even though the platform performs computations at
+the tuple level, bookkeeping is done at the window boundary, making the
+computations within a window an atomic event in the platform.  We can
+think of each window as an  atomic
+micro-batch of tuples, to be processed together as one
+atomic operation (See Figure 2).  
+
+
+
+This atomic batching allows the platform to avoid the very steep
+per tuple bookkeeping cost and instead has a manageable per batch
+bookkeeping cost. This translates to higher throughput, low recovery
+time, and higher scalability. Later in this document we illustrate how
+the atomic micro-batch concept allows more efficient optimization
+algorithms.
+
+
+
+The platform also has in-built support for
+application windows.  An application window is part of the
+application specification, and can be a small or large multiple of the
+streaming window.  An example from our Yahoo! Finance test application
+is the moving average, calculated over a sliding application window of 5
+minutes which equates to 300 (= 5 \* 60) streaming windows.
+
+
+
+Note that these two window concepts are distinct.  A streaming
+window is an abstraction of many tuples into a higher atomic event for
+easier management.  An application window is a group of consecutive
+streaming windows used for data aggregation (e.g. sum, average, maximum,
+minimum) on a per operator level.
+
+![](images/application_development/ApplicationDeveloperGuide.html-image02.png)
+
+Alongside the platform, a set of
+predefined, benchmarked standard library operator templates is provided
+for ease of use and rapid development of application. These
+operators are open sourced to Apache Software Foundation under the
+project name “Malhar” as part of our efforts to foster community
+innovation. These operators can be used in a DAG as is, while others
+have  [properties](#h.32hioqz)
+[ ](#h.32hioqz)that can be set to specify the
+desired computation. Those interested in details, should refer to
+[Apex Malhar Operator Library](apex_malhar.md)
+.
+
+
+
+The platform is a Hadoop YARN native
+application. It runs in a Hadoop cluster just like any
+other YARN application (MapReduce etc.) and is designed to seamlessly
+integrate with rest of Hadoop technology stack. It leverages Hadoop as
+much as possible and relies on it as its distributed operating system.
+Hadoop dependencies include resource management, compute/memory/network
+allocation, HDFS, security, fault tolerance, monitoring, metrics,
+multi-tenancy, logging etc. Hadoop classes/concepts are reused as much
+as possible.  The aim is to enable enterprises
+to leverage their existing Hadoop infrastructure for real time streaming
+applications. The platform is designed to scale with big
+data applications and scale with Hadoop.
+
+
+
+A streaming application is an asynchronous execution of
+computations across distributed nodes. All computations are done in
+parallel on a distributed cluster. The computation model is designed to
+do as many parallel computations as possible in a non-blocking fashion.
+The task of monitoring of the entire application is done on (streaming)
+window boundaries with a streaming window as an atomic entity. A window
+completion is a quantum of work done. There is no assumption that an
+operator can be interrupted at precisely a particular tuple or window.
+
+
+
+
+An operator itself also
+cannot assume or predict the exact time a tuple that it emitted would
+get consumed by downstream operators. The operator processes the tuples
+it gets and simply emits new tuples based on its business logic. The
+only guarantee it has is that the upstream operators are processing
+either the current or some later window, and the downstream operator is
+processing either the current or some earlier window. The completion of
+a window (i.e. propagation of the  end\_window event through an operator) in any
+operator guarantees that all upstream operators have finished processing
+this window. Thus, the end\_window event is blocking on an operator
+with multiple outputs, and is a synchronization point in the DAG. The
+ begin\_window event does not have
+any such restriction, a single begin\_window event from any upstream operator
+triggers the operator to start processing tuples.
+
+Streaming Application Manager (STRAM)
+--------------------------------------------------
+
+Streaming Application Manager (STRAM) is the Hadoop YARN native
+application master. STRAM is the first process that is activated upon
+application launch and orchestrates the streaming application on the
+platform. STRAM is a lightweight controller process. The
+responsibilities of STRAM include
+
+1.  Running the Application
+
+    *  Read the logical plan of the application (DAG) submitted by the client
+    *  Validate the logical plan
+    *  Translate the logical plan into a physical plan, where certain operators may  be partitioned (i.e. replicated) to multiple operators for  handling load.
+    *  Request resources (Hadoop containers) from Resource Manager,
+        per physical plan
+    *  Based on acquired resources and application attributes, create
+        an execution plan by partitioning the DAG into fragments,
+        each assigned to different containers.
+    *  Executes the application by deploying each fragment to
+        its container. Containers then start stream processing and run
+        autonomously, processing one streaming window after another. Each
+        container is represented as an instance of the  StreamingContainer class, which updates
+        STRAM via the heartbeat protocol and processes directions received
+        from STRAM.
+
+2.  Continually monitoring the application via heartbeats from each StreamingContainer
+3.  Collecting Application System Statistics and Logs
+4.  Logging all application-wide decisions taken
+5.  Providing system data on the state of the application via a  Web Service.
+6.  Supporting [Fault Tolerance](#h.2nusc19)
+
+    a.  Detecting a node outage
+    b.  Requesting a replacement resource from the Resource Manager
+        and scheduling state restoration for the streaming operators
+    c.  Saving state to Zookeeper
+
+7.  Supporting [Dynamic
+    Partitioning](#h.3hv69ve)[:](#h.3hv69ve) Periodically
+    evaluating the SLA and modifying the physical plan if required
+    (logical plan does not change).
+8.  Enabling [Security](#h.3q5sasy)[:](#h.3q5sasy) Distributing
+    security tokens for distributed components of the execution engine
+    and securing web service requests.
+9.  Enabling [Dynamic  modification](#h.40ew0vw)[ ](#h.40ew0vw)of
+    DAG: In the future, we intend to allow for user initiated
+    modification of the logical plan to allow for changes to the
+    processing logic and functionality.
+
+
+
+An example of the Yahoo! Finance Quote application scheduled on a
+cluster of 5 Hadoop containers (processes) is shown in Figure 3.
+
+![](images/application_development/ApplicationDeveloperGuide.html-image01.png)
+
+
+
+An example for the translation from a logical plan to a physical
+plan and an execution plan for a subset of the application is shown in
+Figure 4.
+
+
+
+![](images/application_development/ApplicationDeveloperGuide.html-image04.png)
+
+
+
+
+
+Hadoop Components
+------------------------------
+
+In this section we cover some aspects of Hadoop that your
+streaming application interacts with. This section is not meant to
+educate the reader on Hadoop, but just get the reader acquainted with
+the terms. We strongly advise readers to learn Hadoop from other
+sources.
+
+A streaming application runs as a native Hadoop 2.2 application.
+Hadoop 2.2 does not differentiate between a map-reduce job and other
+applications, and hence as far as Hadoop is concerned, the streaming
+application is just another job. This means that your application
+leverages all the bells and whistles Hadoop provides and is fully
+supported within Hadoop technology stack. The platform is responsible
+for properly integrating itself with the relevant components of Hadoop
+that exist today and those that may emerge in the future
+
+
+
+All investments that leverage multi-tenancy (for example quotas
+and queues), security (for example kerberos), data flow integration (for
+example copying data in-out of HDFS), monitoring, metrics collections,
+etc. will require no changes when streaming applications run on
+Hadoop.
+
+### YARN
+
+[YARN](http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site)is
+the core library of Hadoop 2.2 that is tasked with resource management
+and works as a distributed application framework. In this section we
+will walk through Yarn's components. In Hadoop 2.2, the old jobTracker
+has been replaced by a combination of ResourceManager (RM) and
+ApplicationMaster (AM).
+
+#### Resource Manager (RM)
+
+[ResourceManager](http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site/YARN.html)(RM)
+manages all the distributed resources. It allocates and arbitrates all
+the slots and the resources (cpu, memory, network) of these slots. It
+works with per-node NodeManagers (NMs) and per-application
+ApplicationMasters (AMs). Currently memory usage is monitored by RM; in
+upcoming releases it will have CPU as well as network management. RM is
+shared by map-reduce and streaming applications. Running streaming
+applications requires no changes in the RM.
+
+#### Application Master (AM)
+
+The AM is the watchdog or monitoring process for your application
+and has the responsibility of negotiating resources with RM and
+interacting with NodeManagers to get the allocated containers started.
+The AM is the starting point of your application and is considered user
+code (not system Hadoop code). The AM itself runs in one container. All
+resource management within the application are managed by the AM. This
+is a critical feature for Hadoop 2.2 where tasks done by jobTracker in
+Hadoop 1.0 have been distributed allowing Hadoop 2.2 to scale much
+beyond Hadoop 1.0. STRAM is a native YARN ApplicationManager.
+
+#### Node Managers (NM)
+
+There is one [NodeManager](http://hadoop.apache.org/docs/current/hadoop-yarn/hadoop-yarn-site/YARN.html)(NM)
+per node in the cluster. All the containers (i.e. processes) on that
+node are monitored by the NM. It takes instructions from RM and manages
+resources of that node as per RM instructions. NMs interactions are same
+for map-reduce and for streaming applications. Running streaming
+applications requires no changes in the NM.
+
+#### RPC Protocol
+
+Communication among RM, AM, and NM is done via the Hadoop RPC
+protocol. Streaming applications use the same protocol to send their
+data. No changes are needed in RPC support provided by Hadoop to enable
+communication done by components of your application.
+
+### HDFS
+
+Hadoop includes a highly fault tolerant, high throughput
+distributed file system ([HDFS](http://hadoop.apache.org/docs/r1.0.4/hdfs_design.html)).
+It runs on commodity hardware, and your streaming application will, by
+default, use it. There is no difference between files created by a
+streaming application and those created by map-reduce.
+
+Developing An Application
+======================================
+
+In this chapter we describe the methodology to develop an
+application using the Realtime Streaming Platform. The platform was
+designed to make it easy to build and launch sophisticated streaming
+applications with the developer having to deal only with the
+application/business logic. The platform deals with details of where to
+run what operators on which servers and how to correctly route streams
+of data among them.
+
+Development Process
+--------------------------------
+
+While the platform does not mandate a specific methodology or set
+of development tools, we have recommendations to maximize productivity
+for the different phases of application development.
+
+#### Design
+
+-   Identify common, reusable operators. Use a library
+    if possible.
+-   Identify scalability and performance requirements before
+    designing the DAG.
+-   Leverage attributes that the platform supports for scalability
+    and performance.
+-   Use operators that are benchmarked and tested so that later
+    surprises are minimized. If you have glue code, create appropriate
+    unit tests for it.
+-   Use THREAD_LOCAL locality for high throughput streams. If all
+    the operators on that stream cannot fit in one container,
+    try NODE_LOCAL locality. Both THREAD_LOCAL and
+    NODE_LOCAL streams avoid the Network Interface Card (NIC)
+    completly. The former uses intra-process communication to also avoid
+    serialization-deserialization overhead.
+-   The overall throughput and latencies are are not necessarily
+    correlated to the number of operators in a simple way -- the
+    relationship is more nuanced. A lot depends on how much work
+    individual operators are doing, how many are able to operate in
+    parallel, and how much data is flowing through the arcs of the DAG.
+    It is, at times, better to break a computation down into its
+    constituent simple parts and then stitch them together via streams
+    to better utilize the compute resources of the cluster. Decide on a
+    per application basis the fine line between complexity of each
+    operator vs too many streams. Doing multiple computations in one
+    operator does save network I/O, while operators that are too complex
+    are hard to maintain.
+-   Do not use operators that depend on the order of two streams
+    as far as possible. In such cases behavior is not idempotent.
+-   Persist key information to HDFS if possible; it may be useful
+    for debugging later.
+-   Decide on an appropriate fault tolerance mechanism. If some
+    data loss is acceptable, use the at-most-once mechanism as it has
+    fastest recovery.
+
+#### Creating New Project
+
+Please refer to the [Apex Application Packages](application_packages.md) for
+the basic steps for creating a new project.
+
+#### Writing the application code
+
+Preferably use an IDE (Eclipse, Netbeans etc.) that allows you to
+manage dependencies and assists with the Java coding. Specific benefits
+include ease of managing operator library jar files, individual operator
+classes, ports and properties. It will also highlight and assist to
+rectify issues such as type mismatches when adding streams while
+typing.
+
+#### Testing
+
+Write test cases with JUnit or similar test framework so that code
+is tested as it is written. For such testing, the DAG can run in local
+mode within the IDE. Doing this may involve writing mock input or output
+operators for the integration points with external systems. For example,
+instead of reading from a live data stream, the application in test mode
+can read from and write to files. This can be done with a single
+application DAG by instrumenting a test mode using settings in the
+configuration that is passed to the application factory
+interface.
+
+Good test coverage will not only eliminate basic validation errors
+such as missing port connections or property constraint violations, but
+also validate the correct processing of the data. The same tests can be
+re-run whenever the application or its dependencies change (operator
+libraries, version of the platform etc.)
+
+#### Running an application
+
+The platform provides a commandline tool called dtcli for managing applications (launching,
+killing, viewing, etc.). This tool was already discussed above briefly
+in the section entitled Running the Test Application. It will introspect
+the jar file specified with the launch command for applications (classes
+that implement ApplicationFactory) or property files that define
+applications. It will also deploy the dependency jar files from the
+application package to the cluster.
+
+
+
+Dtcli can run the application in local mode (i.e. outside a
+cluster). It is recommended to first run the application in local mode
+in the development environment before launching on the Hadoop cluster.
+This way some of the external system integration and correct
+functionality of the application can be verified in an easier to debug
+environment before testing distributed mode.
+
+
+
+For more details on CLI please refer to the [dtCli Guide](dtcli.md).
+
+Application API
+----------------------------
+
+This section introduces the API to write a streaming application.
+The work involves connecting operators via streams to form the logical
+DAG. The steps are
+
+1.  Instantiate an application (DAG)
+
+2.  (Optional) Set Attributes
+    *  Assign application name
+    *  Set any other attributes as per application requirements
+
+3.  Create/re-use and instantiate operators
+    *  Assign operator name that is unique within the  application
+    *  Declare schema upfront for each operator (and thereby its  [ports](#h.ihv636)[)](#h.ihv636)
+    *  (Optional) Set [properties](#h.32hioqz)[ ](#h.32hioqz) and [attributes](#h.41mghml)[ ](#h.41mghml) on the dag as per specification
+    *  Connect ports of operators via streams
+        *  Each stream connects one output port of an operator to one or  more input ports of other operators.
+        *  (Optional) Set attributes on the streams
+
+4.  Test the application.
+
+
+
+There are two methods to create an application, namely Java, and
+Properties file. Java API is for applications being developed by humans,
+and properties file (Hadoop like) is more suited for DAGs generated by
+tools.
+
+### Java API
+
+The Java API is the most common way to create a streaming
+application. It is meant for application developers who prefer to
+leverage the features of Java, and the ease of use and enhanced
+productivity provided by IDEs like NetBeans or Eclipse. Using Java to
+specify the application provides extra validation abilities of Java
+compiler, such as compile time checks for type safety at the time of
+writing the code. Later in this chapter you can read more about
+validation support in the platform.
+
+The developer specifies the streaming application by implementing
+the ApplicationFactory interface, which is how platform tools (CLI etc.)
+recognize and instantiate applications. Here we show how to create a
+Yahoo! Finance application that streams the last trade price of a ticker
+and computes the high and low price in every 1 min window. Run above
+ test application to execute the
+DAG in local mode within the IDE.
+
+
+
+Let us revisit how the Yahoo! Finance test application constructs the DAG:
+
+
+
+```java
+public class Application implements StreamingApplication
+{
+[...CUT...]
+  @Override
+  public void populateDAG(DAG dag, Configuration conf)
+  {
+    dag.getAttributes().attr(DAG.STRAM_WINDOW_SIZE_MILLIS).set(streamingWindowSizeMilliSeconds);
+
+    StockTickInput tick = getStockTickInputOperator("StockTickInput", dag);
+    SumKeyVal<String, Long> dailyVolume = getDailyVolumeOperator("DailyVolume", dag);
+    ConsolidatorKeyVal<String,Double,Long,String,?,?> quoteOperator = getQuoteOperator("Quote", dag);
+
+    RangeKeyVal<String, Double> highlow = getHighLowOperator("HighLow", dag, appWindowCountMinute);
+    SumKeyVal<String, Long> minuteVolume = getMinuteVolumeOperator("MinuteVolume", dag, appWindowCountMinute);
+    ConsolidatorKeyVal<String,HighLow,Long,?,?,?> chartOperator = getChartOperator("Chart", dag);
+
+    SimpleMovingAverage<String, Double> priceSMA = getPriceSimpleMovingAverageOperator("PriceSMA", dag, appWindowCountSMA);
+
+    dag.addStream("price", tick.price, quoteOperator.in1, highlow.data, priceSMA.data);
+    dag.addStream("vol", tick.volume, dailyVolume.data, minuteVolume.data);
+    dag.addStream("time", tick.time, quoteOperator.in3);
+    dag.addStream("daily_vol", dailyVolume.sum, quoteOperator.in2);
+
+    dag.addStream("quote_data", quoteOperator.out, getConsole("quoteConsole", dag, "QUOTE"));
+
+    dag.addStream("high_low", highlow.range, chartOperator.in1);
+    dag.addStream("vol_1min", minuteVolume.sum, chartOperator.in2);
+    dag.addStream("chart_data", chartOperator.out, getConsole("chartConsole", dag, "CHART"));
+
+    dag.addStream("sma_price", priceSMA.doubleSMA, getConsole("priceSMAConsole", dag, "Price SMA"));
+
+    return dag;
+  }
+}
+```
+
+
+
+
+### Property File API
+
+The platform also supports specification of a DAG via a property
+file. The aim here to make it easy for tools to create and run an
+application. This method of specification does not have the Java
+compiler support of compile time check, but since these applications
+would be created by software, they should be correct by construction.
+The syntax is derived from Hadoop properties and should be easy for
+folks who are used to creating software that integrated with
+Hadoop.
+
+
+
+Create an application (DAG): myApplication.properties
+
+
+```
+# input operator that reads from a file
+dt.operator.inputOp.classname=com.acme.SampleInputOperator
+dt.operator.inputOp.fileName=somefile.txt
+
+# output operator that writes to the console
+dt.operator.outputOp.classname=com.acme.ConsoleOutputOperator
+
+# stream connecting both operators
+dt.stream.inputStream.source=inputOp.outputPort
+dt.stream.inputStream.sinks=outputOp.inputPort
+```
+
+
+
+Above snippet is intended to convey the basic idea of specifying
+the DAG without using Java. Operators would come from a predefined
+library and referenced in the specification by class name and port names
+(obtained from the library providers documentation or runtime
+introspection by tools). For those interested in details, see later
+sections and refer to the  Operation and
+Installation Guide mentioned above.
+
+### Attributes
+
+Attributes impact the runtime behavior of the application. They do
+not impact the functionality. An example of an attribute is application
+name. Setting it changes the application name. Another example is
+streaming window size. Setting it changes the streaming window size from
+the default value to the specified value. Users cannot add new
+attributes, they can only choose from the ones that come packaged and
+pre-supported by the platform. Details of attributes are covered in the
+ Operation and Installation
+Guide.
+
+Operators
+----------------------
+
+Operators are basic compute units.
+Operators process each incoming tuple and emit zero or more tuples on
+output ports as per the business logic. The data flow, connectivity,
+fault tolerance (node outage), etc. is taken care of by the platform. As
+an operator developer, all that is needed is to figure out what to do
+with the incoming tuple and when (and which output port) to send out a
+particular output tuple. Correctly designed operators will most likely
+get reused. Operator design needs care and foresight. For details, refer
+to the  [Operator Developer
+Guide](https://www.datatorrent.com/docs/guides/OperatorDeveloperGuide.html)
+. As an application developer you need to connect operators
+in a way that it implements your business logic. You may also require
+operator customization for functionality and use attributes for
+performance/scalability etc.
+
+
+
+All operators process tuples asynchronously in a distributed
+cluster. An operator cannot assume or predict the exact time a tuple
+that it emitted will get consumed by a downstream operator. An operator
+also cannot predict the exact time when a tuple arrives from an upstream
+operator. The only guarantee is that the upstream operators are
+processing the current or a future window, i.e. the windowId of upstream
+operator is equals or exceeds its own windowId. Conversely the windowId
+of a downstream operator is less than or equals its own windowId. The
+end of a window operation, i.e. the API call to endWindow on an operator
+requires that all upstream operators have finished processing this
+window. This means that completion of processing a window propagates in
+a blocking fashion through an operator. Later sections provides more
+details on streams and data flow of tuples.
+
+
+
+Each operator has a unique name within the DAG as provided by the
+user. This is the name of the operator in the logical plan. The name of
+the operator in the physical plan is an integer assigned to it by STRAM.
+These integers are use the sequence from 1 to N, where N is total number
+of physically unique operators in the DAG.  Following the same rule,
+each partitioned instance of a logical operator has its own integer as
+an id. This id along with the Hadoop container name uniquely identifies
+the operator in the execution plan of the DAG. The logical names and the
+physical names are required for web service support. Operators can be
+accessed via both names. These same names are used while interacting
+with  dtcli to access an operator.
+Ideally these names should be self-descriptive. For example in Figure 1,
+the node named “Daily volume” has a physical identifier of 2.
+
+### Operator Interface
+
+Operator interface in a DAG consists of [ports](#h.ihv636)[,](#h.ihv636) [properties](#h.32hioqz)[,](#h.32hioqz) and
+ [attributes](#h.41mghml)
+[.](#h.41mghml) Operators interact with other
+components of the DAG via ports. Functional behavior of the operators
+can be customized via parameters. Run time performance and physical
+instantiation is controlled by attributes. Ports and parameters are
+fields (variables) of the Operator class/object, while attributes are
+meta information that is attached to the operator object via an
+AttributeMap. An operator must have at least one port. Properties are
+optional. Attributes are provided by the platform and always have a
+default value that enables normal functioning of operators.
+
+#### Ports
+
+Ports are connection points by which an operator receives and
+emits tuples. These should be transient objects instantiated in the
+operator object, that implement particular interfaces. Ports should be
+transient as they contain no state. They have a pre-defined schema and
+can only be connected to other ports with the same schema. An input port
+needs to implement the interface  Operator.InputPort and
+interface Sink. A default
+implementation of these is provided by the abstract class DefaultInputPort. An output port needs to
+implement the interface  Operator.OutputPort. A default implementation
+of this is provided by the concrete class DefaultOutputPort. These two are a quick way to
+implement the above interfaces, but operator developers have the option
+of providing their own implementations.
+
+
+
+Here are examples of an input and an output port from the operator
+Sum.
+
+
+
+```java
+@InputPortFieldAnnotation(name = "data")
+public final transient DefaultInputPort<V> data = new DefaultInputPort<V>() {
+  @Override
+  public void process(V tuple)
+  {
+  	...
+  }
+}
+@OutputPortFieldAnnotation(optional=true)
+public final transient DefaultOutputPort<V> sum = new DefaultOutputPort<V>(){ … };
+```
+
+
+
+
+The process call is in the Sink interface. An emit on an output
+port is done via emit(tuple) call. For the above example it would be
+sum.emit(t), where the type of t is the generic parameter V.
+
+
+
+There is no limit on how many ports an operator can have. However
+any operator must have at least one port. An operator with only one port
+is called an Input Adapter if it has no input port and an Output Adapter
+if it has no output port. These are special operators needed to get/read
+data from outside system/source into the application, or push/write data
+into an outside system/sink. These could be in Hadoop or outside of
+Hadoop. These two operators are in essence gateways for the streaming
+application to communicate with systems outside the application.
+
+
+
+Port connectivity can be validated during compile time by adding
+PortFieldAnnotations shown above. By default all ports have to be
+connected, to allow a port to go unconnected, you need to add
+“optional=true” to the annotation.
+
+
+
+Attributes can be specified for ports that affect the runtime
+behavior. An example of an attribute is parallel partition that specifes
+a parallel computation flow per partition. It is described in detail in
+the [Parallel
+Partitions](#h.3vac5uf)[ ](#h.3vac5uf)section.
+Another example is queue capacity that specifies the buffer size for the
+port. Details of attributes are covered in  Operation and Installation Guide.
+
+#### Properties
+
+Properties are the abstractions by which functional behavior of an
+operator can be customized. They should be non-transient objects
+instantiated in the operator object. They need to be non-transient since
+they are part of the operator state and re-construction of the operator
+object from its checkpointed state must restore the operator to the
+desired state. Properties are optional, i.e. an operator may or may not
+have properties; they are part of user code and their values are not
+interpreted by the platform in any way.
+
+
+
+All non-serializable objects should be declared transient.
+Examples include sockets, session information, etc. These objects should
+be initialized during setup call, which is called every time the
+operator is initialized.
+
+#### Attributes
+
+Attributes are values assigned to the operators that impact
+run-time. This includes things like the number of partitions, at most
+once or at least once or exactly once recovery modes, etc. Attributes do
+not impact functionality of the operator. Users can change certain
+attributes in runtime. Users cannot add attributes to operators; they
+are pre-defined by the platform. They are interpreted by the platform
+and thus cannot be defined in user created code (like properties).
+Details of attributes are covered in  [Operation and Installation Guide](http://docs.google.com/OperationandInstallationGuide.html)
+.
+
+### Operator State
+
+The state of an operator is defined as the data that it transfers
+from one window to a future window. Since the computing model of the
+platform is to treat windows like micro-batches, the operator state can
+be [checkpointed](#h.3mzq4wv)[ ](#h.3mzq4wv)every
+Nth window, or every T units of time, where T is significantly greater
+than the streaming window.  When an operator is checkpointed, the entire
+object is written to HDFS.  The larger the amount of state in an
+operator, the longer it takes to recover from a failure. A stateless
+operator can recover much quicker than a stateful one. The needed
+windows are preserved by the upstream buffer server and are used to
+recompute the lost windows, and also rebuild the buffer server in the
+current container.
+
+
+
+The distinction between Stateless and Stateful is based solely on
+the need to transfer data in the operator from one window to the next.
+The state of an operator is independent of the number of ports.
+
+#### Stateless
+
+A Stateless operator is defined as one where no data is needed to
+be kept at the end of every window. This means that all the computations
+of a window can be derived from all the tuples the operator receives
+within that window. This guarantees that the output of any window can be
+reconstructed by simply replaying the tuples that arrived in that
+window. Stateless operators are more efficient in terms of fault
+tolerance, and cost to achieve SLA.
+
+#### Stateful
+
+A Stateful operator is defined as one where data is needed to be
+stored at the end of a window for computations occurring in later
+window; a common example is the computation of a sum of values in the
+input tuples.
+
+### Operator API
+
+The Operator API consists of methods that operator developers may
+need to override. In this section we will discuss the Operator APIs from
+the point of view of an application developer. Knowledge of how an
+operator works internally is critical for writing an application. Those
+interested in the details should refer to  Malhar Operator Developer Guide.
+
+
+
+The APIs are available in three modes, namely Single Streaming
+Window, Sliding Application Window, and Aggregate Application Window.
+These are not mutually exclusive, i.e. an operator can use single
+streaming window as well as sliding application window. A physical
+instance of an operator is always processing tuples from a single
+window. The processing of tuples is guaranteed to be sequential, no
+matter which input port the tuples arrive on.
+
+
+
+In the later part of this section we will evaluate three common
+uses of streaming windows by applications. They have different
+characteristics and implications on optimization and recovery mechanisms
+(i.e. algorithm used to recover a node after outage) as discussed later
+in the section.
+
+#### Streaming Window
+
+Streaming window is atomic micro-batch computation period. The API
+methods relating to a streaming window are as follows
+
+
+
+[](#) [](#)
+
+<table>
+<colgroup>
+<col width="100%" />
+</colgroup>
+<tbody>
+<tr class="odd">
+<td align="left"><p>public void process(&lt;tuple_type&gt; tuple) // Called on the input port on which the tuple arrives</p>
+<p>public void beginWindow(long windowId) // Called at the start of the window as soon as the first begin_window tuple arrives</p>
+<p>public void endWindow() // Called at the end of the window after end_window tuples arrive on all input ports</p>
+<p>public void setup(OperatorContext context) // Called once during initialization of the operator</p>
+<p>public void teardown() // Called once when the operator is being shutdown</p></td>
+</tr>
+</tbody>
+</table>
+
+
+
+
+
+A tuple can be emitted in any of the three streaming run-time
+calls, namely beginWindow, process, and endWindow but not in setup or
+teardown.
+
+#### Aggregate Application Window
+
+An operator with an aggregate window is stateful within the
+application window timeframe and possibly stateless at the end of that
+application window. An size of an aggregate application window is an
+operator attribute and is defined as a multiple of the streaming window
+size. The platform recognizes this attribute and optimizes the operator.
+The beginWindow, and endWindow calls are not invoked for those streaming
+windows that do not align with the application window. For example in
+case of streaming window of 0.5 second and application window of 5
+minute, an application window spans 600 streaming windows (5\*60\*2 =
+600). At the start of the sequence of these 600 atomic streaming
+windows, a beginWindow gets invoked, and at the end of these 600
+streaming windows an endWindow gets invoked. All the intermediate
+streaming windows do not invoke beginWindow or endWindow. Bookkeeping,
+node recovery, stats, UI, etc. continue to work off streaming windows.
+For example if operators are being checkpointed say on an average every
+30th window, then the above application window would have about 20
+checkpoints.
+
+#### Sliding Application Window
+
+A sliding window is computations that requires previous N
+streaming windows. After each streaming window the Nth past window is
+dropped and the new window is added to the computation. An operator with
+sliding window is a stateful operator at end of any window. The sliding
+window period is an attribute and is a multiple of streaming window. The
+platform recognizes this attribute and leverages it during bookkeeping.
+A sliding aggregate window with tolerance to data loss does not have a
+very high bookkeeping cost. The cost of all three recovery mechanisms,
+ at most once (data loss tolerant),
+at least once (data loss
+intolerant), and exactly once (data
+loss intolerant and no extra computations) is same as recovery
+mechanisms based on streaming window. STRAM is not able to leverage this
+operator for any extra optimization.
+
+### Single vs Multi-Input Operator
+
+A single-input operator by definition has a single upstream
+operator, since there can only be one writing port for a stream.  If an
+operator has a single upstream operator, then the beginWindow on the
+upstream also blocks the beginWindow of the single-input operator. For
+an operator to start processing any window at least one upstream
+operator has to start processing that window. A multi-input operator
+reads from more than one upstream ports. Such an operator would start
+processing as soon as the first begin_window event arrives. However the
+window would not close (i.e. invoke endWindow) till all ports receive
+end\_window events for that windowId. Thus the end of a window is a
+blocking event. As we saw earlier, a multi-input operator is also the
+point in the DAG where windows of all upstream operators are
+synchronized. The windows (atomic micro-batches) from a faster (or just
+ahead in processing) upstream operators are queued up till the slower
+upstream operator catches up. STRAM monitors such bottlenecks and takes
+corrective actions. The platform ensures minimal delay, i.e processing
+starts as long as at least one upstream operator has started
+processing.
+
+### Recovery Mechanisms
+
+Application developers can set any of the recovery mechanisms
+below to deal with node outage. In general, the cost of recovery depends
+on the state of the operator, while data integrity is dependant on the
+application. The mechanisms are per window as the platform treats
+windows as atomic compute units. Three recovery mechanisms are
+supported, namely
+
+-   At-least-once: All atomic batches are processed at least once.
+    No data loss occurs.
+-   At-most-once: All atomic batches are processed at most once.
+    Data loss is possible; this is the most efficient setting.
+-   Exactly-once: All atomic batches are processed exactly once.
+    No data loss occurs; this is the least efficient setting since
+    additional work is needed to ensure proper semantics.
+
+At-least-once is the default. During a recovery event, the
+operator connects to the upstream buffer server and asks for windows to
+be replayed. At-least-once and exactly-once mechanisms start from its
+checkpointed state. At-most-once starts from the next begin-window
+event.
+
+
+
+Recovery mechanisms can be specified per Operator while writing
+the application as shown below.
+
+
+
+Operator o = dag.addOperator(“operator”, …);
+
+dag.setAttribute(o,
+
+                 OperatorContext.PROCESSING\_MODE,
+
+
+               
+ ProcessingMode.AT\_MOST\_ONCE);
+
+
+
+Also note that once an operator is attributed to AT\_MOST\_ONCE,
+all the operators downstream to it have to be AT\_MOST\_ONCE. The client
+will give appropriate warnings or errors if that’s not the case.
+
+
+
+Details are explained in the chapter on Fault Tolerance
+below[.](#h.2nusc19)
+
+Streams
+--------------------
+
+A stream is a connector
+(edge) abstraction, and is a fundamental building block of the platform.
+A stream consists of tuples that flow from one port (called the
+output port) to one or more ports
+on other operators (called  input ports) another -- so note a potentially
+confusing aspect of this terminology: tuples enter a stream through its
+output port and leave via one or more input ports. A stream has the
+following characteristics
+
+-   Tuples are always delivered in the same order in which they
+    were emitted.
+-   Consists of a sequence of windows one after another. Each
+    window being a collection of in-order tuples.
+-   A stream that connects two containers passes through a
+    buffer server.
+-   All streams can be persisted (by default in HDFS).
+-   Exactly one output port writes to the stream.
+-   Can be read by one or more input ports.
+-   Connects operators within an application, not outside
+    an application.
+-   Has an unique name within an application.
+-   Has attributes which act as hints to STRAM.
+-   Streams have four modes, namely in-line, in-node, in-rack,
+    and other. Modes may be overruled (for example due to lack
+    of containers). They are defined as follows:
+
+<!-- -->
+
+-   THREAD\_LOCAL: In the same thread, uses thread
+    stack (intra-thread). This mode can only be used for a downstream
+    operator which has only one input port connected; also called
+    in-line.
+-   CONTAINER\_LOCAL: In the same container (intra-process); also
+    called in-container.
+-   NODE\_LOCAL: In the same Hadoop node (inter processes, skips
+    NIC); also called in-node.
+-   RACK\_LOCAL: On nodes in the same rack; also called
+    in-rack.
+-   unspecified: No guarantee. Could be anywhere within the
+    cluster
+
+
+
+An example of a stream declaration is given below
+
+
+
+[](#) [](#)
+
+<table>
+<colgroup>
+<col width="100%" />
+</colgroup>
+<tbody>
+<tr class="odd">
+<td align="left"><p>DAG dag = new DAG();</p>
+<p> …</p>
+<p>dag.addStream(&quot;views&quot;, viewAggregate.sum, cost.data).setLocality(CONTAINER_LOCAL); // A container local  stream</p>
+<p>dag.addStream(“clicks”, clickAggregate.sum, rev.data); // An example of unspecified locality</p></td>
+</tr>
+</tbody>
+</table>
+
+
+
+The platform guarantees in-order delivery of tuples in a stream.
+STRAM views each stream as collection of ordered windows. Since no tuple
+can exist outside a window, a replay of a stream consists of replay of a
+set of windows. When multiple input ports read the same stream, the
+execution plan of a stream ensures that each input port is logically not
+blocked by the reading of another input port. The schema of a stream is
+same as the schema of the tuple.
+
+
+
+In a stream all tuples emitted by an operator in a window belong
+to that window. A replay of this window would consists of an in-order
+replay of all the tuples. Thus the tuple order within a stream is
+guaranteed. However since an operator may receive multiple streams (for
+example an operator with two input ports), the order of arrival of two
+tuples belonging to different streams is not guaranteed. In general in
+an asynchronous distributed architecture this is expected. Thus the
+operator (specially one with multiple input ports) should not depend on
+the tuple order from two streams. One way to cope with this
+indeterminate order, if necessary, is to wait to get all the tuples of a
+window and emit results in endWindow call. All operator templates
+provided as part of  [standard operator template
+library](#h.3ep43zb) [ ](#h.3ep43zb)follow
+these principles.
+
+
+
+A logical stream gets partitioned into physical streams each
+connecting the partition to the upstream operator. If two different
+attributes are needed on the same stream, it should be split using
+StreamDuplicator operator.
+
+
+
+Modes of the streams are critical for performance. An in-line
+stream is the most optimal as it simply delivers the tuple as-is without
+serialization-deserialization. Streams should be marked
+container\_local, specially in case where there is a large tuple volume
+between two operators which then on drops significantly. Since the
+setLocality call merely provides a hint, STRAM may ignore it. An In-node
+stream is not as efficient as an in-line one, but it is clearly better
+than going off-node since it still avoids the potential bottleneck of
+the network card.
+
+
+
+THREAD\_LOCAL and CONTAINER\_LOCAL streams do not use a buffer
+server as this stream is in a single process. The other two do.
+
+Validating an Application
+--------------------------------------
+
+The platform provides various ways of validating the application
+specification and data input. An understanding of these checks is very
+important for an application developer since it affects productivity.
+Validation of an application is done in three phases, namely
+
+
+
+1.  Compile Time: Caught during application development, and is
+    most cost effective. These checks are mainly done on declarative
+    objects and leverages the Java compiler. An example is checking that
+    the schemas specified on all ports of a stream are
+    mutually compatible.
+2.  Initialization Time: When the application is being
+    initialized, before submitting to Hadoop. These checks are related
+    to configuration/context of an application, and are done by the
+    logical DAG builder implementation. An example is the checking that
+    all non-optional ports are connected to other ports.
+3.  Run Time: Validations done when the application is running.
+    This is the costliest of all checks. These are checks that can only
+    be done at runtime as they involve data. For example divide by 0
+    check as part of business logic.
+
+### Compile Time
+
+Compile time validations apply when an application is specified in
+Java code and include all checks that can be done by Java compiler in
+the development environment (including IDEs like NetBeans or Eclipse).
+Examples include
+
+1.  Schema Validation: The tuples on ports are POJO (plain old
+    java objects) and compiler checks to ensure that all the ports on a
+    stream have the same schema.
+2.  Stream Check: Single Output Port and at least one Input port
+    per stream. A stream can only have one output port writer. This is
+    part of the addStream api. This
+    check ensures that developers only connect one output port to
+    a stream. The same signature also ensures that there is at least one
+    input port for a stream
+3.  Naming: Compile time checks ensures that applications
+    components operators, streams are named
+
+### Initialization/Instantiation Time
+
+Initialization time validations include various checks that are
+done post compile, and before the application starts running in a
+cluster (or local mode). These are mainly configuration/contextual in
+nature. These checks are as critical to proper functionality of the
+application as the compile time validations.
+
+
+
+Examples include
+
+-   [JavaBeans
+    Validation](http://docs.oracle.com/javaee/6/tutorial/doc/gircz.html):
+    Examples include
+
+<!-- -->
+
+-   @Max(): Value must be less than or equal to the number
+
+<!-- -->
+
+-   @Min(): Value must be greater than or equal to the
+    number
+-   @NotNull: The value of the field or property must not be
+    null
+-   @Pattern(regexp = “....”): Value must match the regular
+    expression
+-   Input port connectivity: By default, every non-optional input
+    port must be connected. A port can be declared optional by using an
+    annotation:     @InputPortFieldAnnotation(name = "...", optional
+    = true)
+-   Output Port Connectivity: Similar. The annotation here is:    
+    @OutputPortFieldAnnotation(name = "...", optional = true)
+
+<!-- -->
+
+-   Unique names in application scope: Operators, streams, must have
+    unique names.
+-   Cycles in the dag: DAG cannot have a cycle.
+-   Unique names in operator scope: Ports, properties, annotations
+    must have unique names.
+-   One stream per port: A port can connect to only one stream.
+    This check applies to input as well as output ports even though an
+    output port can technically write to two streams. If you must have
+    two streams originating from a single output port, use  a streamDuplicator operator.
+-   Application Window Period: Has to be an integral multiple the
+    streaming window period.
+
+### Run Time
+
+Run time checks are those that are done when the application is
+running. The real-time streaming platform provides rich run time error
+handling mechanisms. The checks are exclusively done by the application
+business logic, but the platform allows applications to count and audit
+these. Some of these features are in the process of development (backend
+and UI) and this section will be updated as they are developed. Upon
+completion examples will be added to  [demos](#h.upglbi) [t](#h.upglbi)o
+illustrate these.
+
+
+
+Error ports are output ports with error annotations. Since they
+are normal ports, they can be monitored and tuples counted, persisted
+and counts shown in the UI.
+
+------------------------------------------------------------------------
+
+
+
+
+
+Multi-Tenancy and Security
+=======================================
+
+Hadoop is a multi-tenant distributed operating system. Security is
+an intrinsic element of multi-tenancy as without it a cluster cannot be
+reasonably be shared among enterprise applications. Streaming
+applications follow all multi-tenancy security models used in Hadoop as
+they are native Hadoop applications. For details refer to the
+[Operation and Installation
+Guide](https://www.datatorrent.com/docs/guides/OperationandInstallationGuide.html)
+.
+
+Security
+---------------------
+
+The platform includes Kerberos support. Both access points, namely
+STRAM and Bufferserver are secure. STRAM passes the token over to
+StreamingContainer, which then gives it to the Bufferserver. The most
+important aspect for an application developer is to note that STRAM is
+the single point of access to ensure security measures are taken by all
+components of the platform.
+
+Resource Limits
+----------------------------
+
+Hadoop enforces quotas on resources. This includes hard-disk (name
+space and total disk quota) as well as priority queues for schedulers.
+The platform uses Hadoop resource limits to manage a streaming
+application. In addition network I/O quotas can be enforced. An operator
+can be dynamically partitioned if it reaches its resource limits; these
+limits may be expressed in terms of throughput, latency, or just
+aggregate resource utilization of a container.
+
+
+
+
+
+------------------------------------------------------------------------
+
+
+
+
+
+Scalability and Partitioning
+=========================================
+
+Scalability is a foundational element of this platform and is a
+building block for an eco-system where big-data meets real-time.
+Enterprises need to continually meet SLA as data grows. Without the
+ability to scale as load grows, or new applications with higher loads
+come to fruition, enterprise grade SLA cannot be met. A big issue with
+the streaming application space is that, it is not just about high load,
+but also the fluctuations in it. There is no way to guarantee future
+load requirements and there is a big difference between high and low
+load within a day for the same feed. Traditional streaming platforms
+solve these two cases by simply throwing more hardware at the
+problem.
+
+
+
+Daily spikes are managed by ensuring enough hardware for peak
+load, which then idles during low load, and future needs are handled by
+a very costly re-architecture, or investing heavily in building a
+scalable distributed operating system. Another salient and often
+overlooked cost is the need to manage SLA -- let’s call it  buffer capacity. Since this means computing the
+peak load within required time, that translates to allocating enough
+resources over and above peak load as daily peaks fluctuate. For example
+an average peak load of 100 resource units (cpu and/or memory and/or
+network) may mean allocating about 200 resource units to be safe. A
+distributed cluster that cannot dynamically scale up and down, in effect
+pays buffer capacity per application. Another big aspect of streaming
+applications is that the load is not just ingestion rate, more often
+than not, the internal operators produce lot more events than the
+ingestion rate. For example a dimensional data (with, say  d dimensions) computation needs 2\*d -1 computations per ingested event. A lot
+of applications have over 10 dimensions, i.e over 1000 computations per
+incoming event and these need to be distributed across the cluster,
+thereby causing an explosion in the throughput (events/sec) that needs
+to be managed.
+
+
+
+The platform is designed to handle such cases at a very low cost.
+The platform scales linearly with Hadoop. If applications need more
+resources, the enterprise can simply add more commodity nodes to Hadoop
+without any downtime, and the Hadoop native platform will take care of
+the rest. If some nodes go bad, these can be removed without downtime.
+The daily peaks and valleys in the load are managed by the platform by
+dynamically scaling at the peak and then giving the resources back to
+Hadoop during low load. This means that a properly designed Hadoop
+cluster does several things for enterprises: (a) reduces the cost of
+hardware due to use of commodity hardware (b) shares buffer capacity
+across all applications as peaks of all applications may not align and
+(c) raises the average CPU usage on a 24x7 basis. As a general design
+this is similar to scale that a map-reduce application can deliver. In
+the following sections of this chapter we will see how this is
+done.
+
+Partitioning
+-------------------------
+
+If all tuples sent through the stream(s) that are connected to the
+input port(s) of an operator in the DAG are received by a single
+physical instance of that operator, that operator can become a
+performance bottleneck. This leads to scalability issues when
+throughput, memory, or CPU needs exceed the processing capacity of that
+single instance.
+
+
+
+To address the problem, the platform offers the capability to
+partition the inflow of data so that it is divided across multiple
+physical instances of a logical operator in the DAG. There are two
+functional ways to partition
+
+-   Load balance: Incoming load is simply partitioned
+    into stream(s) that go to separate instances of physical operators
+    and scalability is achieved via adding more physical operators. Each
+    tuple is sent to physical operator (partition) based on a
+    round-robin or other similar algorithm. This scheme scales linearly.
+    A lot of key based computations can load balance in the platform due
+    to the ability to insert  Unifiers. For many computations, the
+    endWindow and Unifier setup is similar to the combiner and reducer
+    mechanism in a Map-Reduce computation.
+-   Sticky Key: The key assertion is that distribution of tuples
+    are sticky, i.e the data with
+    same key will always be processed by the same physical operator, no
+    matter how many times it is sent through the stream. This stickiness
+    will continue even if the number of partitions grows dynamically and
+    can eventually be leveraged for advanced features like
+    bucket testing. How this is accomplished and what is required to
+    develop compliant operators will be explained below.
+
+
+
+We plan to add more partitioning mechanisms proactively to the
+platform over time as needed by emerging usage patterns. The aim is to
+allow enterprises to be able to focus on their business logic, and
+significantly reduce the cost of operability. As an enabling technology
+for managing high loads, this platform provides enterprises with a
+significant innovative edge. Scalability and Partitioning is a
+foundational building block for this platform.
+
+### Sticky Partition vs Round Robin
+
+As noted above, partitioning via sticky key is data aware but
+round-robin partitioning is not. An example for non-sticky load
+balancing would be round robin distribution over multiple instances,
+where for example a tuple stream of  A, A,
+A with 3 physical operator
+instances would result in processing of a single A by each of the instances, In contrast, sticky
+partitioning means that exactly one instance of the operators will
+process all of the  Atuples if they
+fall into the same bucket, while B
+may be processed by another operator. Data aware mapping of
+tuples to partitions (similar to distributed hash table) is accomplished
+via Stream Codecs. In later sections we would show how these two
+approaches can be used in combination.
+
+### Stream Codec
+
+The platform does not make assumptions about the tuple
+type, it could be any Java object. The operator developer knows what
+tuple type an input port expects and is capable of processing. Each
+input port has a stream codec  associated thatdefines how data is serialized when transmitted over a socket
+stream; it also defines another
+function that computes the partition hash key for the tuple. The engine
+uses that key to determine which physical instance(s)  (for a
+partitioned operator) receive that  tuple. For this to work, consistent hashing is required.
+The default codec uses the Java Object\#hashCode function, which is
+sufficient for basic types such as Integer, String etc. It will also
+work with custom tuple classes as long as they implement hashCode
+appropriately. Reliance on hashCode may not work when generic containers
+are used that do not hash the actual data, such as standard collection
+classes (HashMap etc.), in which case a custom stream codec must be
+assigned to the input port.
+
+### Static Partitioning
+
+DAG designers can specify at design time how they would like
+certain operators to be partitioned. STRAM then instantiates the DAG
+with the physical plan which adheres to the partitioning scheme defined
+by the design. This plan is the initial partition of the application. In
+other words, Static Partitioning is used to tell STRAM to compute the
+physical DAG from a logical DAG once, without taking into consideration
+runtime states or loads of various operators.
+
+### Dynamic Partitioning
+
+In streaming applications the load changes during the day, thus
+creating situations where the number of partitioned operator instances
+needs to adjust dynamically. The load can be measured in terms of
+processing within the

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