Request Handler / Controller

The code that processes a single request.

Also known as: controller, route handler, endpoint handler

A request handler is the function that runs when a request reaches a particular route, such as GET /orders/42. It reads the input, calls the code that does the real work, and returns a response. Frameworks call it a handler, a controller, an endpoint or a view, depending on the design.

@app.get("/orders/<int:order_id>")
def get_order(order_id):
    order = orders.get(order_id)     # illustrative: the work happens elsewhere
    if order is None:
        return {"error": "not_found"}, 404
    return order_to_json(order), 200

Keep handlers thin. A good one parses and validates input, calls one piece of business logic, and shapes the response. It shouldn’t also query tables, calculate totals and send emails. Thin handlers are easier to read and test.

The classic mistake is letting a handler fail silently or fail loudly in the wrong way. A handler that returns 200 OK with an error message in the body breaks clients that check status codes. A handler that lets an exception escape can leak a stack trace to the user. Return the right HTTP status and a clean error. For the path a request takes through the whole server, see request lifecycle.