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| <section id="polling-pollexecutor"> |
| <h1>Polling: <code class="docutils literal notranslate"><span class="pre">PollExecutor</span></code><a class="headerlink" href="#polling-pollexecutor" title="Permalink to this headline">¶</a></h1> |
| <p>PollExecutor allows producing futures which should be resolved via a |
| custom poll function.</p> |
| <section id="poll-function"> |
| <span id="id1"></span><h2>Poll function<a class="headerlink" href="#poll-function" title="Permalink to this headline">¶</a></h2> |
| <p>The poll function has the following semantics:</p> |
| <ul class="simple"> |
| <li><p>It’s called with a single argument, a list of zero or more |
| <a class="reference internal" href="#more_executors.PollDescriptor" title="more_executors.PollDescriptor"><code class="xref py py-class docutils literal notranslate"><span class="pre">PollDescriptor</span></code></a> objects.</p></li> |
| <li><p>If the poll function can determine that a particular future should be |
| completed, either successfully or in error, it should call the |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">yield_result()</span></code> or <code class="xref py py-meth docutils literal notranslate"><span class="pre">yield_exception()</span></code> methods on the appropriate |
| <a class="reference internal" href="#more_executors.PollDescriptor" title="more_executors.PollDescriptor"><code class="xref py py-class docutils literal notranslate"><span class="pre">PollDescriptor</span></code></a>.</p></li> |
| <li><p>If the poll function raises an exception, all futures depending on that poll |
| will fail with that exception. The poll function will be retried later.</p></li> |
| <li><p>If the poll function returns an int or float, it is used as the delay |
| in seconds until the next poll.</p></li> |
| </ul> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>The poll function should avoid holding a reference to the corresponding |
| <a class="reference internal" href="#more_executors.PollExecutor" title="more_executors.PollExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">PollExecutor</span></code></a>. If it holds a reference to the |
| executor, invoking <a class="reference external" href="https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Executor.shutdown" title="(in Python v3.11)"><code class="xref py py-meth docutils literal notranslate"><span class="pre">shutdown()</span></code></a> becomes |
| mandatory in order to stop the polling thread.</p> |
| </div> |
| </section> |
| <section id="cancel-function"> |
| <span id="id2"></span><h2>Cancel function<a class="headerlink" href="#cancel-function" title="Permalink to this headline">¶</a></h2> |
| <p>The cancel function has the following semantics:</p> |
| <ul class="simple"> |
| <li><p>It’s called when cancel is requested on a future returned by this executor, |
| if and only if the future is currently in the list of futures being polled, |
| i.e. it will not be called if the delegate future has not yet completed.</p></li> |
| <li><p>It’s called with a single argument: the value returned by the delegate |
| future.</p></li> |
| <li><p>It should return <cite>True</cite> if the future can be cancelled.</p></li> |
| <li><p>If the cancel function raises an exception, the future’s cancel method will |
| return <cite>False</cite> and a message will be logged.</p></li> |
| </ul> |
| </section> |
| <section id="example"> |
| <h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2> |
| <p>Consider a web service which executes tasks asynchronously, with an API like |
| this:</p> |
| <ul> |
| <li><p>To create a task: <cite>POST https://myservice/object/:id/publish</cite></p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p>Returns an identifier for a task, such as <cite>{“task_id”: 123}</cite></p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| <li><p>To get status of a single task: <cite>GET https://myservice/tasks/:id</cite></p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p>Returns task status, such as <cite>{“task_id”: 123, “state”: “finished”}</cite></p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| <li><p>To cancel a single task: <cite>DELETE https://myservice/tasks/:id</cite></p></li> |
| <li><p>To search for status of multiple tasks: |
| <cite>POST https://myservice/tasks/search {“task_id”: [123, 456, …]}</cite></p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p>Returns array of task statuses, such as:</p></li> |
| </ul> |
| <div class="highlight-JSON notranslate"><div class="highlight"><pre><span></span><span class="p">[{</span><span class="nt">"task_id"</span><span class="p">:</span><span class="w"> </span><span class="mi">123</span><span class="p">,</span><span class="w"> </span><span class="nt">"state"</span><span class="p">:</span><span class="w"> </span><span class="s2">"finished"</span><span class="p">},</span><span class="w"></span> |
| <span class="w"> </span><span class="p">{</span><span class="nt">"task_id"</span><span class="p">:</span><span class="w"> </span><span class="mi">456</span><span class="p">,</span><span class="w"> </span><span class="nt">"state"</span><span class="p">:</span><span class="w"> </span><span class="s2">"error"</span><span class="p">,</span><span class="w"> </span><span class="nt">"error"</span><span class="p">:</span><span class="w"> </span><span class="s2">"..."</span><span class="p">}]</span><span class="w"></span> |
| </pre></div> |
| </div> |
| </div></blockquote> |
| </li> |
| </ul> |
| <p>Imagine we want to run many tasks concurrently and monitor their status. |
| Two obvious approaches include:</p> |
| <ul> |
| <li><p>We could make a function which starts a task and polls that task until |
| it completes, and have several threads run that function for different tasks.</p> |
| <ul class="simple"> |
| <li><p>This could be easily encapsulated as a future using only the executors |
| from <a class="reference external" href="https://docs.python.org/3/library/concurrent.futures.html#module-concurrent.futures" title="(in Python v3.11)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a>, but is very inefficient - each task in |
| progress will consume a thread for polling.</p></li> |
| </ul> |
| </li> |
| <li><p>We could maintain a list of all outstanding tasks and implement a |
| special-purpose function to wait for all of them to complete.</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p>This is efficient as only one request is needed per poll interval, but |
| since tasks aren’t represented as futures, useful features from |
| <a class="reference external" href="https://docs.python.org/3/library/concurrent.futures.html#module-concurrent.futures" title="(in Python v3.11)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a> cannot be used and different types of |
| asynchronous work will be handled inconsistently.</p></li> |
| </ul> |
| </div></blockquote> |
| </li> |
| </ul> |
| <p><a class="reference internal" href="#more_executors.PollExecutor" title="more_executors.PollExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">PollExecutor</span></code></a> bridges the gap between the two |
| approaches. By providing a custom poll function, the tasks can be monitored |
| by one thread efficiently, while at the same time allowing full usage of the |
| API provided by <cite>Future</cite> objects.</p> |
| <p>In this example, the poll function would look similar to this:</p> |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">poll_tasks</span><span class="p">(</span><span class="n">poll_descriptors</span><span class="p">):</span> |
| <span class="c1"># Collect tasks we'll need to search for</span> |
| <span class="n">task_ids</span> <span class="o">=</span> <span class="p">[</span><span class="n">d</span><span class="o">.</span><span class="n">result</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'task_id'</span><span class="p">)</span> |
| <span class="k">for</span> <span class="n">d</span> <span class="ow">in</span> <span class="n">poll_descriptors</span><span class="p">]</span> |
| |
| <span class="c1"># Search for status of these tasks</span> |
| <span class="n">search</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"task_id"</span><span class="p">:</span> <span class="n">task_ids</span><span class="p">}</span> |
| <span class="n">response</span> <span class="o">=</span> <span class="n">requests</span><span class="o">.</span><span class="n">post</span><span class="p">(</span> |
| <span class="s1">'https://myservice/tasks/search'</span><span class="p">,</span> <span class="n">json</span><span class="o">=</span><span class="n">search</span><span class="p">)</span> |
| |
| <span class="c1"># If request failed then all outstanding futures fail</span> |
| <span class="n">response</span><span class="o">.</span><span class="n">raise_for_status</span><span class="p">()</span> |
| |
| <span class="c1"># Request passed, resolve futures accordingly</span> |
| <span class="n">tasks</span> <span class="o">=</span> <span class="p">{</span><span class="n">task</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'task_id'</span><span class="p">):</span> <span class="n">task</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'state'</span><span class="p">)</span> |
| <span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">response</span><span class="o">.</span><span class="n">json</span><span class="p">()}</span> |
| <span class="k">for</span> <span class="n">d</span> <span class="ow">in</span> <span class="n">poll_descriptors</span><span class="p">:</span> |
| <span class="n">task</span> <span class="o">=</span> <span class="n">tasks</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">d</span><span class="o">.</span><span class="n">result</span><span class="p">,</span> <span class="p">{})</span> |
| <span class="n">state</span> <span class="o">=</span> <span class="n">task</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'state'</span><span class="p">)</span> |
| <span class="k">if</span> <span class="n">state</span> <span class="o">==</span> <span class="s1">'finished'</span><span class="p">:</span> |
| <span class="n">d</span><span class="o">.</span><span class="n">yield_result</span><span class="p">()</span> |
| <span class="k">elif</span> <span class="n">state</span> <span class="o">==</span> <span class="s1">'error'</span><span class="p">:</span> |
| <span class="n">d</span><span class="o">.</span><span class="n">yield_exception</span><span class="p">(</span><span class="n">TaskFailed</span><span class="p">())</span> |
| </pre></div> |
| </div> |
| <p>To ensure that calling <cite>future.cancel()</cite> also cancels tasks on the |
| remote service, we can also provide a cancel function:</p> |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">cancel_task</span><span class="p">(</span><span class="n">task</span><span class="p">):</span> |
| <span class="n">task_id</span> <span class="o">=</span> <span class="n">task</span><span class="p">[</span><span class="s1">'task_id'</span><span class="p">]</span> |
| |
| <span class="c1"># Attempt to DELETE this task</span> |
| <span class="n">response</span> <span class="o">=</span> <span class="n">requests</span><span class="o">.</span><span class="n">delete</span><span class="p">(</span> |
| <span class="s1">'https://myservice/tasks/</span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="n">task_id</span><span class="p">)</span> |
| |
| <span class="c1"># Succeeded only if response was OK.</span> |
| <span class="c1"># Otherwise, we may have been too late to cancel.</span> |
| <span class="k">return</span> <span class="n">response</span><span class="o">.</span><span class="n">ok</span> |
| </pre></div> |
| </div> |
| <p>With the poll and cancel functions in place, futures could be created |
| like this:</p> |
| <div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">publish</span><span class="p">(</span><span class="n">object_id</span><span class="p">):</span> |
| <span class="n">response</span> <span class="o">=</span> <span class="n">requests</span><span class="o">.</span><span class="n">post</span><span class="p">(</span> |
| <span class="s1">'https://myservice/object/</span><span class="si">%s</span><span class="s1">/publish'</span> <span class="o">%</span> <span class="n">object_id</span><span class="p">)</span> |
| <span class="n">response</span><span class="o">.</span><span class="n">raise_for_status</span><span class="p">()</span> |
| <span class="k">return</span> <span class="n">response</span><span class="o">.</span><span class="n">json</span><span class="p">()</span> |
| |
| <span class="c1"># Submit up to 4 HTTP requests at a time,</span> |
| <span class="c1"># and poll for the returned tasks using our poll function.</span> |
| <span class="n">executor</span> <span class="o">=</span> <span class="n">Executors</span><span class="o">.</span>\ |
| <span class="n">threadpool</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span><span class="o">.</span>\ |
| <span class="n">with_poll</span><span class="p">(</span><span class="n">poll_tasks</span><span class="p">,</span> <span class="n">cancel_task</span><span class="p">)</span> |
| <span class="n">futures</span> <span class="o">=</span> <span class="p">[</span><span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">publish</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span> |
| <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">)]</span> |
| |
| <span class="c1"># Now we can use concurrent.futures API to get tasks as</span> |
| <span class="c1"># the poll function determines they've completed</span> |
| <span class="k">for</span> <span class="n">completed</span> <span class="ow">in</span> <span class="n">as_completed</span><span class="p">(</span><span class="n">futures</span><span class="p">):</span> |
| <span class="c1"># ...</span> |
| </pre></div> |
| </div> |
| <dl class="py class"> |
| <dt class="sig sig-object py" id="more_executors.PollExecutor"> |
| <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">more_executors.</span></span><span class="sig-name descname"><span class="pre">PollExecutor</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">delegate</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">poll_fn</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">cancel_fn</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">default_interval</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">5.0</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">logger</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">name</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">'default'</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#more_executors.PollExecutor" title="Permalink to this definition">¶</a></dt> |
| <dd><p>Instances of <cite>PollExecutor</cite> submit callables to a delegate executor |
| and resolve the returned futures via a provided poll function.</p> |
| <p>A cancel function may also be provided to perform additional processing |
| when a returned future is cancelled.</p> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters</dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>delegate</strong> (<a class="reference external" href="https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Executor" title="(in Python v3.11)"><em>Executor</em></a>) – executor to which callables will be submitted</p></li> |
| <li><p><strong>poll_fn</strong> (<em>callable</em>) – a <a class="reference internal" href="#poll-function">poll function</a> used to decide when futures should be resolved</p></li> |
| <li><p><strong>cancel_fn</strong> (<em>callable</em>) – a <a class="reference internal" href="#cancel-function">cancel function</a> invoked when future cancel is required</p></li> |
| <li><p><strong>default_interval</strong> (<a class="reference external" href="https://docs.python.org/3/library/functions.html#float" title="(in Python v3.11)"><em>float</em></a>) – default interval between polls (in seconds)</p></li> |
| <li><p><strong>logger</strong> (<a class="reference external" href="https://docs.python.org/3/library/logging.html#logging.Logger" title="(in Python v3.11)"><em>Logger</em></a>) – a logger used for messages from this executor</p></li> |
| <li><p><strong>name</strong> (<a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#str" title="(in Python v3.11)"><em>str</em></a>) – a name for this executor</p></li> |
| </ul> |
| </dd> |
| </dl> |
| <div class="versionchanged"> |
| <p><span class="versionmodified changed">Changed in version 2.7.0: </span>Introduced <code class="docutils literal notranslate"><span class="pre">name</span></code>.</p> |
| </div> |
| <dl class="py method"> |
| <dt class="sig sig-object py" id="more_executors.PollExecutor.notify"> |
| <span class="sig-name descname"><span class="pre">notify</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#more_executors.PollExecutor.notify" title="Permalink to this definition">¶</a></dt> |
| <dd><p>Request the executor to re-run the polling function as soon as possible.</p> |
| <p>This method may be used to perform the next poll earlier than it would |
| otherwise run.</p> |
| <p>This is useful for cases where futures may be resolved via a mixture of |
| polling and external events.</p> |
| <div class="versionadded"> |
| <p><span class="versionmodified added">New in version 2.2.0.</span></p> |
| </div> |
| </dd></dl> |
| |
| </dd></dl> |
| |
| <dl class="py method"> |
| <dt class="sig sig-object py" id="more_executors.Executors.with_poll"> |
| <em class="property"><span class="pre">classmethod</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">Executors.</span></span><span class="sig-name descname"><span class="pre">with_poll</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">executor</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">poll_fn</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">cancel_fn</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">default_interval</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">5.0</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">logger</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#more_executors.Executors.with_poll" title="Permalink to this definition">¶</a></dt> |
| <dd><dl class="field-list simple"> |
| <dt class="field-odd">Returns</dt> |
| <dd class="field-odd"><p><p>a new executor which produces polled futures.</p> |
| <p>Submitted callables will have their output passed into |
| a poll function.</p> |
| </p> |
| </dd> |
| <dt class="field-even">Return type</dt> |
| <dd class="field-even"><p><a class="reference internal" href="#more_executors.PollExecutor" title="more_executors.PollExecutor"><em>PollExecutor</em></a></p> |
| </dd> |
| </dl> |
| </dd></dl> |
| |
| <dl class="py class"> |
| <dt class="sig sig-object py" id="more_executors.PollDescriptor"> |
| <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">more_executors.</span></span><span class="sig-name descname"><span class="pre">PollDescriptor</span></span><a class="headerlink" href="#more_executors.PollDescriptor" title="Permalink to this definition">¶</a></dt> |
| <dd><p>Represents an unresolved <a class="reference external" href="https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Future" title="(in Python v3.11)"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a>.</p> |
| <p>The poll function used by <a class="reference internal" href="#more_executors.PollExecutor" title="more_executors.PollExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">PollExecutor</span></code></a> will be |
| invoked with a list of PollDescriptor objects.</p> |
| <dl class="py property"> |
| <dt class="sig sig-object py" id="more_executors.PollDescriptor.result"> |
| <em class="property"><span class="pre">property</span><span class="w"> </span></em><span class="sig-name descname"><span class="pre">result</span></span><a class="headerlink" href="#more_executors.PollDescriptor.result" title="Permalink to this definition">¶</a></dt> |
| <dd><p>The result from the delegate executor’s future, which should be used to |
| drive the poll.</p> |
| </dd></dl> |
| |
| <dl class="py method"> |
| <dt class="sig sig-object py" id="more_executors.PollDescriptor.yield_exception"> |
| <span class="sig-name descname"><span class="pre">yield_exception</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">exception</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">traceback</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#more_executors.PollDescriptor.yield_exception" title="Permalink to this definition">¶</a></dt> |
| <dd><p>The poll function can call this function to make the future raise the given exception.</p> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters</dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>exception</strong> (<a class="reference external" href="https://docs.python.org/3/library/exceptions.html#Exception" title="(in Python v3.11)"><em>Exception</em></a>) – An exception to be returned or raised by the future associated with this |
| descriptor</p></li> |
| <li><p><strong>traceback</strong> (<em>traceback</em>) – An optional associated traceback. This argument only has an effect on python 2.x, |
| where exception objects do not include a traceback.</p></li> |
| </ul> |
| </dd> |
| </dl> |
| </dd></dl> |
| |
| <dl class="py method"> |
| <dt class="sig sig-object py" id="more_executors.PollDescriptor.yield_result"> |
| <span class="sig-name descname"><span class="pre">yield_result</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">result</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#more_executors.PollDescriptor.yield_result" title="Permalink to this definition">¶</a></dt> |
| <dd><p>The poll function can call this function to make the future yield the given result.</p> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters</dt> |
| <dd class="field-odd"><p><strong>result</strong> (<a class="reference external" href="https://docs.python.org/3/library/functions.html#object" title="(in Python v3.11)"><em>object</em></a>) – a result to be returned by the future associated with this descriptor</p> |
| </dd> |
| </dl> |
| </dd></dl> |
| |
| </dd></dl> |
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