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Release 5.2.28 on 6/10/2011
2011-09-13 17:44:24 +00:00
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Permanent Message Handling</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<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" />
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<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 11.2.5.2</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">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>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>RepMgrGSG</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
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">DB_REPMGR_ACKS_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">DB_REPMGR_ACKS_ONE</code>
</p>
<p>
At least one replica must acknowledge the permanent
message within the timeout period.
</p>
</li>
<li>
<p>
<code class="literal">DB_REPMGR_ACKS_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">DB_REPMGR_ACKS_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">DB_REPMGR_ACKS_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">DB_REPMGR_ACKS_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">DB_REPMGR_ACKS_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 the
<code class="methodname">DbEnv::rep_set_timeout()</code>
method. When you do this, you provide the
<code class="literal">DB_REP_ACK_TIMEOUT</code> value to
the <code class="literal">which</code> parameter, and the
timeout value in microseconds to the
<code class="literal">timeout</code> parameter.
</p>
<p>
For example:
</p>
<pre class="programlisting"> dbenv-&gt;rep_set_timeout(DB_REP_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>RepMgrGSG</span>
</h3>
</div>
</div>
</div>
<p>
For illustration purposes, we will now update
<code class="literal">RepMgrGSG</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">RepMgrGSG::init()</code>
immediately after we set the application's priority
and before we open our environment handle.
</p>
<pre class="programlisting">int RepMgrGSG::init(RepConfigInfo *config)
{
int ret = 0;
app_config = config;
dbenv.set_errfile(stderr);
dbenv.set_errpfx(progname);
DbSite *dbsite;
dbenv.repmgr_site(app_config-&gt;this_host.host,
app_config-&gt;this_host.port, &amp;dbsite, 0);
dbsite-&gt;set_config(DB_LOCAL_SITE, 1);
if (app_config-&gt;this_host.creator)
dbsite-&gt;set_config(DB_GROUP_CREATOR, 1);
dbsite-&gt;close();
int i = 1;
for ( REP_HOST_INFO *cur = app_config-&gt;other_hosts;
cur != NULL &amp;&amp; i &lt;= app_config-&gt;nrsites;
cur = cur-&gt;next, i++) {
dbenv.repmgr_site(cur-&gt;host, cur-&gt;port, &amp;dbsite, 0);
dbsite-&gt;set_config(DB_BOOTSTRAP_HELPER, 1);
dbsite-&gt;close();
}
dbenv.rep_set_priority(app_config-&gt;priority);
<strong class="userinput"><code>/* Permanent messages require at least one ack */
dbenv.repmgr_set_ack_policy(DB_REPMGR_ACKS_ONE);
/* Give 500 microseconds to receive the ack */
dbenv.rep_set_timeout(DB_REP_ACK_TIMEOUT, 500);</code></strong>
dbenv.set_cachesize(0, CACHESIZE, 0);
dbenv.set_flags(DB_TXN_NOSYNC, 1);
... </pre>
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<td width="40%" align="left" valign="top">Adding the Replication Manager to
<span>RepMgr</span>
 </td>
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