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<div class="section" id="tailable-cursors">
<h1>Tailable Cursors<a class="headerlink" href="#tailable-cursors" title="Permalink to this headline">ΒΆ</a></h1>
<p>By default, MongoDB will automatically close a cursor when the client has
exhausted all results in the cursor. However, for <a class="reference external" href="https://docs.mongodb.org/manual/core/capped-collections/">capped collections</a> you may
use a <a class="reference external" href="https://docs.mongodb.org/manual/reference/glossary/#term-tailable-cursor">tailable cursor</a>
that remains open after the client exhausts the results in the initial cursor.</p>
<p>The following is a basic example of using a tailable cursor to tail the oplog
of a replica set member:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">time</span>
<span class="kn">import</span> <span class="nn">pymongo</span>
<span class="n">client</span> <span class="o">=</span> <span class="n">pymongo</span><span class="o">.</span><span class="n">MongoClient</span><span class="p">()</span>
<span class="n">oplog</span> <span class="o">=</span> <span class="n">client</span><span class="o">.</span><span class="n">local</span><span class="o">.</span><span class="n">oplog</span><span class="o">.</span><span class="n">rs</span>
<span class="n">first</span> <span class="o">=</span> <span class="n">oplog</span><span class="o">.</span><span class="n">find</span><span class="p">()</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="s1">'$natural'</span><span class="p">,</span> <span class="n">pymongo</span><span class="o">.</span><span class="n">ASCENDING</span><span class="p">)</span><span class="o">.</span><span class="n">limit</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">first</span><span class="p">)</span>
<span class="n">ts</span> <span class="o">=</span> <span class="n">first</span><span class="p">[</span><span class="s1">'ts'</span><span class="p">]</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="c1"># For a regular capped collection CursorType.TAILABLE_AWAIT is the</span>
<span class="c1"># only option required to create a tailable cursor. When querying the</span>
<span class="c1"># oplog the oplog_replay option enables an optimization to quickly</span>
<span class="c1"># find the 'ts' value we're looking for. The oplog_replay option</span>
<span class="c1"># can only be used when querying the oplog.</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="n">oplog</span><span class="o">.</span><span class="n">find</span><span class="p">({</span><span class="s1">'ts'</span><span class="p">:</span> <span class="p">{</span><span class="s1">'$gt'</span><span class="p">:</span> <span class="n">ts</span><span class="p">}},</span>
<span class="n">cursor_type</span><span class="o">=</span><span class="n">pymongo</span><span class="o">.</span><span class="n">CursorType</span><span class="o">.</span><span class="n">TAILABLE_AWAIT</span><span class="p">,</span>
<span class="n">oplog_replay</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">while</span> <span class="n">cursor</span><span class="o">.</span><span class="n">alive</span><span class="p">:</span>
<span class="k">for</span> <span class="n">doc</span> <span class="ow">in</span> <span class="n">cursor</span><span class="p">:</span>
<span class="n">ts</span> <span class="o">=</span> <span class="n">doc</span><span class="p">[</span><span class="s1">'ts'</span><span class="p">]</span>
<span class="nb">print</span><span class="p">(</span><span class="n">doc</span><span class="p">)</span>
<span class="c1"># We end up here if the find() returned no documents or if the</span>
<span class="c1"># tailable cursor timed out (no new documents were added to the</span>
<span class="c1"># collection for more than 1 second).</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
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