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+ PDF</a>
+</div>
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ZooKeeper%3A+A+Distributed+Coordination+Service+for+Distributed%0A++++Applications">ZooKeeper: A Distributed Coordination Service for Distributed
+ Applications</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_designGoals">Design Goals</a>
+</li>
+<li>
+<a href="#sc_dataModelNameSpace">Data model and the hierarchical namespace</a>
+</li>
+<li>
+<a href="#Nodes+and+ephemeral+nodes">Nodes and ephemeral nodes</a>
+</li>
+<li>
+<a href="#Conditional+updates+and+watches">Conditional updates and watches</a>
+</li>
+<li>
+<a href="#Guarantees">Guarantees</a>
+</li>
+<li>
+<a href="#Simple+API">Simple API</a>
+</li>
+<li>
+<a href="#Implementation">Implementation</a>
+</li>
+<li>
+<a href="#Uses">Uses</a>
+</li>
+<li>
+<a href="#Performance">Performance</a>
+</li>
+<li>
+<a href="#The+ZooKeeper+Project">The ZooKeeper Project</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+
+<title>ZooKeeper</title>
+
+
+
+
+<a name="N1000A"></a><a name="ZooKeeper%3A+A+Distributed+Coordination+Service+for+Distributed%0A++++Applications"></a>
+<h2 class="h3">ZooKeeper: A Distributed Coordination Service for Distributed
+ Applications</h2>
+<div class="section">
+<p>ZooKeeper is a distributed, open-source coordination service for
+ distributed applications. It exposes a simple set of primitives that
+ distributed applications can build upon to implement higher level services
+ for synchronization, configuration maintenance, and groups and naming. It
+ is designed to be easy to program to, and uses a data model styled after
+ the familiar directory tree structure of file systems. It runs in Java and
+ has bindings for both Java and C.</p>
+<p>Coordination services are notoriously hard to get right. They are
+ especially prone to errors such as race conditions and deadlock. The
+ motivation behind ZooKeeper is to relieve distributed applications the
+ responsibility of implementing coordination services from scratch.</p>
+<a name="N10016"></a><a name="sc_designGoals"></a>
+<h3 class="h4">Design Goals</h3>
+<p>
+<strong>ZooKeeper is simple.</strong> ZooKeeper
+ allows distributed processes to coordinate with each other through a
+ shared hierarchal namespace which is organized similarly to a standard
+ file system. The name space consists of data registers - called znodes,
+ in ZooKeeper parlance - and these are similar to files and directories.
+ Unlike a typical file system, which is designed for storage, ZooKeeper
+ data is kept in-memory, which means ZooKeeper can acheive high
+ throughput and low latency numbers.</p>
+<p>The ZooKeeper implementation puts a premium on high performance,
+ highly available, strictly ordered access. The performance aspects of
+ ZooKeeper means it can be used in large, distributed systems. The
+ reliability aspects keep it from being a single point of failure. The
+ strict ordering means that sophisticated synchronization primitives can
+ be implemented at the client.</p>
+<p>
+<strong>ZooKeeper is replicated.</strong> Like the
+ distributed processes it coordinates, ZooKeeper itself is intended to be
+ replicated over a sets of machines called quorums.</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>ZooKeeper Service</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/zkservice.jpg">
+
+ </td>
+</tr>
+</table>
+<p>The servers that make up the ZooKeeper service must all know about
+ each other. They maintain an in-memory image of state, along with a
+ transaction logs and snapshots in a persistent store. As long as a
+ majority of the servers are available, the ZooKeeper service will be
+ available.</p>
+<p>Clients connect to a single ZooKeeper server. The client maintains
+ a TCP connection through which it sends requests, gets responses, gets
+ watch events, and sends heart beats. If the TCP connection to the server
+ breaks, the client will connect to a different server.</p>
+<p>
+<strong>ZooKeeper is ordered.</strong> ZooKeeper
+ stamps each update with a number that reflects the order of all
+ ZooKeeper transactions. Subsequent operations can use the order to
+ implement higher-level abstractions, such as synchronization
+ primitives.</p>
+<p>
+<strong>ZooKeeper is fast.</strong> It is
+ especially fast in "read-dominant" workloads. ZooKeeper applications run
+ on thousands of machines, and it performs best where reads are more
+ common than writes, at ratios of around 10:1.</p>
+<a name="N10046"></a><a name="sc_dataModelNameSpace"></a>
+<h3 class="h4">Data model and the hierarchical namespace</h3>
+<p>The name space provided by ZooKeeper is much like that of a
+ standard file system. A name is a sequence of path elements separated by
+ a slash (/). Every node in ZooKeeper's name space is identified by a
+ path.</p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>ZooKeeper's Hierarchical Namespace</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/zknamespace.jpg">
+
+ </td>
+</tr>
+</table>
+<a name="N1005C"></a><a name="Nodes+and+ephemeral+nodes"></a>
+<h3 class="h4">Nodes and ephemeral nodes</h3>
+<p>Unlike is standard file systems, each node in a ZooKeeper
+ namespace can have data associated with it as well as children. It is
+ like having a file-system that allows a file to also be a directory.
+ (ZooKeeper was designed to store coordination data: status information,
+ configuration, location information, etc., so the data stored at each
+ node is usually small, in the byte to kilobyte range.) We use the term
+ <em>znode</em> to make it clear that we are talking about
+ ZooKeeper data nodes.</p>
+<p>Znodes maintain a stat structure that includes version numbers for
+ data changes, ACL changes, and timestamps, to allow cache validations
+ and coordinated updates. Each time a znode's data changes, the version
+ number increases. For instance, whenever a client retrieves data it also
+ receives the version of the data.</p>
+<p>The data stored at each znode in a namespace is read and written
+ atomically. Reads get all the data bytes associated with a znode and a
+ write replaces all the data. Each node has an Access Control List (ACL)
+ that restricts who can do what.</p>
+<p>ZooKeeper also has the notion of ephemeral nodes. These znodes
+ exists as long as the session that created the znode is active. When the
+ session ends the znode is deleted. Ephemeral nodes are useful when you
+ want to implement <remark>[tbd]</remark>.</p>
+<a name="N10075"></a><a name="Conditional+updates+and+watches"></a>
+<h3 class="h4">Conditional updates and watches</h3>
+<p>ZooKeeper supports the concept of <em>watches</em>.
+ Clients can set a watch on a znodes. A watch will be triggered and
+ removed when the znode changes. When a watch is triggered the client
+ receives a packet saying that the znode has changed. And if the
+ connection between the client and one of the Zoo Keeper servers is
+ broken, the client will receive a local notification. These can be used
+ to <remark>[tbd]</remark>.</p>
+<a name="N10085"></a><a name="Guarantees"></a>
+<h3 class="h4">Guarantees</h3>
+<p>ZooKeeper is very fast and very simple. Since its goal, though, is
+ to be a basis for the construction of more complicated services, such as
+ synchronization, it provides a set of guarantees. These are:</p>
+<ul>
+
+<li>
+
+<p>Sequential Consistency - Updates from a client will be applied
+ in the order that they were sent.</p>
+
+</li>
+
+
+<li>
+
+<p>Atomicity - Updates either succeed or fail. No partial
+ results.</p>
+
+</li>
+
+
+<li>
+
+<p>Single System Image - A client will see the same view of the
+ service regardless of the server that it connects to.</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>Reliability - Once an update has been applied, it will persist
+ from that time forward until a client overwrites the update.</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>Timeliness - The clients view of the system is guaranteed to
+ be up-to-date within a certain time bound.</p>
+
+</li>
+
+</ul>
+<p>For more information on these, and how they can be used, see
+ <remark>[tbd]</remark>
+</p>
+<a name="N100BB"></a><a name="Simple+API"></a>
+<h3 class="h4">Simple API</h3>
+<p>One of the design goals of ZooKeeper is provide a very simple
+ programming interface. As a result, it supports only these
+ operations:</p>
+<dl>
+
+<dt>
+<term>create</term>
+</dt>
+<dd>
+<p>creates a node at a location in the tree</p>
+</dd>
+
+
+<dt>
+<term>delete</term>
+</dt>
+<dd>
+<p>deletes a node</p>
+</dd>
+
+
+<dt>
+<term>exists</term>
+</dt>
+<dd>
+<p>tests if a node exists at a location</p>
+</dd>
+
+
+<dt>
+<term>get data</term>
+</dt>
+<dd>
+<p>reads the data from a node</p>
+</dd>
+
+
+<dt>
+<term>set data</term>
+</dt>
+<dd>
+<p>writes data to a node</p>
+</dd>
+
+
+<dt>
+<term>get children</term>
+</dt>
+<dd>
+<p>retrieves a list of children of a node</p>
+</dd>
+
+
+<dt>
+<term>sync</term>
+</dt>
+<dd>
+<p>waits for data to be propagated</p>
+</dd>
+
+</dl>
+<p>For a more in-depth discussion on these, and how they can be used
+ to implement higher level operations, please refer to
+ <remark>[tbd]</remark>
+</p>
+<a name="N100FE"></a><a name="Implementation"></a>
+<h3 class="h4">Implementation</h3>
+<p>
+<a href="#fg_zkComponents">ZooKeeper Components</a> shows the high-level components
+ of the ZooKeeper service. With the exception of the request processor,
+ <remark>[tbd: where does the request processor live?]</remark> each of
+ the servers that make up the ZooKeeper service replicates its own copy
+ of each of components. <remark>[tbd: I changed the wording in this
+ sentence from the white paper. Can someone please make sure it is still
+ correct?]</remark>
+</p>
+<p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>ZooKeeper Components</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/zkcomponents.jpg">
+
+ </td>
+</tr>
+</table>
+</p>
+<p>The replicated database is an in-memory database containing the
+ entire data tree. Updates are logged to disk for recoverability, and
+ writes are serialized to disk before they are applied to the in-memory
+ database.</p>
+<p>Every ZooKeeper server services clients. Clients connect to
+ exactly one server to submit irequests. Read requests are serviced from
+ the local replica of each server database. Requests that change the
+ state of the service, write requests, are processed by an agreement
+ protocol.</p>
+<p>As part of the agreement protocol all write requests from clients
+ are forwarded to a single server, called the
+ <em>leader</em>. The rest of the ZooKeeper servers, called
+ <em>followers</em>, receive message proposals from the
+ leader and agree upon message delivery. The messaging layer takes care
+ of replacing leaders on failures and syncing followers with
+ leaders.</p>
+<p>ZooKeeper uses a custom atomic messaging protocol. Since the
+ messaging layer is atomic, ZooKeeper can guarantee that the local
+ replicas never diverge. When the leader receives a write request, it
+ calculates what the state of the system is when the write is to be
+ applied and transforms this into a transaction that captures this new
+ state.</p>
+<a name="N1012F"></a><a name="Uses"></a>
+<h3 class="h4">Uses</h3>
+<p>The programming interface to ZooKeeper is deliberately simple.
+ With it, however, you can implement higher order operations, such as
+ synchronizations primitives, group membership, ownership, etc. Some
+ distributed applications have used it to: <remark>[tbd: add uses from
+ white paper and video presentation.]</remark> For more information, see
+ <remark>[tbd]</remark>
+</p>
+<a name="N1013E"></a><a name="Performance"></a>
+<h3 class="h4">Performance</h3>
+<p>ZooKeeper is designed to be highly performant. But is it? The
+ results of the ZooKeeper's development team at Yahoo! Research indicate
+ that it is. (See <a href="#fg_zkPerfRW">ZooKeeper Throughput as the Read-Write Ratio Varies</a>.) It is especially high
+ performance in applications where reads outnumber writes, since writes
+ involve synchronizing the state of all servers. (Reads outnumbering
+ writes is typically the case for a coordination service.)</p>
+<p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>ZooKeeper Throughput as the Read-Write Ratio Varies</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/zkperfRW.jpg">
+
+ </td>
+</tr>
+</table>Benchmarks also indicate that it is reliable, too. <a href="#fg_zkPerfReliability">Reliability in the Presence of Errors</a> shows how a deployment responds to
+ various failures. The events marked in the figure are the
+ following:</p>
+<ol>
+
+<li>
+
+<p>Failure and recovery of a follower</p>
+
+</li>
+
+
+<li>
+
+<p>Failure and recovery of a different follower</p>
+
+</li>
+
+
+<li>
+
+<p>Failure of the leader</p>
+
+</li>
+
+
+<li>
+
+<p>Failure and recovery of two followers</p>
+
+</li>
+
+
+<li>
+
+<p>Failure of another leader</p>
+
+</li>
+
+</ol>
+<p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>Reliability in the Presence of Errors</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/zkperfreliability.jpg">
+
+ </td>
+</tr>
+</table>
+</p>
+<p>The are a few important observations from this graph. First, if
+ followers fail and recover quickly, then ZooKeeper is able to sustain a
+ high throughput despite the failure. But maybe more importantly, the
+ leader election algorithm allows for the system to recover fast enough
+ to prevent throughput from dropping substantially. In our observations,
+ ZooKeeper takes less than 200ms to elect a new leader. Third, as
+ followers recover, ZooKeeper is able to raise throughput again once they
+ start processing requests.</p>
+<a name="N1018F"></a><a name="The+ZooKeeper+Project"></a>
+<h3 class="h4">The ZooKeeper Project</h3>
+<p>ZooKeeper has been successfully used in industrial applications.
+ It is used at Yahoo! as the coordination and failure recovery service
+ for Yahoo! Message Broker, which is a highly scalable publish-subscribe
+ system managing thousands of topics for replication and data delivery.
+ It is used by the Fetching Service for Yahoo! crawler, where it also
+ manages failure recovery. And it is used by Hadoop On Demand (HOD),
+ which is an open source implementation of the map-reduce model of
+ computation. HOD uses Zookeeper as a communications and control channel
+ between slave and master process. (For more information, see the <a href="http://hadoop.apache.org/core/">Hadoop</a> and <a href="http://hadoop.apache.org/core/docs/current/hod.html">Hadoop on
+ Demand</a> open source projects on Apache.)</p>
+<p>ZooKeeper itself is an open source project, under the Apache Open
+ Source Foundation. It is a subproject of Hadoop. All users and
+ developers are encourged to join the community and contribute their
+ expertise. See the <a href="http://hadoop.apache.org/zookeeper/">Zookeeper Project on
+ Apache</a> for more information.</p>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+ |end content
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+</div>
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+ Copyright ©
+ 2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
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+<a class="dida" href="zookeeperProgrammers.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br>
+ PDF</a>
+</div>
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#The+ZooKeeper+Data+Model">The ZooKeeper Data Model</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_zkDataModel_znodes">ZNodes</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_zkDataMode_watches">Watches</a>
+</li>
+<li>
+<a href="#Data+Access">Data Access</a>
+</li>
+<li>
+<a href="#Ephemeral+Nodes">Ephemeral Nodes</a>
+</li>
+<li>
+<a href="#Unique+Naming">Unique Naming</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#sc_timeInZk">Time in ZooKeeper</a>
+</li>
+<li>
+<a href="#sc_zkStatStructure">ZooKeeper Stat Structure</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#ZooKeeper+Sessions">ZooKeeper Sessions</a>
+</li>
+<li>
+<a href="#ZooKeeper+Watches">ZooKeeper Watches</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_WatchGuarantees">What ZooKeeper Guarantees about Watches</a>
+</li>
+<li>
+<a href="#sc_WatchRememberThese">Things to Remember about Watches</a>
+</li>
+</ul>
+</li>
+<li>
+<a href="#Consistency+Guarantees">Consistency Guarantees</a>
+</li>
+<li>
+<a href="#Bindings">Bindings</a>
+<ul class="minitoc">
+<li>
+<a href="#Java+Binding">Java Binding</a>
+</li>
+<li>
+<a href="#C+Binding">C Binding</a>
+<ul class="minitoc">
+<li>
+<a href="#Installation">Installation</a>
+</li>
+<li>
+<a href="#Using+the+Client">Using the Client</a>
+</li>
+</ul>
+</li>
+</ul>
+</li>
+<li>
+<a href="#Building+Blocks%3A+A+Guide+to+ZooKeeper+Operations">Building Blocks: A Guide to ZooKeeper Operations</a>
+</li>
+<li>
+<a href="#Program+Structure%2C+with+Simple+Example">Program Structure, with Simple Example</a>
+</li>
+<li>
+<a href="#Gotchas%3A+Common+Problems+and+Troubleshooting">Gotchas: Common Problems and Troubleshooting</a>
+</li>
+</ul>
+</div>
+
+<title>ZooKeeper Programmer's Guide</title>
+
+
+<subtitle>Developing Distributed Applications that use ZooKeeper</subtitle>
+
+
+
+
+<a name="_introduction"></a>
+<preface id="_introduction">
+
+<title>Introduction</title>
+
+
+<p>This document is a guide for developers wishing to create
+ distributed applications that take advantage of ZooKeeper's coordination
+ services. It contains conceptual and practical information.</p>
+
+
+<p>The first four chapters of this guide present higher level
+ discussions of various ZooKeeper concepts. These are necessary both for an
+ understanding of how Zookeeper works as well how to work with it. It does
+ not contain source code, but it does assume a familiarity with the
+ problems associated with distributed computing. The chapters in this first
+ group are:</p>
+
+
+<ul>
+
+<li>
+
+<p>
+<a href="#ch_zkDataModel">The ZooKeeper Data Model</a>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_zkSessions">ZooKeeper Sessions</a>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_zkWatches">ZooKeeper Watches</a>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_zkGuarantees">Consistency Guarantees</a>
+</p>
+
+</li>
+
+</ul>
+
+
+<p>The next four chapters of this provided practical programming
+ information. These are:</p>
+
+
+<ul>
+
+<li>
+
+<p>
+<a href="#ch_guideToZkOperations">Building Blocks: A Guide to ZooKeeper Operations</a>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_bindings">Bindings</a>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a>
+
+<remark>[tbd]</remark>
+</p>
+
+</li>
+
+
+<li>
+
+<p>
+<a href="#ch_gotchas">Gotchas: Common Problems and Troubleshooting</a>
+</p>
+
+</li>
+
+</ul>
+
+
+<p>The book concludes with an <a href="#apx_linksToOtherInfo">appendix</a> containing links to other
+ useful, ZooKeeper-related information.</p>
+
+
+<p>Most of information in this document is written to be accessible as
+ stand-alone reference material. However, before starting your first
+ ZooKeeper application, you should probably at least read the chaptes on
+ the <a href="#ch_zkDataModel">ZooKeeper Data Model</a> and <a href="#ch_guideToZkOperations">ZooKeeper Basic Operations</a>. Also,
+ the <a href="#ch_programStructureWithExample">Simple Programmming
+ Example</a>
+<remark>[tbd]</remark> is helpful for understand the basic
+ structure of a ZooKeeper client application.</p>
+
+</preface>
+
+
+<a name="N1007F"></a><a name="The+ZooKeeper+Data+Model"></a>
+<h2 class="h3">The ZooKeeper Data Model</h2>
+<div class="section">
+<p>ZooKeeper has a hierarchal name space, much like a distributed file
+ system. The only difference is that each node in the namespace can have
+ data associated with it as well as children. It is like having a file
+ system that allows a file to also be a directory. Paths to nodes are
+ always expressed as canonical, absolute, slash-separated paths; there are
+ no relative reference. Any unicode character can be used in a path subject
+ to the following constraints:</p>
+<ul>
+
+<li>
+
+<p>The null character (\u0000) cannot be part of a path name. (This
+ causes problems with the C binding.)</p>
+
+</li>
+
+
+<li>
+
+<p>The following characters can't be used because they don't
+ display well, or render in confusing ways: \u0001 - \u0019 and \u007F
+ - \u009F.</p>
+
+</li>
+
+
+<li>
+
+<p>The following characters are not allowed because <remark>[tbd:
+ do we need reasons?]</remark> :\ud800 -uF8FFF, \uFFF0-uFFFF, \uXFFFE -
+ \uXFFFF (where X is an digit 1 - E), \uF0000 - \uFFFFF.</p>
+
+</li>
+
+
+<li>
+
+<p>The "." character can be used as part of another name, but "."
+ and ".." cannot alone make up the whole name of a path location,
+ because ZooKeeper doesn't use relative paths. The following would be
+ invalid: "/a/b/./c" or "/a/b/../c".</p>
+
+</li>
+
+
+<li>
+
+<p>The token "zookeeper" is reserved.</p>
+
+</li>
+
+</ul>
+<a name="N100AC"></a><a name="sc_zkDataModel_znodes"></a>
+<h3 class="h4">ZNodes</h3>
+<p>Every node in a ZooKeeper tree is refered to as a
+ <em>znode</em>. Znodes maintain a stat structure that
+ includes version numbers for data changes, acl changes. The stat
+ structure also has timestamps. The version number, together with the
+ timestamp allow ZooKeeper to validate the cache and to coordinate
+ updates. Each time a znode's data changes, the version number increases.
+ For instance, whenever a client retrieves data, it also receives the
+ version of the data. And when a client performs an update or a delete,
+ it must supply the version of the data of the znode it is changing. If
+ the version it supplies doesn't match the actual version of the data,
+ the update will fail. (This behavior can be overridden. For more
+ information see... <remark>[tbd... reference here to the section
+ describing the special version number -1]</remark>
+</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>In distributed application engineering, the word
+ <em>node</em> can refer to a generic host machine, a
+ server, a member of quorums, a client process, etc. In the ZooKeeper
+ documentatin, <em>znodes</em> refer to the data nodes.
+ <em>Servers</em> to refer to machines that make up the
+ ZooKeeper service; <em>quorum peers</em> refer to the
+ servers that make up a quorum; client refers to any host or process
+ which uses a ZooKeeper service.</p>
+
+</div>
+</div>
+<p>Znodes are the main enitity that a programmer access. They have
+ several characteristics that are worth mentioning here.</p>
+<a name="N100CF"></a><a name="sc_zkDataMode_watches"></a>
+<h4>Watches</h4>
+<p>Clients can set watches on znodes. Changes to that znode trigger
+ the watch and then clear the watch. When a watch triggers, ZooKeeper
+ sends the client a notification. More information about watches can be
+ found in the section
+ <a href="recipes.html#sc_recipes_Locks">
+ Zookeeper Watches</a>.
+ <remark>[tbd: fix this link] [tbd: Ben there is note from to emphasize
+ that "it is queued". What is "it" and is what we have here
+ sufficient?]</remark>
+</p>
+<a name="N100DF"></a><a name="Data+Access"></a>
+<h4>Data Access</h4>
+<p>The data stored at each znode in a namespace is read and written
+ atomically. Reads get all the data bytes associated with a znode and a
+ write replaces all the data. Each node has an Access Control List
+ (ACL) that restricts who can do what.</p>
+<a name="N100E9"></a><a name="Ephemeral+Nodes"></a>
+<h4>Ephemeral Nodes</h4>
+<p>ZooKeeper also has the notion of ephemeral nodes. These znodes
+ exists as long as the session that created the znode is active. When
+ the session ends the znode is deleted. Because of this behavior
+ ephemeral znodes are not allowed to have children.</p>
+<a name="N100F3"></a><a name="Unique+Naming"></a>
+<h4>Unique Naming</h4>
+<p>Finally you create a znode, you can request that ZooKeeper
+ append a monotonicly increasing counter be appended to the path name
+ of the znode to be requested. This counter is unique to the parent
+ znode.</p>
+<a name="N100FE"></a><a name="sc_timeInZk"></a>
+<h3 class="h4">Time in ZooKeeper</h3>
+<p>ZooKeeper tracks time multiple ways:</p>
+<ul>
+
+<li>
+
+<p>
+<strong>Zxid</strong>
+</p>
+
+
+<p>Every change to the ZooKeeper state receives a stamp in the
+ form of a <em>zxid</em> (ZooKeeper Transaction Id).
+ This exposes the total ordering of all changes to ZooKeeper. Each
+ change will have a unique zxid and if zxid1 is smaller than zxid2
+ then zxid1 happened before zxid2.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>Version numbers</strong>
+</p>
+
+
+<p>Every change to a a node will cause an increase to one of the
+ version numbers of that node. The three version numbers are version
+ (number of changes to the data of a znode), cversion (number of
+ changes to the children of a znode), and aversion (number of changes
+ to the ACL of a znode).</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>Ticks</strong>
+</p>
+
+
+<p>When using multi-server ZooKeeper, servers use ticks to define
+ timing of events such as status uploads, session timeouts,
+ connection timeouts between peers, etc. The tick time is only
+ indirectly exposed through the minimum session timeout (2 times the
+ tick time); if a client requests a session timeout less than the
+ minimum session timeout, the server will tell the client that the
+ session timeout is actually the minimum session timeout.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>Real time</strong>
+</p>
+
+
+<p>ZooKeeper doesn't use real time, or clock time, at all except
+ to put timestamps into the stat structure on znode creation and
+ znode modification.</p>
+
+</li>
+
+</ul>
+<a name="N10136"></a><a name="sc_zkStatStructure"></a>
+<h3 class="h4">ZooKeeper Stat Structure</h3>
+<p>The Stat structure for each znode in ZooKeeper is made up of the
+ following fields:</p>
+<ul>
+
+<li>
+
+<p>
+<strong>czxid</strong>
+</p>
+
+
+<p>The zxid of the change that caused this znode to be
+ created.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>mzxid</strong>
+</p>
+
+
+<p>The zxid of the change that last modified this znode.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>ctime</strong>
+</p>
+
+
+<p>The time in milliseconds from epoch when this znode was
+ created.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>mtime</strong>
+</p>
+
+
+<p>The time in milliseconds from epoch when this znode was last
+ modified.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>version</strong>
+</p>
+
+
+<p>The number of changes to the data of this znode.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>cversion</strong>
+</p>
+
+
+<p>The number of changes to the children of this znode.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>aversion</strong>
+</p>
+
+
+<p>The number of changes to the ACL of this znode.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>ephemeralOwner</strong>
+</p>
+
+
+<p>The session id of the owner of this znode if the znode is an
+ ephemeral node. If it is not an ephemeral node, it will be
+ zero.</p>
+
+</li>
+
+</ul>
+</div>
+
+
+<a name="N10194"></a><a name="ZooKeeper+Sessions"></a>
+<h2 class="h3">ZooKeeper Sessions</h2>
+<div class="section">
+<p>When a client gets a handle to the ZooKeeper service, ZooKeeper
+ creates a ZooKeeper session, represented as a 64-bit number, that it
+ assigns to the client. If the client connects to a different ZooKeeper
+ server, it will send the session id as a part of the connection handshake.
+ As a security measure, the server creates a password for the session id
+ that any ZooKeeper server can validate. <remark>[tbd: note from Ben:
+ "perhaps capability is a better word." need clarification on that.]
+ </remark>The password is sent to the client with the session id when the
+ client establishes the session. The client sends this password with the
+ session id whenever it reestablishes the session with a new server.</p>
+<p>One of the parameters to the ZooKeeper client library call to create
+ a ZooKeeper session is the session timeout in milliseconds. The client
+ sends a requested timeout, the server responds with the timeout that it
+ can give the client. The current implementation requires that the timeout
+ be between 2 times the tickTime (as set in the server configuration) and
+ 60 seconds.</p>
+<p>The session is kept alive by requests sent by the client. If the
+ session is idle for a period of time that would timeout the session, the
+ client will send a PING request to keep the session alive. This PING
+ request not only allows the ZooKeeper server to know that the client is
+ still active, but it also allows the client to verify that its connection
+ to the ZooKeeper server is still active. The timing of the PING is
+ conservative enough to ensure reasonable time to detect a dead connection
+ and reconnect to a new server.</p>
+</div>
+
+
+<a name="N101A7"></a><a name="ZooKeeper+Watches"></a>
+<h2 class="h3">ZooKeeper Watches</h2>
+<div class="section">
+<p>All of the read operations in ZooKeeper - <strong>getData()</strong>, <strong>getChildren()</strong>, and <strong>exists()</strong> - have the option of setting a watch as a
+ side effect. Here is ZooKeeper's definition of a watch: a watch event is
+ one-time trigger, sent to the client that set the watch, which occurs when
+ the data for which the watch was set changes. There are three key points
+ to consider in this definition of a watch:</p>
+<ul>
+
+<li>
+
+<p>
+<strong>One-time trigger</strong>
+</p>
+
+
+<p>One watch event will be sent to the client the data has changed.
+ For example, if a client does a getData("/znode1", true) and later the
+ data for /znode1 is changed or deleted, the client will get a watch
+ event for /znode1. If /znode1 changes again, no watch event will be
+ sent unless the client has done another read that sets a new
+ watch.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>Sent to the client</strong>
+</p>
+
+
+<p>This implies that an event is on the way to the client, but may
+ not reach the client before the successful return code to the change
+ operation reaches the client that initiated the change. Watches are
+ sent asynchronously to watchers. ZooKeeper provides an ordering
+ guarantee: a client will never see a change for which it has set a
+ watch until it first sees the watch event. Network delays or other
+ factors may cause different clients to see watches and return codes
+ from updates at different times. The key point is that everything seen
+ by the different clients will have a consistent order.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>The data for which the watch was
+ set</strong>
+</p>
+
+
+<p>This refers to the different ways a node can change. ZooKeeper
+ maintains two lists of watches: data watches and child watches.
+ getData() and exists() set data watches. getChildren() sets child
+ watches. Thus, setData() will trigger data watches for the znode being
+ set (assuming the set is successful). A successful create() will
+ trigger a data watch for the znode being created and a child watch for
+ the parent znode. A successful delete() will trigger both a data watch
+ and a child watch (since there can be no more children) for a znode
+ being deleted as well as a child watch for the parent znode.</p>
+
+</li>
+
+</ul>
+<p>Watches are maintained locally at the ZooKeeper server to which the
+ client is connected. This allows watches to be light weight to set,
+ maintain, and dispatch. It also means if a client connects to a different
+ server, the new server is not going to know about its watches. So, when a
+ client gets a disconnect event, it must consider that an implicit trigger
+ of all watches. When a client reconnects to a new server, the client
+ should re-set any watches that it is still interested in.</p>
+<a name="N101DD"></a><a name="sc_WatchGuarantees"></a>
+<h3 class="h4">What ZooKeeper Guarantees about Watches</h3>
+<p>With regard to watches, ZooKeeper maintains these
+ guarantees:</p>
+<ul>
+
+<li>
+
+<p>Watches are ordered with respect to other events, other
+ watches, and asynchronous replies. The ZooKeeper client libraries
+ ensures that everything is dispatched in order.</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>A client will see a watch event for a znode it is watching
+ before seeing the new data that corresponds to that znode.</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>The order of watch events from ZooKeeper corresponds to the
+ order of the updates as seen by the ZooKeeper service.</p>
+
+</li>
+
+</ul>
+<a name="N10202"></a><a name="sc_WatchRememberThese"></a>
+<h3 class="h4">Things to Remember about Watches</h3>
+<ul>
+
+<li>
+
+<p>Watches are one time triggers; if you get a watch event and
+ you want to get notified of future changes, you must set another
+ watch.</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>Because watches are one time triggers and there is latency
+ between getting the event and sending a new request to get a watch
+ you cannot reliably see every change that happens to a node in
+ ZooKeeper. Be prepared to handle the case where the znode changes
+ multiple times between getting the event and setting the watch
+ again. (You may not care, but at least realize it may
+ happen.)</p>
+
+</li>
+
+</ul>
+<ul>
+
+<li>
+
+<p>When you disconnect from a server (for example, when the
+ server fails), all of the watches you have registered are lost, so
+ you should treat this case as if all your watches were
+ triggered.</p>
+
+</li>
+
+</ul>
+</div>
+
+
+<a name="N10225"></a><a name="Consistency+Guarantees"></a>
+<h2 class="h3">Consistency Guarantees</h2>
+<div class="section">
+<p>ZooKeeper is a high performance, scalable service. Both reads and
+ write operations are designed to be fast, though reads are faster than
+ writes. The reason for this is that in the case of reads, ZooKeeper can
+ serve older data, which in turn is due to ZooKeeper's consistency
+ guarantees:</p>
+<dl>
+
+<dt>
+<term>Sequential Consistency</term>
+</dt>
+<dd>
+<p>Updates from a client will be applied in the order that they
+ were sent.</p>
+</dd>
+
+
+<dt>
+<term>Atomicity</term>
+</dt>
+<dd>
+<p>Updates either succeed or fail -- there are no partial
+ results.</p>
+</dd>
+
+
+<dt>
+<term>Single System Image</term>
+</dt>
+<dd>
+<p>A client will see the same view of the service regardless of
+ the server that it connects to.</p>
+</dd>
+
+
+<dt>
+<term>Reliability</term>
+</dt>
+<dd>
+<p>Once an update has been applied, it will persist from that
+ time forward until a client overwrites the update. This guarantee
+ has two corollaries:</p>
+<ol>
+
+<li>
+
+<p>If a client gets a successful return code, the update will
+ have been applied. On some failures (communication errors,
+ timeouts, etc) the client will not know if the update has
+ applied or not. We take steps to minimize the failures, but the
+ only guarantee is only present with successful return codes.
+ (This is called the _monotonicity condition_ in Paxos.)</p>
+
+</li>
+
+
+<li>
+
+<p>Any updates that are seen by the client, through a read
+ request or successful update, will never be rolled back when
+ recovering from server failures.</p>
+
+</li>
+
+</ol>
+</dd>
+
+
+<dt>
+<term>Timeliness</term>
+</dt>
+<dd>
+<p>The clients view of the system is guaranteed to be up-to-date
+ within a certain time bound. (On the order of tens of seconds.)
+ Either system changes will be seen by a client within this bound, or
+ the client will detect a service outage.</p>
+</dd>
+
+</dl>
+<p>Using these consistency guarantees it is easy to build higher level
+ functions such as leader election, barriers, queues, and read/write
+ revocable locks solely at the ZooKeeper client (no additions needed to
+ ZooKeeper). See <a href="recipes.html">Recipes and Solutions</a>
+ for more details.</p>
+<p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>Sometimes developers mistakenly assume one other guarantee that
+ Zookeeper does <em>not</em> in fact make. This is:</p>
+
+
+<dl>
+
+<dt>
+<term>Simultaneously Conistent Cross-Client Views</term>
+</dt>
+<dd>
+<p>ZooKeeper does not guarantee that at every instance in
+ time, two different clients will have identical views of
+ ZooKeeper data. Due to factors like network delays, one client
+ may perform an update before another client gets notified of the
+ change. Consider the scenario of two clients, A and B. If client
+ A sets the value of a znode /a from 0 to 1, then tells client B
+ to read /a, client B may read the old value of 0, depending on
+ which server in the ZooKeeper quorum it is connected to. If it
+ is important that Client A and Client B read the same value,
+ Client B should should call the <strong>sync()</strong> method from the ZooKeeper API
+ method before it performs its read.</p>
+<p>So, ZooKeeper by itself doesn't guarantee instantaneous,
+ atomic, synchronization across its quorum, but ZooKeeper
+ primitives can be used to construct higher level functions that
+ provide complete client synchronization. (For more information,
+ see the <a href="recipes.html#sc_recipes_Locks">Locks</a>
+
+<remark>[tbd: fix final link target]</remark> in <a href="recipes.html">Zookeeper Recipes</a>.
+ <remark>[tbd: fix final link target]</remark>).</p>
+</dd>
+
+</dl>
+
+</div>
+</div>
+</p>
+</div>
+
+
+<a name="N10291"></a><a name="Bindings"></a>
+<h2 class="h3">Bindings</h2>
+<div class="section">
+<p>The ZooKeeper client libraries come in two languages: Java and C.
+ The following sections describe these.</p>
+<a name="N1029A"></a><a name="Java+Binding"></a>
+<h3 class="h4">Java Binding</h3>
+<p>There are two packages that make up the ZooKeeper Java binding:
+ <strong>org.apache.zookeeper</strong> and <strong>org.apache.zookeeper.data</strong>. The rest of the
+ packages that make up ZooKeeper are used internally or are part of the
+ server implementation. The <strong>org.apache.zookeeper.data</strong> package is made up of
+ generated classes that are used simply as containers.</p>
+<p>The main class used by a ZooKeeper Java client is the <strong>ZooKeeper</strong> class. Its two constructors differ only
+ by an optional session id and password. ZooKeeper supports session
+ recovery accross instances of a process. A Java program may save its
+ session id and password to stable storage, restart, and recover the
+ session that was used by the earlier instance of the program.</p>
+<p>When a ZooKeeper object is created, two threads are created as
+ well: an IO thread and an event thread. All IO happens on the IO thread
+ (using Java NIO). All event callbacks happen on the event thread.
+ Session maintenance such as reconnecting to ZooKeeper servers and
+ maintaining heartbeat is done on the IO thread. Responses for
+ synchronous methods are also processed in the IO thread. All responses
+ to asynchronous methods and watch events are processed on the event
+ thread. There are a few things to notice that result from this
+ design:</p>
+<ul>
+
+<li>
+
+<p>All completions for asynchronous calls and watcher callbacks
+ will be made in order, one at a time. The caller can do any
+ processing they wish, but no other callbacks will be processed
+ during that time.</p>
+
+</li>
+
+
+<li>
+
+<p>Callbacks do not block the processing of the IO thread or the
+ processing of the synchronous calls.</p>
+
+</li>
+
+
+<li>
+
+<p>Synchronous calls may not return in the correct order. For
+ example, assume a client does the following processing: issues an
+ asynchronous read of node <strong>/a</strong> with
+ <em>watch</em> set to true, and then in the completion
+ callback of the read it does a synchronous read of <strong>/a</strong>. (Maybe not good practice, but not illegal
+ either, and it makes for a simple example.)</p>
+
+
+<p>Note that if there is a change to <strong>/a</strong> between the asynchronous read and the
+ synchronous read, the client library will receive the watch event
+ saying <strong>/a</strong> changed before the
+ response for the synchronous read, but because the completion
+ callback is blocking the event queue, the synchronous read will
+ return with the new value of <strong>/a</strong>
+ before the watch event is processed.</p>
+
+</li>
+
+</ul>
+<p>Finally, the rules associated with shutdown are straightforward:
+ once a ZooKeeper object is closed or receives a fatal event
+ (SESSION_EXPIRED and AUTH_FAILED), the ZooKeeper object becomes invalid,
+ the two threads shut down, and any further ZooKeeper calls throw
+ errors.</p>
+<a name="N102E3"></a><a name="C+Binding"></a>
+<h3 class="h4">C Binding</h3>
+<p>The C binding has a single-threaded and multi-threaded library.
+ The multi-threaded library is easiest to use and is most similar to the
+ Java API. This library will create an IO thread and an event dispatch
+ thread for handling connection maintenance and callbacks. The
+ single-threaded library allows ZooKeeper to be used in event driven
+ applications by exposing the event loop used in the multi-threaded
+ library.</p>
+<p>The package includes two shared libraries: zookeeper_st and
+ zookeeper_mt. The former only provides the asynchronous APIs and
+ callbacks for integrating into the application's event loop. The only
+ reason this library exists is to support the platforms were a
+ <em>pthread</em> library is not available or is unstable
+ (i.e. FreeBSD 4.x). In all other cases, application developers should
+ link with zookeeper_mt, as it includes support for both Sync and Async
+ API.</p>
+<a name="N102F2"></a><a name="Installation"></a>
+<h4>Installation</h4>
+<p>If you're building the client from a check-out from the Apache
+ repository, follow the steps outlined below. If you're building from a
+ project source package downloaded from apache, skip to step <strong>3</strong>.</p>
+<ol>
+
+<li>
+
+<p>Run <span class="codefrag command">ant compile_just</span> from the zookeeper
+ top level directory (<span class="codefrag filename">.../trunk/zookeeper</span>).
+ This will create a directory named "generated" under
+ <span class="codefrag filename">zookeeper/c</span>.</p>
+
+</li>
+
+
+<li>
+
+<p>Change directory to the<span class="codefrag filename">zookeeper/c</span> and
+ run <span class="codefrag command">autoreconf -i</span> to bootstrap <strong>autoconf</strong>, <strong>automake</strong> and <strong>libtool</strong>. Make sure you have <strong>autoconf version 2.59</strong> or greater installed.
+ Skip to step<strong> 4</strong>.</p>
+
+</li>
+
+
+<li>
+
+<p>If you are building from a project source package,
+ unzip/untar the source tarball and cd to the<span class="codefrag filename">
+ zookeeper-x.x.x/</span> directory.</p>
+
+</li>
+
+
+<li>
+
+<p>Run <span class="codefrag command">./configure <your-options></span> to
+ generate the makefile. Here are some of options the <strong>configure</strong> utility supports that can be
+ useful in this step:</p>
+
+
+<ul>
+
+<li>
+
+<p>
+<span class="codefrag command">--enable-debug</span>
+</p>
+
+
+<p>Enables optimization and enables debug info compiler
+ options. (Disabled by default.)</p>
+
+</li>
+
+
+<li>
+
+<p>
+<span class="codefrag command">--without-syncapi </span>
+</p>
+
+
+<p>Disables Sync API support; zookeeper_mt library won't be
+ built. (Enabled by default.)</p>
+
+</li>
+
+
+<li>
+
+<p>
+<span class="codefrag command">--disable-static </span>
+</p>
+
+
+<p>Do not build static libraries. (Enabled by
+ default.)</p>
+
+</li>
+
+
+<li>
+
+<p>
+<span class="codefrag command">--disable-shared</span>
+</p>
+
+
+<p>Do not build shared libraries. (Enabled by
+ default.)</p>
+
+</li>
+
+</ul>
+
+
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>See INSTALL for general information about running
+ <strong>configure</strong>. <remark>[tbd: what
+ is INSTALL? a directory? a file?]</remark>
+</p>
+
+</div>
+</div>
+
+</li>
+
+
+<li>
+
+<p>Run <span class="codefrag command">make</span> or <span class="codefrag command">make
+ install</span> to build the libraries and install them.</p>
+
+</li>
+
+
+<li>
+
+<p>To generate doxygen documentation for the ZooKeeper API, run
+ <span class="codefrag command">make doxygen-doc</span>. All documentation will be
+ placed in a new subfolder named docs. By default, this command
+ only generates HTML. For information on other document formats,
+ run <span class="codefrag command">./configure --help</span>
+</p>
+
+</li>
+
+</ol>
+<a name="N1039D"></a><a name="Using+the+Client"></a>
+<h4>Using the Client</h4>
+<p>You can test your client by running a zookeeper server (see
+ instructions on the project wiki page on how to run it) and connecting
+ to it using one of the cli applications that were built as part of the
+ installation procedure. cli_mt (multithreaded, built against
+ zookeeper_mt library) is shown in this example, but you could also use
+ cli_st (singlethreaded, built against zookeeper_st library):</p>
+<p>
+<pre class="code">$ cli_mt zookeeper_host:9876</pre>This
+ is a client application that gives you a shell for executing simple
+ zookeeper commands. Once succesully started and connected to the
+ server it displays a shell prompt. You can now enter zookeeper
+ commands. For example, to create a node:</p>
+<pre class="code">> create /my_new_node</pre>
+<p>To verify that the node's been created:</p>
+<p>You should see a list of node who are children of the root node
+ "/". <remark>[tbd: document all the cli commands (I think this is
+ Ben's tbd? It's from sourceforge)]</remark>
+</p>
+<p>In order to be able to use the ZooKeeper API in your application
+ you have to remember to</p>
+<ol>
+
+<li>
+
+<p>Include zookeeper header: #include
+ <zookeeper/zookeeper.h</p>
+
+</li>
+
+
+<li>
+
+<p>If you are building a multithreaded client, compile with
+ -DTHREADED compiler flag to enable the multi-threaded version of
+ the library, and then link against against the
+ <span class="codefrag varname">zookeeper_mt</span> library. If you are building a
+ single-threaded client, do not compile with -DTHREADED, and be
+ sure to link against the<span class="codefrag varname"> zookeeper_st
+ </span>library.</p>
+
+</li>
+
+</ol>
+<p>Refer to <a href="#ch_programStructureWithExample">Program Structure, with Simple Example</a>for examples of usage in Java and C.
+ <remark>[tbd: some kind of short tutorial would be helpful, I guess
+ (ben's tbd?) ][tbd: whatever the case, make sure that link points to something.]</remark>
+</p>
+</div>
+
+
+<a name="N103DC"></a><a name="Building+Blocks%3A+A+Guide+to+ZooKeeper+Operations"></a>
+<h2 class="h3">Building Blocks: A Guide to ZooKeeper Operations</h2>
+<div class="section">
+<p>
+<remark>[Engineering input needed. This is a new section. The below
+ is just placeholder, and was actually copied from the overview book. There
+ should probably be a subsection on each of those operations, with a little
+ bit of illustrative code for each op.] </remark>
+</p>
+<p>One of the design goals of ZooKeeper is provide a very simple
+ programming interface. As a result, it supports only these
+ operations:</p>
+<dl>
+
+<dt>
+<term>create</term>
+</dt>
+<dd>
+<p>creates a node at a location in the tree</p>
+</dd>
+
+
+<dt>
+<term>delete</term>
+</dt>
+<dd>
+<p>deletes a node</p>
+</dd>
+
+
+<dt>
+<term>exists</term>
+</dt>
+<dd>
+<p>tests if a node exists at a location</p>
+</dd>
+
+
+<dt>
+<term>get data</term>
+</dt>
+<dd>
+<p>reads the data from a node</p>
+</dd>
+
+
+<dt>
+<term>set data</term>
+</dt>
+<dd>
+<p>writes data to a node</p>
+</dd>
+
+
+<dt>
+<term>get children</term>
+</dt>
+<dd>
+<p>retrieves a list of children of a node</p>
+</dd>
+
+
+<dt>
+<term>sync</term>
+</dt>
+<dd>
+<p>waits for data to be propagated.</p>
+</dd>
+
+</dl>
+</div>
+
+
+<a name="N1041E"></a><a name="Program+Structure%2C+with+Simple+Example"></a>
+<h2 class="h3">Program Structure, with Simple Example</h2>
+<div class="section">
+<p>
+<remark>[tbd]</remark>
+</p>
+</div>
+
+
+<a name="N10429"></a><a name="Gotchas%3A+Common+Problems+and+Troubleshooting"></a>
+<h2 class="h3">Gotchas: Common Problems and Troubleshooting</h2>
+<div class="section">
+<p>So now you know ZooKeeper. It's fast, simple, your application
+ works, but wait ... something's wrong. Here are some pitfalls that
+ ZooKeeper users fall into:</p>
+<ol>
+
+<li>
+
+<p>If you are using watches, you must look for the connected watch
+ event. When a ZooKeeper client disconnects from a server, all the
+ watches are removed, so a client must treat the disconnect event as an
+ implicit trigger of watches. The easiest way to deal with this is to
+ act like the connected watch event is a watch trigger for all your
+ watches. The connected event makes a better trigger than the
+ disconnected event because you can access ZooKeeper and reestablish
+ watches when you are connected.</p>
+
+</li>
+
+
+<li>
+
+<p>You must test ZooKeeper server failures. The ZooKeeper service
+ can survive failures as long as a majority of servers are active. The
+ question to ask is: can your application handle it? In the real world
+ a client's connection to ZooKeeper can break. (ZooKeeper server
+ failures and network partitions are common reasons for connection
+ loss.) The ZooKeeper client library takes care of recovering your
+ connection and letting you know what happened, but you must make sure
+ that you recover your state and any outstanding requests that failed.
+ Find out if you got it right in the test lab, not in production - test
+ with a ZooKeeper service made up of a several of servers and subject
+ them to reboots.</p>
+
+</li>
+
+
+<li>
+
+<p>The list of ZooKeeper servers used by the client must match the
+ list of ZooKeeper servers that each ZooKeeper server has. Things can
+ work, although not optimally, if the client list is a subset of the
+ real list of ZooKeeper servers, but not if the client lists ZooKeeper
+ servers not in the ZooKeeper cluster.</p>
+
+</li>
+
+
+<li>
+
+<p>Be careful where you put that transaction log. The most
+ performance-critical part of ZooKeeper is the transaction log.
+ ZooKeeper must sync transactions to media before it returns a
+ response. A dedicated transaction log device is key to consistent good
+ performance. Putting the log on a busy device will adversely effect
+ performance. If you only have one storage device, put trace files on
+ NFS and increase the snapshotCount; it doesn't eliminate the problem,
+ but it can mitigate it.</p>
+
+</li>
+
+
+<li>
+
+<p>Set your Java max heap size correctly. It is very important to
+ <em>avoid swapping.</em> Going to disk unnecessarily will
+ almost certainly degrade your performance unacceptably. Remember, in
+ ZooKeeper, everything is ordered, so if one request hits the disk, all
+ other queued requests hit the disk.</p>
+
+
+<p>To avoid swapping, try to set the heapsize to the amount of
+ physical memory you have, minus the amount needed by the OS and cache.
+ The best way to determine an optimal heap size for your configurations
+ is to <em>run load tests</em>. If for some reason you
+ can't, be conservative in your estimates and choose a number well
+ below the limit that would cause your machine to swap. For example, on
+ a 4G machine, a 3G heap is a conservative estimate to start
+ with.</p>
+
+</li>
+
+</ol>
+</div>
+
+
+<a name="apx_linksToOtherInfo"></a>
+<appendix id="apx_linksToOtherInfo">
+
+<title>Links to Other Information</title>
+
+
+<p>Outside the formal documentation, there're several other sources of
+ information for ZooKeeper developers.</p>
+
+
+<dl>
+
+<dt>
+<term>ZooKeeper Whitepaper <remark>[tbd: find url]</remark>
+</term>
+</dt>
+<dd>
+<p>The definitive discussion of ZooKeeper design and performance,
+ by Yahoo! Research</p>
+</dd>
+
+
+<dt>
+<term>API Reference <remark>[tbd: find url]</remark>
+</term>
+</dt>
+<dd>
+<p>The complete reference to the ZooKeeper API</p>
+</dd>
+
+
+<dt>
+<term>
+<a href="http://us.dl1.yimg.com/download.yahoo.com/dl/ydn/zookeeper.m4v">Zookeeper
+ Talk at the Hadoup Summit 2008</a>
+</term>
+</dt>
+<dd>
+<p>A video introduction to ZooKeeper, by Benjamin Reed of Yahoo!
+ Research</p>
+</dd>
+
+
+<dt>
+<term>
+<a href="http://wiki.apache.org/hadoop/ZooKeeper/Tutorial">Barrier and
+ Queue Tutorial</a>
+</term>
+</dt>
+<dd>
+<p>The excellent Java tutorial by Flavio Junqueira, implementing
+ simple barriers and producer-consumer queues using ZooKeeper.</p>
+</dd>
+
+
+<dt>
+<term>
+<a href="http://wiki.apache.org/hadoop/ZooKeeper/ZooKeeperArticles">ZooKeeper
+ - A Reliable, Scalable Distributed Coordination System</a>
+</term>
+</dt>
+<dd>
+<p>An article by Todd Hoff (07/15/2008)</p>
+</dd>
+
+
+<dt>
+<term>
+<a href="recipes.html">Zookeeper Recipes [tbd: fix
+ linkend for apache site]</a>
+</term>
+</dt>
+<dd>
+<p>Pseudo-level discussion of the implementation of various
+ synchronization solutions with ZooKeeper: Event Handles, Queues,
+ Locks, and Two-phase Commits.</p>
+</dd>
+
+
+<dt>
+<term>
+<remark>[tbd]</remark>
+</term>
+</dt>
+<dd>
+<p>Whatever good sources anyone can think of...</p>
+</dd>
+
+</dl>
+
+</appendix>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+ |end content
+ +-->
+<div class="clearboth"> </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 ©
+ 2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
+ |end bottomstrip
+ +-->
+</div>
+</body>
+</html>
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+<div class="menupagetitle">Getting Started</div>
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+ PDF</a>
+</div>
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#Getting+Started%3A+Coordinating+Distributed+Applications+with%0A++++++ZooKeeper">Getting Started: Coordinating Distributed Applications with
+ ZooKeeper</a>
+<ul class="minitoc">
+<li>
+<a href="#sc_InstallingSingleMode">Installing and Running ZooKeeper in Single Server Mode</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>
+
+<title>ZooKeeper Getting Started Guide</title>
+
+
+
+
+<a name="N1000A"></a><a name="Getting+Started%3A+Coordinating+Distributed+Applications+with%0A++++++ZooKeeper"></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="N10017"></a><a name="sc_InstallingSingleMode"></a>
+<h3 class="h4">Installing and Running ZooKeeper in Single Server Mode</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 copying a JAR file and creating a
+ configuration.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>Zookeeper requires Java 1.5 or more recent.</p>
+
+</div>
+</div>
+<p>[tbd: should we start w/ a word here about were to get the source,
+ exactly what to download, how to unpack it, and where to put it? Also,
+ does the user need to be in sudo, or can they be under their regular
+ login?]</p>
+<p>Once you have downloaded the ZooKeeper source, cd to the root of
+ your ZooKeeper source, and run "ant jar". For example:<pre class="code">$ cd ~/dev/zookeeper
+
+$ ~/dev/zookeeper/: ant jar</pre>
+</p>
+<p>This should generate a JAR file called zookeeper.jar. To start
+ Zookeeper, compile and run zookeeper.jar. <em>[tbd, some more
+ instruction here. Perhaps a command line? Are these two steps or
+ one?]</em>
+</p>
+<p>To start ZooKeeper you need a configuration file. Here is a sample
+ file:</p>
+<p>
+<pre class="code">tickTime=2000
+dataDir=/var/zookeeper/
+clientPort=2181
+</pre>
+</p>
+<p>This file can be called anything, but for the sake of this
+ discussion, call it <strong>zoo.cfg</strong>. 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>
+<p>
+<pre class="code">java -cp zookeeper-dev.jar:java/lib/log4j-1.2.15.jar:conf org.apache.zookeeper.server.quorum.QuorumPeerMain zoo.cfg</pre>
+</p>
+<p>ZooKeeper logs messages using log4j -- more detail available in
+ the <a href="zookeeperProgrammers.html#Logging">Logging</a>
+ section of the Programmer's Guide.<remark revision="include_tbd">[tbd:
+ real reference needed]</remark> You will see log messages coming to the
+ console 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>
+<p></p>
+<a name="N1007A"></a><a name="sc_ConnectingToZooKeeper"></a>
+<h3 class="h4">Connecting to ZooKeeper</h3>
+<p>Once ZooKeeper is running, you have several option for connection
+ to it:</p>
+<ul>
+
+<li>
+
+<p>
+<strong>Java</strong>: Use java -cp
+ zookeeper.jar:java/lib/log4j-1.2.15.jar:conf
+ org.apache.zookeeper.ZooKeeperMain 127.0.0.1:2181</p>
+
+
+<p>This lets you perform simple, file-like operations.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<strong>C</strong>: compile cli_mt
+ (multi-threaded) or cli_st (single-threaded) by running
+ <span class="codefrag command">_make cli_mt_</span> or <span class="codefrag command">_make cli_st_</span>
+ in the c subdirectory in the ZooKeeper sources.</p>
+
+
+<p>You can run the program using <em>LD_LIBRARY_PATH=.
+ cli_mt 127.0.0.1:2181</em> or <em>LD_LIBRARY_PATH=.
+ cli_st 127.0.0.1:2181</em>. This will give you a simple shell
+ to execute file system like operations on ZooKeeper.</p>
+
+</li>
+
+</ul>
+<a name="N100AB"></a><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. <remark>[tbd: what is the messaging loop? Do we talk about it
+ anywyhere? is this too much info for a getting started guide?]</remark>
+ For more information, see the <a href="zookeeperProgrammers.html#ch_programStructureWithExample.html">Programming
+ Examples in the Zookeeper Programmer's Guide</a> for
+ sample code using of the different APIs.</p>
+<a name="N100BC"></a><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. The file is similar to the one used in standalone mode, but with a
+ few differences. Here is an example:</p>
+<p>
+<pre class="code">tickTime=2000
+dataDir=/var/zookeeper/
+clientPort=2181
+initLimit=5
+syncLimit=2
+server.1=zoo1:2888 server.2=zoo2:2888
+server.3=zoo3:2888 </pre>
+</p>
+<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. [TBD: someone please verify that the previous is
+ true.]</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 *myid* in the data
+ directory. That file has the contains the server number, in
+ ASCII.</p>
+<p>Finally, note the "2888" port numbers after each server name.
+ These are the "electionPort" numbers of the servers (as opposed to
+ clientPorts), that is ports for <remark>[tbd: feedback need: what are
+ these ports, exactly?]</remark>.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+
+<p>If you want to test multiple servers on a single machine, define
+ the electionPort for each server in that server's config file, using
+ the line <span class="codefrag command">electionPort=xxxx</span> as means of avoiding
+ clashes.</p>
+
+</div>
+</div>
+<a name="N100F2"></a><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 *myid* file. The
+ dataLogDir parameters indicates a different directory to use for the
+ transaction logs.</p>
+
+</li>
+
+
+<li>
+
+<p>
+<remark>[tbd: feedback need: what is the other config param?
+ (I believe two are mentioned above.)]</remark>
+</p>
+
+</li>
+
+</ul>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
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--- hadoop/zookeeper/trunk/src/docs/forrest.properties (original)
+++ hadoop/zookeeper/trunk/src/docs/forrest.properties Wed Sep 24 18:05:14 2008
@@ -102,3 +102,5 @@
#project.issues-rss-url=
#I18n Property only works for the "forrest run" target.
#project.i18n=true
+
+project.required.plugins=org.apache.forrest.plugin.output.pdf,org.apache.forrest.plugin.input.simplifiedDocbook
|