docs/content/v2024.2/deploy/multi-dc/async-replication/async-transactional-switchover.md
Planned switchover is the process of switching write access from the Primary universe to the Standby universe without losing any data (zero RPO). This operation primarily involves updating metadata and typically completes within 30 seconds. However, because application traffic needs to be redirected to the new universe, it is advisable to perform this operation during a maintenance window.
Assuming universe A is the Primary and universe B is the Standby, use the following steps to perform a planned switchover to change the replication direction from A->B to B->A.
Check existing replication group health
To minimize disruptions and the duration of the switchover, ensure that the existing replication is healthy and the lag is under 5 seconds. For information, refer to Monitor xCluster.
Check prerequisites
Ensure that all prerequisites for setting up xCluster are satisfied for the new replication direction. For more information, refer to the xCluster prerequisites.
Set up xCluster Replication from the Standby universe (B) to Primary universe (A) by following the steps in Set up transactional xCluster.
Skip the bootstrap (backup/restore) step, as the data is already present in both universes.
Ensure that the mode of replication used matches the original setup. Continuously monitor the health of the new replication to prevent any unexpected issues post-switchover.
This step puts both universes in read-only mode, allowing universe B to catch up with universe A.
Wait for any pending updates to propagate to B:
Get the current time on A.
Wait until the xCluster safe time on B is a few seconds greater than the time obtained in the previous step.
{{< note title="Note" >}} The lag and skew values might be non-zero as they are estimates based on the last time the tablets were polled. Because no new writes can occur during this period, you can be certain that no data is lost within this timeframe. {{< /note >}}
{{< note >}} Not applicable for Automatic mode {{< /note >}}
Because xCluster does not replicate sequence data, you need to manually synchronize the sequence next values on universe B after switchover to match those on universe A. This ensures that new writes on universe B do not conflict with existing data.
For example, if you have a SERIAL column in a table and the highest value in that column after switchover is 500, you need to set the sequence associated with that column to a value higher than 500, such as 501. This ensures that new writes on universe B do not conflict with existing data.
Use the nextval function to set the sequence next values appropriately.
Run the following command against A to delete the old replication group.
{{% readfile "includes/transactional-drop.md" %}}
The old Standby (B) is now the new Primary universe, and the old Primary (A) is the new Standby universe. Update the application connection strings to point to the new Primary universe (B).