What causes "already listening on ${addr}" in micro/go-micro?

Close the existing listener before Listen on the same address, or pick a distinct port (or ":0" for an ephemeral port)

already listening on ${addr}

Error message

already listening on ${addr}

What it means

Listen refuses to bind an address that already has a memory listener registered in the same process, returning 'already listening on <addr>'. The in-memory transport keys listeners by the exact normalized host:port, so two Listen calls with the same (normalized) address collide.

Source

Thrown at transport/memory.go:254

		return nil, err
	}

	addr, err = maddr.Extract(host)
	if err != nil {
		return nil, err
	}

	// if zero port then randomly assign one
	if len(port) > 0 && port == "0" {
		i := rand.Intn(20000)
		port = fmt.Sprintf("%d", 10000+i)
	}

	// set addr with port
	addr = mnet.HostPort(addr, port)

	if _, ok := m.listeners[addr]; ok {
		return nil, errors.New("already listening on " + addr)
	}

	listener := &memoryListener{
		lopts: options,
		topts: m.opts,
		addr:  addr,
		conn:  make(chan *memorySocket),
		exit:  make(chan bool),
		ctx:   m.opts.Context,
	}

	m.listeners[addr] = listener

	return listener, nil
}

func (m *memoryTransport) Init(opts ...Option) error {
	for _, o := range opts {

View on GitHub (pinned to 24529f1404)

Solutions

Close the existing listener before Listen on the same address, or pick a distinct port (or ":0" for an ephemeral port)
Track created listeners (map keyed by addr) so setup code never calls Listen twice
Check the error — it names the colliding address — and correct the config so each server has a unique host:port

Example fix

// before
m.Listen("127.0.0.1:8080")
m.Listen("127.0.0.1:8080") // already listening on 127.0.0.1:8080
// after
l, err := m.Listen("127.0.0.1:8080")
if err == nil { defer l.Close() }
l2, err := m.Listen("127.0.0.1:8081") // distinct addr

Defensive patterns

Strategy: validation

Validate before calling

if _, ok := listeners[addr]; ok {
	return fmt.Errorf("already listening on %s", addr)
}

Try / catch

l, err := m.Listen(addr)
if err != nil && strings.HasPrefix(err.Error(), "already listening on ") {
	// reuse existing listener or choose another port
}

Prevention

Track your listeners and Close before re-Listen
Use ":0" ephemeral ports in tests
Keep a registry of ports per test to avoid cross-test collisions

When it happens

Trigger: Calling Listen twice on the same addr string; two components configured with the same fixed port; Listen on 'host:0' resolving to the same port as an existing listener (rare but possible).

Common situations: Starting a server twice due to duplicate init code or a test suite running setup twice; multiple services in one binary configured with the same port; forgetting a previous Close when reusing addresses in tests.

Related errors

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/d5c2b6a92b37ca03. Report an issue: GitHub.