Replaying old messages
We noted previously that with AMQP- and JMS-style message brokers, processing and acknowledging messages is a destructive operation, since it causes the messages to be deleted on the broker. On the other hand, in a log-based message broker, consuming messages is more like reading from a file: it is a read-only operation that does not change the log.
The only side effect of processing, besides any output of the consumer, is that the consumer offset moves forward. But the offset is under the consumer’s control, so it can easily be manipulated if necessary: for example, you can start a copy of a consumer with yesterday’s offsets and write the output to a different location, in order to reprocess the last day’s worth of messages. You can repeat this any number of times, varying the processing code.
This aspect makes log-based messaging more like the batch processes of the last chapter, where derived data is clearly separated from input data through a repeatable transformation process. It allows more experimentation and easier recovery from errors and bugs, making it a good tool for integrating dataflows within an organization .