Exactly once support

There are two versions of the IBM® MQ Kafka Connectors, 1 and 2. The version 2 connectors provide support for exactly-once and at-least-once message delivery, whereas the version 1 connectors provide support for at-least-once message delivery.

At-least-once message delivery means that in the event of a failure, in either IBM MQ, the IBM MQ Kafka Connector, or Kafka:

For the source connector, IBM MQ messages are not lost, but might be delivered to Kafka multiple times resulting in duplicate Kafka messages.
For the sink connector, Kafka messages are not lost, but might be delivered to IBM MQ multiple times resulting in duplicate IBM MQ messages.

Exactly-once message delivery means that in the event of a failure, in either IBM MQ, the IBM MQ Kafka Connector, or Kafka:

For the source connector, IBM MQ messages are not lost, and will be delivered to Kafka with no chance for duplicate Kafka messages.
For the sink connector, Kafka messages are not lost, and will be delivered to IBM MQ with no chance for duplicate IBM MQ messages.

Exactly-once support is only available in the version 2 connector shipped with IBM MQ, or with IBM Event Streams. It is not available in the version 1 connector.

The version 2 connector can run in either at-least-once or exactly-once mode. Exactly-once support is enabled by appropriate configuration of Kafka, and by making use of a "state queue". Each instance of a connector running in exactly-once mode needs its own state queue.

Throughput and scalability of the connectors running in exactly-once mode are less than running in at-least-once mode. Only enable exactly-once mode if your applications aren't designed to work with duplicate messages.

See Running the MQ source connector for details on configuring exactly-once mode in the source connector, and Running the MQ sink connector for details on configuring exactly-once mode in the sink connector.