Question: How can you force a Redis slave to sync with its master?

Answer

To force a Redis slave to sync with its master, you can use the SLAVEOF command. This command makes a server a replica (or slave) of another server (the master), implementing a master-replica model.

If a Redis instance is already a replica of some master, and you want to force a new synchronization, you need to first break the existing replication and then re-establish it. Here's how you can do that:

redis-cli SLAVEOF NO ONE
redis-cli SLAVEOF master_host master_port

In this script, master_host and master_port should be replaced with the hostname and port number of your master Redis instance.

Please note, these commands will cause the current Redis instance to stop accepting write queries until the synchronization process completes. Additionally, if your data set is large, the synchronization may take a while.

Another way to force a resynchronization is by using the DEBUG RELOAD command, which unloads and reloads the database again. However, this technique is not recommended for production environments as it blocks the server during the operation.

redis-cli DEBUG RELOAD

Remember to use these commands cautiously, especially in a production environment. Inappropriate use can lead to data loss or service interruption.