Kafka ordering guarantees

Question

I was going through kafka documentation and came across

Guarantees

At a high-level, Kafka gives the following guarantees:

Messages sent by a producer to a particular topic partition will be appended in the order they are sent. That is, if a record M1 is sent by the same producer as a record M2, and M1 is sent first, then M1 will have a lower offset than M2 and appear earlier in the log. A consumer instance sees records in the order they are stored in the log. For a topic with replication factor N, we will tolerate up to N-1 server failures without losing any records committed to the log.

I had few questions.

1. Is it always guaranteed that M1 will have a lower offset than M2 ? what if M1 is retried later than M2 ?
2. I also understood from various documentations that ordering is not guaranteed, and the consumer has to deal with it.

Answer 1

A possible scenario even with a single partition is:

• Producer sends M1
• Producer sends M2
• M1 is not ack'ed on the first try due to some failure
• M2 is delivered
• M1 is delivered in a subsequent try.

One easy way to avoid this is through the producer config max.in.flight.requests.per.connection=1.

This of course has performance implications, so it should be used with caution.