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+<h1>ZooKeeper</h1>
+<div id="front-matter">
+<div id="minitoc-area">
+<ul class="minitoc">
+<li>
+<a href="#ch_DesignOverview">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="#Reliability">Reliability</a>
+</li>
+<li>
+<a href="#The+ZooKeeper+Project">The ZooKeeper Project</a>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+
+
+
+
+
+<a name="ch_DesignOverview"></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="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 hosts called an ensemble.</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="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="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 <em>[tbd]</em>.</p>
+<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 <em>[tbd]</em>.</p>
+<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
+ <em>[tbd]</em>
+</p>
+<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
+ <em>[tbd]</em>
+</p>
+<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,
+ each of
+ the servers that make up the ZooKeeper service replicates its own copy
+ of each of components.</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>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="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: <em>[tbd: add uses from
+ white paper and video presentation.]</em> For more information, see
+ <em>[tbd]</em>
+</p>
+<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>
+<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-3.2.jpg">
+
+ </td>
+</tr>
+</table>
+<p>The figure <a href="#fg_zkPerfRW">ZooKeeper Throughput as the Read-Write Ratio Varies</a> is a throughput
+ graph of ZooKeeper release 3.2 running on servers with dual 2Ghz
+ Xeon and two SATA 15K RPM drives. One drive was used as a
+ dedicated ZooKeeper log device. The snapshots were written to
+ the OS drive. Write requests were 1K writes and the reads were
+ 1K reads. "Servers" indicate the size of the ZooKeeper
+ ensemble, the number of servers that make up the
+ service. Approximately 30 other servers were used to simulate
+ the clients. The ZooKeeper ensemble was configured such that
+ leaders do not allow connections from clients.</p>
+<div class="note">
+<div class="label">Note</div>
+<div class="content">
+<p>In version 3.2 r/w performance improved by ~2x
+ compared to the <a href="http://hadoop.apache.org/zookeeper/docs/r3.1.1/zookeeperOver.html#Performance">previous
+ 3.1 release</a>.</p>
+</div>
+</div>
+<p>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>
+<a name="Reliability"></a>
+<h3 class="h4">Reliability</h3>
+<p>To show the behavior of the system over time as
+ failures are injected we ran a ZooKeeper service made up of
+ 7 machines. We ran the same saturation benchmark as before,
+ but this time we kept the write percentage at a constant
+ 30%, which is a conservative ratio of our expected
+ workloads.
+ </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>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="The+ZooKeeper+Project"></a>
+<h3 class="h4">The ZooKeeper Project</h3>
+<p>ZooKeeper has been
+ <a href="http://wiki.apache.org/hadoop/ZooKeeper/PoweredBy">
+ successfully used
+ </a>
+ in many 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. A number of
+ Yahoo! advertising systems also use ZooKeeper to implement
+ reliable services.
+ </p>
+<p>All users and developers are encouraged 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
+ +-->
+<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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