Monitoring Streaming Replication

Hi team,

We are currently using the following script on our PostgreSQL 10 standby
server to monitor streaming replication lag:

replication_lag=$(psql -U gateway postgres -tAc "
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())
END;
")

This script appears to measure only the replay lag on the standby. However,
I am concerned that it may not detect certain replication failure scenarios
and could incorrectly report a healthy status.

Specifically, I would like to know how to monitor the following situations:

1. The standby is disconnected from the primary.

2. The WAL receiver process has stopped.

3. The standby cannot continue replication because the required WAL files
have already been removed from the primary (for example, requested WAL
segment ... has already been removed).

Could you suggest the best approach or a more comprehensive monitoring
script for PostgreSQL 10 streaming replication that covers not only
replication lag but also these failure scenarios? If some of these
conditions cannot be detected using SQL alone.

Thanks.

.