Middleware And Listeners

Middleware are functions which are executed before or after requests to the server. They can be used to modify the request to or response from user-defined handler functions.

Additionally, Sanic provides listeners which allow you to run code at various points of your application's lifecycle.

Middleware

There are two types of middleware: request and response. Both are declared using the @app.middleware decorator, with the decorator's parameter being a string representing its type: 'request' or 'response'. Response middleware receives both the request and the response as arguments.

The simplest middleware doesn't modify the request or response at all:

@app.middleware('request')
async def print_on_request(request):
    print("I print when a request is received by the server")

@app.middleware('response')
async def print_on_response(request, response):
    print("I print when a response is returned by the server")

Modifying the request or response

Middleware can modify the request or response parameter it is given, as long as it does not return it. The following example shows a practical use-case for this.

app = Sanic(__name__)

@app.middleware('response')
async def custom_banner(request, response):
    response.headers["Server"] = "Fake-Server"

@app.middleware('response')
async def prevent_xss(request, response):
    response.headers["x-xss-protection"] = "1; mode=block"

app.run(host="0.0.0.0", port=8000)

The above code will apply the two middleware in order. First, the middleware custom_banner will change the HTTP response header Server to Fake-Server, and the second middleware prevent_xss will add the HTTP header for preventing Cross-Site-Scripting (XSS) attacks. These two functions are invoked after a user function returns a response.

Responding early

If middleware returns a HTTPResponse object, the request will stop processing and the response will be returned. If this occurs to a request before the relevant user route handler is reached, the handler will never be called. Returning a response will also prevent any further middleware from running.

@app.middleware('request')
async def halt_request(request):
    return text('I halted the request')

@app.middleware('response')
async def halt_response(request, response):
    return text('I halted the response')

Listeners

If you want to execute startup/teardown code as your server starts or closes, you can use the following listeners:

  • before_server_start
  • after_server_start
  • before_server_stop
  • after_server_stop

These listeners are implemented as decorators on functions which accept the app object as well as the asyncio loop.

For example:

@app.listener('before_server_start')
async def setup_db(app, loop):
    app.db = await db_setup()

@app.listener('after_server_start')
async def notify_server_started(app, loop):
    print('Server successfully started!')

@app.listener('before_server_stop')
async def notify_server_stopping(app, loop):
    print('Server shutting down!')

@app.listener('after_server_stop')
async def close_db(app, loop):
    await app.db.close()

If you want to schedule a background task to run after the loop has started, Sanic provides the add_task method to easily do so.

async def notify_server_started_after_five_seconds():
    await asyncio.sleep(5)
    print('Server successfully started!')

app.add_task(notify_server_started_after_five_seconds())