What happens if the process crashes between saving to the database and committing (offset) in Kafka?

If the process crashes between saving data to the database and committing (offset commit) in Kafka, then upon restart, the consumer may read the same messages again. This is because the offset was not committed, and Kafka considers these messages as not yet processed.

As a result, the following scenarios are possible:

• Duplicate processing: data has already been saved in the database, but the offset was not committed, so re-reading the message may lead to reprocessing.
• Data loss: if the database save was not atomic with the offset commit, desynchronization may occur.

To avoid problems, patterns such as the following are used:

• Transactions: if the database and Kafka support transactions, data can be saved atomically with the offset.
• Idempotency: message processing should be idempotent so that reprocessing does not lead to errors.
• Storing offset with data: for example, storing the offset in the same transaction as the data in the database.

Thus, without special measures, duplication or data loss can occur during a failure between saving and committing the offset.