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Release 5.2.28 on 6/10/2011
2011-09-13 17:44:24 +00:00
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<title>Starting and Stopping Replication</title>
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<link rel="up" href="repapp.html" title="Chapter 3. The DB Replication Manager" />
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<div class="libver">
<p>Library Version 11.2.5.2</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">
Starting and Stopping Replication
</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="repapp.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="repmgr_init_example_c.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="rep_init_code"></a>
Starting and Stopping Replication
</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="rep_init_code.html#election_flags">Managing Election Policies</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_init_code.html#thread_count">Selecting the Number of Threads</a>
</span>
</dt>
</dl>
</div>
<p>
As described above, you introduce replication to an application by
starting with a transactional application, performing some basic
replication configuration, and then starting replication using
<span><code class="methodname">DbEnv::repmgr_start()</code>.</span>
</p>
<p>
You stop replication by closing your environment
cleanly in the same way you would for any DB application.
</p>
<p>
For example, the following code fragment initializes, then
stops and starts replication. Note that other replication
activities are omitted for brevity.
</p>
<pre class="programlisting">#include &lt;db_cxx.h&gt;
/* Use a 10mb cache */
#define CACHESIZE (10 * 1024 * 1024)
...
DbEnv *dbenv; /* Environment handle. */
DbSite *dbsite; /* Replication manager site handle */
const char *progname; /* Program name. */
const char *envHome; /* Environment home directory. */
const char *listen_host; /* A TCP/IP hostname. */
const char *other_host; /* A TCP/IP hostname. */
int is_group_creator; /* A flag */
u_int16 listen_port; /* A TCP/IP port. */
u_int16 other_port; /* A TCP/IP port. */
/* Initialize variables */
dbenv = NULL;
progname = "example_replication";
envHome = "ENVIRONMENT_HOME";
listen_host = "mymachine.sleepycat.com";
listen_port = 5001;
other_host = "anothermachine.sleepycat.com";
other_port = 4555;
is_group_creator = 1; /* This is usually set via a command line
argument or some other external
configuration mechanism. */
try {
/* Create the environment handle */
dbenv = new DbEnv(0);
/*
* Configure the environment handle. Here we configure
* asynchronous transactional commits for performance reasons.
*/
dbenv-&gt;set_errfile(stderr);
dbenv-&gt;set_errpfx(progname);
(void)dbenv-&gt;set_cachesize(0, CACHESIZE, 0);
(void)dbenv-&gt;set_flags(DB_TXN_NOSYNC, 1);
/*
* Configure the local address. This is the local hostname
* and port that this replication environment will use to
* receive incoming replication messages. Note that this can
* be performed only once for the replication environment.
* It is required.
* First: Create a DB_SITE handle to identify the site's
* host/port network address.
*/
dbenv-&gt;repmgr_site(listen_host, listen_port, &amp;dbsite;, 0);
/*
* Second: Configure this site as the local site within the
* replication group.
*/
dbsite-&gt;set_config(DB_LOCAL_SITE, 1);
/*
* Third: Set DB_GROUP_CREATOR if applicable. This can be done
* only for the local site. It should also only be peformed
* for one and only one site in a replication group, so
* typically this is driven by an externally-supplied
* configuration option.
*
* DB_GROUP_CREATOR only has meaning if you are starting the
* very first site for the very first time in a replication
* group. It is otherwise ignored.
*/
if (is_group_creator)
dbsite-&gt;set_config(DB_GROUP_CREATOR, 1);
/*
* Having configured the local site, we can immediately
* deallocate the DB_SITE handle.
*/
dbsite-&gt;close(dbsite);
/*
* Set this replication environment's priority. This is used
* for elections.
*
* Set this number to a positive integer, or 0 if you do not want
* this site to be able to become a master.
*/
dbenv-&gt;rep_set_priority(100);
/*
* Configure a bootstrap helper. This information is used only
* if the site currently exists, and the local site has never
* been started before. Otherwise, this configuration
* information is ignored.
*
*/
if (!is_group_creator) {
dbenv-&gt;repmgr_site(other_host, other_port, &amp;dbsite, 0);
dbsite-&gt;set_config(dbsite, DB_BOOTSTRAP_HELPER, 1);
/*
* Having configured the bootstrap helper site, we can
* immediately deallocate the DB_SITE handle.
*/
dbsite-&gt;close();
}
/* Open the environment handle. Note that we add DB_THREAD and
* DB_INIT_REP to the list of flags. These are required.
*/
dbenv-&gt;open(home, DB_CREATE | DB_RECOVER |
DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN |
DB_THREAD | DB_INIT_REP,
0);
/*
* Start the replication manager such that it uses 3
* threads.
*/
dbenv-&gt;repmgr_start(3, DB_REP_ELECTION);
/* Sleep to give ourselves time to find a master */
sleep(5);
/*
**********************************************************
*** All other application code goes here, including *****
*** database opens *****
**********************************************************
*/
} catch (DbException &amp;de) {
/* Error handling goes here */
}
/* Close out the application here.
try {
/*
* Make sure all your database handles are closed
* (omitted from this example).
*/
/* Close the environment */
if (dbenv != NULL)
(void)dbenv-&gt;close(dbenv, 0);
} catch (DbException &amp;de) {
/* Error handling goes here */
}
/* All done */ </pre>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3>
</div>
</div>
</div>
<p>
Before continuing, it is worth taking a look at the
<span>
startup election flags accepted by
<span><code class="methodname">DbEnv::repgmr_start()</code>.</span>
These flags control how your replication application will
behave when it first starts up.
</span>
</p>
<p>
In the previous example, we specified
<code class="literal">DB_REP_ELECTION</code>
when we started replication. This causes the
application to try to find a master upon startup. If it
cannot, it calls for an election. In the event an
election is held, the environment receiving the most number of
votes will become the master.
</p>
<p>
There's some important points to make here:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
This
<span>flag</span>
only requires that other
environments in the replication group
participate in the vote. There is no
requirement that
<span class="emphasis"><em>all</em></span> such
environments participate. In other
words, if an environment
starts up, it can call for an
election, and select a master, even
if all other environment have not yet
joined the replication group.
</p>
</li>
<li>
<p>
It only requires a simple majority of
participating environments to elect a master.
This is always true of elections held using the Replication Manager.
</p>
</li>
<li>
<p>
As always, the environment participating in the election with the most
up-to-date log files is selected as
master. If an environment with more recent log files
has not yet joined the replication
group, it may not become the master.
</p>
</li>
</ul>
</div>
<p>
Any one of these points may be enough to cause a
less-than-optimum environment to be selected as master.
Therefore, to give you a better degree of control over
which environment becomes a master at application startup,
the Replication Manager offers the following start-up
<span>flags:</span>
</p>
<div class="informaltable">
<table border="1" width="80%">
<colgroup>
<col />
<col />
</colgroup>
<thead>
<tr>
<th>Flag</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<code class="literal">DB_REP_MASTER</code>
</td>
<td>
<p>
The application starts up and declares the environment to be a master
without calling for an election. It is an error for more
than one environment to start up using this flag, or for
an environment
to use this flag when a master already exists.
</p>
<p>
Note that no replication group should
<span class="emphasis"><em>ever</em></span> operate with more than
one master.
</p>
<p>
In the event that a environment attempts to become a
master when a master already exists, the
replication code will resolve the problem by
holding an election. Note, however, that there
is always a possibility of data loss in the face
of duplicate masters, because once a master is
selected, the environment that loses the election will
have to roll back any transactions committed
until it is in sync with the "real" master.
</p>
</td>
</tr>
<tr>
<td>
<code class="literal">DB_REP_CLIENT</code>
</td>
<td>
<p>
The application starts up and declares
the environment to be a replica without calling for
an election. Note that the environment
can still become a master if a subsequent
application starts up, calls for an
election, and this environment is elected
master.
</p>
</td>
</tr>
<tr>
<td>
<code class="literal">DB_REP_ELECTION</code>
</td>
<td>
<p>
As described above, the application starts up,
looks for a master, and if one is not found calls
for an election.
</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3>
</div>
</div>
</div>
<p>
Under the hood, the Replication Manager is threaded and you can
control the number of threads used to process messages received from
other replicas. The threads that the Replication Manager uses are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Incoming message thread. This thread
receives messages from the site's
socket and passes those messages to
message processing threads (see below)
for handling.
</p>
</li>
<li>
<p>
Outgoing message thread. Outgoing
messages are sent from whatever thread
performed a write to the database(s).
That is, the thread that called, for
example,
<code class="methodname">Db::put()</code>
is the thread that writes replication messages
about that fact to the socket.
</p>
<p>
Note that if this write activity would
cause the thread to be blocked due to
some condition on the socket, the Replication Manager
will hand the outgoing message to the
incoming message thread, and it will
then write the message to the socket.
This prevents your database write
threads from blocking due to abnormal
network I/O conditions.
</p>
</li>
<li>
<p>
Message processing threads are
responsible for parsing and then
responding to incoming replication
messages. Typically, a response will
include write activity to your
database(s), so these threads can be
busy performing disk I/O.
</p>
</li>
</ul>
</div>
<p>
Of these threads, the only ones that you have any
configuration control over are the message processing
threads. In this case, you can determine how many
of these threads you want to run.
</p>
<p>
It is always a bit of an art to decide on a thread count,
but the short answer is you probably do not need more
than three threads here, and it is likely that one will
suffice. That said, the best thing to do is set your
thread count to a fairly low number and then increase
it if it appears that your application will benefit
from the additional threads.
</p>
</div>
</div>
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<a accesskey="u" href="repapp.html">Up</a>
</td>
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</tr>
<tr>
<td width="40%" align="left" valign="top">Chapter 3. The DB Replication Manager </td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
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