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Connection Pooling
— SQLAlchemy 0.6.8 Documentation</title>
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<h1>SQLAlchemy 0.6.8 Documentation</h1>
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Version: <span class="versionnum">0.6.8</span> Last Updated: 06/05/2011 13:10:26
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Connection Pooling
<h2>
Connection Pooling
</h2>
<ul>
<li><a class="reference internal" href="#">Connection Pooling</a><ul>
<li><a class="reference internal" href="#connection-pool-configuration">Connection Pool Configuration</a></li>
<li><a class="reference internal" href="#switching-pool-implementations">Switching Pool Implementations</a></li>
<li><a class="reference internal" href="#using-a-custom-connection-function">Using a Custom Connection Function</a></li>
<li><a class="reference internal" href="#constructing-a-pool">Constructing a Pool</a></li>
<li><a class="reference internal" href="#pool-event-listeners">Pool Event Listeners</a></li>
<li><a class="reference internal" href="#builtin-pool-implementations">Builtin Pool Implementations</a></li>
<li><a class="reference internal" href="#pooling-plain-db-api-connections">Pooling Plain DB-API Connections</a></li>
</ul>
</li>
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<div class="section" id="module-sqlalchemy.pool">
<span id="connection-pooling"></span><span id="pooling-toplevel"></span><h1>Connection Pooling<a class="headerlink" href="#module-sqlalchemy.pool" title="Permalink to this headline">¶</a></h1>
<p>The establishment of a
database connection is typically a somewhat expensive operation, and
applications need a way to get at database connections repeatedly
with minimal overhead. Particularly for
server-side web applications, a connection pool is the standard way to
maintain a “pool” of active database connections in memory which are
reused across requests.</p>
<p>SQLAlchemy includes several connection pool implementations
which integrate with the <a class="reference internal" href="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>. They can also be used
directly for applications that want to add pooling to an otherwise
plain DBAPI approach.</p>
<div class="section" id="connection-pool-configuration">
<h2>Connection Pool Configuration<a class="headerlink" href="#connection-pool-configuration" title="Permalink to this headline">¶</a></h2>
<p>The <tt class="xref py py-class docutils literal"><span class="pre">Engine</span></tt> returned by the
<a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a> function in most cases has a <a class="reference internal" href="#sqlalchemy.pool.QueuePool" title="sqlalchemy.pool.QueuePool"><tt class="xref py py-class docutils literal"><span class="pre">QueuePool</span></tt></a>
integrated, pre-configured with reasonable pooling defaults. If
you’re reading this section to simply enable pooling- congratulations!
You’re already done.</p>
<p>The most common <a class="reference internal" href="#sqlalchemy.pool.QueuePool" title="sqlalchemy.pool.QueuePool"><tt class="xref py py-class docutils literal"><span class="pre">QueuePool</span></tt></a> tuning parameters can be passed
directly to <a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a> as keyword arguments:
<tt class="docutils literal"><span class="pre">pool_size</span></tt>, <tt class="docutils literal"><span class="pre">max_overflow</span></tt>, <tt class="docutils literal"><span class="pre">pool_recycle</span></tt> and
<tt class="docutils literal"><span class="pre">pool_timeout</span></tt>. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'postgresql://me@localhost/mydb'</span><span class="p">,</span>
<span class="n">pool_size</span><span class="o">=</span><span class="mi">20</span><span class="p">,</span> <span class="n">max_overflow</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span></pre></div>
</div>
<p>In the case of SQLite, a <a class="reference internal" href="#sqlalchemy.pool.SingletonThreadPool" title="sqlalchemy.pool.SingletonThreadPool"><tt class="xref py py-class docutils literal"><span class="pre">SingletonThreadPool</span></tt></a> is provided instead,
to provide compatibility with SQLite’s restricted threading model, as well
as to provide a reasonable default behavior to SQLite “memory” databases,
which maintain their entire dataset within the scope of a single connection.</p>
<p>All SQLAlchemy pool implementations have in common
that none of them “pre create” connections - all implementations wait
until first use before creating a connection. At that point, if
no additional concurrent checkout requests for more connections
are made, no additional connections are created. This is why it’s perfectly
fine for <a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a> to default to using a <a class="reference internal" href="#sqlalchemy.pool.QueuePool" title="sqlalchemy.pool.QueuePool"><tt class="xref py py-class docutils literal"><span class="pre">QueuePool</span></tt></a>
of size five without regard to whether or not the application really needs five connections
queued up - the pool would only grow to that size if the application
actually used five connections concurrently, in which case the usage of a
small pool is an entirely appropriate default behavior.</p>
</div>
<div class="section" id="switching-pool-implementations">
<h2>Switching Pool Implementations<a class="headerlink" href="#switching-pool-implementations" title="Permalink to this headline">¶</a></h2>
<p>The usual way to use a different kind of pool with <a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a>
is to use the <tt class="docutils literal"><span class="pre">poolclass</span></tt> argument. This argument accepts a class
imported from the <tt class="docutils literal"><span class="pre">sqlalchemy.pool</span></tt> module, and handles the details
of building the pool for you. Common options include specifying
<a class="reference internal" href="#sqlalchemy.pool.QueuePool" title="sqlalchemy.pool.QueuePool"><tt class="xref py py-class docutils literal"><span class="pre">QueuePool</span></tt></a> with SQLite:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">QueuePool</span>
<span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'sqlite:///file.db'</span><span class="p">,</span> <span class="n">poolclass</span><span class="o">=</span><span class="n">QueuePool</span><span class="p">)</span></pre></div>
</div>
<p>Disabling pooling using <a class="reference internal" href="#sqlalchemy.pool.NullPool" title="sqlalchemy.pool.NullPool"><tt class="xref py py-class docutils literal"><span class="pre">NullPool</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">import</span> <span class="n">NullPool</span>
<span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span>
<span class="s">'postgresql+psycopg2://scott:tiger@localhost/test'</span><span class="p">,</span>
<span class="n">poolclass</span><span class="o">=</span><span class="n">NullPool</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="using-a-custom-connection-function">
<h2>Using a Custom Connection Function<a class="headerlink" href="#using-a-custom-connection-function" title="Permalink to this headline">¶</a></h2>
<p>All <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a> classes accept an argument <tt class="docutils literal"><span class="pre">creator</span></tt> which is
a callable that creates a new connection. <a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a>
accepts this function to pass onto the pool via an argument of
the same name:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">as</span> <span class="nn">pool</span>
<span class="kn">import</span> <span class="nn">psycopg2</span>
<span class="k">def</span> <span class="nf">getconn</span><span class="p">():</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">psycopg2</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s">'ed'</span><span class="p">,</span> <span class="n">host</span><span class="o">=</span><span class="s">'127.0.0.1'</span><span class="p">,</span> <span class="n">dbname</span><span class="o">=</span><span class="s">'test'</span><span class="p">)</span>
<span class="c"># do things with 'c' to set up</span>
<span class="k">return</span> <span class="n">c</span>
<span class="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'postgresql+psycopg2://'</span><span class="p">,</span> <span class="n">creator</span><span class="o">=</span><span class="n">getconn</span><span class="p">)</span></pre></div>
</div>
<p>For most “initialize on connection” routines, it’s more convenient
to use a <a class="reference internal" href="interfaces.html#sqlalchemy.interfaces.PoolListener" title="sqlalchemy.interfaces.PoolListener"><tt class="xref py py-class docutils literal"><span class="pre">PoolListener</span></tt></a>, so that the usual URL argument to
<a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a> is still usable. <tt class="docutils literal"><span class="pre">creator</span></tt> is there as
a total last resort for when a DBAPI has some form of <tt class="docutils literal"><span class="pre">connect</span></tt>
that is not at all supported by SQLAlchemy.</p>
</div>
<div class="section" id="constructing-a-pool">
<h2>Constructing a Pool<a class="headerlink" href="#constructing-a-pool" title="Permalink to this headline">¶</a></h2>
<p>To use a <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a> by itself, the <tt class="docutils literal"><span class="pre">creator</span></tt> function is
the only argument that’s required and is passed first, followed
by any additional options:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">as</span> <span class="nn">pool</span>
<span class="kn">import</span> <span class="nn">psycopg2</span>
<span class="k">def</span> <span class="nf">getconn</span><span class="p">():</span>
<span class="n">c</span> <span class="o">=</span> <span class="n">psycopg2</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s">'ed'</span><span class="p">,</span> <span class="n">host</span><span class="o">=</span><span class="s">'127.0.0.1'</span><span class="p">,</span> <span class="n">dbname</span><span class="o">=</span><span class="s">'test'</span><span class="p">)</span>
<span class="k">return</span> <span class="n">c</span>
<span class="n">mypool</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">QueuePool</span><span class="p">(</span><span class="n">getconn</span><span class="p">,</span> <span class="n">max_overflow</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">pool_size</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span></pre></div>
</div>
<p>DBAPI connections can then be procured from the pool using the <a class="reference internal" href="#sqlalchemy.pool.Pool.connect" title="sqlalchemy.pool.Pool.connect"><tt class="xref py py-meth docutils literal"><span class="pre">Pool.connect()</span></tt></a>
function. The return value of this method is a DBAPI connection that’s contained
within a transparent proxy:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># get a connection</span>
<span class="n">conn</span> <span class="o">=</span> <span class="n">mypool</span><span class="o">.</span><span class="n">connect</span><span class="p">()</span>
<span class="c"># use it</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"select foo"</span><span class="p">)</span></pre></div>
</div>
<p>The purpose of the transparent proxy is to intercept the <tt class="docutils literal"><span class="pre">close()</span></tt> call,
such that instead of the DBAPI connection being closed, its returned to the
pool:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># "close" the connection. Returns</span>
<span class="c"># it to the pool.</span>
<span class="n">conn</span><span class="o">.</span><span class="n">close</span><span class="p">()</span></pre></div>
</div>
<p>The proxy also returns its contained DBAPI connection to the pool
when it is garbage collected,
though it’s not deterministic in Python that this occurs immediately (though
it is typical with cPython).</p>
<p>A particular pre-created <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a> can be shared with one or more
engines by passing it to the <tt class="docutils literal"><span class="pre">pool</span></tt> argument of <a class="reference internal" href="engines.html#sqlalchemy.create_engine" title="sqlalchemy.create_engine"><tt class="xref py py-func docutils literal"><span class="pre">create_engine()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">e</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s">'postgresql://'</span><span class="p">,</span> <span class="n">pool</span><span class="o">=</span><span class="n">mypool</span><span class="p">)</span></pre></div>
</div>
</div>
<div class="section" id="pool-event-listeners">
<h2>Pool Event Listeners<a class="headerlink" href="#pool-event-listeners" title="Permalink to this headline">¶</a></h2>
<p>Connection pools support an event interface that allows hooks to execute
upon first connect, upon each new connection, and upon checkout and
checkin of connections. See <a class="reference internal" href="interfaces.html#sqlalchemy.interfaces.PoolListener" title="sqlalchemy.interfaces.PoolListener"><tt class="xref py py-class docutils literal"><span class="pre">PoolListener</span></tt></a> for details.</p>
</div>
<div class="section" id="builtin-pool-implementations">
<h2>Builtin Pool Implementations<a class="headerlink" href="#builtin-pool-implementations" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="sqlalchemy.pool.Pool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">Pool</tt><big>(</big><em>creator</em>, <em>recycle=-1</em>, <em>echo=None</em>, <em>use_threadlocal=False</em>, <em>logging_name=None</em>, <em>reset_on_return=True</em>, <em>listeners=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.Pool" title="Permalink to this definition">¶</a></dt>
<dd><p>Abstract base class for connection pools.</p>
<dl class="method">
<dt id="sqlalchemy.pool.Pool.__init__">
<tt class="descname">__init__</tt><big>(</big><em>creator</em>, <em>recycle=-1</em>, <em>echo=None</em>, <em>use_threadlocal=False</em>, <em>logging_name=None</em>, <em>reset_on_return=True</em>, <em>listeners=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.Pool.__init__" title="Permalink to this definition">¶</a></dt>
<dd><p>Construct a Pool.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>creator</strong> – a callable function that returns a DB-API
connection object. The function will be called with
parameters.</li>
<li><strong>recycle</strong> – If set to non -1, number of seconds between
connection recycling, which means upon checkout, if this
timeout is surpassed the connection will be closed and
replaced with a newly opened connection. Defaults to -1.</li>
<li><strong>logging_name</strong> – String identifier which will be used within
the “name” field of logging records generated within the
“sqlalchemy.pool” logger. Defaults to a hexstring of the object’s
id.</li>
<li><strong>echo</strong> – If True, connections being pulled and retrieved
from the pool will be logged to the standard output, as well
as pool sizing information. Echoing can also be achieved by
enabling logging for the “sqlalchemy.pool”
namespace. Defaults to False.</li>
<li><strong>use_threadlocal</strong> – If set to True, repeated calls to
<a class="reference internal" href="#sqlalchemy.pool.Pool.connect" title="sqlalchemy.pool.Pool.connect"><tt class="xref py py-meth docutils literal"><span class="pre">connect()</span></tt></a> within the same application thread will be
guaranteed to return the same connection object, if one has
already been retrieved from the pool and has not been
returned yet. Offers a slight performance advantage at the
cost of individual transactions by default. The
<tt class="xref py py-meth docutils literal"><span class="pre">unique_connection()</span></tt> method is provided to bypass the
threadlocal behavior installed into <a class="reference internal" href="#sqlalchemy.pool.Pool.connect" title="sqlalchemy.pool.Pool.connect"><tt class="xref py py-meth docutils literal"><span class="pre">connect()</span></tt></a>.</li>
<li><strong>reset_on_return</strong> – If true, reset the database state of
connections returned to the pool. This is typically a
ROLLBACK to release locks and transaction resources.
Disable at your own peril. Defaults to True.</li>
<li><strong>listeners</strong> – A list of
<a class="reference internal" href="interfaces.html#sqlalchemy.interfaces.PoolListener" title="sqlalchemy.interfaces.PoolListener"><tt class="xref py py-class docutils literal"><span class="pre">PoolListener</span></tt></a>-like objects or
dictionaries of callables that receive events when DB-API
connections are created, checked out and checked in to the
pool.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="method">
<dt id="sqlalchemy.pool.Pool.connect">
<tt class="descname">connect</tt><big>(</big><big>)</big><a class="headerlink" href="#sqlalchemy.pool.Pool.connect" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a DBAPI connection from the pool.</p>
<p>The connection is instrumented such that when its
<tt class="docutils literal"><span class="pre">close()</span></tt> method is called, the connection will be returned to
the pool.</p>
</dd></dl>
<dl class="method">
<dt id="sqlalchemy.pool.Pool.dispose">
<tt class="descname">dispose</tt><big>(</big><big>)</big><a class="headerlink" href="#sqlalchemy.pool.Pool.dispose" title="Permalink to this definition">¶</a></dt>
<dd><p>Dispose of this pool.</p>
<p>This method leaves the possibility of checked-out connections
remaining open, It is advised to not reuse the pool once dispose()
is called, and to instead use a new pool constructed by the
recreate() method.</p>
</dd></dl>
<dl class="method">
<dt id="sqlalchemy.pool.Pool.recreate">
<tt class="descname">recreate</tt><big>(</big><big>)</big><a class="headerlink" href="#sqlalchemy.pool.Pool.recreate" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a>, of the same class as this one
and configured with identical creation arguments.</p>
<p>This method is used in conjunection with <a class="reference internal" href="#sqlalchemy.pool.Pool.dispose" title="sqlalchemy.pool.Pool.dispose"><tt class="xref py py-meth docutils literal"><span class="pre">dispose()</span></tt></a>
to close out an entire <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a> and create a new one in
its place.</p>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.pool.QueuePool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">QueuePool</tt><big>(</big><em>creator</em>, <em>pool_size=5</em>, <em>max_overflow=10</em>, <em>timeout=30</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.QueuePool" title="Permalink to this definition">¶</a></dt>
<dd><p>Bases: <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">sqlalchemy.pool.Pool</span></tt></a></p>
<p>A Pool that imposes a limit on the number of open connections.</p>
<dl class="method">
<dt id="sqlalchemy.pool.QueuePool.__init__">
<tt class="descname">__init__</tt><big>(</big><em>creator</em>, <em>pool_size=5</em>, <em>max_overflow=10</em>, <em>timeout=30</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.QueuePool.__init__" title="Permalink to this definition">¶</a></dt>
<dd><p>Construct a QueuePool.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>creator</strong> – a callable function that returns a DB-API
connection object. The function will be called with
parameters.</li>
<li><strong>pool_size</strong> – The size of the pool to be maintained,
defaults to 5. This is the largest number of connections that
will be kept persistently in the pool. Note that the pool
begins with no connections; once this number of connections
is requested, that number of connections will remain.
<tt class="docutils literal"><span class="pre">pool_size</span></tt> can be set to 0 to indicate no size limit; to
disable pooling, use a <a class="reference internal" href="#sqlalchemy.pool.NullPool" title="sqlalchemy.pool.NullPool"><tt class="xref py py-class docutils literal"><span class="pre">NullPool</span></tt></a>
instead.</li>
<li><strong>max_overflow</strong> – The maximum overflow size of the
pool. When the number of checked-out connections reaches the
size set in pool_size, additional connections will be
returned up to this limit. When those additional connections
are returned to the pool, they are disconnected and
discarded. It follows then that the total number of
simultaneous connections the pool will allow is pool_size +
<cite>max_overflow</cite>, and the total number of “sleeping”
connections the pool will allow is pool_size. <cite>max_overflow</cite>
can be set to -1 to indicate no overflow limit; no limit
will be placed on the total number of concurrent
connections. Defaults to 10.</li>
<li><strong>timeout</strong> – The number of seconds to wait before giving up
on returning a connection. Defaults to 30.</li>
<li><strong>recycle</strong> – If set to non -1, number of seconds between
connection recycling, which means upon checkout, if this
timeout is surpassed the connection will be closed and
replaced with a newly opened connection. Defaults to -1.</li>
<li><strong>echo</strong> – If True, connections being pulled and retrieved
from the pool will be logged to the standard output, as well
as pool sizing information. Echoing can also be achieved by
enabling logging for the “sqlalchemy.pool”
namespace. Defaults to False.</li>
<li><strong>use_threadlocal</strong> – If set to True, repeated calls to
<tt class="xref py py-meth docutils literal"><span class="pre">connect()</span></tt> within the same application thread will be
guaranteed to return the same connection object, if one has
already been retrieved from the pool and has not been
returned yet. Offers a slight performance advantage at the
cost of individual transactions by default. The
<tt class="xref py py-meth docutils literal"><span class="pre">unique_connection()</span></tt> method is provided to bypass the
threadlocal behavior installed into <tt class="xref py py-meth docutils literal"><span class="pre">connect()</span></tt>.</li>
<li><strong>reset_on_return</strong> – If true, reset the database state of
connections returned to the pool. This is typically a
ROLLBACK to release locks and transaction resources.
Disable at your own peril. Defaults to True.</li>
<li><strong>listeners</strong> – A list of
<a class="reference internal" href="interfaces.html#sqlalchemy.interfaces.PoolListener" title="sqlalchemy.interfaces.PoolListener"><tt class="xref py py-class docutils literal"><span class="pre">PoolListener</span></tt></a>-like objects or
dictionaries of callables that receive events when DB-API
connections are created, checked out and checked in to the
pool.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.pool.SingletonThreadPool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">SingletonThreadPool</tt><big>(</big><em>creator</em>, <em>pool_size=5</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.SingletonThreadPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Bases: <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">sqlalchemy.pool.Pool</span></tt></a></p>
<p>A Pool that maintains one connection per thread.</p>
<p>Maintains one connection per each thread, never moving a connection to a
thread other than the one which it was created in.</p>
<p>This is used for SQLite, which both does not handle multithreading by
default, and also requires a singleton connection if a :memory: database
is being used.</p>
<p>Options are the same as those of <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a>, as well as:</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><strong>pool_size</strong> – The number of threads in which to maintain connections
at once. Defaults to five.</td>
</tr>
</tbody>
</table>
<dl class="method">
<dt id="sqlalchemy.pool.SingletonThreadPool.__init__">
<tt class="descname">__init__</tt><big>(</big><em>creator</em>, <em>pool_size=5</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.SingletonThreadPool.__init__" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.pool.AssertionPool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">AssertionPool</tt><big>(</big><em>*args</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.AssertionPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Bases: <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">sqlalchemy.pool.Pool</span></tt></a></p>
<p>A Pool that allows at most one checked out connection at any given
time.</p>
<p>This will raise an exception if more than one connection is checked out
at a time. Useful for debugging code that is using more connections
than desired.</p>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.pool.NullPool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">NullPool</tt><big>(</big><em>creator</em>, <em>recycle=-1</em>, <em>echo=None</em>, <em>use_threadlocal=False</em>, <em>logging_name=None</em>, <em>reset_on_return=True</em>, <em>listeners=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.NullPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Bases: <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">sqlalchemy.pool.Pool</span></tt></a></p>
<p>A Pool which does not pool connections.</p>
<p>Instead it literally opens and closes the underlying DB-API connection
per each connection open/close.</p>
<p>Reconnect-related functions such as <tt class="docutils literal"><span class="pre">recycle</span></tt> and connection
invalidation are not supported by this Pool implementation, since
no connections are held persistently.</p>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.pool.StaticPool">
<em class="property">class </em><tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">StaticPool</tt><big>(</big><em>creator</em>, <em>recycle=-1</em>, <em>echo=None</em>, <em>use_threadlocal=False</em>, <em>logging_name=None</em>, <em>reset_on_return=True</em>, <em>listeners=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.StaticPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Bases: <a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">sqlalchemy.pool.Pool</span></tt></a></p>
<p>A Pool of exactly one connection, used for all requests.</p>
<p>Reconnect-related functions such as <tt class="docutils literal"><span class="pre">recycle</span></tt> and connection
invalidation (which is also used to support auto-reconnect) are not
currently supported by this Pool implementation but may be implemented
in a future release.</p>
</dd></dl>
</div>
<div class="section" id="pooling-plain-db-api-connections">
<h2>Pooling Plain DB-API Connections<a class="headerlink" href="#pooling-plain-db-api-connections" title="Permalink to this headline">¶</a></h2>
<p>Any <span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0249"><strong>PEP 249</strong></a> DB-API module can be “proxied” through the connection
pool transparently. Usage of the DB-API is exactly as before, except
the <tt class="docutils literal"><span class="pre">connect()</span></tt> method will consult the pool. Below we illustrate
this with <tt class="docutils literal"><span class="pre">psycopg2</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sqlalchemy.pool</span> <span class="kn">as</span> <span class="nn">pool</span>
<span class="kn">import</span> <span class="nn">psycopg2</span> <span class="kn">as</span> <span class="nn">psycopg</span>
<span class="n">psycopg</span> <span class="o">=</span> <span class="n">pool</span><span class="o">.</span><span class="n">manage</span><span class="p">(</span><span class="n">psycopg</span><span class="p">)</span>
<span class="c"># then connect normally</span>
<span class="n">connection</span> <span class="o">=</span> <span class="n">psycopg</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="n">database</span><span class="o">=</span><span class="s">'test'</span><span class="p">,</span> <span class="n">username</span><span class="o">=</span><span class="s">'scott'</span><span class="p">,</span>
<span class="n">password</span><span class="o">=</span><span class="s">'tiger'</span><span class="p">)</span></pre></div>
</div>
<p>This produces a <tt class="xref py py-class docutils literal"><span class="pre">_DBProxy</span></tt> object which supports the same
<tt class="docutils literal"><span class="pre">connect()</span></tt> function as the original DB-API module. Upon
connection, a connection proxy object is returned, which delegates its
calls to a real DB-API connection object. This connection object is
stored persistently within a connection pool (an instance of
<a class="reference internal" href="#sqlalchemy.pool.Pool" title="sqlalchemy.pool.Pool"><tt class="xref py py-class docutils literal"><span class="pre">Pool</span></tt></a>) that corresponds to the exact connection arguments sent
to the <tt class="docutils literal"><span class="pre">connect()</span></tt> function.</p>
<p>The connection proxy supports all of the methods on the original
connection object, most of which are proxied via <tt class="docutils literal"><span class="pre">__getattr__()</span></tt>.
The <tt class="docutils literal"><span class="pre">close()</span></tt> method will return the connection to the pool, and the
<tt class="docutils literal"><span class="pre">cursor()</span></tt> method will return a proxied cursor object. Both the
connection proxy and the cursor proxy will also return the underlying
connection to the pool after they have both been garbage collected,
which is detected via weakref callbacks (<tt class="docutils literal"><span class="pre">__del__</span></tt> is not used).</p>
<p>Additionally, when connections are returned to the pool, a
<tt class="docutils literal"><span class="pre">rollback()</span></tt> is issued on the connection unconditionally. This is
to release any locks still held by the connection that may have
resulted from normal activity.</p>
<p>By default, the <tt class="docutils literal"><span class="pre">connect()</span></tt> method will return the same connection
that is already checked out in the current thread. This allows a
particular connection to be used in a given thread without needing to
pass it around between functions. To disable this behavior, specify
<tt class="docutils literal"><span class="pre">use_threadlocal=False</span></tt> to the <tt class="docutils literal"><span class="pre">manage()</span></tt> function.</p>
<dl class="function">
<dt id="sqlalchemy.pool.manage">
<tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">manage</tt><big>(</big><em>module</em>, <em>**params</em><big>)</big><a class="headerlink" href="#sqlalchemy.pool.manage" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a proxy for a DB-API module that automatically
pools connections.</p>
<p>Given a DB-API 2.0 module and pool management parameters, returns
a proxy for the module that will automatically pool connections,
creating new connection pools for each distinct set of connection
arguments sent to the decorated module’s connect() function.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>module</strong> – a DB-API 2.0 database module</li>
<li><strong>poolclass</strong> – the class used by the pool module to provide
pooling. Defaults to <a class="reference internal" href="#sqlalchemy.pool.QueuePool" title="sqlalchemy.pool.QueuePool"><tt class="xref py py-class docutils literal"><span class="pre">QueuePool</span></tt></a>.</li>
<li><strong>**params</strong> – will be passed through to <em>poolclass</em></li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.pool.clear_managers">
<tt class="descclassname">sqlalchemy.pool.</tt><tt class="descname">clear_managers</tt><big>(</big><big>)</big><a class="headerlink" href="#sqlalchemy.pool.clear_managers" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove all current DB-API 2.0 managers.</p>
<p>All pools and connections are disposed.</p>
</dd></dl>
</div>
</div>
</div>
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