Listener and route — bind and match — Build a Service Mesh (Envoy / Istio style)

Build a Service Mesh (Envoy / Istio style) (13 scenes)

Scene 04 · Listener and route — bind and match

A listener accepts on a port; an ordered route table picks a destination on the first match. Reorder the rules and the same request lands somewhere else.

Previously

Reading method and path is only useful if the proxy has a structured table that says 'this kind of request goes there' — and that table is two named pieces: a listener and a route.

Scene 04

Listener and route — bind and match

Watch

Diagram

The tab on the left edge of the sidecar is the **listener**: a socket the proxy binds — an `(address, port)` like 0.0.0.0:8080 where requests arrive. Inside the proxy is an ordered list of rows, each one a **route**: a match rule (path prefix, method, or header) and a destination name. The proxy evaluates the rows top-to-bottom and the first one that matches wins — every row below it is unreachable for that request.

↑ listener — socket bound to (0.0.0.0, 8080) where requests arrive

↓ ordered routes — first match wins, every row below is unreachable

← this route matched first → exit toward its destination

A test request `GET /checkout/cart` (with header `x-canary: true`) hits the listener on port 8080. The proxy walks the route table top-to-bottom. The first matching row glows green and the request exits toward its named destination.

Implementation

Listener.bind(address, port)

the socket the proxy listens on — LDS pushes this

1def bind_listener(cfg):
2    sock = listen(addr=cfg.address, port=cfg.port)
3    # filter chain: TLS? HTTP? raw TCP? — picks HCM here.
4    chain = [http_connection_manager(routes=cfg.routes)]
5    while conn := sock.accept():
6        chain.handle(conn)  # parse req, then route_match

RouteTable.match(req) # first-match-wins

walk the rows top-to-bottom; return on the first hit

1def match(req, routes):
2    for route in routes:        # ORDERED — RDS order is policy
3        if matches(req, route.match):
4            return route.destination
5    return DEFAULT_REJECT       # 404 / direct_response
6 
7def matches(req, m):
8    if m.kind == 'wildcard': return True
9    if m.kind == 'prefix':   return req.path.startswith(m.value)
10    if m.kind == 'header':
11        name, want = m.value.split(':', 1)
12        return req.headers.get(name.strip()) == want.strip()

match(GET /checkout/cart, x-canary: true)

same request, three orderings, three destinations

1# ordering 0: wildcard on top
2[wildcard '/*', prefix '/checkout', header 'x-canary'] -> default-svc
3 
4# ordering 1: prefix on top  (canary unreachable below)
5[prefix '/checkout', header 'x-canary', wildcard '/*'] -> checkout-v1
6 
7# ordering 2: header on top  (canary works)
8[header 'x-canary', prefix '/checkout', wildcard '/*'] -> checkout-v2

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