Date: Mon, 08 Sep 2025 09:26:28 +0900 (JST)
Message-Id: <20250908.092628.942175382089863152.ishii@postgresql.org>
To: nadav@tailorbrands.com
Cc: ishii@postgresql.org, pgpool-hackers@lists.postgresql.org
Subject: Re: Proposal: recent access based routing for primary-replica
 setups
From: Tatsuo Ishii <ishii@postgresql.org>
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Hi Nadav,

> Hi Tatsuo,
> 
> Thanks for getting back to me. Let me clarify the ordering concern and
> provide an example to make it clearer:
> 
> Currently, replication_delay_source_cmd executes without awareness of the
> replica list or the order in which Pgpool loads them. For Aurora, since
> we’re bypassing the internal DB tables and fetching lag data directly via
> the AWS CloudWatch API, we need to ensure the returned lag values are
> mapped to the correct instances.
> 
> For example, assume Pgpool has the following configuration:
> 
> primary: db-primary
> replicas: db-replica-a, db-replica-b, db-replica-c
> 
> If the command retrieves lag values [15, 120, 60] from CloudWatch, we need
> to guarantee these are consistently mapped as:
> 
> 
>    -
> 
>    db-replica-a → 15ms
>    -
> 
>    db-replica-b → 120ms
>    -
> 
>    db-replica-c → 60ms
> 
> Without explicitly passing the instance identifiers and their order to the
> command, there’s a risk that mismatched ordering will cause Pgpool to make
> incorrect routing decisions.
> 
> To address this, I suggest extending replication_delay_source_cmd to accept
> an ordered list of instance identifiers as arguments. This way, the command
> can fetch the metrics in the same sequence Pgpool expects, ensuring
> alignment between configuration and returned data.

Thanks for the clarification. Previously I misunderstood that Aurora
only provides "reader endpoint", which made me think your proposal to
be impossible. But after some research , I found that Aurora also
provides "cluster endpoint" which refers to each replica instance.  So
let me check if my understanding is
correct. replication_delay_source_cmd will be invoked as:

replication_delay_source_cmd db-replica-a db-replica-b db-replica-c

> Would you agree this approach makes sense?

Yes.

> If so, I can provide an updated
> patch to demonstrate how the command would handle ordered instance mapping.

Thanks. That would be good.

BTW, There are minor points regarding your previous patch. In the patch

083.external_replication_delay/

is the test directory. This does not fit in with our test
infrastructure tradition. Tests for new features should be added
between 001 and 049. 050 and greater are reserved for tests for bug
fixes. So at this point, 041 is appropreate (if other test for a new
feature is added before your patch is committed, you need to adjust
the number of course).

You need to include a patch for documentation. You don't need to write
Japanese doc (doc.ja). We will create it from the English document
later on.

Best regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp