Handling Reconnects and Errors in Parsed Streams

{ "jsonrpc": "2.0", "id": 99, "method": "ping" }

import WebSocket from "ws";

const URL = "wss://<ENDPOINT>/?api-key=<API_KEY>";
const filters = [
  { programs: ["JUP6LkbZbjS1jKKwapdHNy74zcZ3tLUZoi5QNyVTaV4"] },
];

let lastSlotSeen: number | null = null;
let backoffMs = 1000;

function connect() {
  const ws = new WebSocket(URL);

  ws.on("open", () => {
    backoffMs = 1000;
    filters.forEach((filter, i) => {
      ws.send(JSON.stringify({
        jsonrpc: "2.0",
        id: i + 1,
        method: "parsedTransactionSubscribe",
        params: [filter],
      }));
    });
  });

  ws.on("message", (data) => {
    const msg = JSON.parse(data.toString());
    if (msg.method === "parsedTransactionNotification") {
      const { slot } = msg.params.result.context;

      // Across reconnects, the slot bounds the backfill window.
      if (lastSlotSeen !== null && slot > lastSlotSeen) {
        // backfill candidates: slots lastSlotSeen+1 .. slot-1 while disconnected
      }
      lastSlotSeen = slot;
    }
  });

  ws.on("close", (code) => {
    console.warn(`connection closed (${code}); reconnecting in ${backoffMs}ms`);
    setTimeout(connect, backoffMs);
    backoffMs = Math.min(backoffMs * 2, 30_000);
  });
}

connect();