RE: [Proposal] Add foreign-server health checks infrastructure

Dear Önder,

> As far as I can see this patch is mostly useful for detecting the failures
> on the initial remote command. This is especially common when the remote
> server does a failover/switchover and postgres_fdw uses a cached connection
> to access to the remote server.

Sounds reasonable. Do you mean that we can add additional GUC like "postgres_fdw.initial_check",
wait WL_SOCKET_CLOSED if the conneciton is found in the hash table, and do reconnection if it might be closed, right?

> I think any extension that deals with multiple Postgres nodes can benefit
> from such logic. In fact, the reason I realized this patch is that on the
> Citus extension, we are trying to solve a similar problem [1], [2].

> Thinking even more, I think any extension that uses libpq and WaitEventSets
> can benefit from such a function.

OK, I agreed it may be useful, but where should this function be?
This cannot be used from application, so it should not in interface/libpq dir. So backend/libpq/pqcomm.c?

> I think it also depends on where you decide to put
> pgfdw_connection_check_internal(). If you prefer the postgres_fdw side,
> could we maybe use ConnCacheEntry in contrib/postgres_fdw/connection.c?
> But if you decide to put it into the Postgres side, the API
> for pgfdw_connection_check_internal() -- or equivalent function -- could be
> discussed. Do we pass a WaitEventSet and if it is NULL create a new one,
> else use what is passed to the function? Not sure, maybe you can come up
> with a better API.

Thank you for describing more detail. I can imagine you said.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED