Routing

Mapping URLs and methods to handler code.

Also known as: routing, routes, URL routing, request routing, route handlers

Routing is how a backend decides which code handles a request: it matches the HTTP method and URL path to a handler function.

# Flask
@app.get("/orders")
def list_orders(): ...

@app.post("/orders")
def create_order(): ...

@app.get("/orders/<int:order_id>")        # path parameter
def get_order(order_id): ...

// Express
app.get("/orders/:id", (req, res) => {
  res.json(findOrder(req.params.id));
});

When GET /orders/917 arrives, the router finds the matching route, extracts order_id = 917 and calls the handler (request handler).

What a router does

Matches method + path pattern, including parameters like :id or <int:order_id>.
Responds 404 Not Found if no path matches, and 405 Method Not Allowed if the path exists but not for that method.
Passes request details to the handler: params, query string, headers, body.
Often lets you group routes under a prefix (/api/v1) and attach shared behavior (middleware) such as authentication.

Practical tips

Match order can matter. In some frameworks the first matching route wins, so put specific ones (/orders/export) before general ones (/orders/<id>).
Keep handlers thin. Parse input, call a service, return the response. Don’t put business logic in the route (service layer).
Use consistent, resource-style URLs (endpoints, REST).
Validate path and query parameters, and convert types (input validation).
Version public APIs with a prefix or header (API versioning).

Routing is part of the request lifecycle. Every web framework has one, with different syntax and the same job.