Web Backends
Web backends expose product behavior through HTTP APIs, persistence, queues, background jobs, and integration services. In AI products they also coordinate retrieval, model calls, streaming responses, authorization, audit logs, and model serving. The backend is where client intent becomes an enforceable system contract.
Validate, authorize, execute, observe
A backend request path should validate input, authenticate and authorize, call domain logic, persist state, map errors, emit telemetry, and return a stable response shape. Long-running model calls need timeouts, cancellation, partial failure handling, and idempotency when actions can be repeated. Database work should go through sql queries that enforce server-side ownership.
flowchart LR Request[HTTP request] --> Validate[Validate schema] Validate --> Auth[Authenticate and authorize] Auth --> Domain[Run domain logic] Domain --> Data[SQL jobs or model calls] Data --> Errors[Map errors and fallbacks] Errors --> Telemetry[Emit telemetry] Telemetry --> Response[Stable response]
Authorized endpoint contract
This snippet defines a small FastAPI endpoint and uses a test client to compare the authorized response with the forbidden response.
from fastapi import FastAPI, Header, HTTPException
from fastapi.testclient import TestClient
app = FastAPI()
@app.get("/tickets/{ticket_id}")
def read_ticket(ticket_id: str, x_user: str = Header()):
if x_user != "u1":
raise HTTPException(
status_code=403,
detail={"code": "not_authorized", "ticket_id": ticket_id},
)
return {
"ticket_id": ticket_id,
"status": "open",
"trace_fields": ["model_version", "latency_ms"],
}
client = TestClient(app)
print(client.get("/tickets/T1", headers={"x-user": "u1"}).status_code,
client.get("/tickets/T1", headers={"x-user": "u1"}).json())
print(client.get("/tickets/T1", headers={"x-user": "u2"}).status_code,
client.get("/tickets/T1", headers={"x-user": "u2"}).json())Observed output:
200 {'ticket_id': 'T1', 'status': 'open', 'trace_fields': ['model_version', 'latency_ms']}
403 {'detail': {'code': 'not_authorized', 'ticket_id': 'T1'}}The endpoint demonstrates a backend responsibility that javascript application architecture must not own: authorization. Testing can verify both the 200 and 403 contracts without a browser, and production integration can require the trace fields before launch.
Failure modes
Common failures are leaking authorization to the frontend, doing background work inside request handlers, returning unversioned response shapes, and collapsing provider failures into anonymous 500 errors. A backend that wraps model calls should expose typed fallbacks and telemetry, not just pass through provider responses.
References
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