WebSocket client

This page is a practical guide for building a WebSocket client.

Minimal flow

Pseudo-code (server-side, using Authorization header):

token = "AT_..."
ws = connect("wss://ws.fastpubsub.com/",
             headers={"Authorization": "Bearer " + token})

subscribe(tenant="prod", channel="orders.eu.#")

on_message(msg):
  handle(msg)

publish(tenant="prod", channel="orders.eu.created", payload="...")

Browser example (using Sec-WebSocket-Protocol subprotocol):

const token = "AT_...";
const ws = new WebSocket("wss://ws.fastpubsub.com/", [
  "llps.v1",
  "at." + token
]);

ws.onopen = () => {
  ws.send(JSON.stringify({
    action: "subscribe",
    tenant: "prod",
    channel: "orders.eu.#"
  }));
};

ws.onmessage = (e) => { /* handle message */ };

Where to put the token:

Priority: Authorization header is checked first; Sec-WebSocket-Protocol is used as a fallback.

See WebSocket API for the full protocol description.

Reconnect strategy (recommended)

You should assume:

• disconnects happen,
• subscriptions are not durable,
• there is a propagation delay after resubscribe (join window).

Recommended approach:

• reconnect with exponential backoff (with a max delay),
• after reconnect, re-send all subscriptions,
• consider a small safety delay before you publish critical “first” messages,
• make consumers idempotent (see Delivery semantics).

Common pitfalls

• Browser Origin: if allowed_ws_origin is configured, your Origin must match.
• Slow consumer: if you do heavy work in the receive loop, you may be disconnected.
• Token expiry: expired tokens fail authentication; refresh/rotate as needed.

See also:

• Tokens
• Troubleshooting

WebSocket client