- -

-

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 module approach 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.

-

Working with this Document¶

-

Each of the techniques and examples below results in an application object -that can be run with any WSGI server. For production, see Deployment Options. -For development, Werkzeug provides a builtin server for development available -at werkzeug.serving.run_simple():

-

from werkzeug.serving import run_simple
-run_simple('localhost', 5000, application, use_reloader=True)
-

-

-

Note that run_simple is not intended for -use in production. Use a full-blown WSGI server.

-

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 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 /backend:

-

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 -Application 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(default_app, make_app)
-

-

-

- - -

Application Dispatching¶

Working with this Document¶

Combining Applications¶

Dispatch by Subdomain¶

Dispatch by Path¶

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Application Dispatching¶

Working with this Document¶

Combining Applications¶

Dispatch by Subdomain¶

Dispatch by Path¶

Table Of Contents

Related Topics

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