Re: Better way to monitor for failed replication?

On Fri, Jan 9, 2026 at 10:50 AM Ron Johnson <ronljohnsonjr(at)gmail(dot)com> wrote:

> Currently, in a bash script, I run this SELECT statement against the
> Primary server which is supposed to replicate to multiple servers. If
> active == f, I send an alter email.
>
> postgres=# SELECT rs.slot_name, rs.active, sr.client_hostname
> from pg_replication_slots rs
> left outer join pg_stat_replication sr on rs.active_pid = sr.pid;
> slot_name | active | client_hostname
> --------------+--------+-----------------
> pgstandby1 | t | BBOPITCPGS302B
> replicate_dr | f |
> (2 rows)
>
> Is there a better way to check for replication that's supposed to be
> happening, but isn't (like PG on the replica was stopped for some reason)?
>
> --
> Death to <Redacted>, and butter sauce.
> Don't boil me, I'm still alive.
> <Redacted> lobster!
>

Your example only takes into account if you are using replication slots,
correct? If you're always using those, this is definitely a good metric to
have since the slot going down means WAL buildup, so I'd definitely keep
it. As for general replication monitoring, these have been the two queries
I use

On the Primary:

SELECT client_addr AS replica
, client_hostname AS replica_hostname
, client_port AS replica_port
, pg_wal_lsn_diff(sent_lsn, replay_lsn) AS bytes
FROM pg_catalog.pg_stat_replication;

This checks for byte-lag for all active streaming replicas, physical or
logical. A count of zero or NULL from this metric means all replicas are
down. Can monitor a specific count if you have a known number of replicas.

On any Replica:

SELECT
CASE
WHEN (pg_last_wal_receive_lsn() = pg_last_wal_replay_lsn()) OR
(pg_is_in_recovery() = false) THEN 0
ELSE EXTRACT (EPOCH FROM clock_timestamp() -
pg_last_xact_replay_timestamp())::INTEGER
END
AS replay_time
, CASE
WHEN pg_is_in_recovery() = false THEN 0
ELSE EXTRACT (EPOCH FROM clock_timestamp() -
pg_last_xact_replay_timestamp())::INTEGER
END
AS received_time;

This monitors the lag in seconds from the replica. Technically it monitors
the last time a WAL file was received (received_time) and the last time WAL
was actually replayed (replay_time). The reason for both is that the
received time can be a false positive when there is no write activity on
the primary. If there's always supposed to be write activity, this can be a
another good metric to indicate that something is very wrong. The
replay_time metric avoids the false positive by only being considered when
receive is different than replay. This metric also works when you're doing
WAL-replay replication instead of streaming.