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<h1 title="Package" class="title">Package org.apache.zookeeper.server</h1>
<div class="docSummary">
<div class="block">ZooKeeper server theory of operation</div>
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<p>See: <a href="#package.description">Description</a></p>
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<th class="colFirst" scope="col">Interface</th>
<th class="colLast" scope="col">Description</th>
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<tbody>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ConnectionMXBean.html" title="interface in org.apache.zookeeper.server">ConnectionMXBean</a></td>
<td class="colLast">
<div class="block">This MBean represents a client connection.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DataTreeMXBean.html" title="interface in org.apache.zookeeper.server">DataTreeMXBean</a></td>
<td class="colLast">
<div class="block">Zookeeper data tree MBean.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NodeHashMap.html" title="interface in org.apache.zookeeper.server">NodeHashMap</a></td>
<td class="colLast">
<div class="block">The interface defined to manage the hash based on the entries in the
nodes map.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/RequestProcessor.html" title="interface in org.apache.zookeeper.server">RequestProcessor</a></td>
<td class="colLast">
<div class="block">RequestProcessors are chained together to process transactions.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerStats.Provider.html" title="interface in org.apache.zookeeper.server">ServerStats.Provider</a></td>
<td class="colLast"> </td>
</tr>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SessionTracker.html" title="interface in org.apache.zookeeper.server">SessionTracker</a></td>
<td class="colLast">
<div class="block">This is the basic interface that ZooKeeperServer uses to track sessions.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SessionTracker.Session.html" title="interface in org.apache.zookeeper.server">SessionTracker.Session</a></td>
<td class="colLast"> </td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SessionTracker.SessionExpirer.html" title="interface in org.apache.zookeeper.server">SessionTracker.SessionExpirer</a></td>
<td class="colLast"> </td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerListener.html" title="interface in org.apache.zookeeper.server">ZooKeeperServerListener</a></td>
<td class="colLast">
<div class="block">Listener for the critical resource events.</div>
</td>
</tr>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerMXBean.html" title="interface in org.apache.zookeeper.server">ZooKeeperServerMXBean</a></td>
<td class="colLast">
<div class="block">ZooKeeper server MBean.</div>
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<th class="colFirst" scope="col">Class</th>
<th class="colLast" scope="col">Description</th>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/AuthenticationHelper.html" title="class in org.apache.zookeeper.server">AuthenticationHelper</a></td>
<td class="colLast">
<div class="block">Contains helper methods to enforce authentication</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/BlueThrottle.html" title="class in org.apache.zookeeper.server">BlueThrottle</a></td>
<td class="colLast">
<div class="block">Implements a token-bucket based rate limiting mechanism with optional
probabilistic dropping inspired by the BLUE queue management algorithm [1].</div>
</td>
</tr>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ByteBufferInputStream.html" title="class in org.apache.zookeeper.server">ByteBufferInputStream</a></td>
<td class="colLast"> </td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ByteBufferOutputStream.html" title="class in org.apache.zookeeper.server">ByteBufferOutputStream</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ConnectionBean.html" title="class in org.apache.zookeeper.server">ConnectionBean</a></td>
<td class="colLast">
<div class="block">Implementation of connection MBean interface.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ContainerManager.html" title="class in org.apache.zookeeper.server">ContainerManager</a></td>
<td class="colLast">
<div class="block">Manages cleanup of container ZNodes.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DatadirCleanupManager.html" title="class in org.apache.zookeeper.server">DatadirCleanupManager</a></td>
<td class="colLast">
<div class="block">This class manages the cleanup of snapshots and corresponding transaction
logs by scheduling the auto purge task with the specified
'autopurge.purgeInterval'.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DataNode.html" title="class in org.apache.zookeeper.server">DataNode</a></td>
<td class="colLast">
<div class="block">This class contains the data for a node in the data tree.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DataTree.html" title="class in org.apache.zookeeper.server">DataTree</a></td>
<td class="colLast">
<div class="block">This class maintains the tree data structure.</div>
</td>
</tr>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DataTree.ProcessTxnResult.html" title="class in org.apache.zookeeper.server">DataTree.ProcessTxnResult</a></td>
<td class="colLast"> </td>
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<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DataTreeBean.html" title="class in org.apache.zookeeper.server">DataTreeBean</a></td>
<td class="colLast">
<div class="block">This class implements the data tree MBean.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DigestCalculator.html" title="class in org.apache.zookeeper.server">DigestCalculator</a></td>
<td class="colLast">
<div class="block">Defines how to calculate the digest for a given node.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DumbWatcher.html" title="class in org.apache.zookeeper.server">DumbWatcher</a></td>
<td class="colLast">
<div class="block">A empty watcher implementation used in bench and unit test.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ExpiryQueue.html" title="class in org.apache.zookeeper.server">ExpiryQueue</a><E></td>
<td class="colLast">
<div class="block">ExpiryQueue tracks elements in time sorted fixed duration buckets.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/FinalRequestProcessor.html" title="class in org.apache.zookeeper.server">FinalRequestProcessor</a></td>
<td class="colLast">
<div class="block">This Request processor actually applies any transaction associated with a
request and services any queries.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NettyServerCnxn.html" title="class in org.apache.zookeeper.server">NettyServerCnxn</a></td>
<td class="colLast"> </td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NettyServerCnxnFactory.html" title="class in org.apache.zookeeper.server">NettyServerCnxnFactory</a></td>
<td class="colLast"> </td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NIOServerCnxn.html" title="class in org.apache.zookeeper.server">NIOServerCnxn</a></td>
<td class="colLast">
<div class="block">This class handles communication with clients using NIO.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NIOServerCnxnFactory.html" title="class in org.apache.zookeeper.server">NIOServerCnxnFactory</a></td>
<td class="colLast">
<div class="block">NIOServerCnxnFactory implements a multi-threaded ServerCnxnFactory using
NIO non-blocking socket calls.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NodeHashMapImpl.html" title="class in org.apache.zookeeper.server">NodeHashMapImpl</a></td>
<td class="colLast">
<div class="block">a simple wrapper to ConcurrentHashMap that recalculates a digest after
each mutation.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ObserverBean.html" title="class in org.apache.zookeeper.server">ObserverBean</a></td>
<td class="colLast">
<div class="block">ObserverBean</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/PrepRequestProcessor.html" title="class in org.apache.zookeeper.server">PrepRequestProcessor</a></td>
<td class="colLast">
<div class="block">This request processor is generally at the start of a RequestProcessor
change.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/PurgeTxnLog.html" title="class in org.apache.zookeeper.server">PurgeTxnLog</a></td>
<td class="colLast">
<div class="block">this class is used to clean up the
snapshot and data log dir's.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/RateLogger.html" title="class in org.apache.zookeeper.server">RateLogger</a></td>
<td class="colLast">
<div class="block">This logs the message once in the beginning and once every LOG_INTERVAL.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ReferenceCountedACLCache.html" title="class in org.apache.zookeeper.server">ReferenceCountedACLCache</a></td>
<td class="colLast"> </td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/Request.html" title="class in org.apache.zookeeper.server">Request</a></td>
<td class="colLast">
<div class="block">This is the structure that represents a request moving through a chain of
RequestProcessors.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/RequestThrottler.html" title="class in org.apache.zookeeper.server">RequestThrottler</a></td>
<td class="colLast">
<div class="block">When enabled, the RequestThrottler limits the number of outstanding requests
currently submitted to the request processor pipeline.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ResponseCache.html" title="class in org.apache.zookeeper.server">ResponseCache</a></td>
<td class="colLast"> </td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxn.html" title="class in org.apache.zookeeper.server">ServerCnxn</a></td>
<td class="colLast">
<div class="block">Interface to a Server connection - represents a connection from a client
to the server.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxnFactory.html" title="class in org.apache.zookeeper.server">ServerCnxnFactory</a></td>
<td class="colLast"> </td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxnHelper.html" title="class in org.apache.zookeeper.server">ServerCnxnHelper</a></td>
<td class="colLast"> </td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerConfig.html" title="class in org.apache.zookeeper.server">ServerConfig</a></td>
<td class="colLast">
<div class="block">Server configuration storage.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerMetrics.html" title="class in org.apache.zookeeper.server">ServerMetrics</a></td>
<td class="colLast"> </td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerStats.html" title="class in org.apache.zookeeper.server">ServerStats</a></td>
<td class="colLast">
<div class="block">Basic Server Statistics</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SessionTrackerImpl.html" title="class in org.apache.zookeeper.server">SessionTrackerImpl</a></td>
<td class="colLast">
<div class="block">This is a full featured SessionTracker.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SessionTrackerImpl.SessionImpl.html" title="class in org.apache.zookeeper.server">SessionTrackerImpl.SessionImpl</a></td>
<td class="colLast"> </td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SnapshotComparer.html" title="class in org.apache.zookeeper.server">SnapshotComparer</a></td>
<td class="colLast">
<div class="block">SnapshotComparer is a tool that loads and compares two snapshots with configurable threshold and various filters, and outputs information about the delta.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SnapshotFormatter.html" title="class in org.apache.zookeeper.server">SnapshotFormatter</a></td>
<td class="colLast">
<div class="block">Dump a snapshot file to stdout.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/SyncRequestProcessor.html" title="class in org.apache.zookeeper.server">SyncRequestProcessor</a></td>
<td class="colLast">
<div class="block">This RequestProcessor logs requests to disk.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/TraceFormatter.html" title="class in org.apache.zookeeper.server">TraceFormatter</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/TxnLogEntry.html" title="class in org.apache.zookeeper.server">TxnLogEntry</a></td>
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<div class="block">A helper class to represent the txn entry.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/TxnLogProposalIterator.html" title="class in org.apache.zookeeper.server">TxnLogProposalIterator</a></td>
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<div class="block">This class provides an iterator interface to access Proposal deserialized
from on-disk txnlog.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/UnimplementedRequestProcessor.html" title="class in org.apache.zookeeper.server">UnimplementedRequestProcessor</a></td>
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<div class="block">Manages the unknown requests (i.e.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/WorkerService.html" title="class in org.apache.zookeeper.server">WorkerService</a></td>
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<div class="block">WorkerService is a worker thread pool for running tasks and is implemented
using one or more ExecutorServices.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/WorkerService.WorkRequest.html" title="class in org.apache.zookeeper.server">WorkerService.WorkRequest</a></td>
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<div class="block">Callers should implement a class extending WorkRequest in order to
schedule work with the service.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZKDatabase.html" title="class in org.apache.zookeeper.server">ZKDatabase</a></td>
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<div class="block">This class maintains the in memory database of zookeeper
server states that includes the sessions, datatree and the
committed logs.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperCriticalThread.html" title="class in org.apache.zookeeper.server">ZooKeeperCriticalThread</a></td>
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<div class="block">Represents critical thread.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperSaslServer.html" title="class in org.apache.zookeeper.server">ZooKeeperSaslServer</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServer.html" title="class in org.apache.zookeeper.server">ZooKeeperServer</a></td>
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<div class="block">This class implements a simple standalone ZooKeeperServer.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerBean.html" title="class in org.apache.zookeeper.server">ZooKeeperServerBean</a></td>
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<div class="block">This class implements the ZooKeeper server MBean interface.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerConf.html" title="class in org.apache.zookeeper.server">ZooKeeperServerConf</a></td>
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<div class="block">Configuration data for a <a href="../../../../org/apache/zookeeper/server/ZooKeeperServer.html" title="class in org.apache.zookeeper.server"><code>ZooKeeperServer</code></a>.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerMain.html" title="class in org.apache.zookeeper.server">ZooKeeperServerMain</a></td>
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<div class="block">This class starts and runs a standalone ZooKeeperServer.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServerShutdownHandler.html" title="class in org.apache.zookeeper.server">ZooKeeperServerShutdownHandler</a></td>
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<div class="block">ZooKeeper server shutdown handler which will be used to handle ERROR or
SHUTDOWN server state transitions, which in turn releases the associated
shutdown latch.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperThread.html" title="class in org.apache.zookeeper.server">ZooKeeperThread</a></td>
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<div class="block">This is the main class for catching all the uncaught exceptions thrown by the
threads.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooTrace.html" title="class in org.apache.zookeeper.server">ZooTrace</a></td>
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<div class="block">This class encapsulates and centralizes tracing for the ZooKeeper server.</div>
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<th class="colFirst" scope="col">Enum</th>
<th class="colLast" scope="col">Description</th>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/DatadirCleanupManager.PurgeTaskStatus.html" title="enum in org.apache.zookeeper.server">DatadirCleanupManager.PurgeTaskStatus</a></td>
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<div class="block">Status of the dataDir purge task</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/EphemeralType.html" title="enum in org.apache.zookeeper.server">EphemeralType</a></td>
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<div class="block">
Abstraction that interprets the <code>ephemeralOwner</code> field of a ZNode.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/EphemeralTypeEmulate353.html" title="enum in org.apache.zookeeper.server">EphemeralTypeEmulate353</a></td>
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<div class="block">See https://issues.apache.org/jira/browse/ZOOKEEPER-2901
version 3.5.3 introduced bugs associated with how TTL nodes were implemented.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ExitCode.html" title="enum in org.apache.zookeeper.server">ExitCode</a></td>
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<div class="block">Exit code used to exit server</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/NettyServerCnxn.HandshakeState.html" title="enum in org.apache.zookeeper.server">NettyServerCnxn.HandshakeState</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxn.DisconnectReason.html" title="enum in org.apache.zookeeper.server">ServerCnxn.DisconnectReason</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServer.State.html" title="enum in org.apache.zookeeper.server">ZooKeeperServer.State</a></td>
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<th class="colFirst" scope="col">Exception</th>
<th class="colLast" scope="col">Description</th>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ClientCnxnLimitException.html" title="class in org.apache.zookeeper.server">ClientCnxnLimitException</a></td>
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<div class="block">Indicates that the number of client connections has exceeded some limit.</div>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/RequestProcessor.RequestProcessorException.html" title="class in org.apache.zookeeper.server">RequestProcessor.RequestProcessorException</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxn.CloseRequestException.html" title="class in org.apache.zookeeper.server">ServerCnxn.CloseRequestException</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ServerCnxn.EndOfStreamException.html" title="class in org.apache.zookeeper.server">ServerCnxn.EndOfStreamException</a></td>
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<td class="colFirst"><a href="../../../../org/apache/zookeeper/server/ZooKeeperServer.MissingSessionException.html" title="class in org.apache.zookeeper.server">ZooKeeperServer.MissingSessionException</a></td>
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<h2 title="Package org.apache.zookeeper.server Description">Package org.apache.zookeeper.server Description</h2>
<div class="block"><h1>ZooKeeper server theory of operation</h1>
ZooKeeperServer is designed to work in standalone mode and also
be extensible so that it can be used to implement the quorum based
version of ZooKeeper.
<p>
ZooKeeper maintains a order when processing requests:
<ul>
<li>All requests will be processed in order.
<li>All responses will return in order.
<li>All watches will be sent in the order that the update takes place.
</ul>
<p>
We will explain the three aspects of ZooKeeperServer: request processing, data
structure maintenance, and session tracking.
<h2>Request processing</h2>
Requests are received by the ServerCnxn. Demarshalling of a request is
done by ClientRequestHandler. After a request has been demarshalled,
ClientRequestHandler invokes the relevant method in ZooKeeper and marshals
the result.
<p>
If the request is just a query, it will be processed by ZooKeeper and returned.
Otherwise, the request will be validated and a transaction will be generated
and logged. This the request will then wait until the request has been logged
before continuing processing.
<p>
Requests are logged as a group. Transactions are queued up and the SyncThread
will process them at predefined intervals. (Currently 20ms) The SyncThread
interacts with ZooKeeperServer the txnQueue. Transactions are added to the
txnQueue of SyncThread via queueItem. When the transaction has been synced to
disk, its callback will be invoked which will cause the request processing to
be completed.
<h2>Data structure maintenance</h2>
ZooKeeper data is stored in-memory. Each znode is stored in a DataNode object.
This object is accessed through a hash table that maps paths to DataNodes.
DataNodes also organize themselves into a tree. This tree is only used for
serializing nodes.
<p>
We guarantee that changes to nodes are stored to non-volatile media before
responding to a client. We do this quickly by writing changes as a sequence
of transactions in a log file. Even though we flush transactions as a group,
we need to avoid seeks as much as possible. Also, since the server can fail
at any point, we need to be careful of partial records.
<p>
We address the above problems by
<ul>
<li>Pre-allocating 1M chunks of file space. This allows us to append to the
file without causing seeks to update file size. It also means that we need
to check for the end of the log by looking for a zero length transaction
rather than simply end of file.
<li>Writing a signature at the end of each transaction. When processing
transactions, we only use transactions that have a valid signature at the end.
</ul>
<p>
As the server runs, the log file will grow quite large. To avoid long startup
times we periodically take a snapshot of the tree of DataNodes. We cannot
take the snapshot synchronously as the data takes a while to write out, so
instead we asynchronously write out the tree. This means that we end up
with a "corrupt" snapshot of the data tree. More formally if we define T
to be the real snapshot of the tree at the time we begin taking the snapshot
and l as the sequence of transactions that are applied to the tree between
the time the snapshot begins and the time the snapshot completes, we write
to disk T+l' where l' is a subset of the transactions in l. While we do not
have a way of figuring out which transactions make up l', it doesn't really
matter. T+l'+l = T+l since the transactions we log are idempotent (applying
the transaction multiple times has the same result as applying the transaction
once). So when we restore the snapshot we also play all transactions in the log
that occur after the snapshot was begun. We can easily figure out where to
start the replay because we start a new logfile when we start a snapshot. Both
the snapshot file and log file have a numeric suffix that represent the
transaction id that created the respective files.
<h2>Session tracking</h2>
Rather than tracking sessions exactly, we track them in batches. That are
processed at fixed intervals. This is easier to implement than exact
session tracking and it is more efficient in terms of performance. It also
provides a small grace period for session renewal.</div>
</div>
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