---
title: "Confluent MRC observers: Kafka replicas outside the ISR"
description: "Two sync replicas commit in DC-A with min.insync.replicas=2 while a Confluent MRC observer in DC-B trails, so Kafka acks=all writes never wait on the remote DC."
url: https://monedula.dev/kafka-simulator/scenarios/mrc-observers/
---

# Confluent MRC observers: Kafka replicas outside the ISR

Scenario 10.1.1 · MRC observers. Disaster recovery · Advanced · about 3 min · Apache Kafka 4.3 · Confluent Platform feature.

Confluent MRC on a 2.5-DC stretched cluster: sync replicas commit in DC-A (min.ISR 2, local low latency); an async observer in DC-B trails committed data. Witness DC-C holds the third voter.

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=10.1.1

## What you start with

Confluent Multi-Region Clusters mix two kinds of replica in one stretched cluster: synchronous replicas that must confirm before a write commits, and asynchronous observers that fetch from the leader like followers but never count toward the ISR. Here min.ISR-2 sync replicas commit locally in DC-A while an observer in DC-B trails the committed high-watermark and a witness in DC-C holds the third voter.

## Step by step

1. **2.5-DC ready - DC-A + DC-B hold data, witness DC-C holds the 3rd voter; observer in DC-B** (t+0.000s): orders has two synchronous replicas in DC-A (broker-1 leader, broker-2) with min.ISR 2, and broker-3 in DC-B as an async observer outside the ISR. The controller quorum has one voter per DC, with c-c in DC-C as the witness that holds the third vote. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=10.1.1&t=0)
2. **Committed in DC-A (HW 3, min.ISR 2) - DC-B observer still at 0 (lag 3)** (t+1.000s): O1, O2 and O3 each committed as soon as broker-2 in DC-A copied them, so the high-watermark is 3 and every acks=all ack came back without waiting on DC-B. The DC-B observer is still at LEO 0 (lag 3): observers fetch like followers but outside the ISR, and the simulator delays each observer fetch (observerLagMs 800). [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=10.1.1&t=1)
3. **DC-B observer catching up asynchronously - trails the high-watermark (lag 2)** (t+1.450s): The observer's first delayed fetch has landed: broker-3 is at LEO 1 while the HW stays 3, so the lag has fallen from 3 to 2. The producer is not affected - its writes were already acknowledged by the DC-A ISR. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=10.1.1&t=2)
4. **DC-B observer caught up - async replica settles (lag 0); never counted toward ISR** (t+1.700s): All three committed offsets have now reached the DC-B observer; its LEO 3 equals the HW, so the lag is 0. broker-3 holds a full remote copy for disaster recovery, yet the ISR is still broker-1 and broker-2 only - the observer never added cross-DC latency to a write. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=10.1.1&t=3)

## The takeaway

Observers give you a continuously updated remote copy for disaster recovery without paying cross-DC round-trip latency on every acks=all write.

## Configs in this scenario

- [`min.insync.replicas`](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_min.insync.replicas): The fewest in-sync replicas, leader included, that must have a write for an acks=all produce to succeed (default 1); with fewer in the ISR the leader rejects the write with NOT_ENOUGH_REPLICAS. [How min.insync.replicas works](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)
- [`acks`](https://kafka.apache.org/43/configuration/producer-configs/#producerconfigs_acks): How many replicas must have a write before the producer counts it as sent: 0 (none), 1 (the leader only) or all (every in-sync replica, the default, and required for idempotence). [How acks works](https://monedula.dev/kafka-simulator/configs/acks/)

## Further reading

- [Confluent documentation](https://docs.confluent.io/platform/current/multi-dc-deployments/multi-region-async/index.html)