Kakfa reliable delivery

Reliable delivery means that once a message is sent to Kafka and gets acknowledged, It would survive broker failures.

On Producer Side:

    We need to set ACKS=all and have atleast 3 insync replicas

If messageA was written using same producer before message B in same partition the offset of message A would be greater then messageB.

Messages are assumed to be committed when written to page cache of Insync replicas

Committed messages will not be lost if one insync replica survives

Assuming ACKS=all and min.isync.replicas=2

1. Producer writes to leader

2.  The leader responds only when it has persisted and all followers fetch from leader. [If min.insync.replicas=2, we write to only 1 more follower, even though topic replication factor is 3]

3.  Consumer read above the highwatermark*

Assuming ACKS=1 and min.isync.replicas=2

1. Producer writes to leader

2.  The leader responds immediately.

[If min.insync.replicas=2, we write to only 1 more follower and only then increment highwatermark]

3.  Consumer read above the highwatermark*

On Consumer Side.

      Kafka uses the term high water mark to mark messages which have been fully replicated

Refer: https://rongxinblog.wordpress.com/2016/07/29/kafka-high-watermark/

*High watermark is calculated as the minimum LEO across all the ISR of this partition, and it grows monotonically.

Presto you have a reliable system