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<div class="section" id="application-dispatching">
<span id="app-dispatch"></span><h1>Application Dispatching<a class="headerlink" href="#application-dispatching" title="Permalink to this headline">¶</a></h1>
<p>Application dispatching is the process of combining multiple Flask
applications on the WSGI level. You can not only combine Flask
applications into something larger but any WSGI application. This would
even allow you to run a Django and a Flask application in the same
interpreter side by side if you want. The usefulness of this depends on
how the applications work internally.</p>
<p>The fundamental difference from the <a class="reference internal" href="packages.html#larger-applications"><em>module approach</em></a> is that in this case you are running the same or
different Flask applications that are entirely isolated from each other.
They run different configurations and are dispatched on the WSGI level.</p>
<div class="section" id="working-with-this-document">
<h2>Working with this Document<a class="headerlink" href="#working-with-this-document" title="Permalink to this headline">¶</a></h2>
<p>Each of the techniques and examples below results in an <tt class="docutils literal"><span class="pre">application</span></tt> object
that can be run with any WSGI server. For production, see <a class="reference internal" href="../deploying/index.html#deployment"><em>Deployment Options</em></a>.
For development, Werkzeug provides a builtin server for development available
at <a class="reference external" href="http://werkzeug.pocoo.org/docs/serving/#werkzeug.serving.run_simple" title="(in Werkzeug v0.7)"><tt class="xref py py-func docutils literal"><span class="pre">werkzeug.serving.run_simple()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug.serving</span> <span class="kn">import</span> <span class="n">run_simple</span>
<span class="n">run_simple</span><span class="p">(</span><span class="s">'localhost'</span><span class="p">,</span> <span class="mi">5000</span><span class="p">,</span> <span class="n">application</span><span class="p">,</span> <span class="n">use_reloader</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that <a class="reference external" href="http://werkzeug.pocoo.org/docs/serving/#werkzeug.serving.run_simple" title="(in Werkzeug v0.7)"><tt class="xref py py-func docutils literal"><span class="pre">run_simple</span></tt></a> is not intended for
use in production. Use a <a class="reference internal" href="../deploying/index.html#deployment"><em>full-blown WSGI server</em></a>.</p>
</div>
<div class="section" id="combining-applications">
<h2>Combining Applications<a class="headerlink" href="#combining-applications" title="Permalink to this headline">¶</a></h2>
<p>If you have entirely separated applications and you want them to work next
to each other in the same Python interpreter process you can take
advantage of the <a class="reference external" href="http://werkzeug.pocoo.org/docs/middlewares/#werkzeug.wsgi.DispatcherMiddleware" title="(in Werkzeug v0.7)"><tt class="xref py py-class docutils literal"><span class="pre">werkzeug.wsgi.DispatcherMiddleware</span></tt></a>. The idea
here is that each Flask application is a valid WSGI application and they
are combined by the dispatcher middleware into a larger one that
dispatched based on prefix.</p>
<p>For example you could have your main application run on <cite>/</cite> and your
backend interface on <cite>/backend</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug.wsgi</span> <span class="kn">import</span> <span class="n">DispatcherMiddleware</span>
<span class="kn">from</span> <span class="nn">frontend_app</span> <span class="kn">import</span> <span class="n">application</span> <span class="k">as</span> <span class="n">frontend</span>
<span class="kn">from</span> <span class="nn">backend_app</span> <span class="kn">import</span> <span class="n">application</span> <span class="k">as</span> <span class="n">backend</span>
<span class="n">application</span> <span class="o">=</span> <span class="n">DispatcherMiddleware</span><span class="p">(</span><span class="n">frontend</span><span class="p">,</span> <span class="p">{</span>
<span class="s">'/backend'</span><span class="p">:</span> <span class="n">backend</span>
<span class="p">})</span>
</pre></div>
</div>
</div>
<div class="section" id="dispatch-by-subdomain">
<h2>Dispatch by Subdomain<a class="headerlink" href="#dispatch-by-subdomain" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you might want to use multiple instances of the same application
with different configurations. Assuming the application is created inside
a function and you can call that function to instanciate it, that is
really easy to implement. In order to develop your application to support
creating new instances in functions have a look at the
<a class="reference internal" href="appfactories.html#app-factories"><em>Application Factories</em></a> pattern.</p>
<p>A very common example would be creating applications per subdomain. For
instance you configure your webserver to dispatch all requests for all
subdomains to your application and you then use the subdomain information
to create user-specific instances. Once you have your server set up to
listen on all subdomains you can use a very simple WSGI application to do
the dynamic application creation.</p>
<p>The perfect level for abstraction in that regard is the WSGI layer. You
write your own WSGI application that looks at the request that comes and
and delegates it to your Flask application. If that application does not
exist yet, it is dynamically created and remembered:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">threading</span> <span class="kn">import</span> <span class="n">Lock</span>
<span class="k">class</span> <span class="nc">SubdomainDispatcher</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">domain</span><span class="p">,</span> <span class="n">create_app</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">domain</span> <span class="o">=</span> <span class="n">domain</span>
<span class="bp">self</span><span class="o">.</span><span class="n">create_app</span> <span class="o">=</span> <span class="n">create_app</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">instances</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">def</span> <span class="nf">get_application</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">host</span><span class="p">):</span>
<span class="n">host</span> <span class="o">=</span> <span class="n">host</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">':'</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span>
<span class="k">assert</span> <span class="n">host</span><span class="o">.</span><span class="n">endswith</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">domain</span><span class="p">),</span> <span class="s">'Configuration error'</span>
<span class="n">subdomain</span> <span class="o">=</span> <span class="n">host</span><span class="p">[:</span><span class="o">-</span><span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">domain</span><span class="p">)]</span><span class="o">.</span><span class="n">rstrip</span><span class="p">(</span><span class="s">'.'</span><span class="p">)</span>
<span class="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="p">:</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">instances</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">subdomain</span><span class="p">)</span>
<span class="k">if</span> <span class="n">app</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">create_app</span><span class="p">(</span><span class="n">subdomain</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">instances</span><span class="p">[</span><span class="n">subdomain</span><span class="p">]</span> <span class="o">=</span> <span class="n">app</span>
<span class="k">return</span> <span class="n">app</span>
<span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">):</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_application</span><span class="p">(</span><span class="n">environ</span><span class="p">[</span><span class="s">'HTTP_HOST'</span><span class="p">])</span>
<span class="k">return</span> <span class="n">app</span><span class="p">(</span><span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">)</span>
</pre></div>
</div>
<p>This dispatcher can then be used like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">myapplication</span> <span class="kn">import</span> <span class="n">create_app</span><span class="p">,</span> <span class="n">get_user_for_subdomain</span>
<span class="kn">from</span> <span class="nn">werkzeug.exceptions</span> <span class="kn">import</span> <span class="n">NotFound</span>
<span class="k">def</span> <span class="nf">make_app</span><span class="p">(</span><span class="n">subdomain</span><span class="p">):</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">get_user_for_subdomain</span><span class="p">(</span><span class="n">subdomain</span><span class="p">)</span>
<span class="k">if</span> <span class="n">user</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="c"># if there is no user for that subdomain we still have</span>
<span class="c"># to return a WSGI application that handles that request.</span>
<span class="c"># We can then just return the NotFound() exception as</span>
<span class="c"># application which will render a default 404 page.</span>
<span class="c"># You might also redirect the user to the main page then</span>
<span class="k">return</span> <span class="n">NotFound</span><span class="p">()</span>
<span class="c"># otherwise create the application for the specific user</span>
<span class="k">return</span> <span class="n">create_app</span><span class="p">(</span><span class="n">user</span><span class="p">)</span>
<span class="n">application</span> <span class="o">=</span> <span class="n">SubdomainDispatcher</span><span class="p">(</span><span class="s">'example.com'</span><span class="p">,</span> <span class="n">make_app</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="dispatch-by-path">
<h2>Dispatch by Path<a class="headerlink" href="#dispatch-by-path" title="Permalink to this headline">¶</a></h2>
<p>Dispatching by a path on the URL is very similar. Instead of looking at
the <cite>Host</cite> header to figure out the subdomain one simply looks at the
request path up to the first slash:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">threading</span> <span class="kn">import</span> <span class="n">Lock</span>
<span class="kn">from</span> <span class="nn">werkzeug.wsgi</span> <span class="kn">import</span> <span class="n">pop_path_info</span><span class="p">,</span> <span class="n">peek_path_info</span>
<span class="k">class</span> <span class="nc">PathDispatcher</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">default_app</span><span class="p">,</span> <span class="n">create_app</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">default_app</span> <span class="o">=</span> <span class="n">default_app</span>
<span class="bp">self</span><span class="o">.</span><span class="n">create_app</span> <span class="o">=</span> <span class="n">create_app</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lock</span> <span class="o">=</span> <span class="n">Lock</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">instances</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">def</span> <span class="nf">get_application</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">prefix</span><span class="p">):</span>
<span class="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">lock</span><span class="p">:</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">instances</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">prefix</span><span class="p">)</span>
<span class="k">if</span> <span class="n">app</span> <span class="ow">is</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">create_app</span><span class="p">(</span><span class="n">prefix</span><span class="p">)</span>
<span class="k">if</span> <span class="n">app</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">instances</span><span class="p">[</span><span class="n">prefix</span><span class="p">]</span> <span class="o">=</span> <span class="n">app</span>
<span class="k">return</span> <span class="n">app</span>
<span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">):</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_application</span><span class="p">(</span><span class="n">peek_path_info</span><span class="p">(</span><span class="n">environ</span><span class="p">))</span>
<span class="k">if</span> <span class="n">app</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="n">pop_path_info</span><span class="p">(</span><span class="n">environ</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">app</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">default_app</span>
<span class="k">return</span> <span class="n">app</span><span class="p">(</span><span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">)</span>
</pre></div>
</div>
<p>The big difference between this and the subdomain one is that this one
falls back to another application if the creator function returns <cite>None</cite>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">myapplication</span> <span class="kn">import</span> <span class="n">create_app</span><span class="p">,</span> <span class="n">default_app</span><span class="p">,</span> <span class="n">get_user_for_prefix</span>
<span class="k">def</span> <span class="nf">make_app</span><span class="p">(</span><span class="n">prefix</span><span class="p">):</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">get_user_for_prefix</span><span class="p">(</span><span class="n">prefix</span><span class="p">)</span>
<span class="k">if</span> <span class="n">user</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">return</span> <span class="n">create_app</span><span class="p">(</span><span class="n">user</span><span class="p">)</span>
<span class="n">application</span> <span class="o">=</span> <span class="n">PathDispatcher</span><span class="p">(</span><span class="n">default_app</span><span class="p">,</span> <span class="n">make_app</span><span class="p">)</span>
</pre></div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Application Dispatching</a><ul>
<li><a class="reference internal" href="#working-with-this-document">Working with this Document</a></li>
<li><a class="reference internal" href="#combining-applications">Combining Applications</a></li>
<li><a class="reference internal" href="#dispatch-by-subdomain">Dispatch by Subdomain</a></li>
<li><a class="reference internal" href="#dispatch-by-path">Dispatch by Path</a></li>
</ul>
</li>
</ul>
<h3>Related Topics</h3>
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<li><a href="../index.html">Documentation overview</a><ul>
<li><a href="index.html">Patterns for Flask</a><ul>
<li>Previous: <a href="appfactories.html" title="previous chapter">Application Factories</a></li>
<li>Next: <a href="urlprocessors.html" title="next chapter">Using URL Processors</a></li>
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