NATS: Slow consumer detected

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Increase pending message limits
sub, err := nc.Subscribe("topic", handler)
sub.SetPendingLimits(1000000, 1024*1024*512)
# Use queue groups for parallel processing
nc.QueueSubscribe("topic", "workers", handler)
# Use JetStream for persistence and replay

How to diagnose MessageQueue errors

Queue errors are usually about the balance between producers and consumers. Growing lag means consumers cannot keep up. Endless rebalancing usually means consumers are being evicted for exceeding a poll or heartbeat interval, not that the cluster is unhealthy. A poison message, one that always fails processing, will block an ordered partition forever unless there is a dead-letter path, which is why a DLQ is not optional in production.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Measure lag over time, not once: kafka-consumer-groups --describe --group <g>. The trend distinguishes a throughput problem from a stuck consumer.
2. For repeated rebalancing, compare processing time per batch against max.poll.interval.ms. Slow processing looks exactly like a dead consumer to the coordinator.
3. Always configure a dead-letter queue with a bounded retry count. Without one, a single malformed message stops the partition indefinitely.
4. Check message size against the broker limit before assuming a network failure: SQS caps at 256 KB, Kafka at message.max.bytes.
5. Verify consumer ACLs and topic authorisation separately from connectivity; an authorisation failure often surfaces as a metadata error.

Tools worth reaching for

• kafka-consumer-groups
• rabbitmqctl list_queues
• aws sqs get-queue-attributes
• kafkacat / kcat
• flower (Celery)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 docs.nats.io

Related MessageQueue errors

• AWS SQS: Message too large (256KB limit)The SQS message body exceeds the 256KB maximum size limit.
• Celery: WorkerLostErrorA Celery worker process died unexpectedly during task execution, often due to OOM kill…
• Kafka: LEADER_NOT_AVAILABLEPartition has no elected leader. Broker down, ISR empty, or topic just created and metadata…
• Kafka: Offset out of rangeConsumer trying to read from offset that doesn't exist. Data retention deleted old messages.
• Kafka: RecordTooLargeExceptionA message exceeded a size limit. Three separate limits must agree: the producer's…
• Kafka: Topic authorization failedThe Kafka client does not have the required ACL permissions to produce to or consume from the…
• Kafka: UNKNOWN_TOPIC_OR_PARTITIONThe client asked for a topic the broker does not know about. Either the topic genuinely does…
• Message Queue: Consumer lag too highMessages accumulating faster than consumers can process. Queue backlog growing.

See all 14 MessageQueue errors →

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This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.