Nats try to add a consumer attached to $JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES

Question

I'm using jetstream with interest approach.

I see that to create a dead-letter, one of the approach is binding to event of $JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES and call to get particular message to save it.

I try it using the subscribe function and work's ok, but I'm interested to create a consumer to avoid losing messages, I cannot loss any message.

When I try to create a consumer with tolling specifying the token event like (it's only a sample):

 nats consumer add DOMAIN -s nats://nats:4222 deadletter --pull --deliver=all --wait=10s --ack=explicit --replay=instant --filter="$JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES.MYSTREAM.my_consumer" --max-deliver=1 --max-pending=1000 --backoff=none --no-headers-only

It's possible to create a consumer of nats events?

Answer 1

You need to create a stream that consumes the special subject $JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES.{stream_name}.{consumer_name}:

nats stream add MYSTREAM_dlq \
  --subjects '$JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES.MYSTREAM.*' \
  --defaults

Then you can create a pull consumer to consume messages in the dead-letter stream:

nats consumer add MYSTREAM_dlq MYSTREAM_dlq_consumer --pull --defaults

Messages in the dead-letter stream have the following format:

{
  "type": "io.nats.jetstream.advisory.v1.max_deliver",
  "id": "EgGS1FReM5Yy1I7L9UXs4W",
  "timestamp": "2023-12-24T14:38:13.107632319Z",
  "stream": "MYSTREAM",
  "consumer": "MYSTREAM_CONSUMER",
  "stream_seq": 6,
  "deliveries": 4,
  "domain": "ngs"
}

You can use stream and stream_seq fields to retrieve the original failed message:

nats stream get MYSTREAM 6