Troubleshooting "Not Enough Replicas" problems in AMQ Streams

Issue

Finding message production blocked with a "Not Enough Replicas" error message is quite a common problem in AMQ Streams installations. This is a typical error message that might be seen in a client's log:

[2023-05-02 09:53:50,911] ERROR Error when sending message to topic test with key: null, value: 9 bytes with error:(org.apache.kafka.clients.producer.internals.ErrorLoggingCallback) org.apache.kafka.common.errors.NotEnoughReplicasException: Messages are rejected since there are fewer in-sync replicas than required.

This article explains why this problem arises, and some general approaches to investigating the cause. Because the list of potential causes is so extensive, the article does not attempt to give step-by-step instructions -- it is just a starting point for troubleshooting.