# Idempotent Producers and Transactions — Apache Kafka: Event Streaming from Basics to Production

Source: https://www.geekswithgeeks.com/en/kafka/rel-exactly-once-basics

> Understand what exactly-once means in Kafka and its boundaries.

## Exactly-once inside Kafka

Kafka's **transactions** let a producer write to several partitions and commit **consumer offsets** atomically: either all of it becomes visible or none of it. A consume-process-produce application can therefore read from topic A and write to topic B with **exactly-once semantics** within Kafka: set a `transactional.id`, wrap the work in begin/commit, and have downstream consumers use **`isolation.level=read_committed`** so they never see aborted data. The guarantee covers Kafka reads and writes; if you also update an external database, you still need an idempotent write or a transactional outbox, because Kafka cannot roll back systems it does not control. Frameworks such as Kafka Streams and Flink can provide end-to-end exactly-once for supported sinks.

## A transactional producer (illustrative)

Reads from `orders`, writes to `orders-enriched` and commits offsets in one transaction. `enrich` stands for your own function.

```python
p = Producer({"bootstrap.servers": "localhost:9092", "transactional.id": "enricher-1"})
c = Consumer({"bootstrap.servers": "localhost:9092", "group.id": "enricher",
              "enable.auto.commit": False, "isolation.level": "read_committed"})
p.init_transactions()
c.subscribe(["orders"])
while True:
    msg = c.poll(1.0)
    if msg is None or msg.error(): continue
    p.begin_transaction()
    p.produce("orders-enriched", key=msg.key(), value=enrich(msg.value()))
    p.send_offsets_to_transaction(c.position(c.assignment()), c.consumer_group_metadata())
    p.commit_transaction()
```

**Quiz:** What does `isolation.level=read_committed` do for a consumer?

- [x] Hides records from aborted or open transactions
- [ ] Encrypts the records
- [ ] Deletes old records
- [ ] Speeds up the broker

*Answer:* Hides records from aborted or open transactions. Only committed transactional data is delivered to the consumer.