Middleware

The middleware pipeline is configured with the MIDDLEWARE configuration property. It must contain a list of classes that inherit from selva.web.middleware.Middleware.

Usage

To demonstrate the middleware system, we will create a timing middleware that will output to the console the time spent in the processing of the request:

application/controller.pyapplication/middleware.pyconfiguration/settings.yaml

from asgikit.requests import Request
from asgikit.responses import respond_json
from selva.web import controller, get


@controller
class HelloController:
    @get
    async def hello(self, request: Request):
        await respond_json(request.response, {"greeting": "Hello, World!"})

from collections.abc import Callable
from datetime import datetime

from asgikit.requests import Request
import structlog
from selva.di import service
from selva.web.middleware import Middleware, CallNext

logger = structlog.get_logger()


@service
class TimingMiddleware(Middleware):
    async def __call__(self, call_next: CallNext, request: Request):
        request_start = datetime.now()
        await call_next(request) # (1)
        request_end = datetime.now()

        delta = request_end - request_start
        logger.info("request duration", duration=str(delta))

1. Invoke the middleware chain to process the request

middleware:
  - application.middleware.TimingMiddleware

Middleware dependencies

Middleware instances are created using the same machinery as services, and therefore can have services of their own. Our TimingMiddleware, for instance, could persist the timings using a service instead of printing to the console:

application/service.pyapplication/middleware.py

from datetime import datetime

from selva.di import service

@service
class TimingService:
    async def save(start: datetime, end: datetime):
        ...

from collections.abc import Callable
from datetime import datetime
from typing import Annotated

from asgikit.requests import Request
from selva.di import service, Inject
from selva.web.middleware import Middleware, CallNext

from application.service import TimingService


@service
class TimingMiddleware:
    timing_service: Annotated[TimingService, Inject]

    async def __call__(self, call_next: CallNext, request: Request):
        request_start = datetime.now()
        await call_next(request)
        request_end = datetime.now()

        await self.timing_service.save(request_start, request_end)