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From af...@apache.org
Subject [01/45] zookeeper git commit: Add 3.4.12 release and update organization for afine
Date Tue, 01 May 2018 04:30:10 GMT
Repository: zookeeper
Updated Branches:
  refs/heads/website e39a1eb26 -> 8a71ac966


http://git-wip-us.apache.org/repos/asf/zookeeper/blob/8a71ac96/_released_docs/r3.4.12/zookeeperStarted.html
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+        PDF</a>
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+<h1>ZooKeeper Getting Started Guide</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_GettingStarted">Getting Started: Coordinating Distributed Applications with
+      ZooKeeper</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_Prerequisites">Pre-requisites</a>
+</li>
+<li>
+<a href="#sc_Download">Download</a>
+</li>
+<li>
+<a href="#sc_InstallingSingleMode">Standalone Operation</a>
+</li>
+<li>
+<a href="#sc_FileManagement">Managing ZooKeeper Storage</a>
+</li>
+<li>
+<a href="#sc_ConnectingToZooKeeper">Connecting to ZooKeeper</a>
+</li>
+<li>
+<a href="#sc_ProgrammingToZooKeeper">Programming to ZooKeeper</a>
+</li>
+<li>
+<a href="#sc_RunningReplicatedZooKeeper">Running Replicated ZooKeeper</a>
+</li>
+<li>
+<a href="#Other+Optimizations">Other Optimizations</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_GettingStarted"></a>
+<h2 class="h3">Getting Started: Coordinating Distributed Applications with
+      ZooKeeper</h2>
+<div class="section">
+<p>This document contains information to get you started quickly with
+    ZooKeeper. It is aimed primarily at developers hoping to try it out, and
+    contains simple installation instructions for a single ZooKeeper server, a
+    few commands to verify that it is running, and a simple programming
+    example. Finally, as a convenience, there are a few sections regarding
+    more complicated installations, for example running replicated
+    deployments, and optimizing the transaction log. However for the complete
+    instructions for commercial deployments, please refer to the <a href="zookeeperAdmin.html">ZooKeeper
+    Administrator's Guide</a>.</p>
+<a name="sc_Prerequisites"></a>
+<h3 class="h4">Pre-requisites</h3>
+<p>See <a href="zookeeperAdmin.html#sc_systemReq">
+          System Requirements</a> in the Admin guide.</p>
+<a name="sc_Download"></a>
+<h3 class="h4">Download</h3>
+<p>To get a ZooKeeper distribution, download a recent
+        <a href="http://zookeeper.apache.org/releases.html">
+          stable</a> release from one of the Apache Download
+        Mirrors.</p>
+<a name="sc_InstallingSingleMode"></a>
+<h3 class="h4">Standalone Operation</h3>
+<p>Setting up a ZooKeeper server in standalone mode is
+      straightforward. The server is contained in a single JAR file,
+      so installation consists of creating a configuration.</p>
+<p>Once you've downloaded a stable ZooKeeper release unpack
+      it and cd to the root</p>
+<p>To start ZooKeeper you need a configuration file. Here is a sample,
+      create it in <strong>conf/zoo.cfg</strong>:</p>
+<pre class="code">
+tickTime=2000
+dataDir=/var/lib/zookeeper
+clientPort=2181
+</pre>
+<p>This file can be called anything, but for the sake of this
+      discussion call
+      it <strong>conf/zoo.cfg</strong>. Change the
+      value of <strong>dataDir</strong> to specify an
+      existing (empty to start with) directory.  Here are the meanings
+      for each of the fields:</p>
+<dl>
+        
+<dt>
+<term>
+<strong>tickTime</strong>
+</term>
+</dt>
+<dd>
+<p>the basic time unit in milliseconds used by ZooKeeper. It is
+            used to do heartbeats and the minimum session timeout will be
+            twice the tickTime.</p>
+</dd>
+      
+</dl>
+<dl>
+        
+<dt>
+<term>
+<strong>dataDir</strong>
+</term>
+</dt>
+<dd>
+<p>the location to store the in-memory database snapshots and,
+            unless specified otherwise, the transaction log of updates to the
+            database.</p>
+</dd>
+
+        
+<dt>
+<term>
+<strong>clientPort</strong>
+</term>
+</dt>
+<dd>
+<p>the port to listen for client connections</p>
+</dd>
+      
+</dl>
+<p>Now that you created the configuration file, you can start
+      ZooKeeper:</p>
+<pre class="code">bin/zkServer.sh start</pre>
+<p>ZooKeeper logs messages using log4j -- more detail
+      available in the
+      <a href="zookeeperProgrammers.html#Logging">Logging</a>
+      section of the Programmer's Guide. You will see log messages
+      coming to the console (default) and/or a log file depending on
+      the log4j configuration.</p>
+<p>The steps outlined here run ZooKeeper in standalone mode. There is
+      no replication, so if ZooKeeper process fails, the service will go down.
+      This is fine for most development situations, but to run ZooKeeper in
+      replicated mode, please see <a href="#sc_RunningReplicatedZooKeeper">Running Replicated
+      ZooKeeper</a>.</p>
+<a name="sc_FileManagement"></a>
+<h3 class="h4">Managing ZooKeeper Storage</h3>
+<p>For long running production systems ZooKeeper storage must
+      be managed externally (dataDir and logs). See the section on
+      <a href="zookeeperAdmin.html#sc_maintenance">maintenance</a> for
+      more details.</p>
+<a name="sc_ConnectingToZooKeeper"></a>
+<h3 class="h4">Connecting to ZooKeeper</h3>
+<pre class="code">$ bin/zkCli.sh -server 127.0.0.1:2181</pre>
+<p>This lets you perform simple, file-like operations.</p>
+<p>Once you have connected, you should see something like:
+        </p>
+<pre class="code">
+
+Connecting to localhost:2181
+log4j:WARN No appenders could be found for logger (org.apache.zookeeper.ZooKeeper).
+log4j:WARN Please initialize the log4j system properly.
+Welcome to ZooKeeper!
+JLine support is enabled
+[zkshell: 0]
+        </pre>
+<p>
+        From the shell, type <span class="codefrag command">help</span> to get a listing of commands that can be executed from the client, as in:
+      </p>
+<pre class="code">
+
+[zkshell: 0] help
+ZooKeeper host:port cmd args
+        get path [watch]
+        ls path [watch]
+        set path data [version]
+        delquota [-n|-b] path
+        quit
+        printwatches on|off
+        createpath data acl
+        stat path [watch]
+        listquota path
+        history
+        setAcl path acl
+        getAcl path
+        sync path
+        redo cmdno
+        addauth scheme auth
+        delete path [version]
+        setquota -n|-b val path
+
+        </pre>
+<p>From here, you can try a few simple commands to get a feel for this simple command line interface.  First, start by issuing the list command, as
+      in <span class="codefrag command">ls</span>, yielding:
+      </p>
+<pre class="code">
+
+[zkshell: 8] ls /
+[zookeeper]
+        </pre>
+<p>Next, create a new znode by running <span class="codefrag command">create /zk_test my_data</span>. This creates a new znode and associates the string "my_data" with the node.
+      You should see:</p>
+<pre class="code">
+
+[zkshell: 9] create /zk_test my_data
+Created /zk_test
+      </pre>
+<p>  Issue another <span class="codefrag command">ls /</span> command to see what the directory looks like:
+        </p>
+<pre class="code">
+
+[zkshell: 11] ls /
+[zookeeper, zk_test]
+
+        </pre>
+<p>
+      Notice that the zk_test directory has now been created.
+      </p>
+<p>Next, verify that the data was associated with the znode by running the <span class="codefrag command">get</span> command, as in:
+      </p>
+<pre class="code">
+
+[zkshell: 12] get /zk_test
+my_data
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 5
+mtime = Fri Jun 05 13:57:06 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 0
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 7
+numChildren = 0
+        </pre>
+<p>We can change the data associated with zk_test by issuing the <span class="codefrag command">set</span> command, as in:
+        </p>
+<pre class="code">
+
+[zkshell: 14] set /zk_test junk
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 6
+mtime = Fri Jun 05 14:01:52 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 1
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 4
+numChildren = 0
+[zkshell: 15] get /zk_test
+junk
+cZxid = 5
+ctime = Fri Jun 05 13:57:06 PDT 2009
+mZxid = 6
+mtime = Fri Jun 05 14:01:52 PDT 2009
+pZxid = 5
+cversion = 0
+dataVersion = 1
+aclVersion = 0
+ephemeralOwner = 0
+dataLength = 4
+numChildren = 0
+      </pre>
+<p>
+       (Notice we did a <span class="codefrag command">get</span> after setting the data and it did, indeed, change.</p>
+<p>Finally, let's <span class="codefrag command">delete</span> the node by issuing:
+      </p>
+<pre class="code">
+
+[zkshell: 16] delete /zk_test
+[zkshell: 17] ls /
+[zookeeper]
+[zkshell: 18]
+</pre>
+<p>That's it for now.  To explore more, continue with the rest of this document and see the <a href="zookeeperProgrammers.html">Programmer's Guide</a>. </p>
+<a name="sc_ProgrammingToZooKeeper"></a>
+<h3 class="h4">Programming to ZooKeeper</h3>
+<p>ZooKeeper has a Java bindings and C bindings. They are
+      functionally equivalent. The C bindings exist in two variants: single
+      threaded and multi-threaded. These differ only in how the messaging loop
+      is done. For more information, see the <a href="zookeeperProgrammers.html#ch_programStructureWithExample">Programming
+      Examples in the ZooKeeper Programmer's Guide</a> for
+      sample code using of the different APIs.</p>
+<a name="sc_RunningReplicatedZooKeeper"></a>
+<h3 class="h4">Running Replicated ZooKeeper</h3>
+<p>Running ZooKeeper in standalone mode is convenient for evaluation,
+      some development, and testing. But in production, you should run
+      ZooKeeper in replicated mode. A replicated group of servers in the same
+      application is called a <em>quorum</em>, and in replicated
+      mode, all servers in the quorum have copies of the same configuration
+      file.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+      
+<p>
+         For replicated mode, a minimum of three servers are required,
+         and it is strongly recommended that you have an odd number of
+         servers. If you only have two servers, then you are in a
+         situation where if one of them fails, there are not enough
+         machines to form a majority quorum. Two servers is inherently
+         <strong>less</strong>
+         stable than a single server, because there are two single
+         points of failure.
+      </p>
+   
+</div>
+</div>
+<p>
+      The required
+      <strong>conf/zoo.cfg</strong>
+      file for replicated mode is similar to the one used in standalone
+      mode, but with a few differences. Here is an example:
+   </p>
+<pre class="code">
+tickTime=2000
+dataDir=/var/lib/zookeeper
+clientPort=2181
+initLimit=5
+syncLimit=2
+server.1=zoo1:2888:3888
+server.2=zoo2:2888:3888
+server.3=zoo3:2888:3888
+</pre>
+<p>The new entry, <strong>initLimit</strong> is
+      timeouts ZooKeeper uses to limit the length of time the ZooKeeper
+      servers in quorum have to connect to a leader. The entry <strong>syncLimit</strong> limits how far out of date a server can
+      be from a leader.</p>
+<p>With both of these timeouts, you specify the unit of time using
+      <strong>tickTime</strong>. In this example, the timeout
+      for initLimit is 5 ticks at 2000 milleseconds a tick, or 10
+      seconds.</p>
+<p>The entries of the form <em>server.X</em> list the
+      servers that make up the ZooKeeper service. When the server starts up,
+      it knows which server it is by looking for the file
+      <em>myid</em> in the data directory. That file has the 
+      contains the server number, in ASCII.</p>
+<p>Finally, note the two port numbers after each server
+       name: " 2888" and "3888". Peers use the former port to connect
+       to other peers. Such a connection is necessary so that peers
+       can communicate, for example, to agree upon the order of
+       updates. More specifically, a ZooKeeper server uses this port
+       to connect followers to the leader. When a new leader arises, a
+       follower opens a TCP connection to the leader using this
+       port. Because the default leader election also uses TCP, we
+       currently require another port for leader election. This is the
+       second port in the server entry.
+       </p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+        
+<p>If you want to test multiple servers on a single
+        machine, specify the servername
+        as <em>localhost</em> with unique quorum &amp;
+        leader election ports (i.e. 2888:3888, 2889:3889, 2890:3890 in
+        the example above) for each server.X in that server's config
+        file. Of course separate <em>dataDir</em>s and
+        distinct <em>clientPort</em>s are also necessary
+        (in the above replicated example, running on a
+        single <em>localhost</em>, you would still have
+        three config files).</p>
+        
+<p>Please be aware that setting up multiple servers on a single
+            machine will not create any redundancy. If something were to
+            happen which caused the machine to die, all of the zookeeper
+            servers would be offline. Full redundancy requires that each
+            server have its own machine. It must be a completely separate
+            physical server. Multiple virtual machines on the same physical
+            host are still vulnerable to the complete failure of that host.</p>
+      
+</div>
+</div>
+<a name="Other+Optimizations"></a>
+<h3 class="h4">Other Optimizations</h3>
+<p>There are a couple of other configuration parameters that can
+      greatly increase performance:</p>
+<ul>
+        
+<li>
+          
+<p>To get low latencies on updates it is important to
+          have a dedicated transaction log directory. By default
+          transaction logs are put in the same directory as the data
+          snapshots and <em>myid</em> file. The dataLogDir
+          parameters indicates a different directory to use for the
+          transaction logs.</p>
+        
+</li>
+
+        
+<li>
+          
+<p>
+<em>[tbd: what is the other config param?]</em>
+</p>
+        
+</li>
+      
+</ul>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
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+        PDF</a>
+</div>
+<h1>Programming with ZooKeeper - A basic tutorial</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_Introduction">Introduction</a>
+</li>
+<li>
+<a href="#sc_barriers">Barriers</a>
+</li>
+<li>
+<a href="#sc_producerConsumerQueues">Producer-Consumer Queues</a>
+</li>
+<li>
+<a href="#Complete+example">Complete example</a>
+<ul class="minitoc">
+<li>
+<a href="#Queue+test">Queue test</a>
+</li>
+<li>
+<a href="#Barrier+test">Barrier test</a>
+</li>
+<li>
+<a href="#sc_sourceListing">Source Listing</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_Introduction"></a>
+<h2 class="h3">Introduction</h2>
+<div class="section">
+<p>In this tutorial, we show simple implementations of barriers and 
+    producer-consumer queues using ZooKeeper. We call the respective classes Barrier and Queue. 
+    These examples assume that you have at least one ZooKeeper server running.</p>
+<p>Both primitives use the following common excerpt of code:</p>
+<pre class="code">
+    static ZooKeeper zk = null;
+    static Integer mutex;
+
+    String root;
+
+    SyncPrimitive(String address) {
+        if(zk == null){
+            try {
+                System.out.println("Starting ZK:");
+                zk = new ZooKeeper(address, 3000, this);
+                mutex = new Integer(-1);
+                System.out.println("Finished starting ZK: " + zk);
+            } catch (IOException e) {
+                System.out.println(e.toString());
+                zk = null;
+            }
+        }
+    }
+
+    synchronized public void process(WatchedEvent event) {
+        synchronized (mutex) {
+            mutex.notify();
+        }
+    }
+</pre>
+<p>Both classes extend SyncPrimitive. In this way, we execute steps that are 
+common to all primitives in the constructor of SyncPrimitive. To keep the examples 
+simple, we create a ZooKeeper object the first time we instantiate either a barrier 
+object or a queue object, and we declare a static variable that is a reference 
+to this object. The subsequent instances of Barrier and Queue check whether a 
+ZooKeeper object exists. Alternatively, we could have the application creating a
+ZooKeeper object and passing it to the constructor of Barrier and Queue.</p>
+<p>
+We use the process() method to process notifications triggered due to watches. 
+In the following discussion, we present code that sets watches. A watch is internal 
+structure that enables ZooKeeper to notify a client of a change to a node. For example, 
+if a client is waiting for other clients to leave a barrier, then it can set a watch and 
+wait for modifications to a particular node, which can indicate that it is the end of the wait. 
+This point becomes clear once we go over the examples.
+</p>
+</div>
+   
+ 
+<a name="sc_barriers"></a>
+<h2 class="h3">Barriers</h2>
+<div class="section">
+<p>
+ A barrier is a primitive that enables a group of processes to synchronize the 
+ beginning and the end of a computation. The general idea of this implementation 
+ is to have a barrier node that serves the purpose of being a parent for individual 
+ process nodes. Suppose that we call the barrier node "/b1". Each process "p" then 
+ creates a node "/b1/p". Once enough processes have created their corresponding 
+ nodes, joined processes can start the computation.
+ </p>
+<p>In this example, each process instantiates a Barrier object, and its constructor takes as parameters:</p>
+<ul>
+<li>
+<p>the address of a ZooKeeper server (e.g., "zoo1.foo.com:2181")</p>
+</li>
+
+<li>
+<p>the path of the barrier node on ZooKeeper (e.g., "/b1")</p>
+</li>
+
+<li>
+<p>the size of the group of processes</p>
+</li>
+
+</ul>
+<p>The constructor of Barrier passes the address of the Zookeeper server to the 
+constructor of the parent class. The parent class creates a ZooKeeper instance if 
+one does not exist. The constructor of Barrier then creates a 
+barrier node on ZooKeeper, which is the parent node of all process nodes, and 
+we call root (<strong>Note:</strong> This is not the ZooKeeper root "/").</p>
+<pre class="code">
+        /**
+         * Barrier constructor
+         *
+         * @param address
+         * @param root
+         * @param size
+         */
+        Barrier(String address, String root, int size) {
+            super(address);
+            this.root = root;
+            this.size = size;
+
+            // Create barrier node
+            if (zk != null) {
+                try {
+                    Stat s = zk.exists(root, false);
+                    if (s == null) {
+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                                CreateMode.PERSISTENT);
+                    }
+                } catch (KeeperException e) {
+                    System.out
+                            .println("Keeper exception when instantiating queue: "
+                                    + e.toString());
+                } catch (InterruptedException e) {
+                    System.out.println("Interrupted exception");
+                }
+            }
+
+            // My node name
+            try {
+                name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
+            } catch (UnknownHostException e) {
+                System.out.println(e.toString());
+            }
+
+        }
+</pre>
+<p>
+To enter the barrier, a process calls enter(). The process creates a node under 
+the root to represent it, using its host name to form the node name. It then wait 
+until enough processes have entered the barrier. A process does it by checking 
+the number of children the root node has with "getChildren()", and waiting for 
+notifications in the case it does not have enough. To receive a notification when 
+there is a change to the root node, a process has to set a watch, and does it 
+through the call to "getChildren()". In the code, we have that "getChildren()" 
+has two parameters. The first one states the node to read from, and the second is
+a boolean flag that enables the process to set a watch. In the code the flag is true.
+</p>
+<pre class="code">
+        /**
+         * Join barrier
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+
+        boolean enter() throws KeeperException, InterruptedException{
+            zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                    CreateMode.EPHEMERAL_SEQUENTIAL);
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+
+                    if (list.size() &lt; size) {
+                        mutex.wait();
+                    } else {
+                        return true;
+                    }
+                }
+            }
+        }
+</pre>
+<p>
+Note that enter() throws both KeeperException and InterruptedException, so it is 
+the reponsability of the application to catch and handle such exceptions.</p>
+<p>
+Once the computation is finished, a process calls leave() to leave the barrier. 
+First it deletes its corresponding node, and then it gets the children of the root 
+node. If there is at least one child, then it waits for a notification (obs: note 
+that the second parameter of the call to getChildren() is true, meaning that 
+ZooKeeper has to set a watch on the the root node). Upon reception of a notification, 
+it checks once more whether the root node has any child.</p>
+<pre class="code">
+        /**
+         * Wait until all reach barrier
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+
+        boolean leave() throws KeeperException, InterruptedException{
+            zk.delete(root + "/" + name, 0);
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+                        if (list.size() &gt; 0) {
+                            mutex.wait();
+                        } else {
+                            return true;
+                        }
+                    }
+                }
+        }
+    }
+</pre>
+</div>
+
+<a name="sc_producerConsumerQueues"></a>
+<h2 class="h3">Producer-Consumer Queues</h2>
+<div class="section">
+<p>
+A producer-consumer queue is a distributed data estructure thata group of processes 
+use to generate and consume items. Producer processes create new elements and add 
+them to the queue. Consumer processes remove elements from the list, and process them. 
+In this implementation, the elements are simple integers. The queue is represented 
+by a root node, and to add an element to the queue, a producer process creates a new node, 
+a child of the root node.
+</p>
+<p>
+The following excerpt of code corresponds to the constructor of the object. As 
+with Barrier objects, it first calls the constructor of the parent class, SyncPrimitive, 
+that creates a ZooKeeper object if one doesn't exist. It then verifies if the root 
+node of the queue exists, and creates if it doesn't.
+</p>
+<pre class="code">
+        /**
+         * Constructor of producer-consumer queue
+         *
+         * @param address
+         * @param name
+         */
+        Queue(String address, String name) {
+            super(address);
+            this.root = name;
+            // Create ZK node name
+            if (zk != null) {
+                try {
+                    Stat s = zk.exists(root, false);
+                    if (s == null) {
+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                                CreateMode.PERSISTENT);
+                    }
+                } catch (KeeperException e) {
+                    System.out
+                            .println("Keeper exception when instantiating queue: "
+                                    + e.toString());
+                } catch (InterruptedException e) {
+                    System.out.println("Interrupted exception");
+                }
+            }
+        }
+</pre>
+<p>
+A producer process calls "produce()" to add an element to the queue, and passes 
+an integer as an argument. To add an element to the queue, the method creates a 
+new node using "create()", and uses the SEQUENCE flag to instruct ZooKeeper to 
+append the value of the sequencer counter associated to the root node. In this way, 
+we impose a total order on the elements of the queue, thus guaranteeing that the 
+oldest element of the queue is the next one consumed.
+</p>
+<pre class="code">
+        /**
+         * Add element to the queue.
+         *
+         * @param i
+         * @return
+         */
+
+        boolean produce(int i) throws KeeperException, InterruptedException{
+            ByteBuffer b = ByteBuffer.allocate(4);
+            byte[] value;
+
+            // Add child with value i
+            b.putInt(i);
+            value = b.array();
+            zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
+                        CreateMode.PERSISTENT_SEQUENTIAL);
+
+            return true;
+        }
+</pre>
+<p>
+To consume an element, a consumer process obtains the children of the root node, 
+reads the node with smallest counter value, and returns the element. Note that 
+if there is a conflict, then one of the two contending processes won't be able to 
+delete the node and the delete operation will throw an exception.</p>
+<p>
+A call to getChildren() returns the list of children in lexicographic order. 
+As lexicographic order does not necessary follow the numerical order of the counter 
+values, we need to decide which element is the smallest. To decide which one has 
+the smallest counter value, we traverse the list, and remove the prefix "element" 
+from each one.</p>
+<pre class="code">
+        /**
+         * Remove first element from the queue.
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+        int consume() throws KeeperException, InterruptedException{
+            int retvalue = -1;
+            Stat stat = null;
+
+            // Get the first element available
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+                    if (list.size() == 0) {
+                        System.out.println("Going to wait");
+                        mutex.wait();
+                    } else {
+                        Integer min = new Integer(list.get(0).substring(7));
+                        for(String s : list){
+                            Integer tempValue = new Integer(s.substring(7));
+                            //System.out.println("Temporary value: " + tempValue);
+                            if(tempValue &lt; min) min = tempValue;
+                        }
+                        System.out.println("Temporary value: " + root + "/element" + min);
+                        byte[] b = zk.getData(root + "/element" + min,
+                                    false, stat);
+                        zk.delete(root + "/element" + min, 0);
+                        ByteBuffer buffer = ByteBuffer.wrap(b);
+                        retvalue = buffer.getInt();
+
+                        return retvalue;
+                    }
+                }
+            }
+        }
+    }
+</pre>
+</div>
+
+
+<a name="Complete+example"></a>
+<h2 class="h3">Complete example</h2>
+<div class="section">
+<p>
+In the following section you can find a complete command line application to demonstrate the above mentioned
+recipes. Use the following command to run it.
+</p>
+<pre class="code">
+ZOOBINDIR="[path_to_distro]/bin"
+. "$ZOOBINDIR"/zkEnv.sh
+java SyncPrimitive [Test Type] [ZK server] [No of elements] [Client type]
+</pre>
+<a name="Queue+test"></a>
+<h3 class="h4">Queue test</h3>
+<p>Start a producer to create 100 elements</p>
+<pre class="code">
+java SyncPrimitive qTest localhost 100 p
+</pre>
+<p>Start a consumer to consume 100 elements</p>
+<pre class="code">
+java SyncPrimitive qTest localhost 100 c
+</pre>
+<a name="Barrier+test"></a>
+<h3 class="h4">Barrier test</h3>
+<p>Start a barrier with 2 participants (start as many times as many participants you'd like to enter)</p>
+<pre class="code">
+java SyncPrimitive bTest localhost 2
+</pre>
+<a name="sc_sourceListing"></a>
+<h3 class="h4">Source Listing</h3>
+<div class="note example">
+<div class="label">SyncPrimitive.Java</div>
+<div class="content">
+
+<title>SyncPrimitive.Java</title>
+
+<pre class="code">
+import java.io.IOException;
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.nio.ByteBuffer;
+import java.util.List;
+import java.util.Random;
+
+import org.apache.zookeeper.CreateMode;
+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.ZooKeeper;
+import org.apache.zookeeper.ZooDefs.Ids;
+import org.apache.zookeeper.data.Stat;
+
+public class SyncPrimitive implements Watcher {
+
+    static ZooKeeper zk = null;
+    static Integer mutex;
+
+    String root;
+
+    SyncPrimitive(String address) {
+        if(zk == null){
+            try {
+                System.out.println("Starting ZK:");
+                zk = new ZooKeeper(address, 3000, this);
+                mutex = new Integer(-1);
+                System.out.println("Finished starting ZK: " + zk);
+            } catch (IOException e) {
+                System.out.println(e.toString());
+                zk = null;
+            }
+        }
+        //else mutex = new Integer(-1);
+    }
+
+    synchronized public void process(WatchedEvent event) {
+        synchronized (mutex) {
+            //System.out.println("Process: " + event.getType());
+            mutex.notify();
+        }
+    }
+
+    /**
+     * Barrier
+     */
+    static public class Barrier extends SyncPrimitive {
+        int size;
+        String name;
+
+        /**
+         * Barrier constructor
+         *
+         * @param address
+         * @param root
+         * @param size
+         */
+        Barrier(String address, String root, int size) {
+            super(address);
+            this.root = root;
+            this.size = size;
+
+            // Create barrier node
+            if (zk != null) {
+                try {
+                    Stat s = zk.exists(root, false);
+                    if (s == null) {
+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                                CreateMode.PERSISTENT);
+                    }
+                } catch (KeeperException e) {
+                    System.out
+                            .println("Keeper exception when instantiating queue: "
+                                    + e.toString());
+                } catch (InterruptedException e) {
+                    System.out.println("Interrupted exception");
+                }
+            }
+
+            // My node name
+            try {
+                name = new String(InetAddress.getLocalHost().getCanonicalHostName().toString());
+            } catch (UnknownHostException e) {
+                System.out.println(e.toString());
+            }
+
+        }
+
+        /**
+         * Join barrier
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+
+        boolean enter() throws KeeperException, InterruptedException{
+            zk.create(root + "/" + name, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                    CreateMode.EPHEMERAL_SEQUENTIAL);
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+
+                    if (list.size() &lt; size) {
+                        mutex.wait();
+                    } else {
+                        return true;
+                    }
+                }
+            }
+        }
+
+        /**
+         * Wait until all reach barrier
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+
+        boolean leave() throws KeeperException, InterruptedException{
+            zk.delete(root + "/" + name, 0);
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+                        if (list.size() &gt; 0) {
+                            mutex.wait();
+                        } else {
+                            return true;
+                        }
+                    }
+                }
+        }
+    }
+
+    /**
+     * Producer-Consumer queue
+     */
+    static public class Queue extends SyncPrimitive {
+
+        /**
+         * Constructor of producer-consumer queue
+         *
+         * @param address
+         * @param name
+         */
+        Queue(String address, String name) {
+            super(address);
+            this.root = name;
+            // Create ZK node name
+            if (zk != null) {
+                try {
+                    Stat s = zk.exists(root, false);
+                    if (s == null) {
+                        zk.create(root, new byte[0], Ids.OPEN_ACL_UNSAFE,
+                                CreateMode.PERSISTENT);
+                    }
+                } catch (KeeperException e) {
+                    System.out
+                            .println("Keeper exception when instantiating queue: "
+                                    + e.toString());
+                } catch (InterruptedException e) {
+                    System.out.println("Interrupted exception");
+                }
+            }
+        }
+
+        /**
+         * Add element to the queue.
+         *
+         * @param i
+         * @return
+         */
+
+        boolean produce(int i) throws KeeperException, InterruptedException{
+            ByteBuffer b = ByteBuffer.allocate(4);
+            byte[] value;
+
+            // Add child with value i
+            b.putInt(i);
+            value = b.array();
+            zk.create(root + "/element", value, Ids.OPEN_ACL_UNSAFE,
+                        CreateMode.PERSISTENT_SEQUENTIAL);
+
+            return true;
+        }
+
+
+        /**
+         * Remove first element from the queue.
+         *
+         * @return
+         * @throws KeeperException
+         * @throws InterruptedException
+         */
+        int consume() throws KeeperException, InterruptedException{
+            int retvalue = -1;
+            Stat stat = null;
+
+            // Get the first element available
+            while (true) {
+                synchronized (mutex) {
+                    List&lt;String&gt; list = zk.getChildren(root, true);
+                    if (list.size() == 0) {
+                        System.out.println("Going to wait");
+                        mutex.wait();
+                    } else {
+                        Integer min = new Integer(list.get(0).substring(7));
+                        String minNode = list.get(0);
+                        for(String s : list){
+                            Integer tempValue = new Integer(s.substring(7));
+                            //System.out.println("Temporary value: " + tempValue);
+                            if(tempValue &lt; min) {
+                                min = tempValue;
+                                minNode = s;
+                            }
+                        }
+                        System.out.println("Temporary value: " + root + "/" + minNode);
+                        byte[] b = zk.getData(root + "/" + minNode,
+                        false, stat);
+                        zk.delete(root + "/" + minNode, 0);
+                        ByteBuffer buffer = ByteBuffer.wrap(b);
+                        retvalue = buffer.getInt();
+
+                        return retvalue;
+                    }
+                }
+            }
+        }
+    }
+
+    public static void main(String args[]) {
+        if (args[0].equals("qTest"))
+            queueTest(args);
+        else
+            barrierTest(args);
+
+    }
+
+    public static void queueTest(String args[]) {
+        Queue q = new Queue(args[1], "/app1");
+
+        System.out.println("Input: " + args[1]);
+        int i;
+        Integer max = new Integer(args[2]);
+
+        if (args[3].equals("p")) {
+            System.out.println("Producer");
+            for (i = 0; i &lt; max; i++)
+                try{
+                    q.produce(10 + i);
+                } catch (KeeperException e){
+
+                } catch (InterruptedException e){
+
+                }
+        } else {
+            System.out.println("Consumer");
+
+            for (i = 0; i &lt; max; i++) {
+                try{
+                    int r = q.consume();
+                    System.out.println("Item: " + r);
+                } catch (KeeperException e){
+                    i--;
+                } catch (InterruptedException e){
+
+                }
+            }
+        }
+    }
+
+    public static void barrierTest(String args[]) {
+        Barrier b = new Barrier(args[1], "/b1", new Integer(args[2]));
+        try{
+            boolean flag = b.enter();
+            System.out.println("Entered barrier: " + args[2]);
+            if(!flag) System.out.println("Error when entering the barrier");
+        } catch (KeeperException e){
+
+        } catch (InterruptedException e){
+
+        }
+
+        // Generate random integer
+        Random rand = new Random();
+        int r = rand.nextInt(100);
+        // Loop for rand iterations
+        for (int i = 0; i &lt; r; i++) {
+            try {
+                Thread.sleep(100);
+            } catch (InterruptedException e) {
+
+            }
+        }
+        try{
+            b.leave();
+        } catch (KeeperException e){
+
+        } catch (InterruptedException e){
+
+        }
+        System.out.println("Left barrier");
+    }
+}
+</pre>
+</div>
+</div>
+</div>
+
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
+<div class="clearboth">&nbsp;</div>
+</div>
+<div id="footer">
+<!--+
+    |start bottomstrip
+    +-->
+<div class="lastmodified">
+<script type="text/javascript"><!--
+document.write("Last Published: " + document.lastModified);
+//  --></script>
+</div>
+<div class="copyright">
+        Copyright &copy;
+          <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
+    |end bottomstrip
+    +-->
+</div>
+</body>
+</html>

http://git-wip-us.apache.org/repos/asf/zookeeper/blob/8a71ac96/_released_docs/r3.4.12/zookeeperTutorial.pdf
----------------------------------------------------------------------
diff --git a/_released_docs/r3.4.12/zookeeperTutorial.pdf b/_released_docs/r3.4.12/zookeeperTutorial.pdf
new file mode 100644
index 0000000..70df263
Binary files /dev/null and b/_released_docs/r3.4.12/zookeeperTutorial.pdf differ

http://git-wip-us.apache.org/repos/asf/zookeeper/blob/8a71ac96/credits.md
----------------------------------------------------------------------
diff --git a/credits.md b/credits.md
index d88c3f6..e4b732a 100644
--- a/credits.md
+++ b/credits.md
@@ -51,7 +51,7 @@ ZooKeeper's active committers are
 |rgs|Raul Gutierrez Segales|Pinterest|-8|
 |hanm|Michael Han|Cloudera Inc.|-8|
 |arshad|Mohammad Arshad|Huawei|+5:30|
-|afine|Abraham Fine|Cloudera Inc.|-8|
+|afine|Abraham Fine|IFTTT|-8|
 
 ## Contributors
 

http://git-wip-us.apache.org/repos/asf/zookeeper/blob/8a71ac96/documentation.md
----------------------------------------------------------------------
diff --git a/documentation.md b/documentation.md
index 15ef3fc..593cf23 100644
--- a/documentation.md
+++ b/documentation.md
@@ -10,6 +10,7 @@ Setup instructions, programming guides, and other documentation are available fo
 * [ZooKeeper 3.5.2-alpha](doc/r3.5.2-alpha/index.html)
 * [ZooKeeper 3.5.1-alpha](doc/r3.5.1-alpha/index.html)
 * [ZooKeeper 3.5.0-alpha](doc/r3.5.0-alpha/index.html)
+* [ZooKeeper 3.4.12](doc/r3.4.12/index.html)
 * [ZooKeeper 3.4.11](doc/r3.4.11/index.html)
 * [ZooKeeper 3.4.10](doc/r3.4.10/index.html)
 * [ZooKeeper 3.4.9](doc/r3.4.9/index.html)

http://git-wip-us.apache.org/repos/asf/zookeeper/blob/8a71ac96/releases.md
----------------------------------------------------------------------
diff --git a/releases.md b/releases.md
index 0db0ac7..e7e0e01 100644
--- a/releases.md
+++ b/releases.md
@@ -26,6 +26,10 @@ Release notes for Apache Zookeeper releases are available in Jira: [Browse relea
 
 ## News {#news}
 
+### 1 May, 2018: release 3.4.12 available
+
+This release fixes 22 issues, including issues that affect incorrect handling of the dataDir and the dataLogDir.  See [ZooKeeper 3.4.12 Release Notes](https://zookeeper.apache.org/doc/r3.4.12/releasenotes.html) for details.
+
 ### 9 November, 2017: release 3.4.11 available
 
 This release fixes 53 issues, it includes support for Java 9 and other critical bug fixes.  See [ZooKeeper 3.4.11 Release Notes](https://zookeeper.apache.org/doc/r3.4.11/releasenotes.html) for details.


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