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-<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>+“/”+<em>p</em></em>
-</p>
-
-</li>
-
-
-<li>
-
-<p>Set watch: <strong>exists(<em>b</em> + ‘‘/ready’’,
- 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 + ‘‘/ready’’,
- 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 L</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(n²). 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 < 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 < 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>
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http://git-wip-us.apache.org/repos/asf/zookeeper/blob/cf24deb2/docs/recipes.pdf
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