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From ph...@apache.org
Subject [04/42] zookeeper git commit: Website update for release 3.5.4-beta
Date Thu, 17 May 2018 22:56:47 GMT
http://git-wip-us.apache.org/repos/asf/zookeeper/blob/ec4ec140/content/doc/r3.5.4-beta/zookeeperOver.html
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class="skin"><br>
+        PDF</a>
+</div>
+<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 achieve 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 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 znode. 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. 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 the 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://zookeeper.apache.org/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="https://cwiki.apache.org/confluence/display/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://zookeeper.apache.org/">
+          Zookeeper Project on Apache
+        </a>
+        for more information.
+      </p>
+</div>
+
+<p align="right">
+<font size="-2"></font>
+</p>
+</div>
+<!--+
+    |end content
+    +-->
+<div class="clearboth">&nbsp;</div>
+</div>
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+<!--+
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+<script type="text/javascript"><!--
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+//  --></script>
+</div>
+<div class="copyright">
+        Copyright &copy;
+          <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
+</div>
+<!--+
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+</html>

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