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  <div class="section" id="frequently-asked-questions">
<h1>Frequently Asked Questions<a class="headerlink" href="#frequently-asked-questions" title="Permalink to this headline">¶</a></h1>
<div class="section" id="questions">
<h2>Questions<a class="headerlink" href="#questions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="mysql-is-throwing-deadlock-errors-what-can-i-do">
<h3>MySQL is throwing deadlock errors, what can I do?<a class="headerlink" href="#mysql-is-throwing-deadlock-errors-what-can-i-do" title="Permalink to this headline">¶</a></h3>
<p><strong>Answer:</strong> MySQL has default isolation level set to <tt class="docutils literal"><span class="pre">REPEATABLE-READ</span></tt>,
if you don&#8217;t really need that, set it to <tt class="docutils literal"><span class="pre">READ-COMMITTED</span></tt>.
You can do that by adding the following to your <tt class="docutils literal"><span class="pre">my.cnf</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="p">[</span><span class="n">mysqld</span><span class="p">]</span>
<span class="n">transaction</span><span class="o">-</span><span class="n">isolation</span> <span class="o">=</span> <span class="n">READ</span><span class="o">-</span><span class="n">COMMITTED</span>
</pre></div>
</div>
<p>For more information about InnoDBs transaction model see <a class="reference external" href="http://dev.mysql.com/doc/refman/5.1/en/innodb-transaction-model.html">MySQL - The InnoDB
Transaction Model and Locking</a> in the MySQL user manual.</p>
<p>(Thanks to Honza Kral and Anton Tsigularov for this solution)</p>
</div>
<div class="section" id="celeryd-is-not-doing-anything-just-hanging">
<h3>celeryd is not doing anything, just hanging<a class="headerlink" href="#celeryd-is-not-doing-anything-just-hanging" title="Permalink to this headline">¶</a></h3>
<p><strong>Answer:</strong> See <a class="reference internal" href="#mysql-is-throwing-deadlock-errors-what-can-i-do">MySQL is throwing deadlock errors, what can I do?</a>.</p>
</div>
<div class="section" id="can-i-send-some-tasks-to-only-some-servers">
<h3>Can I send some tasks to only some servers?<a class="headerlink" href="#can-i-send-some-tasks-to-only-some-servers" title="Permalink to this headline">¶</a></h3>
<p>As of now there is only one use-case that works like this, and that is
tasks of type <tt class="docutils literal"><span class="pre">A</span></tt> can be sent to servers <tt class="docutils literal"><span class="pre">x</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>, while tasks
of type <tt class="docutils literal"><span class="pre">B</span></tt> can be sent to server <tt class="docutils literal"><span class="pre">z</span></tt>. One server can&#8217;t handle more than
one routing_key, but this is coming in a later release.</p>
<p>Say you have two servers, <tt class="docutils literal"><span class="pre">x</span></tt>, and <tt class="docutils literal"><span class="pre">y</span></tt> that handles regular tasks,
and one server <tt class="docutils literal"><span class="pre">z</span></tt>, that only handles feed related tasks, you can use this
configuration:</p>
<blockquote>
<ul class="simple">
<li>Servers <tt class="docutils literal"><span class="pre">x</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>: settings.py:</li>
</ul>
<div class="highlight-python"><div class="highlight"><pre><span class="n">AMQP_SERVER</span> <span class="o">=</span> <span class="s">&quot;rabbit&quot;</span>
<span class="n">AMQP_PORT</span> <span class="o">=</span> <span class="mf">5678</span>
<span class="n">AMQP_USER</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
<span class="n">AMQP_PASSWORD</span> <span class="o">=</span> <span class="s">&quot;secret&quot;</span>
<span class="n">AMQP_VHOST</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>

<span class="n">CELERY_AMQP_CONSUMER_QUEUE</span> <span class="o">=</span> <span class="s">&quot;regular_tasks&quot;</span>
<span class="n">CELERY_AMQP_EXCHANGE</span> <span class="o">=</span> <span class="s">&quot;tasks&quot;</span>
<span class="n">CELERY_AMQP_PUBLISHER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.regular&quot;</span>
<span class="n">CELERY_AMQP_CONSUMER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.#&quot;</span>
<span class="n">CELERY_AMQP_EXCHANGE_TYPE</span> <span class="o">=</span> <span class="s">&quot;topic&quot;</span>
</pre></div>
</div>
<ul class="simple">
<li>Server <tt class="docutils literal"><span class="pre">z</span></tt>: settings.py:</li>
</ul>
<div class="highlight-python"><div class="highlight"><pre><span class="n">AMQP_SERVER</span> <span class="o">=</span> <span class="s">&quot;rabbit&quot;</span>
<span class="n">AMQP_PORT</span> <span class="o">=</span> <span class="mf">5678</span>
<span class="n">AMQP_USER</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
<span class="n">AMQP_PASSWORD</span> <span class="o">=</span> <span class="s">&quot;secret&quot;</span>
<span class="n">AMQP_VHOST</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>

<span class="n">CELERY_AMQP_CONSUMER_QUEUE</span> <span class="o">=</span> <span class="s">&quot;feed_tasks&quot;</span>
<span class="n">CELERY_AMQP_EXCHANGE</span> <span class="o">=</span> <span class="s">&quot;tasks&quot;</span>
<span class="n">CELERY_AMQP_PUBLISHER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.regular&quot;</span>
<span class="n">CELERY_AMQP_CONSUMER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.feed.#&quot;</span>
<span class="n">CELERY_AMQP_EXCHANGE_TYPE</span> <span class="o">=</span> <span class="s">&quot;topic&quot;</span>
</pre></div>
</div>
</blockquote>
<p>Now to make a Task run on the <tt class="docutils literal"><span class="pre">z</span></tt> server you need to set its
<tt class="docutils literal"><span class="pre">routing_key</span></tt> attribute so it starts with the words <tt class="docutils literal"><span class="pre">&quot;task.feed.&quot;</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">feedaggregator.models</span> <span class="kn">import</span> <span class="n">Feed</span>
<span class="kn">from</span> <span class="nn">celery.task</span> <span class="kn">import</span> <span class="n">Task</span>

<span class="k">class</span> <span class="nc">FeedImportTask</span><span class="p">(</span><span class="n">Task</span><span class="p">):</span>
    <span class="n">name</span> <span class="o">=</span> <span class="s">&quot;import_feed&quot;</span>
    <span class="n">routing_key</span> <span class="o">=</span> <span class="s">&quot;task.feed.importer&quot;</span>

    <span class="k">def</span> <span class="nf">run</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">feed_url</span><span class="p">):</span>
        <span class="c"># something importing the feed</span>
        <span class="n">Feed</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">import_feed</span><span class="p">(</span><span class="n">feed_url</span><span class="p">)</span>
</pre></div>
</div>
<p>You can also override this using the <tt class="docutils literal"><span class="pre">routing_key</span></tt> argument to
<a title="celery.task.apply_async" class="reference external" href="reference/celery.task.html#celery.task.apply_async"><tt class="xref docutils literal"><span class="pre">celery.task.apply_async()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">celery.task</span> <span class="kn">import</span> <span class="n">apply_async</span>
<span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">myapp.tasks</span> <span class="kn">import</span> <span class="n">RefreshFeedTask</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">apply_async</span><span class="p">(</span><span class="n">RefreshFeedTask</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="s">&quot;http://cnn.com/rss&quot;</span><span class="p">],</span>
<span class="gp">... </span>            <span class="n">routing_key</span><span class="o">=</span><span class="s">&quot;task.feed.importer&quot;</span><span class="p">)</span>
</pre></div>
</div>
</div>
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