Re: [PATCH] postgres_fdw connection caching - cause remote sessions linger till the local session exit

On Mon, 22 Jun 2020 at 14:56, Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> Hi,
>
> When a query on foreign table is executed from a local session using
> postgres_fdw, as expected the local postgres backend opens a
> connection which causes a remote session/backend to be opened on the
> remote postgres server for query execution.
>
> One observation is that, even after the query is finished, the remote
> session/backend still persists on the remote postgres server. Upon
> researching, I found that there is a concept of Connection Caching for
> the remote connections made using postgres_fdw. Local backend/session
> can cache up to 8 different connections per backend. This caching is
> useful as it avoids the cost of reestablishing new connections per
> foreign query.
>
> However, at times, there may be situations where the long lasting
> local sessions may execute very few foreign queries and remaining all
> are local queries, in this scenario, the remote sessions opened by the
> local sessions/backends may not be useful as they remain idle and eat
> up the remote server connections capacity. This problem gets even
> worse(though this use case is a bit imaginary) if all of
> max_connections(default 100 and each backend caching 8 remote
> connections) local sessions open remote sessions and they are cached
> in the local backend.
>
> I propose to have a new session level GUC called
> "enable_connectioncache"(name can be changed if it doesn't correctly
> mean the purpose) with the default value being true which means that
> all the remote connections are cached. If set to false, the
> connections are not cached and so are remote sessions closed by the local backend/session at
> the end of each remote transaction.

I've not looked at your patch deeply but if this problem is talking
only about postgres_fdw I think we should improve postgres_fdw, not
adding a GUC to the core. It’s not that all FDW plugins use connection
cache and postgres_fdw’s connection cache is implemented within
postgres_fdw, I think we should focus on improving postgres_fdw. I
also think it’s not a good design that the core manages connections to
remote servers connected via FDW. I wonder if we can add a
postgres_fdw option for this purpose, say keep_connection [on|off].
That way, we can set it per server so that remote connections to the
particular server don’t remain idle.

Regards,

--
Masahiko Sawada            http://www.2ndQuadrant.com/
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