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Subject svn commit: r1767604 [6/10] - in /helix/site-content: ./ 0.6.1-incubating-docs/ 0.6.2-incubating-docs/ 0.6.3-docs/ 0.6.4-docs/ 0.6.5-docs/ 0.6.6-docs/ 0.6.6-docs/css/ 0.6.6-docs/images/ 0.6.6-docs/img/ 0.6.6-docs/js/ 0.6.6-docs/recipes/ 0.6.6-docs/rele...
Date Wed, 02 Nov 2016 04:11:37 GMT
Added: helix/site-content/0.6.6-docs/recipes/rsync_replicated_file_store.html
URL: http://svn.apache.org/viewvc/helix/site-content/0.6.6-docs/recipes/rsync_replicated_file_store.html?rev=1767604&view=auto
==============================================================================
--- helix/site-content/0.6.6-docs/recipes/rsync_replicated_file_store.html (added)
+++ helix/site-content/0.6.6-docs/recipes/rsync_replicated_file_store.html Wed Nov  2 04:11:36 2016
@@ -0,0 +1,359 @@
+
+<!DOCTYPE html>
+<!--
+ Generated by Apache Maven Doxia at 2016-11-01
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+		<title>Apache Helix - Near-Realtime Rsync Replicated File System</title>
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+				<li>Near-Realtime Rsync Replicated File System</li>
+				<li class="publishDate version-date pull-right">Last Published: 2016-11-01</li>
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+<div class="section"> 
+ <div class="page-header">
+  <h2 id="Near-Realtime_Rsync_Replicated_File_System">Near-Realtime Rsync Replicated File System</h2>
+ </div> 
+ <div class="section"> 
+  <h3 id="Quick_Demo">Quick Demo</h3> 
+  <ul> 
+   <li>This demo starts 3 instances with id’s as <tt>localhost_12001, localhost_12002, localhost_12003</tt></li> 
+   <li>Each instance stores its files under <tt>/tmp/&lt;id&gt;/filestore</tt></li> 
+   <li><tt>localhost_12001</tt> is designated as the master, and <tt>localhost_12002</tt> and <tt>localhost_12003</tt> are the slaves</li> 
+   <li>Files written to the master are replicated to the slaves automatically. In this demo, a.txt and b.txt are written to <tt>/tmp/localhost_12001/filestore</tt> and they get replicated to other folders.</li> 
+   <li>When the master is stopped, <tt>localhost_12002</tt> is promoted to master.</li> 
+   <li>The other slave <tt>localhost_12003</tt> stops replicating from <tt>localhost_12001</tt> and starts replicating from new master <tt>localhost_12002</tt></li> 
+   <li>Files written to new master <tt>localhost_12002</tt> are replicated to <tt>localhost_12003</tt></li> 
+   <li>In the end state of this quick demo, <tt>localhost_12002</tt> is the master and <tt>localhost_12003</tt> is the slave. Manually create files under <tt>/tmp/localhost_12002/filestore</tt> and see that appear in <tt>/tmp/localhost_12003/filestore</tt></li> 
+   <li>Ignore the interrupted exceptions on the console :-)</li> 
+  </ul> 
+  <div class="source"> 
+   <pre>git clone https://git-wip-us.apache.org/repos/asf/helix.git
+cd helix
+git checkout tags/helix-0.6.6
+cd recipes/rsync-replicated-file-system/
+mvn clean install package -DskipTests
+cd target/rsync-replicated-file-system-pkg/bin
+chmod +x *
+./quickdemo
+
+</pre> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Overview">Overview</h3> 
+  <p>There are many applications that require storage for storing large number of relatively small data files. Examples include media stores to store small videos, images, mail attachments etc. Each of these objects is typically kilobytes, often no larger than a few megabytes. An additional distinguishing feature of these use cases is that files are typically only added or deleted, rarely updated. When there are updates, they do not have any concurrency requirements.</p> 
+  <p>These are much simpler requirements than what general purpose distributed file system have to satisfy; these would include concurrent access to files, random access for reads and updates, posix compliance, and others. To satisfy those requirements, general DFSs are also pretty complex that are expensive to build and maintain.</p> 
+  <p>A different implementation of a distributed file system includes HDFS which is inspired by Google’s GFS. This is one of the most widely used distributed file system that forms the main data storage platform for Hadoop. HDFS is primary aimed at processing very large data sets and distributes files across a cluster of commodity servers by splitting up files in fixed size chunks. HDFS is not particularly well suited for storing a very large number of relatively tiny files.</p> 
+ </div> 
+ <div class="section"> 
+  <h3 id="File_Store">File Store</h3> 
+  <p>It’s possible to build a vastly simpler system for the class of applications that have simpler requirements as we have pointed out.</p> 
+  <ul> 
+   <li>Large number of files but each file is relatively small</li> 
+   <li>Access is limited to create, delete and get entire files</li> 
+   <li>No updates to files that are already created (or it’s feasible to delete the old file and create a new one)</li> 
+  </ul> 
+  <p>We call this system a Partitioned File Store (PFS) to distinguish it from other distributed file systems. This system needs to provide the following features:</p> 
+  <ul> 
+   <li>CRD access to large number of small files</li> 
+   <li>Scalability: Files should be distributed across a large number of commodity servers based on the storage requirement</li> 
+   <li>Fault-tolerance: Each file should be replicated on multiple servers so that individual server failures do not reduce availability</li> 
+   <li>Elasticity: It should be possible to add capacity to the cluster easily</li> 
+  </ul> 
+  <p>Apache Helix is a generic cluster management framework that makes it very easy to provide scalability, fault-tolerance and elasticity features. rsync can be easily used as a replication channel between servers so that each file gets replicated on multiple servers.</p> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Design">Design</h3> 
+  <div class="section"> 
+   <h4 id="High_Level">High Level</h4> 
+   <ul> 
+    <li>Partition the file system based on the file name</li> 
+    <li>At any time a single writer can write, we call this a master</li> 
+    <li>For redundancy, we need to have additional replicas called slave. Slaves can optionally serve reads</li> 
+    <li>Slave replicates data from the master</li> 
+    <li>When a master fails, a slave gets promoted to master</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Transaction_Log">Transaction Log</h4> 
+   <p>Every write on the master will result in creation/deletion of one or more files. In order to maintain timeline consistency slaves need to apply the changes in the same order To facilitate this, the master logs each transaction in a file and each transaction is associated with an 64 bit ID in which the 32 LSB represents a sequence number and MSB represents the generation number The sequence number gets incremented on every transaction and the generation is incremented when a new master is elected</p> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Replication">Replication</h4> 
+   <p>Replication is required for slaves to keep up with changes on the master. Every time the slave applies a change it checkpoints the last applied transaction ID. During restarts, this allows the slave to pull changes from the last checkpointed ID. Similar to master, the slave logs each transaction to the transaction logs but instead of generating new transaction ID, it uses the same ID generated by the master.</p> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Failover">Failover</h4> 
+   <p>When a master fails, a new slave will be promoted to master. If the previous master node is reachable, then the new master will flush all the changes from previous the master before taking up mastership. The new master will record the end transaction ID of the current generation and then start a new generation with sequence starting from 1. After this the master will begin accepting writes.</p> 
+   <p><img src="../images/PFS-Generic.png" alt="Partitioned File Store" /></p> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Rsync-based_Solution">Rsync-based Solution</h3> 
+  <p><img src="../images/RSYNC_BASED_PFS.png" alt="Rsync based File Store" /></p> 
+  <p>This application demonstrates a file store that uses rsync as the replication mechanism. One can envision a similar system where instead of using rsync, one can implement a custom solution to notify the slave of the changes and also provide an api to pull the change files.</p> 
+  <div class="section"> 
+   <h4 id="Concepts">Concepts</h4> 
+   <ul> 
+    <li>file_store_dir: Root directory for the actual data files</li> 
+    <li>change_log_dir: The transaction logs are generated under this folder</li> 
+    <li>check_point_dir: The slave stores the check points ( last processed transaction) here</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Master">Master</h4> 
+   <ul> 
+    <li>File server: This component supports file uploads and downloads and writes the files to <tt>file_store_dir</tt>. This is not included in this application. The idea is that most applications have different ways of implementing this component and have some associated business logic. It is not hard to come up with such a component if needed.</li> 
+    <li>File store watcher: This component watches the <tt>file_store_dir</tt> directory on the local file system for any changes and notifies the registered listeners of the changes</li> 
+    <li>Change log generator: This registers as a listener of the file store watcher and on each notification logs the changes into a file under <tt>change_log_dir</tt></li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Slave">Slave</h4> 
+   <ul> 
+    <li>File server: This component on the slave will only support reads</li> 
+    <li>Cluster state observer: Slave observes the cluster state and is able to know who is the current master</li> 
+    <li>Replicator: This has two subcomponents 
+     <ul> 
+      <li>Periodic rsync of change log: This is a background process that periodically rsyncs the <tt>change_log_dir</tt> of the master to its local directory</li> 
+      <li>Change Log Watcher: This watches the <tt>change_log_dir</tt> for changes and notifies the registered listeners of the change</li> 
+      <li>On demand rsync invoker: This is registered as a listener to change log watcher and on every change invokes rsync to sync only the changed file</li> 
+     </ul></li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Coordination">Coordination</h4> 
+   <p>The coordination between nodes is done by Helix. Helix does the partition management and assigns the partition to multiple nodes based on the replication factor. It elects one the nodes as master and designates others as slaves. It provides notifications to each node in the form of state transitions (Offline to Slave, Slave to Master). It also provides notifications when there is change is cluster state. This allows the slave to stop replicating from current master and start replicating from new master.</p> 
+   <p>In this application, we have only one partition but its very easy to extend it to support multiple partitions. By partitioning the file store, one can add new nodes and Helix will automatically re-distribute partitions among the nodes. To summarize, Helix provides partition management, fault tolerance and facilitates automated cluster expansion.</p> 
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Added: helix/site-content/0.6.6-docs/recipes/service_discovery.html
URL: http://svn.apache.org/viewvc/helix/site-content/0.6.6-docs/recipes/service_discovery.html?rev=1767604&view=auto
==============================================================================
--- helix/site-content/0.6.6-docs/recipes/service_discovery.html (added)
+++ helix/site-content/0.6.6-docs/recipes/service_discovery.html Wed Nov  2 04:11:36 2016
@@ -0,0 +1,404 @@
+
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+				<li class="divider">/</li>
+				<li>Service Discovery</li>
+				<li class="publishDate version-date pull-right">Last Published: 2016-11-01</li>
+			</ul>
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+	</header>
+
+	<div class="main-body">
+	<div class="row">
+		<div class="span12">
+			<div class="body-content">
+<!-- -
+Licensed to the Apache Software Foundation (ASF) under one
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+<div class="section"> 
+ <div class="page-header">
+  <h2 id="Service_Discovery">Service Discovery</h2>
+ </div> 
+ <p>One of the common usage of ZooKeeper is to enable service discovery. The basic idea is that when a server starts up it advertises its configuration/metadata such as its hostname and port on ZooKeeper. This allows clients to dynamically discover the servers that are currently active. One can think of this like a service registry to which a server registers when it starts and is automatically deregistered when it shutdowns or crashes. In many cases it serves as an alternative to VIPs.</p> 
+ <p>The core idea behind this is to use ZooKeeper ephemeral nodes. The ephemeral nodes are created when the server registers and all its metadata is put into a ZNode. When the server shutdowns, ZooKeeper automatically removes this ZNode.</p> 
+ <p>There are two ways the clients can dynamically discover the active servers:</p> 
+ <div class="section"> 
+  <h3 id="ZooKeeper_Watch">ZooKeeper Watch</h3> 
+  <p>Clients can set a child watch under specific path on ZooKeeper. When a new service is registered/deregistered, ZooKeeper notifies the client via a watch event and the client can read the list of services. Even though this looks trivial, there are lot of things one needs to keep in mind like ensuring that you first set the watch back on ZooKeeper before reading data.</p> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Poll">Poll</h3> 
+  <p>Another approach is for the client to periodically read the ZooKeeper path and get the list of services.</p> 
+  <p>Both approaches have pros and cons, for example setting a watch might trigger herd effect if there are large number of clients. This is problematic, especially when servers are starting up. But the advantage to setting watches is that clients are immediately notified of a change which is not true in case of polling. In some cases, having both watches and polls makes sense; watch allows one to get notifications as soon as possible while poll provides a safety net if a watch event is missed because of code bug or ZooKeeper fails to notify.</p> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Other_Developer_Considerations">Other Developer Considerations</h3> 
+  <ul> 
+   <li>What happens when the ZooKeeper session expires? All the watches and ephemeral nodes previously added or created by this server are lost. One needs to add the watches again, recreate the ephemeral nodes, and so on.</li> 
+   <li>Due to network issues or Java GC pauses session expiry might happen again and again; this phenomenon is known as flapping. It's important for the server to detect this and deregister itself.</li> 
+  </ul> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Other_Operational_Considerations">Other Operational Considerations</h3> 
+  <ul> 
+   <li>What if the node is behaving badly? One might kill the server, but it will lose the ability to debug. It would be nice to have the ability to mark a server as disabled and clients know that a node is disabled and will not contact that node.</li> 
+  </ul> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Configuration_Ownership">Configuration Ownership</h3> 
+  <p>This is an important aspect that is often ignored in the initial stages of your development. Typically, the service discovery pattern means that servers start up with some configuration which it simply puts into ZooKeeper. While this works well in the beginning, configuration management becomes very difficult since the servers themselves are statically configured. Any change in server configuration implies restarting the server. Ideally, it will be nice to have the ability to change configuration dynamically without having to restart a server.</p> 
+  <p>Ideally you want a hybrid solution, a node starts with minimal configuration and gets the rest of configuration from ZooKeeper.</p> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Using_Helix_for_Service_Discovery">Using Helix for Service Discovery</h3> 
+  <p>Even though Helix has a higher-level abstraction in terms of state machines, constraints and objectives, service discovery is one of things has been a prevalent use case from the start. The controller uses the exact mechanism we described above to discover when new servers join the cluster. We create these ZNodes under /CLUSTERNAME/LIVEINSTANCES. Since at any time there is only one controller, we use a ZK watch to track the liveness of a server.</p> 
+  <p>This recipe simply demonstrates how one can re-use that part for implementing service discovery. This demonstrates multiple modes of service discovery:</p> 
+  <ul> 
+   <li>POLL: The client reads from zookeeper at regular intervals 30 seconds. Use this if you have 100’s of clients</li> 
+   <li>WATCH: The client sets up watcher and gets notified of the changes. Use this if you have 10’s of clients</li> 
+   <li>NONE: This does neither of the above, but reads directly from zookeeper when ever needed</li> 
+  </ul> 
+  <p>Helix provides these additional features compared to other implementations available elsewhere:</p> 
+  <ul> 
+   <li>It has the concept of disabling a node which means that a badly behaving node can be disabled using the Helix admin API</li> 
+   <li>It automatically detects if a node connects/disconnects from zookeeper repeatedly and disables the node</li> 
+   <li>Configuration management 
+    <ul> 
+     <li>Allows one to set configuration via the admin API at various granulaties like cluster, instance, resource, partition</li> 
+     <li>Configurations can be dynamically changed</li> 
+     <li>The server is notified when configurations change</li> 
+    </ul></li> 
+  </ul> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Checkout_and_Build">Checkout and Build</h3> 
+  <div class="source"> 
+   <pre>git clone https://git-wip-us.apache.org/repos/asf/helix.git
+cd helix
+git checkout tags/helix-0.6.6
+mvn clean install package -DskipTests
+cd recipes/service-discovery/target/service-discovery-pkg/bin
+chmod +x *
+</pre> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Start_ZooKeeper">Start ZooKeeper</h3> 
+  <div class="source"> 
+   <pre>./start-standalone-zookeeper 2199
+</pre> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Run_the_Demo">Run the Demo</h3> 
+  <div class="source"> 
+   <pre>./service-discovery-demo.sh
+</pre> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Output">Output</h3> 
+  <div class="source"> 
+   <pre>START:Service discovery demo mode:WATCH
+	Registering service
+		host.x.y.z_12000
+		host.x.y.z_12001
+		host.x.y.z_12002
+		host.x.y.z_12003
+		host.x.y.z_12004
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12000
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12002
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Deregistering service:
+		host.x.y.z_12002
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12000
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Registering service:host.x.y.z_12002
+END:Service discovery demo mode:WATCH
+=============================================
+START:Service discovery demo mode:POLL
+	Registering service
+		host.x.y.z_12000
+		host.x.y.z_12001
+		host.x.y.z_12002
+		host.x.y.z_12003
+		host.x.y.z_12004
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12000
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12002
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Deregistering service:
+		host.x.y.z_12002
+	Sleeping for poll interval:30000
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12000
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Registering service:host.x.y.z_12002
+END:Service discovery demo mode:POLL
+=============================================
+START:Service discovery demo mode:NONE
+	Registering service
+		host.x.y.z_12000
+		host.x.y.z_12001
+		host.x.y.z_12002
+		host.x.y.z_12003
+		host.x.y.z_12004
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12000
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12002
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Deregistering service:
+		host.x.y.z_12000
+	SERVICES AVAILABLE
+		SERVICENAME 	HOST 			PORT
+		myServiceName 	host.x.y.z 		12001
+		myServiceName 	host.x.y.z 		12002
+		myServiceName 	host.x.y.z 		12003
+		myServiceName 	host.x.y.z 		12004
+	Registering service:host.x.y.z_12000
+END:Service discovery demo mode:NONE
+=============================================
+</pre> 
+  </div> 
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+++ helix/site-content/0.6.6-docs/recipes/task_dag_execution.html Wed Nov  2 04:11:36 2016
@@ -0,0 +1,409 @@
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+<div class="section"> 
+ <div class="page-header">
+  <h2 id="Distributed_Task_Execution">Distributed Task Execution</h2>
+ </div> 
+ <p>This recipe is intended to demonstrate how task dependencies can be modeled using primitives provided by Helix. A given task can be run with the desired amount of parallelism and will start only when upstream dependencies are met. The demo executes the task DAG described below using 10 workers. Although the demo starts the workers as threads, there is no requirement that all the workers need to run in the same process. In reality, these workers run on many different boxes on a cluster. When worker fails, Helix takes care of re-assigning a failed task partition to a new worker.</p> 
+ <p>Redis is used as a result store. Any other suitable implementation for TaskResultStore can be plugged in.</p> 
+ <div class="section"> 
+  <h3 id="Workflow">Workflow</h3> 
+  <div class="section"> 
+   <h4 id="Input">Input</h4> 
+   <p>10000 impression events and around 100 click events are pre-populated in task result store (redis).</p> 
+   <ul> 
+    <li> <p><b>ImpEvent</b>: format: id,isFraudulent,country,gender</p></li> 
+    <li> <p><b>ClickEvent</b>: format: id,isFraudulent,impEventId</p></li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Stages">Stages</h4> 
+   <ul> 
+    <li> <p><b>FilterImps</b>: Filters impression where isFraudulent=true.</p></li> 
+    <li> <p><b>FilterClicks</b>: Filters clicks where isFraudulent=true</p></li> 
+    <li> <p><b>impCountsByGender</b>: Generates impression counts grouped by gender. It does this by incrementing the count for ‘impression_gender_counts:&lt;gender_value&gt;’ in the task result store (redis hash). Depends on: <b>FilterImps</b></p></li> 
+    <li> <p><b>impCountsByCountry</b>: Generates impression counts grouped by country. It does this by incrementing the count for ‘impression_country_counts:&lt;country_value&gt;’ in the task result store (redis hash). Depends on: <b>FilterClicks</b></p></li> 
+    <li> <p><b>impClickJoin</b>: Joins clicks with corresponding impression event using impEventId as the join key. Join is needed to pull dimensions not present in click event. Depends on: <b>FilterImps, FilterClicks</b></p></li> 
+    <li> <p><b>clickCountsByGender</b>: Generates click counts grouped by gender. It does this by incrementing the count for click_gender_counts:&lt;gender_value&gt; in the task result store (redis hash). Depends on: <b>impClickJoin</b></p></li> 
+    <li> <p><b>clickCountsByGender</b>: Generates click counts grouped by country. It does this by incrementing the count for click_country_counts:&lt;country_value&gt; in the task result store (redis hash). Depends on: <b>impClickJoin</b></p></li> 
+    <li> <p><b>report</b>: Reads from all aggregates generated by previous stages and prints them. Depends on: <b>impCountsByGender, impCountsByCountry, clickCountsByGender,clickCountsByGender</b></p></li> 
+   </ul> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Creating_a_DAG">Creating a DAG</h3> 
+  <p>Each stage is represented as a Node along with the upstream dependency and desired parallelism. Each stage is modeled as a resource in Helix using OnlineOffline state model. As part of an Offline to Online transition, we watch the external view of upstream resources and wait for them to transition to the online state. See Task.java for additional info.</p> 
+  <div class="source"> 
+   <pre>Dag dag = new Dag();
+dag.addNode(new Node(&quot;filterImps&quot;, 10, &quot;&quot;));
+dag.addNode(new Node(&quot;filterClicks&quot;, 5, &quot;&quot;));
+dag.addNode(new Node(&quot;impClickJoin&quot;, 10, &quot;filterImps,filterClicks&quot;));
+dag.addNode(new Node(&quot;impCountsByGender&quot;, 10, &quot;filterImps&quot;));
+dag.addNode(new Node(&quot;impCountsByCountry&quot;, 10, &quot;filterImps&quot;));
+dag.addNode(new Node(&quot;clickCountsByGender&quot;, 5, &quot;impClickJoin&quot;));
+dag.addNode(new Node(&quot;clickCountsByCountry&quot;, 5, &quot;impClickJoin&quot;));
+dag.addNode(new Node(&quot;report&quot;,1,&quot;impCountsByGender,impCountsByCountry,clickCountsByGender,clickCountsByCountry&quot;));
+</pre> 
+  </div> 
+ </div> 
+ <div class="section"> 
+  <h3 id="Demo">Demo</h3> 
+  <p>In order to run the demo, use the following steps</p> 
+  <p>See <a class="externalLink" href="http://redis.io/topics/quickstart">http://redis.io/topics/quickstart</a> on how to install redis server</p> 
+  <div class="source"> 
+   <pre>Start redis e.g:
+./redis-server --port 6379
+
+git clone https://git-wip-us.apache.org/repos/asf/helix.git
+cd helix
+git checkout helix-0.6.6
+cd recipes/task-execution
+mvn clean install package -DskipTests
+cd target/task-execution-pkg/bin
+chmod +x task-execution-demo.sh
+./task-execution-demo.sh 2181 localhost 6379
+
+</pre> 
+  </div> 
+  <p>Here's a visual representation of the DAG.</p> 
+  <div class="source"> 
+   <pre>                       +-----------------+       +----------------+
+                       |   filterImps    |       |  filterClicks  |
+                       | (parallelism=10)|       | (parallelism=5)|
+                       +----------+-----++       +-------+--------+
+                       |          |     |                |
+                       |          |     |                |
+                       |          |     |                |
+                       |          |     +-------&gt;--------v------------+
+      +--------------&lt;-+   +------v-------+    |  impClickJoin        |
+      |impCountsByGender   |impCountsByCountry | (parallelism=10)     |
+      |(parallelism=10)    |(parallelism=10)   ++-------------------+-+
+      +-----------+--+     +---+----------+     |                   |
+                  |            |                |                   |
+                  |            |                |                   |
+                  |            |       +--------v---------+       +-v-------------------+
+                  |            |       |clickCountsByGender       |clickCountsByCountry |
+                  |            |       |(parallelism=5)   |       |(parallelism=5)      |
+                  |            |       +----+-------------+       +---------------------+
+                  |            |            |                     |
+                  |            |            |                     |
+                  |            |            |                     |
+                  +-----&gt;+-----+&gt;-----------v----+&lt;---------------+
+                         | report                |
+                         |(parallelism=1)        |
+                         +-----------------------+
+
+</pre> 
+  </div> 
+  <p>(credit for above ascii art: <a class="externalLink" href="http://www.asciiflow.com">http://www.asciiflow.com</a>)</p> 
+  <div class="section"> 
+   <h4 id="Output">Output</h4> 
+   <div class="source"> 
+    <pre>Done populating dummy data
+Executing filter task for filterImps_3 for impressions_demo
+Executing filter task for filterImps_2 for impressions_demo
+Executing filter task for filterImps_0 for impressions_demo
+Executing filter task for filterImps_1 for impressions_demo
+Executing filter task for filterImps_4 for impressions_demo
+Executing filter task for filterClicks_3 for clicks_demo
+Executing filter task for filterClicks_1 for clicks_demo
+Executing filter task for filterImps_8 for impressions_demo
+Executing filter task for filterImps_6 for impressions_demo
+Executing filter task for filterClicks_2 for clicks_demo
+Executing filter task for filterClicks_0 for clicks_demo
+Executing filter task for filterImps_7 for impressions_demo
+Executing filter task for filterImps_5 for impressions_demo
+Executing filter task for filterClicks_4 for clicks_demo
+Executing filter task for filterImps_9 for impressions_demo
+Running AggTask for impCountsByGender_3 for filtered_impressions_demo gender
+Running AggTask for impCountsByGender_2 for filtered_impressions_demo gender
+Running AggTask for impCountsByGender_0 for filtered_impressions_demo gender
+Running AggTask for impCountsByGender_9 for filtered_impressions_demo gender
+Running AggTask for impCountsByGender_1 for filtered_impressions_demo gender
+Running AggTask for impCountsByGender_4 for filtered_impressions_demo gender
+Running AggTask for impCountsByCountry_4 for filtered_impressions_demo country
+Running AggTask for impCountsByGender_5 for filtered_impressions_demo gender
+Executing JoinTask for impClickJoin_2
+Running AggTask for impCountsByCountry_3 for filtered_impressions_demo country
+Running AggTask for impCountsByCountry_1 for filtered_impressions_demo country
+Running AggTask for impCountsByCountry_0 for filtered_impressions_demo country
+Running AggTask for impCountsByCountry_2 for filtered_impressions_demo country
+Running AggTask for impCountsByGender_6 for filtered_impressions_demo gender
+Executing JoinTask for impClickJoin_1
+Executing JoinTask for impClickJoin_0
+Executing JoinTask for impClickJoin_3
+Running AggTask for impCountsByGender_8 for filtered_impressions_demo gender
+Executing JoinTask for impClickJoin_4
+Running AggTask for impCountsByGender_7 for filtered_impressions_demo gender
+Running AggTask for impCountsByCountry_5 for filtered_impressions_demo country
+Running AggTask for impCountsByCountry_6 for filtered_impressions_demo country
+Executing JoinTask for impClickJoin_9
+Running AggTask for impCountsByCountry_8 for filtered_impressions_demo country
+Running AggTask for impCountsByCountry_7 for filtered_impressions_demo country
+Executing JoinTask for impClickJoin_5
+Executing JoinTask for impClickJoin_6
+Running AggTask for impCountsByCountry_9 for filtered_impressions_demo country
+Executing JoinTask for impClickJoin_8
+Executing JoinTask for impClickJoin_7
+Running AggTask for clickCountsByCountry_1 for joined_clicks_demo country
+Running AggTask for clickCountsByCountry_0 for joined_clicks_demo country
+Running AggTask for clickCountsByCountry_2 for joined_clicks_demo country
+Running AggTask for clickCountsByCountry_3 for joined_clicks_demo country
+Running AggTask for clickCountsByGender_1 for joined_clicks_demo gender
+Running AggTask for clickCountsByCountry_4 for joined_clicks_demo country
+Running AggTask for clickCountsByGender_3 for joined_clicks_demo gender
+Running AggTask for clickCountsByGender_2 for joined_clicks_demo gender
+Running AggTask for clickCountsByGender_4 for joined_clicks_demo gender
+Running AggTask for clickCountsByGender_0 for joined_clicks_demo gender
+Running reports task
+Impression counts per country
+{CANADA=1940, US=1958, CHINA=2014, UNKNOWN=2022, UK=1946}
+Click counts per country
+{US=24, CANADA=14, CHINA=26, UNKNOWN=14, UK=22}
+Impression counts per gender
+{F=3325, UNKNOWN=3259, M=3296}
+Click counts per gender
+{F=33, UNKNOWN=32, M=35}
+</pre> 
+   </div> 
+  </div> 
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+				<li>Release Notes for Apache Helix 0.6.6</li>
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+<div class="section"> 
+ <div class="page-header">
+  <h2 id="Release_Notes_for_Apache_Helix_0.6.6">Release Notes for Apache Helix 0.6.6</h2>
+ </div> 
+ <p>The Apache Helix team would like to announce the release of Apache Helix 0.6.6.</p> 
+ <p>This is the eighth release under the Apache umbrella, and the fourth as a top-level project.</p> 
+ <p>Helix is a generic cluster management framework used for the automatic management of partitioned, replicated and distributed resources hosted on a cluster of nodes. Helix provides the following features:</p> 
+ <ul> 
+  <li>Automatic assignment of resource/partition to nodes</li> 
+  <li>Node failure detection and recovery</li> 
+  <li>Dynamic addition of Resources</li> 
+  <li>Dynamic addition of nodes to the cluster</li> 
+  <li>Pluggable distributed state machine to manage the state of a resource via state transitions</li> 
+  <li>Automatic load balancing and throttling of transitions</li> 
+ </ul> 
+ <div class="section"> 
+  <h3 id="Changes">Changes</h3> 
+  <div class="section"> 
+   <h4 id="Bug__HELIX-543_Avoid_moving_partitions_unnecessarily_when_auto-rebalancing.">Bug * [HELIX-543] Avoid moving partitions unnecessarily when auto-rebalancing.</h4> 
+   <ul> 
+    <li>Check Workflow is JobQueue before doing parallel jobs logics.</li> 
+    <li>[HELIX-631] AutoRebalanceStrategy does not work correctly all the time.</li> 
+    <li>Fix NPE when first time call WorkflowRebalancer.</li> 
+    <li>Fix Workflow and Job metrics Counters.</li> 
+    <li>[HELIX-633] AutoRebalancer should ignore disabled instance and all partitions on disabled instances should be dropped in FULL_AUTO rebalance mode.</li> 
+    <li>Fix task assignment in instance group tag check.</li> 
+    <li>Fix missing workflowtype assign in builder.</li> 
+    <li>TaskUtil.getWorkflowCfg throws NPE if workflow doesn't exist.</li> 
+    <li>Fix the statemodelFactories in Example Process.</li> 
+    <li>[HELIX-618] Job hung if the target resource does not exist anymore at the time when it is scheduled.</li> 
+    <li>[Helix-624] Fix NPE in TestMessageService.TestMultiMessageCriteria.</li> 
+    <li>[HELIX-615] Naming problem of scheduled jobs from recurrent queue.</li> 
+    <li>[HELIX-613] Fix thread leaking problems in TaskStateModel by sharing one thread pool among all tasks and timeout tasks from TaskStateModels created from the same TaskStateModelFactory.</li> 
+    <li>[Helix-612] Bump up the version of zkClient and zookeeper to avoid NPE.</li> 
+    <li>[HELIX-600] Task scheduler fails to schedule a recurring workflow if the startTime is set to a future timestamp.</li> 
+    <li>[HELIX-592] addCluster should respect overwriteExisitng when adding stateModel Definations.</li> 
+    <li>[HELIX-589] Delete job API throws NPE if the job does not exist in last scheduled workflow.</li> 
+    <li>[HELIX-587] Fix NPE in ClusterStateVerifier.</li> 
+    <li>[HELIX-584] SimpleDateFormat should not be used as singleton due to its race conditions.</li> 
+    <li>[HELIX-578] NPE while deleting a job from a recurrent job queue.</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Improvement__Add_AbortedJobCount_in_JobMonior.">Improvement * Add AbortedJobCount in JobMonior.</h4> 
+   <ul> 
+    <li>Job Config and logic refactoring with 1)Support identical task initialization with job command and number of tasks, 2)Remove unused MaxForcedReassignmentPerTask field and 3)Refactor logics of failure.</li> 
+    <li>[HELIX-635] GenericTaskAssignmentCalculator rebalance with consistent hashing. 1) Implement consistent hashing mapping calculation, 2) Remove reassign logics and applied in consistent hashing.</li> 
+    <li>Refactor TaskAssignmentCalculator API.</li> 
+    <li>Monitors for Task framework. 1) Add workflow and job monitor MBeans and implementations, and 2) Add tests for MBean existing checking.</li> 
+    <li>Check whether instance is disable when assigning tasks to instances in TaskRebalancer.</li> 
+    <li>Add Partition task start time.</li> 
+    <li>Add WorkflowType and JobType in WorkflowConfig and JobConfig.</li> 
+    <li>Added method to TaskDriver to get all workflows from a cluster. Added methods to convert HelixProperty to WorkflowConfig and JobConfig.</li> 
+    <li>More cleanup on workflow and workflowConfig builders.</li> 
+    <li>Add Builder class for TaskConfig, and add unit test for testing generic jobs.</li> 
+    <li>Add static methods into TaskDriver for getting configuration/context for jobs and workflows.</li> 
+    <li>Refactor Workflow and Jobqueue builders to make the builder API more clean.</li> 
+    <li>Add methods in TaskDriver for getting Workflow/Job configuration and context. External users should call these methods instead of TaskUtil.</li> 
+    <li>[HELIX-623] Do not expose internal configuration field name. Client should use JobConfig.Builder to create jobConfig.</li> 
+    <li>[HELIX-617] Job IdealState is generated even the job is not running and not removed when it is completed.</li> 
+    <li>[HELIX-616] Change JobQueue to be subclass of Workflow instead of WorkflowConfig.</li> 
+    <li>[HELIX-614] Fix the bug when job expiry time is shorter than job schedule interval in recurring job queue.</li> 
+    <li>[Helix-606] Add an option in IdealState to allow a resource to disable showing external view.</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="New_Feature__HELIX-636_Add_Java_API_and_REST_API_for_clean_up_JobQueue.">New Feature * [HELIX-636] Add Java API and REST API for clean up JobQueue.</h4> 
+   <ul> 
+    <li>Add ABORT state in TaskState and set tasks IN_PROGRESS to ABORT when workflow fails.</li> 
+    <li>[HELIX-568] Add new topology aware (rack-aware) rebalance strategy based on CRUSH algorithm. Design doc is available at: https://cwiki.apache.org/confluence/display/HELIX/Helix+Topology-aware+Rebalance+Strategy.</li> 
+    <li>[HELIX-634] Refactor AutoRebalancer to allow configuable placement strategy.</li> 
+    <li>Support user defined content store per workflow/job/task layer, 1) Add feature to support workflow/job/task layer key value user defined content store, and 2) Add test case for workflow/job/task layer key-value pair store and verify.</li> 
+    <li>Allow an instance group tag to be configured for a job, so all tasks of the job can only be running on the instances containing the tag. 1) Add instance group tag for jobs, and 2) Add a test for job assignment when the only instance can be assigned instance is disabled.</li> 
+    <li>Add pollForJobState and pollForWorkflowState function in TaskDriver.</li> 
+    <li>Populate Error message from running client's task and persist it into JobContext for better error reporting.</li> 
+    <li>Add a new task state (TASK_ABORTED) to TaskResult. This allows client to abort a task and let Helix not retry it even Task RetryCount is bigger than 1.</li> 
+    <li>Add new job option to allow contining a job even its direct dependent job fails.</li> 
+    <li>Support changes of workflow configuration.</li> 
+    <li>[HELIX-622] Add new resource configuration option to allow resource to disable emmiting monitoring bean.</li> 
+    <li>[HELIX-599] Support creating/maintaining/routing resources with same names in different instance groups.</li> 
+    <li>[HELIX-601] Allow workflow to schedule dependency jobs in parallel.</li> 
+    <li>[HELIX-591] Provide getResourcesWithTag in HelixAdmin to retrieve all resources with a group tag.</li> 
+    <li>[HELIX-583] support deleting recurring job queue.</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Task__Upgrade_maven_release_plugin_version.">Task * Upgrade maven release plugin version.</h4> 
+   <ul> 
+    <li>Update Apache POM version.</li> 
+    <li>Make sure all dependant service using zookeeper 3.4.9 version.</li> 
+    <li>Bump Zookeeper client version to 3.4.9 to catch the fix of session reestablish failure due to large set of watches.</li> 
+   </ul> 
+  </div> 
+  <div class="section"> 
+   <h4 id="Test__Add_integration_test_for_running_task_with_unregistered_command.">Test * Add integration test for running task with unregistered command.</h4> 
+   <ul> 
+    <li>Refactor redundant code TestTaskRebalancerRetryLimit.</li> 
+    <li>Add test to test task retry with and without delay.</li> 
+    <li>Add unit tests to retrieve all workflows and job info from a cluster.</li> 
+    <li>Add unit test to retrieve workflow and job configurations.</li> 
+    <li>Add job dependency workflow test.</li> 
+    <li>Refactor tests with TaskTestBase and remove duplicated code.</li> 
+    <li>Add TaskTestBase and refactor 2 tests.</li> 
+    <li>Fix task framework unit test failure.</li> 
+    <li>Refactor TaskUtil class to move as many as methods out of the class, and make other methods in it as internal API as possible. Expose necessary APIs in TaskDriver instead.</li> 
+    <li>More fixes and cleanup on task unit tests.</li> 
+    <li>Fix task framework unit tests.</li> 
+    <li>Clean up unit tests for task framework.</li> 
+   </ul> 
+  </div> 
+ </div> 
+</div> 
+<div class="section"> 
+ <h2 id="Cheers_--_The_Apache_Helix_Team">Cheers, -- The Apache Helix Team</h2> 
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