Node.js example

import { io } from "socket.io-client";

let cursor = 0;
let epoch = 0;
let pending = 0;
let queue = Promise.resolve();
const socket = io(process.env.PHOTON_API_ORIGIN ?? "https://api.photon.codes", {
  path: `/v1/projects/${process.env.PHOTON_PROJECT_ID}/events/ws`,
  addTrailingSlash: false,
  transports: ["websocket"],
  extraHeaders: { Authorization: `Bearer ${process.env.PHOTON_API_KEY}` },
  autoConnect: false,
});

socket.on("connect", () => {
  socket.timeout(10_000).emit("subscribe", { startSeq: cursor }, (error, result) => {
    if (error) socket.io.engine.close();
    else if (!result.ok) console.error(result.error.code);
  });
});
socket.on("disconnect", () => { epoch++; });
socket.on("delivery", (event, { seq }) => {
  const generation = epoch;
  if (++pending > 128) {
    pending--;
    epoch++;
    socket.io.engine.close();
    return;
  }
  queue = queue.then(async () => {
    if (generation !== epoch || seq <= cursor) return;
    await handleEvent(event); // Your idempotent application handler.
    // Persist seq here before advancing the in-memory checkpoint.
    cursor = seq;
  }).catch(error => {
    console.error("Event processing failed", error);
    if (generation === epoch) {
      epoch++;
      socket.io.engine.close();
    }
  }).finally(() => { pending--; });
});
socket.on("delivery_error", error => console.error(error.code));
socket.on("connect_error", error => console.error(error.message));
socket.connect();