Kif jaħdem il-middleware f'FastAPI?

Question

Accepted Answer

**Middleware** f'FastAPI huwa kod li jaħdem għal **kull request u response** — il-wrapping tal-endpoints tiegħek biex iħedded concerns bħal logging, timing, CORS, u li żżid headers. FastAPI juża s-sistema middleware ta' Starlette (ibbazata fuq ASGI).

## Kitba middleware custom

```python
import time

@app.middleware("http")
async def add_timing(request: Request, call_next):
    start = time.time()
    response = await call_next(request)        # call the endpoint (and any inner middleware)
    response.headers["X-Process-Time"] = str(time.time() - start)   # modify the response
    return response
```

Il-pattern: il-kod qabel `await call_next(request)` jaħdem **ġo** (qabel l-endpoint), `call_next` jinvoka l-endpoint, u l-kod wara jaħdem **barra** (jibbodifica r-response). Dan iwrappa kull request — hawnhekk, żżid header ta' timing għall-responses kollha.

## Il-flussu tar-request/response

```text
Request  → MW1 → MW2 → endpoint
Response ← MW1 ← MW2 ← endpoint
Middleware wraps the endpoint; runs in order on the way in, reverse on the way out.
```

## Middleware integrat / komuni

```python
from fastapi.middleware.cors import CORSMiddleware
from fastapi.middleware.gzip import GZipMiddleware

app.add_middleware(CORSMiddleware, allow_origins=["https://app.example.com"], ...)  # CORS
app.add_middleware(GZipMiddleware, minimum_size=1000)                               # compression
```

Il-middleware ġeneralment jiġi miżjud permezz ta' `add_middleware` — CORS (ippermetti requests cross-origin), GZip (kompriż responses), u trusted-host checks huma integrati tipiċi.

## Użi komuni tal-middleware

```text
✓ Logging / request timing / metrics
✓ CORS (cross-origin request handling)
✓ Adding security or custom headers
✓ Response compression (GZip)
✓ Request ID / correlation ID injection
```

## Middleware vs dependencies

```text
Middleware   → runs for ALL requests; works at the raw request/response level (headers,
               timing, the whole response). Best for truly global concerns.
Dependencies → per-endpoint (or grouped) logic with access to parsed params, DI, and
               typed values. Best for auth, validation, shared parameters.
→ Use middleware for global cross-cutting concerns; dependencies for endpoint logic.
```

## Għaliex dan importanti

Il-middleware hu l-mekkaniżmu t-tajjeb għal **concerns globali u cross-cutting** f'FastAPI — il-logika li għandha tiġi applikata għal kull request u response, bħal logging, request timing, CORS, compression, u headers custom.

L-għarfien tiegħu importanti għall-istruttura ta' applikazzjonijiet korretti: il-middleware iwrappa l-endpoints kollha (jaħdem qabel u wara, bid-dħul ta' modifika request u response), li jagħmlu ideali għall-imġiba ta' livell app li tkun teddjaħa u error-prone li tittirripetutha fi kull endpoint.

Il-għarfien tal-distinzjoni bejn **middleware** (globali, livell request/response raw) u **dependencies** (per-endpoint, b'aċċess għal valuri parsed/typed u DI — aħjar għal auth u validation) huwa importanti biex taħtar l-għodda t-tajba.

L-bżonnata komuni bħal CORS u compression jiġu mħaddma minn middleware integrat, filwaqt li middleware custom jiccu logging u instrumentation — li jagħmlu dan għarfien pratiku u spiss-użat għall-bini ta' APIs strutturati tajjeb u osservabbli.