Configuration

Every setting can be passed as an option to Socketelx.start_link/1 (or as {Socketelx, opts} in a supervision tree) or set as an environment variable. An explicit option wins over the environment variable.

Environment variables are named SOCKETELX_*. The older PUSHER_* spelling of the same names (PUSHER_APP_ID, PUSHER_REDIS_URL, ...) is still accepted, so an existing deployment keeps working; if both are set, SOCKETELX_* wins.

In Docker, cluster mode also uses the release's own variables, which are not Socketelx settings: RELEASE_COOKIE (shared secret, required in practice), RELEASE_NODE and RELEASE_DISTRIBUTION. See Multi-server.

One app, or several

By default a server has one app (one id, key and secret). To serve several apps from one port, see Several apps in one server.

Allowed origins

Browsers send an Origin header when opening a WebSocket. By default every origin is accepted. To accept only your own sites:

SOCKETELX_ALLOWED_ORIGINS=https://chat.example.com,*.example.org

Each entry is one of:

A refused connection receives pusher:error 4009 and is closed. Connections without an Origin header (backend scripts, mobile apps) are not affected: this stops other websites from using your visitors' browsers, it is not a login. In Elixir, pass a list: allowed_origins: ["https://chat.example.com"].

Per-IP limit

SOCKETELX_MAX_CONNECTIONS_PER_IP=20 allows each client address at most 20 simultaneous WebSocket connections (more get pusher:error 4100). Behind a proxy or tunnel every connection looks like it comes from the proxy, so also set SOCKETELX_CLIENT_IP_HEADER to the header your proxy fills in with the real address (cf-connecting-ip on Cloudflare, x-forwarded-for for most others; the first address in a list is used). Only set it when the proxy overwrites that header, otherwise clients can fake it.

Built-in TLS

Set SOCKETELX_TLS_CERTFILE and SOCKETELX_TLS_KEYFILE (PEM files) and the server speaks HTTPS and WSS on its port. Most deployments terminate TLS at a proxy or Cloudflare instead, which is fine; use this when nothing sits in front. Certificates are read at start, so restart to renew.

User sign-in and events to one user

pusher.signin() in pusher-js identifies the person behind a connection. Point userAuthentication.endpoint at a route that calls Socketelx.authenticate_user/4 (or signs socket_id::user::user_data with HMAC-SHA256 in any language). Once signed in, an event triggered on the channel #server-to-user-<id> reaches every connection of that user, on any node.

Client events

Clients can send client-* events on private and presence channels, only to channels they are subscribed to, and never on public channels. Set SOCKETELX_ENABLE_CLIENT_MESSAGES=false to refuse them all (the client receives error 4301).

Webhooks

With SOCKETELX_WEBHOOK_URL set, the server POSTs JSON for these events:

• channel_occupied, channel_vacated
• member_added, member_removed (presence channels)
• client_event

Each request carries X-Pusher-Key and X-Pusher-Signature (HMAC-SHA256 of the raw body with your app secret), the same as Pusher's webhooks. Verify the signature before trusting a request.

Delivery runs in the background and never blocks a connection. A failed delivery (network error, timeout, HTTP 5xx, 408 or 429) is retried up to SOCKETELX_WEBHOOK_RETRIES times (default 3) with a doubling delay: 1 s, 2 s, 4 s. Other 4xx responses are not retried, because the receiver rejected the payload. Two things to design for:

• By default retries are held in memory, so a webhook still waiting for a retry is lost if the node stops. Set SOCKETELX_WEBHOOK_QUEUE_PATH to a directory (a mounted volume in Docker) and a webhook that failed its first attempt is saved there and sent again after a restart. It protects against the node restarting, not against losing the disk.
• Retried events can arrive after later events, so make your receiver idempotent and don't rely on order.

Rate limits and capacity

All are off by default.

• SOCKETELX_REST_RATE_LIMIT=100 allows 100 REST API requests per second for the whole instance (the health check and /metrics are exempt). Extra requests get 429 with Retry-After: 1. Most server libraries treat that as a failed publish, so set it above your real peak.
• SOCKETELX_CLIENT_EVENT_RATE_LIMIT=10 allows each connection 10 client events per second. Extra events are dropped and the client gets pusher:error 4301.
• SOCKETELX_MAX_CONNECTIONS=10000 refuses new WebSocket connections on this node beyond that count with pusher:error 4004. Clients reconnect, and with a load balancer they may land on another node.

The limits are per instance and per node. With several nodes, each node applies its own limit.

Metrics

Set SOCKETELX_METRICS=true and scrape GET /metrics (Prometheus text format). Counters are per node and reset on restart; scrape every node.

Set SOCKETELX_METRICS_TOKEN to require Authorization: Bearer <token>. Without a token anyone who can reach the port can read /metrics, so block that path at your proxy.

Redis

See Multi-server. Redis is used only when :redis_url is set.