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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Overview — Alembic 0.8.3 documentation</title> <link rel="stylesheet" href="../_static/nature_override.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="stylesheet" href="../_static/changelog.css" type="text/css" /> <link rel="stylesheet" href="../_static/sphinx_paramlinks.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '0.8.3', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="top" title="Alembic 0.8.3 documentation" href="../index.html" /> <link rel="up" title="API Details" href="index.html" /> <link rel="next" title="Runtime Objects" href="runtime.html" /> <link rel="prev" title="API Details" href="index.html" /> </head> <body role="document"> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="runtime.html" title="Runtime Objects" accesskey="N">next</a> |</li> <li class="right" > <a href="index.html" title="API Details" accesskey="P">previous</a> |</li> <li class="nav-item nav-item-0"><a href="../index.html">Alembic 0.8.3 documentation</a> »</li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">API Details</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="overview"> <h1>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</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’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’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’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 “offline” 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 “operation directive” 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 “autogenerate” 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> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="index.html" title="previous chapter">API Details</a></p> <h4>Next topic</h4> <p class="topless"><a href="runtime.html" title="next chapter">Runtime Objects</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/api/overview.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="runtime.html" title="Runtime Objects" >next</a> |</li> <li class="right" > <a href="index.html" title="API Details" >previous</a> |</li> <li class="nav-item nav-item-0"><a href="../index.html">Alembic 0.8.3 documentation</a> »</li> <li class="nav-item nav-item-1"><a href="index.html" >API Details</a> »</li> </ul> </div> <div class="footer" role="contentinfo"> © Copyright 2010-2015, Mike Bayer. 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