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<title>Load balancing</title>
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<h2 class="title">Load balancing</h2>
<p class="para">
Four load balancing strategies are supported to distribute
statements over the configured MySQL slave servers:
</p>
<p class="para">
<dl>
<dt>
<span class="term">random</span>
<dd>
<p class="para">
Chooses a random server whenever a statement is executed.
</p>
</dd>
</dt>
<dt>
<span class="term">random once (default)</span>
<dd>
<p class="para">
Chooses a random server after the first statement is executed,
and uses the decision for the rest of the PHP request.
</p>
<p class="para">
It is the default, and the lowest impact on the connection state.
</p>
</dd>
</dt>
<dt>
<span class="term">round robin</span>
<dd>
<p class="para">
Iterates over the list of configured servers.
</p>
</dd>
</dt>
<dt>
<span class="term">user-defined via callback</span>
<dd>
<p class="para">
Is used to implement any other strategy.
</p>
</dd>
</dt>
</dl>
</p>
<p class="para">
The load balancing policy is configured in the plugins configuration
file using the
<a href="mysqlnd-ms.plugin-ini-json.html#ini.mysqlnd-ms-plugin-config-v2.filter-random" class="link">random</a>,
<a href="mysqlnd-ms.plugin-ini-json.html#ini.mysqlnd-ms-plugin-config-v2.filter-roundrobin" class="link">roundrobin</a>,
and <a href="mysqlnd-ms.plugin-ini-json.html#ini.mysqlnd-ms-plugin-config-v2.filter-user" class="link">user</a>
<a href="mysqlnd-ms.filter.html" class="link">filters</a>.
</p>
<p class="para">
Servers can be prioritized assigning a weight. A server that has been given
a weight of two will get twice as many requests as a server that has been
given the default weight of one. Prioritization can be handy in heterogenous
environments. For example, you may want to assign more requests to
a powerful machine than to a less powerful. Or, you may have configured
servers that are close or far from the client, thus expose different latencies.
</p>
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