topic.js

const streams = {};
function callback(topic, message) {
  const [type, cluster] = topic.split("/");
  if (type === "log") {
    let stream = streams[topic];
    if (!stream) {
      stream = streams[topic] = fs.createWriteStream(`log_${cluster}.txt`, {
        flags: "a",
      });
    }
    const ts = new Date().toISOString();
    stream.write(`${ts} > ${readMessage(message)}\n`);
  }
}