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From maha...@apache.org
Subject svn commit: r1411035 [36/43] - in /zookeeper/site/trunk: content/ content/doc/r3.4.5/ content/doc/r3.4.5/api/ content/doc/r3.4.5/api/org/ content/doc/r3.4.5/api/org/apache/ content/doc/r3.4.5/api/org/apache/zookeeper/ content/doc/r3.4.5/api/org/apache/...
Date Mon, 19 Nov 2012 00:34:45 GMT
Added: zookeeper/site/trunk/content/doc/r3.4.5/recipes.html
URL: http://svn.apache.org/viewvc/zookeeper/site/trunk/content/doc/r3.4.5/recipes.html?rev=1411035&view=auto
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--- zookeeper/site/trunk/content/doc/r3.4.5/recipes.html (added)
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+</div>
+<h1>ZooKeeper Recipes and Solutions</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_recipes">A Guide to Creating Higher-level Constructs with ZooKeeper</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_outOfTheBox">Out of the Box Applications: Name Service, Configuration,
Group
+    Membership</a>
+</li>
+<li>
+<a href="#sc_recipes_eventHandles">Barriers</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_doubleBarriers">Double Barriers</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#sc_recipes_Queues">Queues</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_recipes_priorityQueues">Priority Queues</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#sc_recipes_Locks">Locks</a>
+<ul class="minitoc">
+<li>
+<a href="#Shared+Locks">Shared Locks</a>
+</li>
+<li>
+<a href="#sc_recoverableSharedLocks">Recoverable Shared Locks</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#sc_recipes_twoPhasedCommit">Two-phased Commit</a>
+</li>
+<li>
+<a href="#sc_leaderElection">Leader Election</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+  
+
+  
+
+  
+<a name="ch_recipes"></a>
+<h2 class="h3">A Guide to Creating Higher-level Constructs with ZooKeeper</h2>
+<div class="section">
+<p>In this article, you'll find guidelines for using
+    ZooKeeper to implement higher order functions. All of them are conventions
+    implemented at the client and do not require special support from
+    ZooKeeper. Hopfully the community will capture these conventions in client-side libraries

+    to ease their use and to encourage standardization.</p>
+<p>One of the most interesting things about ZooKeeper is that even
+    though ZooKeeper uses <em>asynchronous</em> notifications, you
+    can use it to build <em>synchronous</em> consistency
+    primitives, such as queues and locks. As you will see, this is possible
+    because ZooKeeper imposes an overall order on updates, and has mechanisms
+    to expose this ordering.</p>
+<p>Note that the recipes below attempt to employ best practices. In
+    particular, they avoid polling, timers or anything else that would result
+    in a "herd effect", causing bursts of traffic and limiting
+    scalability.</p>
+<p>There are many useful functions that can be imagined that aren't
+    included here - revocable read-write priority locks, as just one example.
+    And some of the constructs mentioned here - locks, in particular -
+    illustrate certain points, even though you may find other constructs, such
+    as event handles or queues, a more practical means of performing the same
+    function. In general, the examples in this section are designed to
+    stimulate thought.</p>
+<a name="sc_outOfTheBox"></a>
+<h3 class="h4">Out of the Box Applications: Name Service, Configuration, Group
+    Membership</h3>
+<p>Name service and configuration are two of the primary applications
+    of ZooKeeper. These two functions are provided directly by the ZooKeeper
+    API.</p>
+<p>Another function directly provided by ZooKeeper is <em>group
+    membership</em>. The group is represented by a node. Members of the
+    group create ephemeral nodes under the group node. Nodes of the members
+    that fail abnormally will be removed automatically when ZooKeeper detects
+    the failure.</p>
+<a name="sc_recipes_eventHandles"></a>
+<h3 class="h4">Barriers</h3>
+<p>Distributed systems use <em>barriers</em>
+      to block processing of a set of nodes until a condition is met
+      at which time all the nodes are allowed to proceed. Barriers are
+      implemented in ZooKeeper by designating a barrier node. The
+      barrier is in place if the barrier node exists. Here's the
+      pseudo code:</p>
+<ol>
+      
+<li>
+        
+<p>Client calls the ZooKeeper API's <strong>exists()</strong> function
on the barrier node, with
+        <em>watch</em> set to true.</p>
+      
+</li>
+
+      
+<li>
+        
+<p>If <strong>exists()</strong> returns false, the
+        barrier is gone and the client proceeds</p>
+      
+</li>
+
+      
+<li>
+        
+<p>Else, if <strong>exists()</strong> returns true,
+        the clients wait for a watch event from ZooKeeper for the barrier
+        node.</p>
+      
+</li>
+
+      
+<li>
+        
+<p>When the watch event is triggered, the client reissues the
+        <strong>exists( )</strong> call, again waiting until
+        the barrier node is removed.</p>
+      
+</li>
+    
+</ol>
+<a name="sc_doubleBarriers"></a>
+<h4>Double Barriers</h4>
+<p>Double barriers enable clients to synchronize the beginning and
+      the end of a computation. When enough processes have joined the barrier,
+      processes start their computation and leave the barrier once they have
+      finished. This recipe shows how to use a ZooKeeper node as a
+      barrier.</p>
+<p>The pseudo code in this recipe represents the barrier node as
+      <em>b</em>. Every client process <em>p</em>
+      registers with the barrier node on entry and unregisters when it is
+      ready to leave. A node registers with the barrier node via the <strong>Enter</strong>
procedure below, it waits until
+      <em>x</em> client process register before proceeding with
+      the computation. (The <em>x</em> here is up to you to
+      determine for your system.)</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+        
+            
+<tr>
+              
+<td><strong>Enter</strong></td>
+
+              <td><strong>Leave</strong></td>
+            
+</tr>
+
+            
+<tr>
+              
+<td>
+<ol>
+                  
+<li>
+                    
+<p>Create a name <em><em>n</em> =
+                        <em>b</em>+&ldquo;/&rdquo;+<em>p</em></em>
+</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Set watch: <strong>exists(<em>b</em> + &lsquo;&lsquo;/ready&rsquo;&rsquo;,
+                        true)</strong>
+</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Create child: <strong>create(
+                        <em>n</em>, EPHEMERAL)</strong>
+</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>
+<strong>L = getChildren(b,
+                        false)</strong>
+</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>if fewer children in L than<em>
+                        x</em>, wait for watch event</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>else <strong>create(b + &lsquo;&lsquo;/ready&rsquo;&rsquo;,
+                        REGULAR)</strong>
+</p>
+                  
+</li>
+                
+</ol>
+</td>
+
+              <td>
+<ol>
+                  
+<li>
+                    
+<p>
+<strong>L = getChildren(b,
+                        false)</strong>
+</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>if no children, exit</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>if <em>p</em> is only process node in
+                      L, delete(n) and exit</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>if <em>p</em> is the lowest process
+                      node in L, wait on highest process node in P</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>else <strong>delete(<em>n</em>) </strong>if
+                      still exists and wait on lowest process node in L</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>goto 1</p>
+                  
+</li>
+                
+</ol>
+</td>
+            
+</tr>
+          
+      
+</table>
+<p>On entering, all processes watch on a ready node and
+        create an ephemeral node as a child of the barrier node. Each process
+        but the last enters the barrier and waits for the ready node to appear
+        at line 5. The process that creates the xth node, the last process, will
+        see x nodes in the list of children and create the ready node, waking up
+        the other processes. Note that waiting processes wake up only when it is
+        time to exit, so waiting is efficient.
+      </p>
+<p>On exit, you can't use a flag such as <em>ready</em>
+      because you are watching for process nodes to go away. By using
+      ephemeral nodes, processes that fail after the barrier has been entered
+      do not prevent correct processes from finishing. When processes are
+      ready to leave, they need to delete their process nodes and wait for all
+      other processes to do the same.</p>
+<p>Processes exit when there are no process nodes left as children of
+      <em>b</em>. However, as an efficiency, you can use the
+      lowest process node as the ready flag. All other processes that are
+      ready to exit watch for the lowest existing process node to go away, and
+      the owner of the lowest process watches for any other process node
+      (picking the highest for simplicity) to go away. This means that only a
+      single process wakes up on each node deletion except for the last node,
+      which wakes up everyone when it is removed.</p>
+<a name="sc_recipes_Queues"></a>
+<h3 class="h4">Queues</h3>
+<p>Distributed queues are a common data structure. To implement a
+    distributed queue in ZooKeeper, first designate a znode to hold the queue,
+    the queue node. The distributed clients put something into the queue by
+    calling create() with a pathname ending in "queue-", with the
+    <em>sequence</em> and <em>ephemeral</em> flags in
+    the create() call set to true. Because the <em>sequence</em>
+    flag is set, the new pathnames will have the form
+    _path-to-queue-node_/queue-X, where X is a monotonic increasing number. A
+    client that wants to be removed from the queue calls ZooKeeper's <strong>getChildren(
)</strong> function, with
+    <em>watch</em> set to true on the queue node, and begins
+    processing nodes with the lowest number. The client does not need to issue
+    another <strong>getChildren( )</strong> until it exhausts
+    the list obtained from the first <strong>getChildren(
+    )</strong> call. If there are are no children in the queue node, the
+    reader waits for a watch notification to check the queue again.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+      
+<p>There now exists a Queue implementation in ZooKeeper
+      recipes directory. This is distributed with the release --
+      src/recipes/queue directory of the release artifact.
+      </p>
+    
+</div>
+</div>
+<a name="sc_recipes_priorityQueues"></a>
+<h4>Priority Queues</h4>
+<p>To implement a priority queue, you need only make two simple
+      changes to the generic <a href="#sc_recipes_Queues">queue
+      recipe</a> . First, to add to a queue, the pathname ends with
+      "queue-YY" where YY is the priority of the element with lower numbers
+      representing higher priority (just like UNIX). Second, when removing
+      from the queue, a client uses an up-to-date children list meaning that
+      the client will invalidate previously obtained children lists if a watch
+      notification triggers for the queue node.</p>
+<a name="sc_recipes_Locks"></a>
+<h3 class="h4">Locks</h3>
+<p>Fully distributed locks that are globally synchronous, meaning at
+    any snapshot in time no two clients think they hold the same lock. These
+    can be implemented using ZooKeeeper. As with priority queues, first define
+    a lock node.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+      
+<p>There now exists a Lock implementation in ZooKeeper
+      recipes directory. This is distributed with the release --
+      src/recipes/lock directory of the release artifact.
+      </p>
+    
+</div>
+</div>
+<p>Clients wishing to obtain a lock do the following:</p>
+<ol>
+      
+<li>
+        
+<p>Call <strong>create( )</strong> with a pathname
+        of "_locknode_/lock-" and the <em>sequence</em> and
+        <em>ephemeral</em> flags set.</p>
+      
+</li>
+
+      
+<li>
+        
+<p>Call <strong>getChildren( )</strong> on the lock
+        node <em>without</em> setting the watch flag (this is
+        important to avoid the herd effect).</p>
+      
+</li>
+
+      
+<li>
+        
+<p>If the pathname created in step <strong>1</strong> has the lowest sequence
number suffix, the
+        client has the lock and the client exits the protocol.</p>
+      
+</li>
+
+      
+<li>
+        
+<p>The client calls <strong>exists( )</strong> with
+        the watch flag set on the path in the lock directory with the next
+        lowest sequence number.</p>
+      
+</li>
+
+      
+<li>
+        
+<p>if <strong>exists( )</strong> returns false, go
+        to step <strong>2</strong>. Otherwise, wait for a
+        notification for the pathname from the previous step before going to
+        step <strong>2</strong>.</p>
+      
+</li>
+    
+</ol>
+<p>The unlock protocol is very simple: clients wishing to release a
+    lock simply delete the node they created in step 1.</p>
+<p>Here are a few things to notice:</p>
+<ul>
+      
+<li>
+        
+<p>The removal of a node will only cause one client to wake up
+        since each node is watched by exactly one client. In this way, you
+        avoid the herd effect.</p>
+      
+</li>
+    
+</ul>
+<ul>
+      
+<li>
+        
+<p>There is no polling or timeouts.</p>
+      
+</li>
+    
+</ul>
+<ul>
+      
+<li>
+        
+<p>Because of the way you implement locking, it is easy to see the
+        amount of lock contention, break locks, debug locking problems,
+        etc.</p>
+      
+</li>
+    
+</ul>
+<a name="Shared+Locks"></a>
+<h4>Shared Locks</h4>
+<p>You can implement shared locks by with a few changes to the lock
+      protocol:</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+        
+            
+<tr>
+              
+<td><strong>Obtaining a read
+              lock:</strong></td>
+
+              <td><strong>Obtaining a write
+              lock:</strong></td>
+            
+</tr>
+
+            
+<tr>
+              
+<td>
+<ol>
+                  
+<li>
+                    
+<p>Call <strong>create( )</strong> to
+                    create a node with pathname
+                    "<span class="codefrag filename">_locknode_/read-</span>".
This is the
+                    lock node use later in the protocol. Make sure to set both
+                    the <em>sequence</em> and
+                    <em>ephemeral</em> flags.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Call <strong>getChildren( )</strong>
+                    on the lock node <em>without</em> setting the
+                    <em>watch</em> flag - this is important, as it
+                    avoids the herd effect.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>If there are no children with a pathname starting
+                    with "<span class="codefrag filename">write-</span>" and
having a lower
+                    sequence number than the node created in step <strong>1</strong>,
the client has the lock and can
+                    exit the protocol. </p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Otherwise, call <strong>exists(
+                    )</strong>, with <em>watch</em> flag, set on
+                    the node in lock directory with pathname staring with
+                    "<span class="codefrag filename">write-</span>" having the
next lowest
+                    sequence number.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>If <strong>exists( )</strong>
+                    returns <em>false</em>, goto step <strong>2</strong>.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Otherwise, wait for a notification for the pathname
+                    from the previous step before going to step <strong>2</strong>
+</p>
+                  
+</li>
+                
+</ol>
+</td>
+
+              <td>
+<ol>
+                  
+<li>
+                    
+<p>Call <strong>create( )</strong> to
+                    create a node with pathname
+                    "<span class="codefrag filename">_locknode_/write-</span>".
This is the
+                    lock node spoken of later in the protocol. Make sure to
+                    set both <em>sequence</em> and
+                    <em>ephemeral</em> flags.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Call <strong>getChildren( )
+                    </strong> on the lock node <em>without</em>
+                    setting the <em>watch</em> flag - this is
+                    important, as it avoids the herd effect.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>If there are no children with a lower sequence
+                    number than the node created in step <strong>1</strong>,
the client has the lock and the
+                    client exits the protocol.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>Call <strong>exists( ),</strong>
+                    with <em>watch</em> flag set, on the node with
+                    the pathname that has the next lowest sequence
+                    number.</p>
+                  
+</li>
+
+                  
+<li>
+                    
+<p>If <strong>exists( )</strong>
+                    returns <em>false</em>, goto step <strong>2</strong>.
Otherwise, wait for a
+                    notification for the pathname from the previous step
+                    before going to step <strong>2</strong>.</p>
+                  
+</li>
+                
+</ol>
+</td>
+            
+</tr>
+          
+      
+</table>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+        
+<p>It might appear that this recipe creates a herd effect:
+          when there is a large group of clients waiting for a read
+          lock, and all getting notified more or less simultaneously
+          when the "<span class="codefrag filename">write-</span>" node with
the lowest
+          sequence number is deleted. In fact. that's valid behavior:
+          as all those waiting reader clients should be released since
+          they have the lock. The herd effect refers to releasing a
+          "herd" when in fact only a single or a small number of
+          machines can proceed.
+        </p>
+      
+</div>
+</div>
+<a name="sc_recoverableSharedLocks"></a>
+<h4>Recoverable Shared Locks</h4>
+<p>With minor modifications to the Shared Lock protocol, you make
+      shared locks revocable by modifying the shared lock protocol:</p>
+<p>In step <strong>1</strong>, of both obtain reader
+      and writer lock protocols, call <strong>getData(
+      )</strong> with <em>watch</em> set, immediately after the
+      call to <strong>create( )</strong>. If the client
+      subsequently receives notification for the node it created in step
+      <strong>1</strong>, it does another <strong>getData( )</strong>
on that node, with
+      <em>watch</em> set and looks for the string "unlock", which
+      signals to the client that it must release the lock. This is because,
+      according to this shared lock protocol, you can request the client with
+      the lock give up the lock by calling <strong>setData()
+      </strong> on the lock node, writing "unlock" to that node.</p>
+<p>Note that this protocol requires the lock holder to consent to
+      releasing the lock. Such consent is important, especially if the lock
+      holder needs to do some processing before releasing the lock. Of course
+      you can always implement <em>Revocable Shared Locks with Freaking
+      Laser Beams</em> by stipulating in your protocol that the revoker
+      is allowed to delete the lock node if after some length of time the lock
+      isn't deleted by the lock holder.</p>
+<a name="sc_recipes_twoPhasedCommit"></a>
+<h3 class="h4">Two-phased Commit</h3>
+<p>A two-phase commit protocol is an algorithm that lets all clients in
+    a distributed system agree either to commit a transaction or abort.</p>
+<p>In ZooKeeper, you can implement a two-phased commit by having a
+    coordinator create a transaction node, say "/app/Tx", and one child node
+    per participating site, say "/app/Tx/s_i". When coordinator creates the
+    child node, it leaves the content undefined. Once each site involved in
+    the transaction receives the transaction from the coordinator, the site
+    reads each child node and sets a watch. Each site then processes the query
+    and votes "commit" or "abort" by writing to its respective node. Once the
+    write completes, the other sites are notified, and as soon as all sites
+    have all votes, they can decide either "abort" or "commit". Note that a
+    node can decide "abort" earlier if some site votes for "abort".</p>
+<p>An interesting aspect of this implementation is that the only role
+    of the coordinator is to decide upon the group of sites, to create the
+    ZooKeeper nodes, and to propagate the transaction to the corresponding
+    sites. In fact, even propagating the transaction can be done through
+    ZooKeeper by writing it in the transaction node.</p>
+<p>There are two important drawbacks of the approach described above.
+    One is the message complexity, which is O(n&sup2;). The second is the
+    impossibility of detecting failures of sites through ephemeral nodes. To
+    detect the failure of a site using ephemeral nodes, it is necessary that
+    the site create the node.</p>
+<p>To solve the first problem, you can have only the coordinator
+    notified of changes to the transaction nodes, and then notify the sites
+    once coordinator reaches a decision. Note that this approach is scalable,
+    but it's is slower too, as it requires all communication to go through the
+    coordinator.</p>
+<p>To address the second problem, you can have the coordinator
+    propagate the transaction to the sites, and have each site creating its
+    own ephemeral node.</p>
+<a name="sc_leaderElection"></a>
+<h3 class="h4">Leader Election</h3>
+<p>A simple way of doing leader election with ZooKeeper is to use the
+    <strong>SEQUENCE|EPHEMERAL</strong> flags when creating
+    znodes that represent "proposals" of clients. The idea is to have a znode,
+    say "/election", such that each znode creates a child znode "/election/n_"
+    with both flags SEQUENCE|EPHEMERAL. With the sequence flag, ZooKeeper
+    automatically appends a sequence number that is greater that any one
+    previously appended to a child of "/election". The process that created
+    the znode with the smallest appended sequence number is the leader.
+    </p>
+<p>That's not all, though. It is important to watch for failures of the
+    leader, so that a new client arises as the new leader in the case the
+    current leader fails. A trivial solution is to have all application
+    processes watching upon the current smallest znode, and checking if they
+    are the new leader when the smallest znode goes away (note that the
+    smallest znode will go away if the leader fails because the node is
+    ephemeral). But this causes a herd effect: upon of failure of the current
+    leader, all other processes receive a notification, and execute
+    getChildren on "/election" to obtain the current list of children of
+    "/election". If the number of clients is large, it causes a spike on the
+    number of operations that ZooKeeper servers have to process. To avoid the
+    herd effect, it is sufficient to watch for the next znode down on the
+    sequence of znodes. If a client receives a notification that the znode it
+    is watching is gone, then it becomes the new leader in the case that there
+    is no smaller znode. Note that this avoids the herd effect by not having
+    all clients watching the same znode. </p>
+<p>Here's the pseudo code:</p>
+<p>Let ELECTION be a path of choice of the application. To volunteer to
+    be a leader: </p>
+<ol>
+      
+<li>
+        
+<p>Create znode z with path "ELECTION/n_" with both SEQUENCE and
+        EPHEMERAL flags;</p>
+      
+</li>
+
+      
+<li>
+        
+<p>Let C be the children of "ELECTION", and i be the sequence
+        number of z;</p>
+      
+</li>
+
+      
+<li>
+        
+<p>Watch for changes on "ELECTION/n_j", where j is the largest
+        sequence number such that j &lt; i and n_j is a znode in C;</p>
+      
+</li>
+    
+</ol>
+<p>Upon receiving a notification of znode deletion: </p>
+<ol>
+      
+<li>
+        
+<p>Let C be the new set of children of ELECTION; </p>
+      
+</li>
+
+      
+<li>
+        
+<p>If z is the smallest node in C, then execute leader
+        procedure;</p>
+      
+</li>
+
+      
+<li>
+        
+<p>Otherwise, watch for changes on "ELECTION/n_j", where j is the
+        largest sequence number such that j &lt; i and n_j is a znode in C;
+        </p>
+      
+</li>
+    
+</ol>
+<p>Note that the znode having no preceding znode on the list of
+    children does not imply that the creator of this znode is aware that it is
+    the current leader. Applications may consider creating a separate znode
+    to acknowledge that the leader has executed the leader procedure. </p>
+</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;
+         2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
+    |end bottomstrip
+    +-->
+</div>
+</body>
+</html>

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