S> Hi all;
S> Does PostgreSQL store a unique cluster or instance identifier that
S> we can access to identify our instances?
Yes. But it's not easy to get at: it's the "Database system identifier"
shown in the output of pg_controldata. (pg12+ will have a function to
get it from SQL, but released versions don't.)
e.g.
Database system identifier: 6659016342798875639
The number is mostly generated from the time the instance was created,
with some bits modified. So for example:
$ perl -le 'print scalar gmtime +(shift >> 32)' 6659016342798875639
Sun Feb 17 17:04:21 2019
PG uses this internally to make sure that replication never tries to
apply WAL records generated on one system to a different system.
Physical replication secondary systems share the primary's system
identifier since they always originate as a copy (via base backup) of
the primary.
