# min.insync.replicas and Availability — Apache Kafka: Event Streaming from Basics to Production

Source: https://www.geekswithgeeks.com/en/kafka/rel-min-isr

> Combine replication factor, min.insync.replicas and acks=all to trade availability against durability.

## The classic 3/2/all recipe

With `acks=all`, a write succeeds only when at least **`min.insync.replicas`** replicas (including the leader) have it; if fewer are in sync the producer gets an error rather than silently weakening the guarantee. The usual production recipe is **replication factor 3, `min.insync.replicas=2`, `acks=all`**: you can lose **one** broker and still write, and acknowledged data exists on at least two brokers. Losing two brokers makes the partition unavailable for writes (reads continue if a replica is in sync). That is the deliberate trade-off: consistency over availability. Keep `unclean.leader.election.enable=false` (the default) so an out-of-date replica never becomes leader and silently drops acknowledged records.

## Surviving broker failures

Replication, ISR and the right producer settings decide whether acknowledged data survives a failure.

![Three layers: replicate, acknowledge, elect.](assets/figures/kafka/section-5-map.svg) — Figure 5.1 — Replicate, acknowledge and elect.

## How many failures can writes survive, run

I ran this for replication factor 3 and `min.insync.replicas=2`: writes work with 0 or 1 failed brokers (True, True) and stop at 2 or 3 failures (False, False).

```python
def can_write(rf, min_isr, failed):
    return rf - failed >= min_isr

print([can_write(3, 2, f) for f in range(4)])
```

Output:

```
[True, True, False, False]
```

## Setting it on a topic (illustrative)

Create a durable topic in one command.

```bash
kafka-topics.sh --bootstrap-server broker1:9092 --create --topic payments \
  --partitions 12 --replication-factor 3 --config min.insync.replicas=2
```

**Quiz:** With replication factor 3 and min.insync.replicas=2, how many broker failures can writes survive?

- [ ] Three
- [ ] Two
- [x] One
- [ ] None

*Answer:* One. Writes need two in-sync replicas, so only one of the three may be down.