How to properly ensure idempotency when processing a batch of documents from Kafka if the service might crash during processing?

To ensure idempotency when processing a batch of documents from Kafka, especially if the service might crash in the middle, you should:

1. Use unique identifiers for each document.
2. Before processing, check if the document has already been processed (for example, by storing the status in a database).
3. Process documents within transactions or with confirmation of successful processing.
4. After a service crash and restart, re-read messages from Kafka but skip those already processed.

Example approach:

• When receiving a message, check if there is a record with such an ID in the database and a status of "processed".
• If not, process and mark it as processed.
• If yes, skip.

This guarantees that even with reprocessing, the result will not change (idempotency).