## TL;DR
Exactly-once in Kafka needs all three pieces: an idempotent transactional producer (enable.idempotence=true and a transactional.id), a consumer with isolation.level=read_committed, and offsets committed transactionally (or an idempotent sink). Miss any one piece and you silently drop to at-least-once. There is no single config switch; it is a producer, consumer, and sink discipline together.

## The query
```text
kafka end to end exactly once semantics
```

## Use this when
- a pipeline must not duplicate or lose messages, ever
- you are auditing whether an existing pipeline is really exactly-once
- duplicates appear downstream and you need to find the broken link

## Not for
- throughput-first pipelines where occasional duplicates are fine
- exactly-once inside a single non-Kafka database, which has its own transactions

## Steps
1. Enable idempotence on the producer and give it a transactional.id. Without a transactional id there are no transactions.
   Expected output: The producer config includes both settings and starts without error.

2. Wrap produce calls in a transaction (init, begin, send, commit) so a batch either lands fully or not at all.
   Expected output: Partial failures roll back instead of leaving half a batch in the topic.

3. Set the consumer to isolation.level=read_committed so it skips aborted transactions.
   Expected output: The consumer never sees messages from rolled-back transactions.

4. Commit offsets transactionally with the downstream write, or make the sink idempotent on the record key.
   Expected output: A consumer restart never reprocesses an already-applied record.

5. Test with a failure injection: kill a producer mid-batch and confirm no duplicates or gaps downstream.
   Expected output: The test shows every input exactly once in the sink.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_gTiLIFsP6Rqi2y-pJQblQQ