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<div class="chapter" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title"><a id="repapp"></a>Chapter 3. The DB Replication Framework</h2>
</div>
</div>
<div></div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="repapp.html#rep_init_code">
Starting and Stopping Replication
</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="repapp.html#election_flags">Managing Election Policies</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="repapp.html#thread_count">Selecting the Number of Threads</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="repmgr_init_example_c.html">Adding the Replication Framework to
simple_txn
</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="fwrkpermmessage.html">Permanent Message Handling</a>
</span>
</dt>
<dd>
<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
rep_mgr
</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="electiontimes.html">Managing Election Times</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="electiontimes.html#electiontimeout">Managing Election Timeouts</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="electiontimes.html#electretrytime">Managing Election Retry Times</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="fmwrkconnectretry.html">Managing Connection Retries</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="heartbeats.html">Managing Heartbeats</a>
</span>
</dt>
</dl>
</div>
<p>
The easiest way to add replication to your transactional
application is to use the replication framework. The replication framework provides a comprehensive
communications layer that enables replication. For a brief listing
of the replication framework's feature set, see
<a href="apioverview.html#repframeworkoverview">Replication Framework Overview</a>.
</p>
<p>
To use the replication framework, you make use of special methods off the
<span><tt class="classname">DB_ENV</tt> class.</span>
That is:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Create an environment handle as normal.
</p>
</li>
<li>
<p>
Configure your environment handle as
needed (e.g. set the error file and
error prefix values, if desired).
</p>
</li>
<li>
<p>
Use the replication framework replication methods to
configure the replication framework. Using these
methods causes DB to know that you
are using the replication framework.
</p>
<p>
Configuring the replication framework
entails setting its replication
priority, setting the TCP/IP address
that this replication environment will use for
incoming replication messages, identify
TCP/IP addresses of other replication
environments, setting the number of
replication environments in the
replication group, and so forth. These actions are
discussed throughout the remainder of
this chapter.
</p>
</li>
<li>
<p>
Open your environment handle. When you
do this, be sure to specify
<span><tt class="literal">DB_INIT_REP</tt> and
<tt class="literal">DB_THREAD</tt> to your
open flags. (This is in addition to the
flags that you normally use for a
single-threaded transactional
application). The first of these causes
replication to be initialized for the
application. The second causes your
environment handle to be free-threaded
(thread safe). Both flags are required
for replication framework usage.
</span>
</p>
</li>
<li>
<p>
Start replication by calling
<span><tt class="methodname">DB_ENV->repmgr_start()</tt>.</span>
</p>
</li>
<li>
<p>
Open your databases as needed. Masters
must open their databases for read
and write activity. Replicas can open
their databases for read-only activity, but
doing so means they must re-open the
databases if the replica ever becomes a
master. Either way, replicas should never attempt to
write to the database(s) directly.
</p>
</li>
</ol>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
The replication framework allows you to only use one
environment handle per process.
</p>
</div>
<p>
When you are ready to shut down your application:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Close your databases
</p>
</li>
<li>
<p>
Close your environment. This causes
replication to stop as well.
</p>
</li>
</ol>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Before you can use the replication framework, you may have to
enable it in your DB library. This is
<span class="emphasis"><em>not</em></span> a requirement for
Microsoft Windows systems, or Unix systems that
use pthread mutexes by default. Other systems,
notably BSD and BSD-derived systems (such as
Mac OS X), must enable the replication framework when you
configure the DB build.
</p>
<p>
You do this by <span class="emphasis"><em>not</em></span>
disabling replication and by configuring the
library with POSIX threads support. In other
words, replication must be turned on in the
build (it is by default), and POSIX thread
support must be enabled if it is not already by
default. To do this, use the
<tt class="literal">--enable-pthread_api</tt> switch
on the configure script.
</p>
<p>
For example:
</p>
<pre class="programlisting">../dist/configure --enable-pthread-api</pre>
</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>
</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><tt class="methodname">DB_ENV->repmgr_start()</tt>.</span>
</p>
<p>
You stop replication by closing your environment
cleanly, as is normal for an 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 <db.h>
/* Use a 10mb cache */
#define CACHESIZE (10 * 1024 * 1024)
...
DB_ENV *dbenv; /* Environment 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 ret; /* Error return code. */
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;
ret = 0;
/* Create the environment handle */
if ((ret = db_env_create(&dbenv, 0)) != 0 ) {
fprintf(stderr, "Error creating environment handle: %s\n",
db_strerror(ret));
goto err;
}
/*
* Configure the environment handle. Here we configure asynchronous
* transactional commits for performance reasons.
*/
dbenv->set_errfile(dbenv, stderr);
dbenv->set_errpfx(dbenv, progname);
(void)dbenv->set_cachesize(dbenv, 0, CACHESIZE, 0);
(void)dbenv->set_flags(dbenv, DB_TXN_NOSYNC, 1);
/*
* Configure the local address. This is the local hostname and port
* that this replication participant will use to receive incoming
* replication messages. Note that this can be performed only once for
* the application. It is required.
*/
if ((ret = dbenv->repmgr_set_local_site(dbenv, listen_host,
listen_port, 0)) != 0) {
fprintf(stderr, "Could not set local address (%d).\n", ret);
goto err;
}
/*
* Set this application'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->rep_set_priority(dbenv, 100);
/*
* Add a site to the list of replication environments known to this
* application.
*/
if (dbenv->repmgr_add_remote_site(dbenv, other_host, other_port,
NULL, 0) != 0) {
fprintf(stderr, "Could not add site %s.\n", other_host);
goto err;
}
/*
* Identify the number of sites in the replication group. This is
* necessary so that elections and permanent message handling can be
* performed correctly.
*/
if (dbenv->repmgr_set_nsites(dbenv, 2) != 0) {
fprintf(stderr, "Could not set the number of sites.\n";
goto err;
}
/* Open the environment handle. Note that we add DB_THREAD and
* DB_INIT_REP to the list of flags. These are required.
*/
if ((ret = dbenv->open(dbenv, home, DB_CREATE | DB_RECOVER |
DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN |
DB_THREAD | DB_INIT_REP,
0)) != 0) {
goto err;
}
/* Start the replication framework such that it uses 3 threads. */
if ((ret = dbenv->repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
goto err;
/* Sleep to give ourselves time to find a master */
sleep(5);
/*
**********************************************************
*** All other application code goes here, including *****
*** database opens *****
**********************************************************
*/
err: /*
* Make sure all your database handles are closed
* (omitted from this example).
*/
/* Close the environment */
if (dbenv != NULL)
(void)dbenv->close(dbenv, 0);
/* All done */
return (ret); </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></div>
</div>
<p>
Before continuing, it is worth taking a look at the
<span>
startup election flags accepted by
<span><tt class="methodname">DB_ENV->repgmr_start()</tt>.</span>
These flags control how your replication application will
behave when it first starts up.
</span>
</p>
<p>
In the previous example, we specified
<tt class="literal">DB_REP_ELECTION</tt>
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. The number of
environments used to calculate the simple
majority is based on the value set for
<span><tt class="methodname">DB_ENV->rep_set_nsites()</tt>.</span>
This is always true of elections held using the replication framework.
</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 better 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 framework 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>
<tt class="literal">DB_REP_MASTER</tt>
</td>
<td>
<p>
The application starts up and declares itself 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>
<tt class="literal">DB_REP_CLIENT</tt>
</td>
<td>
<p>
The application starts up and declares
itself to be a replica without calling for
an election. Note that the application
can still become a master if a subsequent
application starts up, calls for an
election, and this application is elected
master.
</p>
</td>
</tr>
<tr>
<td>
<tt class="literal">DB_REP_ELECTION</tt>
</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>
<tr>
<td>
<tt class="literal">DB_REP_FULL_ELECTION</tt>
</td>
<td>
<p>
Identical to
<tt class="literal">DB_REP_ELECTION</tt>
except that the election requires all
known members of the replication group to
participate. If a given environment has not yet
started but it is included in the
replication group count (using
<span><tt class="methodname">DB_ENV->rep_set_nsites()</tt>)</span>
then a master can not be elected.
</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></div>
</div>
<p>
Under the hood, the replication framework is threaded and you can
control the number of threads used to process messages received from
other replicas. The threads that the replication framework 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
are performed in whatever thread
performed a write to the database(s).
That is, the thread that called, for
example,
<tt class="methodname">DB->put()</tt>
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 framework
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>
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