RE: Transactions involving multiple postgres foreign servers, take 2

From: Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com>
> > If so, can't we stipulate that the FDW implementor should ensure that the
> commit function always returns control to the caller?
>
> How can the FDW implementor ensure that? Since even palloc could call
> ereport(ERROR) I guess it's hard to require that to all FDW
> implementors.

I think the what FDW commit routine will do is to just call xa_commit(), or PQexec("COMMIT PREPARED") in postgres_fdw.

> It's still a rough idea but I think we can use TMASYNC flag and
> xa_complete explained in the XA specification. The core transaction
> manager call prepare, commit, rollback APIs with the flag, requiring
> to execute the operation asynchronously and to return a handler (e.g.,
> a socket taken by PQsocket in postgres_fdw case) to the transaction
> manager. Then the transaction manager continues polling the handler
> until it becomes readable and testing the completion using by
> xa_complete() with no wait, until all foreign servers return OK on
> xa_complete check.

Unfortunately, even Oracle and Db2 don't support XA asynchronous execution for years. Our DBMS Symfoware doesn't, either. I don't expect other DBMSs support it.

Hmm, I'm afraid this may be one of the FDW's intractable walls for a serious scale-out DBMS. If we define asynchronous FDW routines for 2PC, postgres_fdw would be able to implement them by using libpq asynchronous functions. But other DBMSs can't ...

> > Maybe we can consider VOLATILE functions update data. That may be
> overreaction, though.
>
> Sorry I don't understand that. The volatile functions are not pushed
> down to the foreign servers in the first place, no?

Ah, you're right. Then, the choices are twofold: (1) trust users in that their functions don't update data or the user's claim (specification) about it, and (2) get notification through FE/BE protocol that the remote transaction may have updated data.

Regards
Takayuki Tsunakawa