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I just built a plpgsql function called
is_replication_lag_more_than_10mts()
using Solution #1 this morning…. I run this from crontab every 5 minutes and sends an email if the replication lags behind more than 10 minutes. I understand the limitation, but in my case it is ok. My primary is constantly active.
I have other databases that are not active where I am thinking of creating a simple table and keep truncating it may be every 5 minutes depending on what my
archive_timeout is… This I know is not really elegant but it is robust in my opinion. From: pgsql-admin-owner@xxxxxxxxxxxxxx [mailto:pgsql-admin-owner@xxxxxxxxxxxxxx]
On Behalf Of Strahinja Kustudic 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: The Postgres Wiki http://wiki.postgresql.org/wiki/Streaming_Replication recommends to run the following two queries: 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. Real solution? Looking at all four solutions, I think the best one would be #2 combined with 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. Does anyone have a better idea?
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