Monitoring PostgreSQL Streaming Replication Delay

Looking at the documentation and all the blog posts about how to monitor replication delay I don't think there is one good and most importantly safe solution which works all the time. 

Solution 1:

I used to check replication delay/lag by running the following query on the

slave:

SELECT EXTRACT(EPOCH FROM (now() - pg_last_xact_replay_timestamp()))::INT;

This query works great and it is a very good query to give you the lag in seconds. The problem is if the master is not active, it doesn't mean a thing. So you need to first check if two servers are in sync and if they are, return 0.

Solution 2:

This can be achieved by comparing pg_last_xlog_receive_location() and pg_last_xlog_replay_location() on the slave, and if they are the same it returns 0, otherwise it runs the above query again:

SELECT
CASE
WHEN pg_last_xlog_receive_location() = pg_last_xlog_replay_location() THEN 0
ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp())::INTEGER
END
AS replication_lag;

This query is all good, but the problem is that it is not safe. If for some reason the master stops sending transaction logs, this query will continue to return 0 and you will think the replication is working, when it is not.

Solution 3:

Master:

SELECT pg_current_xlog_location();

Slave:

SELECT pg_last_xlog_receive_location();

and by comparing these two values you could see if the servers are in sync. The problem yet again is that if streaming replication fails, both of these functions will continue to return same values and you could still end up thinking the replication is working. But also you need to query both the master and slave to be able to monitor this, which is not that easy on monitoring systems, and you still don't have the information about the actual lag in seconds, so you would still need to run the first query.

Solution 4:

You could query pg_stat_replication on the master, compare sent_location and replay_location, and if they are the same, the replication is in sync. One more good thing about pg_stat_replication is that if streaming replication fails it will return an empty result, so you will know it failed. But the biggest problem with this system view is that only the postgres user can read it, so it's not that monitoring friendly since you don't want to give your monitoring system super user privileges, and you still don't have the delay in seconds. I think the best one would be 2 combinedwith a check if the wal receiver process is running before running that

query with something like:

$ ps aux | egrep 'wal\sreceiver'
postgres  3858  0.0  0.0 2100112 3312 ?        Ss   19:35   0:01 postgres:
wal receiver process   streaming 36/900A738

This solution would only be run on the slave and it is pretty easy to setup.