Question: How can you check the Redis replication status?

Answer

You can check the status of Redis replication using the INFO command, particularly the INFO REPLICATION section. This command provides information about Redis's master or slave state along with other details such as connected slaves, master link status etc.

Here is an example of how to do this:

$ redis-cli
127.0.0.1:6379> INFO REPLICATION

The output will provide a lot of useful information. If your instance is set as a master, it will show something similar to:

# Replication
role:master
connected_slaves:3
slave0:ip=127.0.0.1,port=6380,state=online,offset=1456,lag=1
slave1:ip=127.0.0.1,port=6381,state=online,offset=1456,lag=1
slave2:ip=127.0.0.1,port=6382,state=online,offset=1456,lag=1
master_replid:<some id>
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:1456
second_repl_offset:-1
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:1
repl_backlog_histlen:1456

If your instance is set as a slave, it will show:

# Replication
role:slave
master_host:127.0.0.1
master_port:6379
master_link_status:up
master_last_io_seconds_ago:6
master_sync_in_progress:0
slave_repl_offset:184823
slave_priority:100
slave_read_only:1
connected_slaves:0
master_replid:<some id>
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:184823
second_repl_offset:-1
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:69868
repl_backlog_histlen:114956

In this way, you can easily check the replication status of your Redis instances.