<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Replication Manager Permanent Message Handling</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> <link rel="start" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" /> <link rel="up" href="repapp.html" title="Chapter 3. The DB Replication Manager" /> <link rel="prev" href="repmgr_init_example_c.html" title="Adding the Replication Manager to SimpleTxn" /> <link rel="next" href="electiontimes.html" title="Managing Election Times" /> </head> <body> <div xmlns="" class="navheader"> <div class="libver"> <p>Library Version 12.1.6.1</p> </div> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Replication Manager Permanent Message Handling</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="repmgr_init_example_c.html">Prev</a> </td> <th width="60%" align="center">Chapter 3. The DB Replication Manager</th> <td width="20%" align="right"> <a accesskey="n" href="electiontimes.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="fwrkpermmessage"></a>Replication Manager Permanent Message Handling</h2> </div> </div> </div> <div class="toc"> <dl> <dt> <span class="sect2"> <a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a> </span> </dt> <dt> <span class="sect2"> <a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a> </span> </dt> <dt> <span class="sect2"> <a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to <span>RepQuoteExampleGSG</span> </a> </span> </dt> </dl> </div> <p> As described in <a class="xref" href="permmessages.html" title="Permanent Message Handling">Permanent Message Handling</a>, messages are marked permanent if they contain database modifications that should be committed at the replica. DB's replication code decides if it must flush its transaction logs to disk depending on whether it receives sufficient permanent message acknowledgments from the participating replicas. More importantly, the thread performing the transaction commit blocks until it either receives enough acknowledgments, or the acknowledgment timeout expires. </p> <p> The Replication Manager is fully capable of managing permanent messages for you if your application requires it (most do). Almost all of the details of this are handled by the Replication Manager for you. However, you do have to set some policies that tell the Replication Manager how to handle permanent messages. </p> <p> There are two things that you have to do: </p> <div class="itemizedlist"> <ul type="disc"> <li> <p> Determine how many acknowledgments must be received by the master. </p> </li> <li> <p> Identify the amount of time that replicas have to send their acknowledgments. </p> </li> </ul> </div> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h3 class="title"><a id="fmwrkpermpolicy"></a>Identifying Permanent Message Policies</h3> </div> </div> </div> <p> You identify permanent message policies using the <span> <code class="classname">ReplicationManagerAckPolicy</code> class which you pass to the environment using the <code class="methodname">EnvironmentConfig.setReplicationManagerAckPolicy</code> method. </span> Note that you can set permanent message policies at any time during the life of the application. </p> <p> The following permanent message policies are available when you use the Replication Manager: </p> <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> <h3 class="title">Note</h3> <p> The following list mentions <span class="emphasis"><em>electable peer</em></span> several times. This is simply another environment that can be elected to be a master (that is, it has a priority greater than 0). Do not confuse this with the concept of a peer as used for client to client transfers. See <a class="xref" href="c2ctransfer.html" title="Client to Client Transfer">Client to Client Transfer</a> for more information on client to client transfers. </p> </div> <div class="itemizedlist"> <ul type="disc"> <li> <p> <code class="literal">ReplicationManagerAckPolicy.ALL</code> </p> <p> All replicas must acknowledge the message within the timeout period. This policy should be selected only if your replication group has a small number of replicas, and those replicas are on extremely reliable networks and servers. </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.ALL_AVAILABLE</code> </p> <p> All currently connected replication clients must acknowledge the message. This policy will invoke the <code class="literal">DB_EVENT_REP_PERM_FAILED</code> event if fewer than a quorum of clients acknowledged during that time. </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.ALL_PEERS</code> </p> <p> All electable peers must acknowledge the message within the timeout period. This policy should be selected only if your replication group is small, and its various environments are on extremely reliable networks and servers. </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.NONE</code> </p> <p> No permanent message acknowledgments are required. If this policy is selected, permanent message handling is essentially "turned off." That is, the master will never wait for replica acknowledgments. In this case, transaction log data is either flushed or not strictly depending on the type of commit that is being performed (synchronous or asynchronous). </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.ONE</code> </p> <p> At least one replica must acknowledge the permanent message within the timeout period. </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.ONE_PEER</code> </p> <p> At least one electable peer must acknowledge the permanent message within the timeout period. </p> </li> <li> <p> <code class="literal">ReplicationManagerAckPolicy.QUORUM</code> </p> <p> A quorum of electable peers must acknowledge the message within the timeout period. A quorum is reached when acknowledgments are received from the minimum number of environments needed to ensure that the record remains durable if an election is held. That is, the master wants to hear from enough electable replicas that they have committed the record so that if an election is held, the master knows the record will exist even if a new master is selected. </p> </li> </ul> </div> <p> By default, a quorum of electable peers must must acknowledge a permanent message in order for it considered to have been successfully transmitted. </p> </div> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h3 class="title"><a id="fmwrkpermtimeout"></a>Setting the Permanent Message Timeout</h3> </div> </div> </div> <p> The permanent message timeout represents the maximum amount of time the committing thread will block waiting for message acknowledgments. If sufficient acknowledgments arrive before this timeout has expired, the thread continues operations as normal. However, if this timeout expires, the committing thread flushes its transaction log buffer before continuing with normal operations. </p> <p> You set the timeout value using <code class="methodname">Environment.setReplicationTimeout()</code>. You pass this method the <code class="methodname">ReplicationTimeoutType.ACK_TIMEOUT</code> constant and a timeout value in microseconds. </p> <p> For example: </p> <pre class="programlisting"> dbenv.setReplicationTimeout(ReplicationTimeoutType.ACK_TIMEOUT, 100); </pre> <p> This timeout value can be set at anytime during the life of the application. </p> </div> <div class="sect2" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h3 class="title"><a id="perm2fmwrkexample"></a>Adding a Permanent Message Policy to <span>RepQuoteExampleGSG</span> </h3> </div> </div> </div> <p> For illustration purposes, we will now update <code class="literal">RepQuoteExampleGSG</code> such that it requires only one acknowledgment from a replica on transactional commits. Also, we will give this acknowledgment a 500 microsecond timeout value. This means that our application's main thread will block for up to 500 microseconds waiting for an acknowledgment. If it does not receive at least one acknowledgment in that amount of time, DB will flush the transaction logs to disk before continuing on. </p> <p> This is a very simple update. We can perform the entire thing in <code class="methodname">RepQuoteExampleGSG.init()</code> immediately after we set the application's priority and before we open our environment handle. </p> <pre class="programlisting"> public int init(RepConfig config) throws DatabaseException { int ret = 0; repConfig = config; EnvironmentConfig envConfig = new EnvironmentConfig(); envConfig.setErrorStream(System.err); envConfig.setErrorPrefix(RepConfig.progname); envConfig.addReplicationManagerSite(repConfig.getThisHost()); for (ReplicationHostAddress host = repConfig.getFirstOtherHost(); host != null; host = repConfig.getNextOtherHost()){ ReplicationManagerSiteConfig repmgrRemoteSiteConfig = new ReplicationManagerSiteConfig(host.host, host.port); repmgrRemoteSiteConfig.setBootstrapHelper(true); envConfig.addReplicationManagerSite( repmgrRemoteSiteConfig); } envConfig.setReplicationPriority(appConfig.priority); <strong class="userinput"><code>envConfig.setReplicationManagerAckPolicy( ReplicationManagerAckPolicy.ALL); envConfig.setReplicationTimeout(ReplicationTimeoutType.ACK_TIMEOUT, 500); </code></strong> envConfig.setCacheSize(RepConfig.CACHESIZE); envConfig.setTxnNoSync(true); ... </pre> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="repmgr_init_example_c.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="repapp.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="electiontimes.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Adding the Replication Manager to <span>SimpleTxn</span> </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Managing Election Times</td> </tr> </table> </div> </body> </html>