Postgres inside Docker container, how to log JSON to Loki via Promtail?

Question

I have Postgres16.1 running in a container in Docker Swarm, alongside inter alia Promtail, Loki and Grafana. I want to direct Postgres logs in JSON format towards Promtail for easy processing in Promtail pipeline stages (especially of multiline logs). The setting Postgres config log_destination=jsonlog will create JSON logs in log/postgresql-<DATE>.json (see documentation here). But Docker stdout/stderr logs (which Promtail reads) will remain in default config.

docker-compose.yaml:

(...)
  db:
    container_name: postgres
    image: <from my local registry>
    command: ["postgres", "-c", "log_statement=all", "-c", "logging_collector=on", "-c", "log_destination=jsonlog"]
    labels:
      logging: "promtail"
(...)

How can I configure Postgres to log to stderr in JSON format? Also, is this a good idea/good practice in the first place?

Not having stderr as log_destination also does not seem to influence the stderr logging behaviour of the container.

Answer 1

Found a straightforward solution that does not require volume mounting or other workarounds here.

After having set logging_collector=onand log_destination=jsonlog, configure postgres log_filename to something static (exluding date/time as per default config) like postgresql.json (see doc here).

Then run the container with ln -sf /dev/stdout /var/lib/postgresql/data/log/postgresql.json.

This way, you shouldn't even have to worry about postgres log rotation, as the link will reroute to stdout directly and thus to docker log management.