PostgreSQL Services

Question

Can someone give a detailed explanation of how the postgresql service(s) works on Ubuntu [16.04]? The behavior that leads me to ask is the following. I use the command sudo systemctl start postgresql to start my postgresql service. When I run systemctl list-units | grep post I see

UNIT                           LOAD   ACTIVE SUB       DESCRIPTION
postgresql.service             loaded active exited    PostgreSQL RDBMS
postgresql@9.6-main.service    loaded active running   PostgreSQL Cluster 9.6-main
system-postgresql.slice        loaded active active    system-postgresql.slice

and when I run sudo systemctl stop postgresql both postgresql.service and postgresql@9.6-main.service disappear. What is each of these three services doing?

Answer 1

I assume postgresql@9.6-main.service is registered service for PostgreSQL Cluster 9.6-main cluster. This is the one that actually controls postgres on your system.

I assume postgresql.service is the effect desribed here

I assume system-postgresql.slice is a postgres slice unit

none of above are critical for running postgres. You can start/stop the cluser with just pg_ctl, without registering it with your services. But if you wonder which service runs it in your case - then it is postgresql@9.6-main.service