Web Server Advanced

Web Handler Cancellation


web-handler execution could be canceled on every await if client drops connection without reading entire response’s BODY.

The behavior is very different from classic WSGI frameworks like Flask and Django.

Sometimes it is a desirable behavior: on processing GET request the code might fetch data from database or other web resource, the fetching is potentially slow.

Canceling this fetch is very good: the peer dropped connection already, there is no reason to waste time and resources (memory etc) by getting data from DB without any chance to send it back to peer.

But sometimes the cancellation is bad: on POST request very often is needed to save data to DB regardless to peer closing.

Cancellation prevention could be implemented in several ways:

  • Applying asyncio.shield() to coroutine that saves data into DB.
  • Spawning a new task for DB saving
  • Using aiojobs or other third party library.

asyncio.shield() works pretty good. The only disadvantage is you need to split web handler into exactly two async functions: one for handler itself and other for protected code.

For example the following snippet is not safe:

async def handler(request):
    await asyncio.shield(write_to_redis(request))
    await asyncio.shield(write_to_postgres(request))
    return web.Response('OK')

Cancellation might be occurred just after saving data in REDIS, write_to_postgres will be not called.

Spawning a new task is much worse: there is no place to await spawned tasks:

async def handler(request):
    return web.Response('OK')

In this case errors from write_to_redis are not awaited, it leads to many asyncio log messages Future exception was never retrieved and Task was destroyed but it is pending!.

Moreover on Graceful shutdown phase aiohttp don’t wait for these tasks, you have a great chance to loose very important data.

On other hand aiojobs provides an API for spawning new jobs and awaiting their results etc. It stores all scheduled activity in internal data structures and could terminate them gracefully:

from aiojobs.aiohttp import setup, spawn

async def coro(timeout):
    await asyncio.sleep(timeout)  # do something in background

async def handler(request):
    await spawn(request, coro())
    return web.Response()

app = web.Application()
app.router.add_get('/', handler)

All not finished jobs will be terminated on aiohttp.web.Application.on_cleanup signal.

To prevent cancellation of the whole web-handler use @atomic decorator:

from aiojobs.aiohttp import atomic

async def handler(request):
    await write_to_db()
    return web.Response()

app = web.Application()
app.router.add_post('/', handler)

It prevents all handler async function from cancellation, write_to_db will be never interrupted.

Custom Routing Criteria

Sometimes you need to register handlers on more complex criteria than simply a HTTP method and path pair.

Although UrlDispatcher does not support any extra criteria, routing based on custom conditions can be accomplished by implementing a second layer of routing in your application.

The following example shows custom routing based on the HTTP Accept header:

class AcceptChooser:

    def __init__(self):
        self._accepts = {}

    async def do_route(self, request):
        for accept in request.headers.getall('ACCEPT', []):
            acceptor = self._accepts.get(accept)
            if acceptor is not None:
                return (await acceptor(request))
        raise HTTPNotAcceptable()

    def reg_acceptor(self, accept, handler):
        self._accepts[accept] = handler

async def handle_json(request):
    # do json handling

async def handle_xml(request):
    # do xml handling

chooser = AcceptChooser()
app.router.add_get('/', chooser.do_route)

chooser.reg_acceptor('application/json', handle_json)
chooser.reg_acceptor('application/xml', handle_xml)

Static file handling

The best way to handle static files (images, JavaScripts, CSS files etc.) is using Reverse Proxy like nginx or CDN services.

But for development it’s very convenient to handle static files by aiohttp server itself.

To do it just register a new static route by UrlDispatcher.add_static() call:

app.router.add_static('/prefix', path_to_static_folder)

When a directory is accessed within a static route then the server responses to client with HTTP/403 Forbidden by default. Displaying folder index instead could be enabled with show_index parameter set to True:

app.router.add_static('/prefix', path_to_static_folder, show_index=True)

When a symlink from the static directory is accessed, the server responses to client with HTTP/404 Not Found by default. To allow the server to follow symlinks, parameter follow_symlinks should be set to True:

app.router.add_static('/prefix', path_to_static_folder, follow_symlinks=True)

When you want to enable cache busting, parameter append_version can be set to True

Cache busting is the process of appending some form of file version hash to the filename of resources like JavaScript and CSS files. The performance advantage of doing this is that we can tell the browser to cache these files indefinitely without worrying about the client not getting the latest version when the file changes:

app.router.add_static('/prefix', path_to_static_folder, append_version=True)

Template Rendering

aiohttp.web does not support template rendering out-of-the-box.

However, there is a third-party library, aiohttp_jinja2, which is supported by the aiohttp authors.

Using it is rather simple. First, setup a jinja2 environment with a call to aiohttp_jinja2.setup():

app = web.Application()

After that you may use the template engine in your handlers. The most convenient way is to simply wrap your handlers with the aiohttp_jinja2.template() decorator:

def handler(request):
    return {'name': 'Andrew', 'surname': 'Svetlov'}

If you prefer the Mako template engine, please take a look at the aiohttp_mako library.

Reading from the same task in WebSockets

Reading from the WebSocket (await ws.receive()) must only be done inside the request handler task; however, writing (ws.send_str(...)) to the WebSocket, closing (await ws.close()) and canceling the handler task may be delegated to other tasks. See also FAQ section.

aiohttp.web creates an implicit asyncio.Task for handling every incoming request.


While aiohttp.web itself only supports WebSockets without downgrading to LONG-POLLING, etc., our team supports SockJS, an aiohttp-based library for implementing SockJS-compatible server code.


Parallel reads from websocket are forbidden, there is no possibility to call aiohttp.web.WebSocketResponse.receive() from two tasks.

See FAQ section for instructions how to solve the problem.

Data Sharing aka No Singletons Please

aiohttp.web discourages the use of global variables, aka singletons. Every variable should have its own context that is not global.

So, aiohttp.web.Application and aiohttp.web.Request support a collections.abc.MutableMapping interface (i.e. they are dict-like objects), allowing them to be used as data stores.

For storing global-like variables, feel free to save them in an Application instance:

app['my_private_key'] = data

and get it back in the web-handler:

async def handler(request):
    data = request.app['my_private_key']

Variables that are only needed for the lifetime of a Request, can be stored in a Request:

async def handler(request):
  request['my_private_key'] = "data"

This is mostly useful for Middlewares and Signals handlers to store data for further processing by the next handlers in the chain.

aiohttp.web.StreamResponse and aiohttp.web.Response objects also support collections.abc.MutableMapping interface. This is useful when you want to share data with signals and middlewares once all the work in the handler is done:

async def handler(request):
  [ do all the work ]
  response['my_metric'] = 123
  return response

To avoid clashing with other aiohttp users and third-party libraries, please choose a unique key name for storing data.

If your code is published on PyPI, then the project name is most likely unique and safe to use as the key. Otherwise, something based on your company name/url would be satisfactory (i.e. org.company.app).


aiohttp.web provides a powerful mechanism for customizing request handlers via middlewares.

A middleware is a coroutine that can modify either the request or response. For example, here’s a simple middleware which appends ' wink' to the response:

from aiohttp.web import middleware

async def middleware(request, handler):
    resp = await handler(request)
    resp.text = resp.text + ' wink'
    return resp

(Note: this example won’t work with streamed responses or websockets)

Every middleware should accept two parameters, a request instance and a handler, and return the response.

When creating an Application, these middlewares are passed to the keyword-only middlewares parameter:

app = web.Application(middlewares=[middleware_1,

Internally, a single request handler is constructed by applying the middleware chain to the original handler in reverse order, and is called by the RequestHandler as a regular handler.

Since middlewares are themselves coroutines, they may perform extra await calls when creating a new handler, e.g. call database etc.

Middlewares usually call the handler, but they may choose to ignore it, e.g. displaying 403 Forbidden page or raising HTTPForbidden exception if the user does not have permissions to access the underlying resource. They may also render errors raised by the handler, perform some pre- or post-processing like handling CORS and so on.

The following code demonstrates middlewares execution order:

from aiohttp import web

def test(request):
    print('Handler function called')
    return web.Response(text="Hello")

async def middleware1(request, handler):
    print('Middleware 1 called')
    response = await handler(request)
    print('Middleware 1 finished')
    return response

async def middleware2(request, handler):
    print('Middleware 2 called')
    response = await handler(request)
    print('Middleware 2 finished')
    return response

app = web.Application(middlewares=[middleware1, middleware2])
app.router.add_get('/', test)

Produced output:

Middleware 1 called
Middleware 2 called
Handler function called
Middleware 2 finished
Middleware 1 finished


A common use of middlewares is to implement custom error pages. The following example will render 404 errors using a JSON response, as might be appropriate a JSON REST service:

from aiohttp import web

async def error_middleware(request, handler):
        response = await handler(request)
        if response.status != 404:
            return response
        message = response.message
    except web.HTTPException as ex:
        if ex.status != 404:
        message = ex.reason
    return web.json_response({'error': message})

app = web.Application(middlewares=[error_middleware])

Old Style Middleware

Deprecated since version 2.3: Prior to v2.3 middleware required an outer middleware factory which returned the middleware coroutine. Since v2.3 this is not required; instead the @middleware decorator should be used.

Old style middleware (with an outer factory and no @middleware decorator) is still supported. Furthermore, old and new style middleware can be mixed.

A middleware factory is simply a coroutine that implements the logic of a middleware. For example, here’s a trivial middleware factory:

async def middleware_factory(app, handler):
    async def middleware_handler(request):
        resp = await handler(request)
        resp.text = resp.text + ' wink'
        return resp
    return middleware_handler

A middleware factory should accept two parameters, an app instance and a handler, and return a new handler.


Both the outer middleware_factory coroutine and the inner middleware_handler coroutine are called for every request handled.

Middleware factories should return a new handler that has the same signature as a request handler. That is, it should accept a single Request instance and return a Response, or raise an exception.


Although middlewares can customize request handlers before or after a Response has been prepared, they can’t customize a Response while it’s being prepared. For this aiohttp.web provides signals.

For example, a middleware can only change HTTP headers for unprepared responses (see prepare()), but sometimes we need a hook for changing HTTP headers for streamed responses and WebSockets. This can be accomplished by subscribing to the on_response_prepare signal:

async def on_prepare(request, response):
    response.headers['My-Header'] = 'value'


Additionally, the on_startup and on_cleanup signals can be subscribed to for application component setup and tear down accordingly.

The following example will properly initialize and dispose an aiopg connection engine:

from aiopg.sa import create_engine

async def create_aiopg(app):
    app['pg_engine'] = await create_engine(

async def dispose_aiopg(app):
    await app['pg_engine'].wait_closed()


Signal handlers should not return a value but may modify incoming mutable parameters.

Signal handlers will be run sequentially, in order they were added. If handler is asynchronous, it will be awaited before calling next one.


Signals API has provisional status, meaning it may be changed in future releases.

Signal subscription and sending will most likely be the same, but signal object creation is subject to change. As long as you are not creating new signals, but simply reusing existing ones, you will not be affected.

Nested applications

Sub applications are designed for solving the problem of the big monolithic code base. Let’s assume we have a project with own business logic and tools like administration panel and debug toolbar.

Administration panel is a separate application by its own nature but all toolbar URLs are served by prefix like /admin.

Thus we’ll create a totally separate application named admin and connect it to main app with prefix by add_subapp():

admin = web.Application()
# setup admin routes, signals and middlewares

app.add_subapp('/admin/', admin)

Middlewares and signals from app and admin are chained.

It means that if URL is '/admin/something' middlewares from app are applied first and admin.middlewares are the next in the call chain.

The same is going for on_response_prepare signal – the signal is delivered to both top level app and admin if processing URL is routed to admin sub-application.

Common signals like on_startup, on_shutdown and on_cleanup are delivered to all registered sub-applications. The passed parameter is sub-application instance, not top-level application.

Third level sub-applications can be nested into second level ones – there are no limitation for nesting level.

Url reversing for sub-applications should generate urls with proper prefix.

But for getting URL sub-application’s router should be used:

admin = web.Application()
admin.router.add_get('/resource', handler, name='name')

app.add_subapp('/admin/', admin)

url = admin.router['name'].url_for()

The generated url from example will have a value URL('/admin/resource').

If main application should do URL reversing for sub-application it could use the following explicit technique:

admin = web.Application()
admin.router.add_get('/resource', handler, name='name')

app.add_subapp('/admin/', admin)
app['admin'] = admin

async def handler(request):  # main application's handler
    admin = request.app['admin']
    url = admin.router['name'].url_for()

Expect Header

aiohttp.web supports Expect header. By default it sends HTTP/1.1 100 Continue line to client, or raises HTTPExpectationFailed if header value is not equal to “100-continue”. It is possible to specify custom Expect header handler on per route basis. This handler gets called if Expect header exist in request after receiving all headers and before processing application’s Middlewares and route handler. Handler can return None, in that case the request processing continues as usual. If handler returns an instance of class StreamResponse, request handler uses it as response. Also handler can raise a subclass of HTTPException. In this case all further processing will not happen and client will receive appropriate http response.


A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status. The server MUST respond with a 417 (Expectation Failed) status if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status.


If all checks pass, the custom handler must write a HTTP/1.1 100 Continue status code before returning.

The following example shows how to setup a custom handler for the Expect header:

async def check_auth(request):
    if request.version != aiohttp.HttpVersion11:

    if request.headers.get('EXPECT') != '100-continue':
        raise HTTPExpectationFailed(text="Unknown Expect: %s" % expect)

    if request.headers.get('AUTHORIZATION') is None:
        raise HTTPForbidden()

    request.transport.write(b"HTTP/1.1 100 Continue\r\n\r\n")

async def hello(request):
    return web.Response(body=b"Hello, world")

app = web.Application()
app.router.add_get('/', hello, expect_handler=check_auth)

Custom resource implementation

To register custom resource use UrlDispatcher.register_resource(). Resource instance must implement AbstractResource interface.

Application runners

run_app() provides a simple blocking API for running an Application.

For starting the application asynchronously on serving on multiple HOST/PORT AppRunner exists.

The simple startup code for serving HTTP site on 'localhost', port 8080 looks like:

runner = web.AppRunner(app)
await runner.setup()
site = web.TCPSite(runner, 'localhost', 8080)
await site.start()

To stop serving call AppRunner.cleanup():

await runner.cleanup()

New in version 3.0.

Graceful shutdown

Stopping aiohttp web server by just closing all connections is not always satisfactory.

The problem is: if application supports websockets or data streaming it most likely has open connections at server shutdown time.

The library has no knowledge how to close them gracefully but developer can help by registering Application.on_shutdown signal handler and call the signal on web server closing.

Developer should keep a list of opened connections (Application is a good candidate).

The following websocket snippet shows an example for websocket handler:

app = web.Application()
app['websockets'] = []

async def websocket_handler(request):
    ws = web.WebSocketResponse()
    await ws.prepare(request)

        async for msg in ws:

    return ws

Signal handler may look like:

async def on_shutdown(app):
    for ws in app['websockets']:
        await ws.close(code=WSCloseCode.GOING_AWAY,
                       message='Server shutdown')


Both run_app() and AppRunner.cleanup() call shutdown signal handlers.

Background tasks

Sometimes there’s a need to perform some asynchronous operations just after application start-up.

Even more, in some sophisticated systems there could be a need to run some background tasks in the event loop along with the application’s request handler. Such as listening to message queue or other network message/event sources (e.g. ZeroMQ, Redis Pub/Sub, AMQP, etc.) to react to received messages within the application.

For example the background task could listen to ZeroMQ on zmq.SUB socket, process and forward retrieved messages to clients connected via WebSocket that are stored somewhere in the application (e.g. in the application['websockets'] list).

To run such short and long running background tasks aiohttp provides an ability to register Application.on_startup signal handler(s) that will run along with the application’s request handler.

For example there’s a need to run one quick task and two long running tasks that will live till the application is alive. The appropriate background tasks could be registered as an Application.on_startup signal handlers as shown in the example below:

async def listen_to_redis(app):
        sub = await aioredis.create_redis(('localhost', 6379), loop=app.loop)
        ch, *_ = await sub.subscribe('news')
        async for msg in ch.iter(encoding='utf-8'):
            # Forward message to all connected websockets:
            for ws in app['websockets']:
                ws.send_str('{}: {}'.format(ch.name, msg))
    except asyncio.CancelledError:
        await sub.unsubscribe(ch.name)
        await sub.quit()

async def start_background_tasks(app):
    app['redis_listener'] = app.loop.create_task(listen_to_redis(app))

async def cleanup_background_tasks(app):
    await app['redis_listener']

app = web.Application()

The task listen_to_redis() will run forever. To shut it down correctly Application.on_cleanup signal handler may be used to send a cancellation to it.

Handling error pages

Pages like 404 Not Found and 500 Internal Error could be handled by custom middleware, see Middlewares for details.

Deploying behind a Proxy

As discussed in Server Deployment the preferable way is deploying aiohttp web server behind a Reverse Proxy Server like nginx for production usage.

In this way properties like scheme host and remote are incorrect.

Real values should be given from proxy server, usually either Forwarded or old-fashion X-Forwarded-For, X-Forwarded-Host, X-Forwarded-Proto HTTP headers are used.

aiohttp does not take forwarded headers into account by default because it produces security issue: HTTP client might add these headers too, pushing non-trusted data values.

That’s why aiohttp server should setup forwarded headers in custom middleware in tight conjunction with reverse proxy configuration.

For changing scheme host and remote the middleware might use clone().

See also

https://github.com/aio-libs/aiohttp-remotes provides secure helpers for modifying scheme, host and remote attributes according to Forwarded and X-Forwarded-* HTTP headers.

Swagger support

aiohttp-swagger is a library that allow to add Swagger documentation and embed the Swagger-UI into your aiohttp.web project.

CORS support

aiohttp.web itself does not support Cross-Origin Resource Sharing, but there is an aiohttp plugin for it: aiohttp_cors.

Debug Toolbar

aiohttp-debugtoolbar is a very useful library that provides a debugging toolbar while you’re developing an aiohttp.web application.

Install it via pip:

$ pip install aiohttp_debugtoolbar

After that attach the aiohttp_debugtoolbar middleware to your aiohttp.web.Application and call aiohttp_debugtoolbar.setup():

import aiohttp_debugtoolbar
from aiohttp_debugtoolbar import toolbar_middleware_factory

app = web.Application(middlewares=[toolbar_middleware_factory])

The toolbar is ready to use. Enjoy!!!

Dev Tools

aiohttp-devtools provides a couple of tools to simplify development of aiohttp.web applications.

Install via pip:

  $ pip install aiohttp-devtools

 * ``runserver`` provides a development server with auto-reload,
live-reload, static file serving and aiohttp_debugtoolbar_
 * ``start`` is a `cookiecutter command which does the donkey work
of creating new :mod:`aiohttp.web` Applications.

Documentation and a complete tutorial of creating and running an app locally are available at aiohttp-devtools.