Subject: Re: Unique cluster / instance identifier?
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: pgsql-admin <pgsql-admin@postgresql.org>
References: <92d53982-cf1d-ce3f-c541-60a252f85aef@quadratum-braccas.com>
 <87r2c5htm3.fsf@news-spur.riddles.org.uk>
From: "S. Bob" <sbob@quadratum-braccas.com>
Message-ID: <a1476f35-afe1-24fe-e86c-4e20fae0f4f5@quadratum-braccas.com>
Date: Mon, 18 Feb 2019 11:07:29 -0700
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Perfect!  Thanks


On 2/18/19 11:03 AM, Andrew Gierth wrote:
>>>>>> "S" == S Bob <sbob@quadratum-braccas.com> writes:
>   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.
>