Re: Add Option To Check All Addresses For Matching target_session_attr

Hi,

Thank you for the review!

Review Response

- Made a first pass at a real commit message
- Fixed the condition on the if statement to use strcmp
- Added a test suite in the files `src/interfaces/libpq/t/
006_target_session_attr_dns.pl` and `src/interfaces/libpq/t/
007_load_balance_dns_check_all_addrs.pl` which checks the
target_session_attrs as when used with and without load balancing.

Regarding the name of the variable itself I am definitely open to opinions
on this. I didn't put too much thought initially and just chose
`check_all_addrs`. I feel like given that it modifies the behaviour of
`target_session_attrs` ideally it should reference that in the name but
that would make that variable name very long: something akin to
`target_session_attrs_check_all_addrs`.

Context

I tested some drivers as well and found that pgx, psycopg, and
rust-postgres all traverse every IP address when looking for a matching
target_session_attrs. Asyncpg and psycopg2 on the other hand follow libpq
and terminate additional attempts after the first failure. Given this it
seems like there is a decent amount of fragmentation in the ecosystem as to
how exactly to implement this feature. I believe some drivers choose to
traverse all addresses because they have users target the same use case
outlined above.

Thanks again,
Andrew Jackson

On Sun, Feb 16, 2025 at 6:03 AM Andrey Borodin <x4mmm@yandex-team.ru> wrote:

> Hi Andrew!
>
> cc Jelte, I suspect he might be interested.
>
> > On 20 Nov 2024, at 20:51, Andrew Jackson <andrewjackson947@gmail.com>
> wrote:
> >
> > Would appreciate any feedback on the applicability/relevancy of the goal
> here or the implementation.
>
> Thank you for raising the issue. Following our discussion in Discord I'm
> putting my thoughts to list.
>
>
> Context
>
> A DNS record might return several IPs. Consider we have a connection
> string with "host=A,B", A is resolved to 1.1.1.1,2.2.2.2, B to
> 3.3.3.3,4.4.4.4.
> If we connect with "target_session_attrs=read-write" IPs 1.1.1.1 and
> 3.3.3.3 will be probed, but 2.2.2.2 and 4.4.4.4 won't (if 1.1.1.1 and
> 3.3.3.3 responded).
>
> If we enable libpq load balancing some random 2 IPs will be probed.
>
> IMO it's a bug, at least when load balancing is enabled. Let's consider if
> we can change default behavior here. I suspect we can't do it for
> "load_balance_hosts=disable". And even for load balancing this might be too
> unexpected change for someone.
>
> Further I only consider proposal not as a bug fix, but as a feature.
>
> In Discord we have surveyed some other drivers.
> pgx treats all IPs as different servers [1]. npgsql goes through all IPs
> one-by-one always [2]. PGJDBC are somewhat in a decision process [3] (cc
> Dave and Vladimir, if they would like to provide some input).
>
>
> Review
>
> The patch needs a rebase. It's trivial, so please fine attached. The patch
> needs real commit message, it's not trivial :)
>
> We definitely need to adjust tests [0]. We need to change
> 004_load_balance_dns.pl so that it tests target_session_attrs too.
>
> Some documentation would be nice.
>
> I do not like how this check is performed
> +                                               if (conn->check_all_addrs
> && conn->check_all_addrs[0] == '1')
> Let's make it like load balancing is done [4].
>
> Finally, let's think about naming alternatives for "check_all_addrs".
>
> I think that's enough for a first round of the review. If it's not a bug,
> but a feature - it's a very narrow window to get to 18. But we might be
> lucky...
>
> Thank you!
>
>
> Best regards, Andrey Borodin.
>
> [0]
> https://github.com/postgres/postgres/commit/7f5b19817eaf38e70ad1153db4e644ee9456853e#diff-b05b74d2a97d7f74d4311ba1702d732f0df1b101c6ac99c146b51215174cf3ffR94
> [1] https://github.com/jackc/pgx/blob/master/pgconn/pgconn.go#L177
> [2]
> https://github.com/npgsql/npgsql/blob/7f1a59fa8dc1ccc34a70154f49a768e1abf826ba/src/Npgsql/Internal/NpgsqlConnector.cs#L986
> [3] https://github.com/pgjdbc/pgjdbc/pull/3012#discussion_r1408069450
> [4]
> https://github.com/postgres/postgres/commit/7f5b19817eaf38e70ad1153db4e644ee9456853e#diff-8d819454e061b9d4cdae9c8922ded05753a629d70f2ac1de1d4f6d5a4aeb7f68R1660
>