Question: What is the Redis replication backlog and how does it work?

Answer

The Redis replication backlog is a crucial component of Redis's master-slave replication system. It is essentially a buffer in memory on the master node that stores all write operations. This buffer serves as a source of data for connected slave nodes to synchronize their data with the master.

Here's how it works:

1. Master Node Operations: Whenever a write operation is performed on the master node, it logs the command into the replication backlog. Subsequent commands overwrite older ones when the buffer limit is reached.

2. Slave Node Synchronization: When a slave node connects to the master, it specifies the offset in the backlog at which it has previously synchronized data. The master then sends all commands from that offset onwards, allowing the slave to update its dataset.

3. Partial Resynchronization: In case of network disconnection, when the slave node reconnects, it attempts a partial resynchronization. If the required data is still available in the replication backlog, the master can send only the missed data instead of initiating a full resynchronization.

4. Full Resynchronization: If the master's replication backlog does not contain the data needed by the slave (perhaps because it's too old and has been overwritten), a full resynchronization is triggered. During this process, the master creates an RDB snapshot of its data, sends it to the slave, and continues to append write commands to the replication backlog.

The size of the replication backlog is configurable and should be set considering aspects like the amount of write traffic, number of slaves, network stability, and capacity of master to handle full resynchronizations.

Here's an example of configuring the backlog size in Redis:

redis-cli CONFIG SET repl-backlog-size 1048576

This command sets the replication backlog size to 1MB. The larger the backlog, the more historical commands it can store, and the greater chance a disconnected slave has of performing a partial rather than full resynchronization.