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  51.   <div class="section" id="frequently-asked-questions">
    52. 

  52. <h1>Frequently Asked Questions<a class="headerlink" href="#frequently-asked-questions" title="Permalink to this headline">¶</a></h1>
    53. 

  53. <div class="section" id="questions">
    54. 

  54. <h2>Questions<a class="headerlink" href="#questions" title="Permalink to this headline">¶</a></h2>
    55. 

  55. <div class="section" id="mysql-is-throwing-deadlock-errors-what-can-i-do">
    56. 

  56. <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>
    57. 

  57. <p><strong>Answer:</strong> MySQL has default isolation level set to <tt class="docutils literal"><span class="pre">REPEATABLE-READ</span></tt>,
    58. 

  58. if you don&#8217;t really need that, set it to <tt class="docutils literal"><span class="pre">READ-COMMITTED</span></tt>.
    59. 

  59. You can do that by adding the following to your <tt class="docutils literal"><span class="pre">my.cnf</span></tt>:</p>
    60. 

  60. <div class="highlight-python"><div class="highlight"><pre><span class="p">[</span><span class="n">mysqld</span><span class="p">]</span>
    61. 

  61. <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>
    62. 

  62. </pre></div>
    63. 

  63. </div>
    64. 

  64. <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
    65. 

  65. Transaction Model and Locking</a> in the MySQL user manual.</p>
    66. 

  66. <p>(Thanks to Honza Kral and Anton Tsigularov for this solution)</p>
    67. 

  67. </div>
    68. 

  68. <div class="section" id="celeryd-is-not-doing-anything-just-hanging">
    69. 

  69. <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>
    70. 

  70. <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>
    71. 

  71. </div>
    72. 

  72. <div class="section" id="i-m-having-integrityerror-duplicate-key-errors-why">
    73. 

  73. <h3>I&#8217;m having <tt class="docutils literal"><span class="pre">IntegrityError:</span> <span class="pre">Duplicate</span> <span class="pre">Key</span></tt> errors. Why?<a class="headerlink" href="#i-m-having-integrityerror-duplicate-key-errors-why" title="Permalink to this headline">¶</a></h3>
    74. 

  74. <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>.
    75. 

  75. Thanks to howsthedotcom.</p>
    76. 

  76. </div>
    77. 

  77. <div class="section" id="why-won-t-my-task-run">
    78. 

  78. <h3>Why won&#8217;t my Task run?<a class="headerlink" href="#why-won-t-my-task-run" title="Permalink to this headline">¶</a></h3>
    79. 

  79. <p><strong>Answer:</strong> Did you register the task in the applications <tt class="docutils literal"><span class="pre">tasks.py</span></tt> module?
    80. 

  80. (or in some other module Django loads by default, like <tt class="docutils literal"><span class="pre">models.py</span></tt>?).
    81. 

  81. Also there might be syntax errors preventing the tasks module being imported.</p>
    82. 

  82. <p>You can find out if the celery daemon is able to run the task by executing the
    83. 

  83. task manually:</p>
    84. 

  84. <div class="highlight-python"><div class="highlight"><pre><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">MyPeriodicTask</span>
    85. 

  85. <span class="gp">&gt;&gt;&gt; </span><span class="n">MyPeriodicTask</span><span class="o">.</span><span class="n">delay</span><span class="p">()</span>
    86. 

  86. </pre></div>
    87. 

  87. </div>
    88. 

  88. <p>Watch celery daemons logfile (or output if not running as a daemon), to see
    89. 

  89. if it&#8217;s able to find the task, or if some other error is happening.</p>
    90. 

  90. </div>
    91. 

  91. <div class="section" id="why-won-t-my-periodic-task-run">
    92. 

  92. <h3>Why won&#8217;t my Periodic Task run?<a class="headerlink" href="#why-won-t-my-periodic-task-run" title="Permalink to this headline">¶</a></h3>
    93. 

  93. <p>See <a class="reference internal" href="#why-won-t-my-task-run">Why won&#8217;t my Task run?</a>.</p>
    94. 

  94. </div>
    95. 

  95. <div class="section" id="can-i-send-some-tasks-to-only-some-servers">
    96. 

  96. <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>
    97. 

  97. <p>As of now there is only one use-case that works like this, and that is
    98. 

  98. 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
    99. 

  99. 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
    100. 

  100. one routing_key, but this is coming in a later release.</p>
    101. 

  101. <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,
    102. 

  102. and one server <tt class="docutils literal"><span class="pre">z</span></tt>, that only handles feed related tasks, you can use this
    103. 

  103. configuration:</p>
    104. 

  104. <blockquote>
    105. 

  105. <ul class="simple">
    106. 

  106. <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>
    107. 

  107. </ul>
    108. 

  108. <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>
    109. 

  109. <span class="n">AMQP_PORT</span> <span class="o">=</span> <span class="mf">5678</span>
    110. 

  110. <span class="n">AMQP_USER</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
    111. 

  111. <span class="n">AMQP_PASSWORD</span> <span class="o">=</span> <span class="s">&quot;secret&quot;</span>
    112. 

  112. <span class="n">AMQP_VHOST</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
    113. 

  113. 

  114. <span class="n">CELERY_AMQP_CONSUMER_QUEUE</span> <span class="o">=</span> <span class="s">&quot;regular_tasks&quot;</span>
    115. 

  115. <span class="n">CELERY_AMQP_EXCHANGE</span> <span class="o">=</span> <span class="s">&quot;tasks&quot;</span>
    116. 

  116. <span class="n">CELERY_AMQP_PUBLISHER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.regular&quot;</span>
    117. 

  117. <span class="n">CELERY_AMQP_CONSUMER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.#&quot;</span>
    118. 

  118. <span class="n">CELERY_AMQP_EXCHANGE_TYPE</span> <span class="o">=</span> <span class="s">&quot;topic&quot;</span>
    119. 

  119. </pre></div>
    120. 

  120. </div>
    121. 

  121. <ul class="simple">
    122. 

  122. <li>Server <tt class="docutils literal"><span class="pre">z</span></tt>: settings.py:</li>
    123. 

  123. </ul>
    124. 

  124. <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>
    125. 

  125. <span class="n">AMQP_PORT</span> <span class="o">=</span> <span class="mf">5678</span>
    126. 

  126. <span class="n">AMQP_USER</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
    127. 

  127. <span class="n">AMQP_PASSWORD</span> <span class="o">=</span> <span class="s">&quot;secret&quot;</span>
    128. 

  128. <span class="n">AMQP_VHOST</span> <span class="o">=</span> <span class="s">&quot;myapp&quot;</span>
    129. 

  129. 

  130. <span class="n">CELERY_AMQP_CONSUMER_QUEUE</span> <span class="o">=</span> <span class="s">&quot;feed_tasks&quot;</span>
    131. 

  131. <span class="n">CELERY_AMQP_EXCHANGE</span> <span class="o">=</span> <span class="s">&quot;tasks&quot;</span>
    132. 

  132. <span class="n">CELERY_AMQP_PUBLISHER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.regular&quot;</span>
    133. 

  133. <span class="n">CELERY_AMQP_CONSUMER_ROUTING_KEY</span> <span class="o">=</span> <span class="s">&quot;task.feed.#&quot;</span>
    134. 

  134. <span class="n">CELERY_AMQP_EXCHANGE_TYPE</span> <span class="o">=</span> <span class="s">&quot;topic&quot;</span>
    135. 

  135. </pre></div>
    136. 

  136. </div>
    137. 

  137. </blockquote>
    138. 

  138. <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
    139. 

  139. <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>
    140. 

  140. <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>
    141. 

  141. <span class="kn">from</span> <span class="nn">celery.task</span> <span class="kn">import</span> <span class="n">Task</span>
    142. 

  142. 

  143. <span class="k">class</span> <span class="nc">FeedImportTask</span><span class="p">(</span><span class="n">Task</span><span class="p">):</span>
    144. 

  144.     <span class="n">name</span> <span class="o">=</span> <span class="s">&quot;import_feed&quot;</span>
    145. 

  145.     <span class="n">routing_key</span> <span class="o">=</span> <span class="s">&quot;task.feed.importer&quot;</span>
    146. 

  146. 

  147.     <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>
    148. 

  148.         <span class="c"># something importing the feed</span>
    149. 

  149.         <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>
    150. 

  150. </pre></div>
    151. 

  151. </div>
    152. 

  152. <p>You can also override this using the <tt class="docutils literal"><span class="pre">routing_key</span></tt> argument to
    153. 

  153. <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>
    154. 

  154. <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>
    155. 

  155. <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>
    156. 

  156. <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>
    157. 

  157. <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>
    158. 

  158. </pre></div>
    159. 

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    160. 

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