Merge branch 'appdispatch-docs'

docs/patterns/appdispatch.rst

@@ -0,0 +1,154 @@
+.. _app-dispatch:
+
+Application Dispatching
+=======================
+
+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.
+
+The fundamental difference from the :ref:`module approach
+<larger-applications>` 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.
+
+Combining Applications
+----------------------
+
+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 :class:`werkzeug.wsgi.DispatcherMiddleware`.  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.
+
+For example you could have your main application run on `/` and your
+backend interface on `/admin`::
+
+    from werkzeug.wsgi import DispatcherMiddleware
+    from frontend_app import application as frontend
+    from backend_app import application as backend
+
+    application = DispatcherMiddleware(frontend, {
+        '/backend':     backend
+    })
+
+
+Dispatch by Subdomain
+---------------------
+
+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
+:ref:`app-factories` pattern.
+
+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.
+
+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::
+
+    from threading import Lock
+
+    class SubdomainDispatcher(object):
+
+        def __init__(self, domain, create_app):
+            self.domain = domain
+            self.create_app = create_app
+            self.lock = Lock()
+            self.instances = {}
+
+        def get_application(self, host):
+            host = host.split(':')[0]
+            assert host.endswith(self.domain), 'Configuration error'
+            subdomain = host[:-len(self.domain)].rstrip('.')
+            with self.lock:
+                app = self.instances.get(subdomain)
+                if app is None:
+                    app = self.create_app(subdomain)
+                    self.instances[subdomain] = app
+                return app
+
+        def __call__(self, environ, start_response):
+            app = self.get_application(environ['HTTP_HOST'])
+            return app(environ, start_response)
+
+
+This dispatcher can then be used like this::
+
+    from myapplication import create_app, get_user_for_subdomain
+    from werkzeug.exceptions import NotFound
+
+    def make_app(subdomain):
+        user = get_user_for_subdomain(subdomain)
+        if user is None:
+            # if there is no user for that subdomain we still have
+            # to return a WSGI application that handles that request.
+            # We can then just return the NotFound() exception as
+            # application which will render a default 404 page.
+            # You might also redirect the user to the main page then
+            return NotFound()
+
+        # otherwise create the application for the specific user
+        return create_app(user)
+
+    application = SubdomainDispatcher('example.com', make_app)
+
+
+Dispatch by Path
+----------------
+
+Dispatching by a path on the URL is very similar.  Instead of looking at
+the `Host` header to figure out the subdomain one simply looks at the
+request path up to the first slash::
+
+    from threading import Lock
+    from werkzeug.wsgi import pop_path_info, peek_path_info
+
+    class PathDispatcher(object):
+
+        def __init__(self, default_app, create_app):
+            self.default_app = default_app
+            self.create_app = create_app
+            self.lock = Lock()
+            self.instances = {}
+
+        def get_application(self, prefix):
+            with self.lock:
+                app = self.instances.get(prefix)
+                if app is None:
+                    app = self.create_app(prefix)
+                    if app is not None:
+                        self.instances[prefix] = app
+                return app
+
+        def __call__(self, environ, start_response):
+            app = self.get_application(peek_path_info(environ))
+            if app is not None:
+                pop_path_info(environ)
+            else:
+                app = self.default_app
+            return app(environ, start_response)
+
+The big difference between this and the subdomain one is that this one
+falls back to another application if the creator function returns `None`::
+
+    from myapplication import create_app, default_app, get_user_for_prefix
+
+    def make_app(prefix):
+        user = get_user_for_prefix(prefix)
+        if user is not None:
+            return create_app(user)
+
+    application = PathDispatcher('example.com', default_app, make_app)

docs/patterns/index.rst

@@ -18,6 +18,7 @@ Snippet Archives <http://flask.pocoo.org/snippets/>`_.
 
    packages
    appfactories
+   appdispatch
    distribute
    fabric
    sqlite3

tests/flask_tests.py

@@ -47,6 +47,10 @@ SECRET_KEY = 'devkey'
 def catch_warnings():
     """Catch warnings in a with block in a list"""
     import warnings
+
+    # make sure deprecation warnings are active in tests
+    warnings.simplefilter('default', category=DeprecationWarning)
+
     filters = warnings.filters
     warnings.filters = filters[:]
     old_showwarning = warnings.showwarning