1. Introduction

The graceful switchover is a zero data loss process for role reversal between the primary and standby clusters. At the end of this process, the standby and primary clusters change roles, and the standby cluster becomes the new primary, and the primary cluster becomes the new standby. Even though this process involves a little downtime, but is seamless and does not require any manual intervention.

2. Technical Explanation

Database Status
1. The primary cluster should be “ONLINE“ and the standby cluster should be “HOT STANDBY“/”WARM STANDBY.”

Trigger Graceful Switchover
Disable Control Center Behaviors
- Disable Observer: Turn off the observer to ensure a smooth switchover without conflicting activities
- Disable Auto Update: Suspend any automatic update processes during the switchover procedure
Prepare for Sync Between Primary and Standby 1
- If the backup mode is set to "Archive," generate a new WAL file as a reference point for synchronization checks
```
SELECT * from pg_switch_wal()
```
- Otherwise skip
Stop Original Primary Cluster
Prepare for Sync Between Primary and Standby 2
- If the backup mode is "Archive," skip this step as it's already done in step 3
- If the backup mode is "Log Shipping," transmit all unapplied WAL files to the standby server via dbvnet
```
start_lsn:
can be retrieved from backupInfo

end_lsn:
pg_controldata -D dataDir

Any wal file with LSN: LSN > start_lsn && LSN <= end_lsn 
```
- If the backup mode is "Streaming," there's no need to sync as the WAL sync is accomplished before the primary cluster shutdown
Update Original Primary PostgreSQL Configuration
- Adjust the original primary PostgreSQL configuration based on the backup mode. Convert its role to become the new standby database.
Sync Between Primary and Standby for Switchover 1
- If the backup mode is "Archive," wait for the WAL file generated in step 3 to be applied in the original standby database.
- Otherwise skip
Sync Between Primary and Standby for Switchover 2
- If the backup mode is "Log Shipping," apply the WAL file transferred in step 5 to the original standby database.
- Otherwise skip
Stop Original Standby Cluster
Update Original Standby PostgreSQL Configuration
- Modify the original standby PostgreSQL configuration according to the backup mode, converting its role to the new primary database
Switch Role and Persist New Configuration Data
Start New Primary Cluster
Refresh Replication Slot on Primary Cluster (if Streaming Backup)

SELECT * FROM pg_drop_replication_slot($1) WHERE EXISTS (SELECT * FROM pg_replication_slots WHERE slot_name = SLOT_NAME)
SELECT * FROM pg_create_physical_replication_slot(SLOT_NAME, false, false)

Start New Standby Cluster
Prepare for Sync validation Between Primary and Standby
- If the backup mode is "Archive," create a new WAL file as a reference for synchronization checks.
- If the backup mode is "Log Shipping," generate a new WAL file and transfer it to the new standby cluster.
- If the backup mode is "Streaming," verify that the current LSN on the new primary is applied in the new standby database.
Sync Between Primary and Standby for Validation 1
- If the backup mode is "Archive," wait for the WAL file generated in step 15 to be applied in the new standby database.
- Otherwise,
- skip
Sync Between Primary and Standby for Validation 2
- If the backup mode is "Log Shipping," apply the WAL file transferred in step 15 to the new standby database.
- Otherwise, skip
Enable Control Center Behaviors
- Enable Observer
- Enable Auto Update: Restore the automatic update processes
Execute User Script on New Primary and Standby Servers
- Optionally run any necessary user scripts on the new primary and standby servers to complete the switchover process.

3. Dbvisit Switchover

3.1 Switchover with WAL Streaming Mode

The first and foremost step is to ensure that the status of the configuration is In Sync before proceeding with the switchover. It is also important that the application/users are not making any changes to the production cluster.

The streaming mode configuration is In Sync.
The message indicates that the configuration is ready for switchover.
User scripts that can be executed post the switchover process on new primary and standby.
Click Start to start the switchover process.

The time taken for the switchover process to complete.
The steps for the switchover process.

The configuration is In Sync with WAL Streaming method on the reverse direction.
The observer is enabled once the switchover is completed and is now monitored.
The new primary server and Cluster status show ONLINE on port 5435.
The Streaming method is used by the cluster to sync between standby and primary.
The new standby server and cluster status showing HOT STANDBY on port 5435

3.2 Switchover with the WAL Archive method.