Common SaaS Problem to Production

Headway's platform receives webhook callbacks from an insurance eligibility verification vendor whenever a client's coverage check finishes. The vendor retries the same webhook if it doesn't get a 200 response within 3 seconds. Below is the handler a Headway engineer wrote for the FastAPI service that updates the client's coverage status.

from fastapi import FastAPI
import requests

app = FastAPI()
coverage_cache = {}

@app.post("/webhooks/eligibility")
def handle_eligibility(payload: dict):
    client_id = payload["client_id"]
    status = payload["status"]
    coverage_cache[client_id] = status

    if status == "verified":
        requests.post("https://internal.headway.co/notify", json={
            "client_id": client_id,"message": "Your coverage has been verified!",
        })

    update_database(client_id, status)
    return {"ok": True}

def update_database(client_id, status):
    db.execute(f"UPDATE clients SET coverage_status = '{status}' WHERE id = {client_id}")

Work through, in writing, what happens the first time this handles a real webhook, what happens if the vendor retries it, and how you'd redesign it to run safely and correctly in production.