This is the promise the whole project was built to keep: a real load balancer that makes every routing decision on purpose and can prove it. The scripted stream exercises every layer at once - round-robin selection walking the pool, the lease raising and lowering active counts, the transport returning success or failure, passive ejection counting B’s failures until its third in a row pulls it from rotation, and the cycle silently reshaping from three backends to two the moment it happens. Tracing the ever-climbing round-robin counter by hand is the only way to be sure of the exact A,B,C,A,B,C,A,B,A,C,A sequence, which is why the spec pins the whole stream.

Then the endgame closes every loop: draining A removes it from new work while its counts stay honest, an all-down moment returns ErrNoHealthyBackend instead of panicking or spinning, recovery brings C back, and the final assertion that every Active() count is 0 proves not a single connection leaked. From a bare backend struct you have built the honest core of a real balancer - every selection algorithm, connection tracking, and a passive-and-active health state machine - the same design that sits inside HAProxy, NGINX, and Envoy, minus the real sockets, concurrency, and richer routing they layer on top. The finalize pass puts this core behind a live TCP reverse proxy; the decisions it makes are the ones you wrote.