# Acknowledgements, Retries and Idempotence — Apache Kafka: Event Streaming from Basics to Production

Source: https://www.geekswithgeeks.com/en/kafka/kp-acks-idempotence

> Configure acks, retries and the idempotent producer to avoid loss and duplicates.

## How sure do you want to be?

The **`acks`** setting controls when a write counts as successful: `0` (do not wait; fastest, may lose data), `1` (wait for the leader only), `all` (wait for all in-sync replicas; strongest). Combined with the topic setting `min.insync.replicas`, `acks=all` protects acknowledged data from a broker failure. The producer **retries** transient errors, which can create **duplicates** if a retry follows a write that actually succeeded. Setting **`enable.idempotence=true`** gives each producer an ID and sequence numbers so the broker discards duplicates within a partition; in current Kafka clients this is on by default together with `acks=all`. Use `delivery.timeout.ms` to bound the total time spent retrying.

## A durable producer config

Illustrative property names from the Java/librdkafka clients. `max.in.flight.requests.per.connection` up to 5 keeps ordering with idempotence.

```properties
acks=all
enable.idempotence=true
retries=2147483647
delivery.timeout.ms=120000
max.in.flight.requests.per.connection=5
linger.ms=10
compression.type=lz4
```

## How the broker drops duplicates, run

I ran a small model of the sequence check: each producer ID must send sequence numbers 0, 1, 2…; a repeated number is ignored as a duplicate and a gap is an error.

```python
seen = {}
def accept(pid, seq):
    last = seen.get(pid, -1)
    if seq == last + 1:
        seen[pid] = seq
        return "append"
    if seq <= last:
        return "duplicate-ignored"
    return "out-of-order-error"

print([accept(7, s) for s in (0, 1, 1, 2, 4)])
```

Output:

```
['append', 'append', 'duplicate-ignored', 'append', 'out-of-order-error']
```

**Quiz:** Which setting makes the producer wait for all in-sync replicas?

- [ ] compression.type=none
- [ ] acks=0
- [ ] linger.ms=0
- [x] acks=all

*Answer:* acks=all. acks=all acknowledges only after every in-sync replica has the record.