---
summary: "Gateway singleton guard using the WebSocket listener bind"
read_when:
  - Running or debugging the gateway process
  - Investigating single-instance enforcement
title: "Gateway lock"
---

## Why

- Ensure only one gateway instance runs per base port on the same host; additional gateways must use isolated profiles and unique ports.
- Survive crashes/SIGKILL without leaving stale lock files.
- Fail fast with a clear error when the control port is already occupied.

## Mechanism

- The gateway first acquires a per-config lock file under the state lock directory and probes the configured port for an existing listener.
- If the recorded lock owner is gone, the port is free, or the lock is stale, startup reclaims the lock and continues.
- The gateway then binds the HTTP/WebSocket listener (default `ws://127.0.0.1:18789`) using an exclusive TCP listener.
- If the bind fails with `EADDRINUSE`, startup throws `GatewayLockError("another gateway instance is already listening on ws://127.0.0.1:<port>")`.
- On shutdown the gateway closes the HTTP/WebSocket server and removes the lock file.

## Error surface

- If another process holds the port, startup throws `GatewayLockError("another gateway instance is already listening on ws://127.0.0.1:<port>")`.
- Other bind failures surface as `GatewayLockError("failed to bind gateway socket on ws://127.0.0.1:<port>: …")`.

## Operational notes

- If the port is occupied by _another_ process, the error is the same; free the port or choose another with `mnemo gateway --port <port>`.
- Under a service supervisor, a new gateway process that sees an existing healthy `/healthz` responder leaves that process in control. On systemd, the duplicate starter exits with code 78 so the default `RestartPreventExitStatus=78` stops `Restart=always` from looping on a lock or `EADDRINUSE` conflict. If the existing process never becomes healthy, retries are bounded and startup fails with a clear lock error instead of looping forever.
- The macOS app still maintains its own lightweight PID guard before spawning the gateway; the runtime lock is enforced by the lock file plus HTTP/WebSocket bind.

## Related

- [Multiple Gateways](/gateway/multiple-gateways) — running multiple instances with unique ports
- [Troubleshooting](/gateway/troubleshooting) — diagnosing `EADDRINUSE` and port conflicts
