Everybody know

Also everybody know that ‘Seconds_Behind_Master’ shows difference in seconds between the slave SQL thread and the slave I/O thread.

Sometime it shows nonsense, and if you build monitoring It is not good practice to use ‘Seconds_Behind_Master’
Example from real life:
If replication becomes stalled due to connectivity problems, Seconds_Behind_Master shows 0 while replica is far away from master, changing timeout values not help. 🙁 i mean:

slave_net_timeout=300

So we implement following monitoring – every 3 second write current timestamp at master.
and check replica delay.
Or You can use percona heartbeat, It do almost the same.
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