STAGE 1 · THE PORT JACK

Dial Tone

Before Redis is a database, it's a program listening on a port.

What you're wiring up

A TCP connection is a two-way byte pipe: bytes arrive in arbitrary chunks — not "one message per read" — and your server's whole job is a loop: read some bytes, recognize a command, write some bytes back. This stage builds that loop with exactly one command, PING, and teaches the single most important socket lesson: the network gives you a stream, not messages.

Redis clients speak RESP even for PING, but this stage lets you cheat: you may answer anything that contains PING with +PONG\r\n. Honest parsing is Stage 2's job. We listen on port 6389 — never 6379 — so a real Redis on your machine can't interfere.

# client → server
*1\r\n$4\r\nPING\r\n

# server → client
+PONG\r\n

Assembly steps

[ 01 ]

Bind and listen on 127.0.0.1:6389; accept connections in a loop.

hint

net.Listen("tcp", "127.0.0.1:6389") then l.Accept() in a for loop.

hint

Print "listening" to stderr only — stdout stays clean; the harness watches the port, not your logs.

[ 02 ]

Read from the connection and reply +PONG\r\n every time a PING arrives; loop until the client hangs up.

hint

conn.Read returning io.EOF means the client left — close and move on, don't crash.

[ 03 ]

Handle multiple PINGs on one connection — the reply loop must not exit after the first answer.

hint

If your test only gets one PONG back, you probably returned after the first write instead of looping.

[ 04 ]

Handle sequential connections — after one client disconnects, the next Accept must work.

hint

Wrap the per-connection loop in a function; call it from the accept loop. Concurrent clients come in Stage 4 — sequential is fine here.

Go deeper (after it passes)

This is the socket layer every server you've ever used sits on. Go deeper: Beej's Guide to Network Programming (free), or just read your own accept loop again — that's nginx's inner shape too.