In Kafka, when there is only one partition and multiple consumers within a consumer group, by default, each message within the partition will be delivered to only one consumer within the group. This behavior is managed by the group coordination and the way offsets are committed.
Each consumer in the consumer group receives a portion of the partition's messages. Kafka ensures that messages within a partition are processed in order. Each message in the partition is identified by its unique offset.
The duplication of messages can be handled in the following ways:
As messages are consumed, the offsets (message positions) are committed to Kafka.
Kafka tracks the last committed offset for each consumer group/partition combination.
If a consumer fails or leaves the group and rejoins, it uses the last committed offset to continue from where it left off.
Kafka delivers each message in the partition to only one consumer within the same consumer group.
Once a message is processed and its offset is committed by a consumer, it will not be delivered to other consumers in the same group.
However, if you're concerned about potential scenarios where duplicates could arise due to consumer failures or processing issues, you can employ strategies within your consumer applications to handle duplicates:
Idempotent Processing: Design your consumer application to handle messages in an idempotent manner, ensuring that processing the same message multiple times won't lead to unintended side effects.
Use Message Keys: If possible, use message keys while producing messages to ensure that messages with the same key go to the same partition. This way, even with multiple consumers, messages with the same key will be processed by the same consumer, reducing the likelihood of processing duplicates.
While Kafka's default behavior ensures that each message within a partition is consumed by only one consumer in a consumer group, it's crucial to consider fault tolerance and potential processing scenarios within your consumer applications to handle cases where duplicates might occur due to failures or processing errors.