Why does Promise.race not resolve in kafkajs eachMessage callback

Question

I have defined a promise like this ...

    const result = await Promise.race([
      new Promise(resolve => {
        consumer.run({
          eachMessage: ({ message }) => {
            const data = JSON.parse(message.value.toString());
            if (data.payload.template
              && data.payload.template.id === '...'
              && data.payload.to[0].email === email) {
              console.log('Should resolve!')
              resolve(data.payload.template.variables.link);
              console.log('resolved');

              consumer.pause();
              consumer.disconnect();
            }
          },
        });
      }),
      new Promise((_, reject) => setTimeout(reject, 3000))
    ]);
    console.log('result is ', result);
    return result;

I can get to resolved but it doesnt print the result at the end, it seems like neither did the timeout nor the actual promise work as expected? Why is that? I suspect its something to do with using resolve inside the kafka js callback?

UPDATE: Seems like its Promise.race() thats not resolving, but why?

*" Seems like its Promise.race() thats not resolving"* - is it rejecting and you're swallowing the error silently somehow? — Tomalak, May 15 '21 at 09:36
Unrelated observation: You should probably move the `consumer.pause()` and `consumer.disconnect()` into the timeout promise handler. This way the consumer would will eventually get paused and disconnected no matter what. In your current implementation, it only will pause and disconnect in the success case. (This might or might not be intentional, I only noticed it.) — Tomalak, May 15 '21 at 09:38

score 1 · Accepted Answer · answered May 15 '21 at 10:10

My suspicion is that your "success-side" promise inadvertently throws and you're swallowing the error silently.

Using a mock-up minimal implementation of the consumer (that succeeds or fails 50/50), the following code works.

Run the code sample a couple of times to see both cases.

var consumer = {
  interval: null,
  counter: 0,
  run: function (config) {
    this.interval = setInterval(() => {
      this.counter++;
      console.log(`Consumer: message #${this.counter}`);
      config.eachMessage({message: this.counter});
    }, 250);
  },
  pause: function () {
    console.log('Consumer: paused');
    clearInterval(this.interval);
  },
  disconnect: function () {
    console.log('Consumer: disconnected');    
  }
};

Promise.race([
  new Promise(resolve => {
    const expectedMsg = Math.random() < 0.5 ? 3 : 4;
    consumer.run({
      eachMessage: ({ message }) => {
        if (message === expectedMsg) resolve("success");
      }
    });
  }),
  new Promise((_, reject) => setTimeout(() => {
    reject('timeout');
    consumer.pause();
    consumer.disconnect();
  }, 1000))
]).then((result) => {
  console.log(`Result: ${result}`);
}).catch((err) => {
  console.log(`ERROR: ${err}`);
});

I have also moved consumer.pause() and consumer.disconnect() to the "timeout-side" promise, this way the consumer is guaranteed to disconnect, albeit it might run a tiny bit longer than necessary in the success case.

Why does Promise.race not resolve in kafkajs eachMessage callback

1 Answers1