Re: Transactions involving multiple postgres foreign servers, take 2 - Mailing list pgsql-hackers

At Tue, 8 Jun 2021 08:45:24 +0000, "tsunakawa.takay@fujitsu.com" <tsunakawa.takay@fujitsu.com> wrote in 
> From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
> > I think the discussion is based the behavior that any process that is
> > responsible for finishing the 2pc-commit continue retrying remote
> > commits until all of the remote-commits succeed.
> 
> Thank you for coming back.  We're talking about the first attempt to prepare and commit in each transaction, not the
retrycase.
 

If we accept each elementary-commit (via FDW connection) to fail, the
parent(?) there's no way the root 2pc-commit can succeed.  How can we
ignore the fdw-error in that case?

> > > Throws: HeuristicMixedException
> > > Thrown to indicate that a heuristic decision was made and that some
> > relevant updates have been
> > > committed while others have been rolled back.
> 
> > I'm not sure about how JTA works in detail, but doesn't
> > UserTransaction.commit() return HeuristicMixedExcpetion when some of
> > relevant updates have been committed but other not? Isn't it the same
> > state with the case where some of the remote servers failed on
> > remote-commit while others are succeeded?
> 
> No.  Taking the description literally and considering the relevant XA specification, it's not about the remote commit
failure. The remote server is not allowed to fail the commit once it has reported successful prepare, which is the
contractof 2PC.  HeuristicMixedException is about the manual resolution, typically by the DBA, using the DBMS-specific
toolor the standard commit()/rollback() API.
 

Mmm. The above seems as if saying that 2pc-comit does not interact
with remotes.  The interface contract does not cover everything that
happens in the real world. If remote-commit fails, that is just an
issue outside of the 2pc world.  In reality remote-commit may fail for
all reasons.

https://www.ibm.com/docs/ja/db2-for-zos/11?topic=support-example-distributed-transaction-that-uses-jta-methods

>      }      catch (javax.transaction.xa.XAException xae)
>      { // Distributed transaction failed, so roll it back.
>        // Report XAException on prepare/commit.

This suggests that both XAResoruce.prepare() and commit() can throw a
exception.

> > (I guess that
> > UserTransaction.commit() would throw RollbackException if
> > remote-prepare has been failed for any of the remotes.)
> 
> Correct.

So UserTransaction.commit() does not throw the same exception if
remote-commit fails.  Isn't the HeuristicMixedExcpetion the exception
thrown in that case?

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center