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Horizontal Shard
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Horizontal Shard
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<h2>
Horizontal Shard
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<ul>
<li><a class="reference internal" href="#">Horizontal Shard</a><ul>
<li><a class="reference internal" href="#api-documentation">API Documentation</a></li>
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<div class="section" id="module-sqlalchemy.ext.horizontal_shard">
<span id="horizontal-shard"></span><h1>Horizontal Shard<a class="headerlink" href="#module-sqlalchemy.ext.horizontal_shard" title="Permalink to this headline">¶</a></h1>
<p>Horizontal sharding support.</p>
<p>Defines a rudimental ‘horizontal sharding’ system which allows a Session to
distribute queries and persistence operations across multiple databases.</p>
<p>For a usage example, see the <a class="reference internal" href="../../examples.html#examples-sharding"><em>Horizontal Sharding</em></a> example included in
the source distrbution.</p>
<div class="section" id="api-documentation">
<h2>API Documentation<a class="headerlink" href="#api-documentation" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="sqlalchemy.ext.horizontal_shard.ShardedSession">
<em class="property">class </em><tt class="descclassname">sqlalchemy.ext.horizontal_shard.</tt><tt class="descname">ShardedSession</tt><big>(</big><em>shard_chooser</em>, <em>id_chooser</em>, <em>query_chooser</em>, <em>shards=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.horizontal_shard.ShardedSession" title="Permalink to this definition">¶</a></dt>
<dd><dl class="method">
<dt id="sqlalchemy.ext.horizontal_shard.ShardedSession.__init__">
<tt class="descname">__init__</tt><big>(</big><em>shard_chooser</em>, <em>id_chooser</em>, <em>query_chooser</em>, <em>shards=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.horizontal_shard.ShardedSession.__init__" title="Permalink to this definition">¶</a></dt>
<dd><p>Construct a ShardedSession.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
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<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>shard_chooser</strong> – A callable which, passed a Mapper, a mapped instance, and possibly a
SQL clause, returns a shard ID. This id may be based off of the
attributes present within the object, or on some round-robin
scheme. If the scheme is based on a selection, it should set
whatever state on the instance to mark it in the future as
participating in that shard.</li>
<li><strong>id_chooser</strong> – A callable, passed a query and a tuple of identity values, which
should return a list of shard ids where the ID might reside. The
databases will be queried in the order of this listing.</li>
<li><strong>query_chooser</strong> – For a given Query, returns the list of shard_ids where the query
should be issued. Results from all shards returned will be combined
together into a single listing.</li>
<li><strong>shards</strong> – A dictionary of string shard names to <a class="reference internal" href="../sqlalchemy/connections.html#sqlalchemy.engine.base.Engine" title="sqlalchemy.engine.base.Engine"><tt class="xref py py-class docutils literal"><span class="pre">Engine</span></tt></a>
objects.</li>
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<dl class="class">
<dt id="sqlalchemy.ext.horizontal_shard.ShardedQuery">
<em class="property">class </em><tt class="descclassname">sqlalchemy.ext.horizontal_shard.</tt><tt class="descname">ShardedQuery</tt><big>(</big><em>*args</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.horizontal_shard.ShardedQuery" title="Permalink to this definition">¶</a></dt>
<dd><dl class="method">
<dt id="sqlalchemy.ext.horizontal_shard.ShardedQuery.__init__">
<tt class="descname">__init__</tt><big>(</big><em>*args</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.horizontal_shard.ShardedQuery.__init__" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="method">
<dt id="sqlalchemy.ext.horizontal_shard.ShardedQuery.set_shard">
<tt class="descname">set_shard</tt><big>(</big><em>shard_id</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.horizontal_shard.ShardedQuery.set_shard" title="Permalink to this definition">¶</a></dt>
<dd><p>return a new query, limited to a single shard ID.</p>
<p>all subsequent operations with the returned query will
be against the single shard regardless of other state.</p>
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