All 7 Redis Replication questions
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What is Redis replication, and how does it help improve availability and read scalability? How do you configure a Redis instance to become a replica of another Redis instance? What is replication lag, and what factors can cause a replica to fall noticeably behind its primary server? How do you check the current replication status and role of a specific Redis instance, whether it is acting as a primary or a replica? How would you design a read scaling strategy using multiple Redis replicas for an application with a very read heavy workload? What happens to a Redis Cluster's replicas during a failover event, and how does the cluster decide which replica gets promoted?
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What is the difference between full resynchronization and partial resynchronization when a replica reconnects to its primary after a temporary disconnection?

Intermediate

Answer

Full resynchronization requires the replica to receive an entirely new complete copy of the primary's dataset, which can be slow and resource intensive for large datasets, while partial resynchronization allows the replica to catch up by receiving only the specific changes it missed during the disconnection, using a backlog buffer the primary maintains, which is much faster and less resource intensive when the disconnection was brief.
-- Check the replication backlog size, which affects
-- how long a disconnection can be tolerated before
-- requiring a full resync instead of a partial one
CONFIG GET repl-backlog-size
Real-world example A replica that briefly loses network connectivity for a few seconds successfully performs a partial resynchronization once reconnected, quickly catching up without needing the time consuming full resync that would have been required for a longer outage.

Common follow-ups: How do you configure the size of the replication backlog buffer?;What determines whether a reconnecting replica can use partial versus full resynchronization?

Redis Architecture & Installation;Redis Sentinel & High Availability
All 7 Redis Replication questions