---
title: "Run an active/passive pair and fail over"
description: "Start the two-region active/passive sandbox, watch the mirror copy records to the DR region, take the primary region down and fail over."
url: https://monedula.dev/flock/docs/kafka-simulator/how-to/run-active-passive/
---
The **Active / passive (mirror)** sandbox models two regions, each its own Kafka cluster: `primary` takes the writes and `dr` holds a mirror copy.

## Steps

### 1. Start the sandbox

In the left rail of the [playground](/kafka-simulator/playground/), set **cluster type** to **Active / passive (mirror)**. On a narrow screen, pick it from the **scenario** menu in the top bar, under **✦ Free play**. The sandbox starts with:

- two regions, `primary` with `broker-1` and `broker-2`, and `dr` with `broker-3` and `broker-4`, each region with its own three-voter KRaft quorum;
- the topic `orders` in `primary`, with one partition and replication factor 2;
- a mirror link, `mm-1`, that copies `orders` from `primary` to `dr`, where it arrives as `primary.orders`;
- the producer `p1` and the consumer group `cg1`, on `orders`.

### 2. Watch the mirror

Select the arrow between the two regions. The Inspector opens on **Mirror link · mm-1**: its kind, **MirrorMaker 2 (re-produces)**, its direction, the destination topic, and for each partition the source high watermark (**src HW**), the last **mirrored** offset and the **lag** between them. The copy trails the source by a short delay, so the lag rises while records flow and falls back as they arrive in `dr`.

Below that, the **failover (dr)** section shows the **rpo**, in records not yet mirrored.

The **Why** tab ends with a **DR posture** card. For this sandbox it reads "Active-passive · async mirror", with the RPO in records and the RTO as "manual failover".

### 3. Take the primary region down

Select the `primary` region band. The Inspector opens on **Region · primary**, with its brokers and topics. Select **kill · hard primary**: every broker and controller in the region goes down, and the producer can no longer write.

### 4. Fail over

Select the mirror link again and, under **failover (dr)**, select **fail over to dr**. The `dr` region becomes the primary: the producer writes to `primary.orders` in `dr`, and the consumer group resumes there. The **rpo** row now counts the records as lost, never mirrored, and the **DR posture** card notes the failover and how many records were lost. The mirror link now offers **fail back to primary** in place of **fail over to dr**.

### 5. Fail back

Select the `primary` region band and **restart primary**. Then, on the mirror link, select **fail back to primary**. The original region is the primary again and forward mirroring resumes.


The **Failure** tab has the same controls in its **Failover (DR)** section, plus two ways to widen the loss window before you fail over: **cut inter-region link** under **Inter-region link**, and the replication speed buttons, **normal**, **slow**, **very slow** and **stopped**, which throttle the mirror. **kill active DC · primary** takes the active region down from there too.



In the terminal, each broker belongs to its region's cluster. On `broker-1`, `kafka-topics --list` prints `orders`; after `ssh broker-3` it prints `primary.orders`, and `cluster-info` names the region.


## Related

- [Share a session as a link](/flock/docs/kafka-simulator/how-to/share-by-url/) keeps the failover you just ran.


- [Run an active/active pair](/flock/docs/kafka-simulator/how-to/run-active-active/) has both regions take writes, with no standby to promote.



- The [Active-passive mirror](/kafka-simulator/scenarios/dr-mirror/), [Failover + RPO](/kafka-simulator/scenarios/dr-failover/) and [Failback after the primary recovers](/kafka-simulator/scenarios/failback-after-primary-recovers/) scenario pages walk through the same steps as scripted scenarios.



- [Mirror between regions](/flock/docs/kafka-simulator/how-to/mirror-between-regions/) renames the regions and switches the link to Cluster Linking.