Re: How to simulate sync/async standbys being closer/farther (network distance) to primary in core postgres?

On Tue, Apr 5, 2022 at 9:23 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> Hi,
>
> I'm thinking if there's a way in core postgres to achieve $subject. In
> reality, the sync/async standbys can either be closer/farther (which
> means sync/async standbys can receive WAL at different times) to
> primary, especially in cloud HA environments with primary in one
> Availability Zone(AZ)/Region and standbys in different AZs/Regions.
> $subject may not be possible on dev systems (say, for testing some HA
> features) unless we can inject a delay in WAL senders before sending
> WAL.
>
> How about having two developer-only GUCs {async,
> sync}_wal_sender_delay? When set, the async and sync WAL senders will
> delay sending WAL by {async, sync}_wal_sender_delay
> milliseconds/seconds? Although, I can't think of any immediate use, it
> will be useful someday IMO, say for features like [1], if it gets in.
> With this set of GUCs, one can even add core regression tests for HA
> features.
>
> Thoughts?

I think this is a common problem, people run into. Once way to
simulate network delay is what you suggest, yes. But I was wondering
if there are tools/libraries that can help us to do that. Googling
gives OS specific tools but nothing like a C or perl library which can
be used for this purpose.

--
Best Wishes,
Ashutosh Bapat