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<title>Connection Pooling (version 1.2.0-1.2.12 *only*)</title>
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<h2 class="title">Connection Pooling (version 1.2.0-1.2.12 *only*)</h2>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
This section is no longer relevant as of the 1.3.0 release of the driver
and only serves as a historical information on how the pooling used to
work.
</p>
<p class="para">
The latest versions of the driver have no concept of pooling anymore and
will maintain only one connection per process, for each connection type
(ReplicaSet/standalone/mongos), for each credentials combination.
</p>
</p></blockquote>
<p class="para">
Creating connections is one of the most heavyweight things that the driver
does. It can take hundreds of milliseconds to set up a connection correctly,
even on a fast network. Thus, the driver tries to minimize the number of new
connections created by reusing connections from a pool.
</p>
<p class="para">
When a user creates a new instance of <a href="class.mongoclient.html" class="classname">MongoClient</a>, all
necessary connections will be taken from their pools (replica sets may
require multiple connections, one for each member of the set). When the
<a href="class.mongoclient.html" class="classname">MongoClient</a> instance goes out of scope, the
connections will be returned to the pool. When the PHP process exits, all
connections in the pools will be closed.
</p>
<div class="section">
<h2 class="title">"Why do I have so many open connections?"</h2>
<p class="para">
Connection pools can generate a large number of connections. This is
expected and, using a little arithmetic, you can figure out how many
connections will be created. There are three factors in determining the
total number of connections:
</p>
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<em>
connections_per_pool
</em>
</p>
<p class="para">
Each connection pool will create, by default, an unlimited number of
connections. One might assume that this is a problem: if it can create an
unlimited number of connections, couldn't it create thousands and the
server would run out of file descriptors? In practice, this is unlikely,
as unused connections are returned to the pool to be used later, so future
connections will use the same connection instead of creating a new one.
Unless you create thousands of connections at once without letting any go
out of scope, the number of connections open should stay at a reasonable
number.
</p>
<p class="para">
You can see how many connections you have in a pool using the
<span class="function"><a href="mongopool.info.html" class="function">MongoPool::info()</a></span> function. Add up the "in use" and
"in pool" fields for a given server. That is the total number of
connections for that pool.
</p>
</li>
<li class="listitem">
<p class="para">
<em>
pools_per_process
</em>
</p>
<p class="para">
Each MongoDB server address you're connecting to gets its own connection
pool. For example, if your local hostname is "example.net", connecting
to "example.net:27017", "localhost:27017", and "/tmp/mongodb-27017.sock"
will create three connection pools. You can see how many connection pools
you have open using <span class="function"><a href="mongopool.info.html" class="function">MongoPool::info()</a></span>.
</p>
</li>
<li class="listitem">
<p class="para">
<em>
processes
</em>
</p>
<p class="para">
Each PHP process has a separate set of pools. PHP-FPM and Apache
generally create between 6 and a couple of dozen PHP worker children.
Check your settings to see what the max number of PHP processes is that
can be spawned.
</p>
<p class="para">
If you are using PHP-FPM, estimating the number of connections can be
tricky because it will spawn more PHP-FPM workers under heavy load. To be
on the safe side, look at the <em>max_children</em> parameter or
add up <em>spare_servers</em> + <em>start_servers</em>
(choose whichever number is higher). That will indicate how many PHP
processes (i.e. sets of pools) you should plan for.
</p>
</li>
</ul>
<p class="para">
The three variables above can be multiplied together to give the max
number of connections expected:
<em>connections_per_pool</em> *
<em>pools_per_process</em> *
<em>processes</em>. Note that
<em>connections_per_pool</em> can be different for different
pools, so <em>connections_per_pool</em> should be the max.
</p>
<p class="para">
For example, suppose we're getting 30 connections per pool, 10 pools per PHP
process, and 128 PHP processes. Then we can expect 38400 connections from
this machine. Thus, we should set this machine's file descriptor limit to
be high enough to handle all of these connections or it may run out of file
descriptors.
</p>
<p class="para">
See <a href="class.mongopool.html" class="classname">MongoPool</a> for more information on connection
pooling.
</p>
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