pg_exporter_last_scrape_error returns 1 on replica unit of the standby cluster when async-replication is enabled

[ggouzi]

Steps to reproduce

• 2 Postgresql clusters of 2 units each
• Enable async-replication with the create-replication action
• Enable monitoring with grafana-agent subordinates
• Check alerts in prometheus (COS)

Expected behavior

Alert PostgresqlExporterError is not firing

Actual behavior

Alert PostgresqlExporterError is firing

Indeed the pg_exporter_last_scrape_error returns 1 on the replica unit of the standby cluster

curl -sS http://127.0.0.1:9187/metrics | grep pg_exporter_last_scrape_error
# HELP pg_exporter_last_scrape_error Whether the last scrape of metrics from PostgreSQL resulted in an error (1 for error, 0 for success).
# TYPE pg_exporter_last_scrape_error gauge
pg_exporter_last_scrape_error 1

The curl command above shows there is no scrape error.

Versions

Juju 3.6.11
OS 24.04 Noble
Postgresql machine charm: 16/stable rev 952

Additional context

This issue seems to be well known at upstream level: prometheus-community/postgres_exporter#957

https://discuss.prometheus.io/t/pg-exporter-last-scrape-error-provides-1-on-crunchy-cluster-replica/1981

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-8888.

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