High Watermark and Message Visibility

High Watermark Concept

The high watermark (HW) is a crucial mechanism in Kafka that:

1. Controls message visibility to consumers
2. Ensures consistency across replicas
3. Prevents reading uncommitted messages

Message Visibility Rules

1. Producer Side

   • Messages written to leader
   • Replicated based on acks setting
   • HW advances after ISR replication

2. Consumer Side

   • Can only read up to HW
   • May see delay with acks=1
   • Guarantees consistent view across consumers

Impact on Reliability

1. Data Consistency

   • Prevents reading uncommitted data
   • Ensures all consumers see same data
   • Handles replica failures gracefully

2. Failure Scenarios

   • Leader failures don't lose committed data
   • Follower catch-up uses HW
   • Replication maintains consistency

References

Flashcards

When does the high watermark advance?:: When all in-sync replicas have replicated the latest offsets

Under what conditions will consumers see messages with acks=1?:: When the high watermark has advanced, even if followers haven't replicated yet

What's the relationship between high watermark and committed messages?:: Consumers can only read up to the high watermark, which represents committed messages