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  <div class="section" id="overview">
<h1>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">ΒΆ</a></h1>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">this section is a technical overview of the
<strong>internal API of Alembic</strong>.
This section is only useful for developers who wish to extend the
capabilities of Alembic; for regular users, reading this section
is <strong>not necessary</strong>.</p>
</div>
<p>A visualization of the primary features of Alembic&#8217;s internals is presented
in the following figure.   The module and class boxes do not list out
all the operations provided by each unit; only a small set of representative
elements intended to convey the primary purpose of each system.</p>
<img alt="../_images/api_overview.png" src="../_images/api_overview.png" />
<p>The script runner for Alembic is present in the <a class="reference internal" href="config.html#alembic-config-toplevel"><span>Configuration</span></a> module.
This module produces a <a class="reference internal" href="config.html#alembic.config.Config" title="alembic.config.Config"><code class="xref py py-class docutils literal"><span class="pre">Config</span></code></a> object and passes it to the
appropriate function in <a class="reference internal" href="commands.html#alembic-command-toplevel"><span>Commands</span></a>.   Functions within
<a class="reference internal" href="commands.html#alembic-command-toplevel"><span>Commands</span></a> will typically instantiate an
<a class="reference internal" href="script.html#alembic.script.ScriptDirectory" title="alembic.script.ScriptDirectory"><code class="xref py py-class docutils literal"><span class="pre">ScriptDirectory</span></code></a> instance, which represents the collection of
version files, and an <a class="reference internal" href="runtime.html#alembic.runtime.environment.EnvironmentContext" title="alembic.runtime.environment.EnvironmentContext"><code class="xref py py-class docutils literal"><span class="pre">EnvironmentContext</span></code></a>, which is a configurational
facade passed to the environment&#8217;s <code class="docutils literal"><span class="pre">env.py</span></code> script.</p>
<p>The <a class="reference internal" href="runtime.html#alembic.runtime.environment.EnvironmentContext" title="alembic.runtime.environment.EnvironmentContext"><code class="xref py py-class docutils literal"><span class="pre">EnvironmentContext</span></code></a> object is the primary object used within
the <code class="docutils literal"><span class="pre">env.py</span></code> script, whose main purpose is that of a facade for creating and using
a <a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> object, which is the actual migration engine
that refers to a database implementation.   The primary method called
on this object within an <code class="docutils literal"><span class="pre">env.py</span></code> script is the
<a class="reference internal" href="runtime.html#alembic.runtime.environment.EnvironmentContext.configure" title="alembic.runtime.environment.EnvironmentContext.configure"><code class="xref py py-meth docutils literal"><span class="pre">EnvironmentContext.configure()</span></code></a> method, which sets up the
<a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> with database connectivity and behavioral
configuration.  It also supplies methods for transaction demarcation and
migration running, but these methods ultimately call upon the
<a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> that&#8217;s been configured.</p>
<p><a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> is the gateway to the database
for other parts of the application, and produces a <a class="reference internal" href="ddl.html#alembic.ddl.impl.DefaultImpl" title="alembic.ddl.impl.DefaultImpl"><code class="xref py py-class docutils literal"><span class="pre">DefaultImpl</span></code></a>
object which does the actual database communication, and knows how to
create the specific SQL text of the various DDL directives such as
ALTER TABLE; <a class="reference internal" href="ddl.html#alembic.ddl.impl.DefaultImpl" title="alembic.ddl.impl.DefaultImpl"><code class="xref py py-class docutils literal"><span class="pre">DefaultImpl</span></code></a> has subclasses that are per-database-backend.
In &#8220;offline&#8221; mode (e.g. <code class="docutils literal"><span class="pre">--sql</span></code>), the <a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> will
produce SQL to a file output stream instead of a database.</p>
<p>During an upgrade or downgrade operation, a specific series of migration
scripts are invoked starting with the <a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a> in conjunction
with the <a class="reference internal" href="script.html#alembic.script.ScriptDirectory" title="alembic.script.ScriptDirectory"><code class="xref py py-class docutils literal"><span class="pre">ScriptDirectory</span></code></a>; the actual scripts themselves make use
of the <a class="reference internal" href="../ops.html#alembic.operations.Operations" title="alembic.operations.Operations"><code class="xref py py-class docutils literal"><span class="pre">Operations</span></code></a> object, which provide the end-user interface to
specific database operations.   The <a class="reference internal" href="../ops.html#alembic.operations.Operations" title="alembic.operations.Operations"><code class="xref py py-class docutils literal"><span class="pre">Operations</span></code></a> object is generated
based on a series of &#8220;operation directive&#8221; objects that are user-extensible,
and start out in the <span class="xref std std-ref">alembic.operations.ops.toplevel</span> module.</p>
<p>Another prominent feature of Alembic is the &#8220;autogenerate&#8221; feature, which
produces new migration scripts that contain Python code.  The autogenerate
feature starts in <a class="reference internal" href="autogenerate.html#alembic-autogenerate-toplevel"><span>Autogeneration</span></a>, and is used exclusively
by the <a class="reference internal" href="commands.html#alembic.command.revision" title="alembic.command.revision"><code class="xref py py-func docutils literal"><span class="pre">alembic.command.revision()</span></code></a> command when the <code class="docutils literal"><span class="pre">--autogenerate</span></code>
flag is passed.  Autogenerate refers to the <a class="reference internal" href="runtime.html#alembic.runtime.migration.MigrationContext" title="alembic.runtime.migration.MigrationContext"><code class="xref py py-class docutils literal"><span class="pre">MigrationContext</span></code></a>
and <a class="reference internal" href="ddl.html#alembic.ddl.impl.DefaultImpl" title="alembic.ddl.impl.DefaultImpl"><code class="xref py py-class docutils literal"><span class="pre">DefaultImpl</span></code></a> in order to access database connectivity and
access per-backend rules for autogenerate comparisons.  It also makes use
of <span class="xref std std-ref">alembic.operations.ops.toplevel</span> in order to represent the operations that
it will render into scripts.</p>
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