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

Dear Önder,

Thank you for being interest to my patch! Your suggestions will be included to newer version.

> In other words, what is the trade-off for calling
> pgfdw_connection_check_internal() inside GetConnection() when we are about
> to use a "cached" connection? I think that might simplify the patch as well

If the checking function is called not periodically but GetConnection(),
it means that the health of foreign servers will be check only when remote connections are used.
So following workload described in [1] cannot handle the issue.

BEGIN --- remote operations--- local operations --- COMMIT

But, yes, I perfectly agreed that it could simplify the code
because we can reduce the timer part. This is second plan of this patch,
I may move on this approach if it is still useful.

> Can we have this function/logic on Postgres core, so that other extensions
> can also use?

I was not sure about any use-case, but I think it can because it does quite general things.
Is there any good motivation to do that?

> What if PQsocket(conn) returns -1? Maybe we move certain connection state
> checks into pgfdw_connection_check_internal() such that it is more generic?
> I can think of checks like: conn!=NULL, PQsocket(conn) != PGINVALID_SOCKET,
> PQstatus == CONNECTION_OK

ereport(ERROR) will be thrown if PQsocket(conn) returns -1.
All of you said should be handled here. I will modify it.

> Do you see any performance implications of creating/freeing waitEventSets
> per check? I wonder if we can somehow re-use the same waitEventSet by
> modifyWaitEvent? I guess no, but still, if this check causes a performance
> implication, can we somehow cache 1 waitEventSet per connection?

I have not tested yet, but I agreed this will be caused performance decrease.
In next version first I will re-use the event set anyway, and it must be considered later.
Actually I'm not sure your suggestion,
but you mean to say that we can add a hash table that associates PGconn and WaitEventSet, right?

[1]: https://www.postgresql.org/message-id/TYAPR01MB58662809E678253B90E82CE5F5889%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED