Re: Transactions involving multiple postgres foreign servers, take 2

On 2020/07/15 15:06, Masahiko Sawada wrote:
> On Tue, 14 Jul 2020 at 09:08, Masahiro Ikeda <ikedamsh(at)oss(dot)nttdata(dot)com> wrote:
>>
>>> I've attached the latest version patches. I've incorporated the review
>>> comments I got so far and improved locking strategy.
>>
>> Thanks for updating the patch!
>> I have three questions about the v23 patches.
>>
>>
>> 1. messages related to user canceling
>>
>> In my understanding, there are two messages
>> which can be output when a user cancels the COMMIT command.
>>
>> A. When prepare is failed, the output shows that
>> committed locally but some error is occurred.
>>
>> ```
>> postgres=*# COMMIT;
>> ^CCancel request sent
>> WARNING: canceling wait for resolving foreign transaction due to user
>> request
>> DETAIL: The transaction has already committed locally, but might not
>> have been committed on the foreign server.
>> ERROR: server closed the connection unexpectedly
>> This probably means the server terminated abnormally
>> before or while processing the request.
>> CONTEXT: remote SQL command: PREPARE TRANSACTION
>> 'fx_1020791818_519_16399_10'
>> ```
>>
>> B. When prepare is succeeded,
>> the output show that committed locally.
>>
>> ```
>> postgres=*# COMMIT;
>> ^CCancel request sent
>> WARNING: canceling wait for resolving foreign transaction due to user
>> request
>> DETAIL: The transaction has already committed locally, but might not
>> have been committed on the foreign server.
>> COMMIT
>> ```
>>
>> In case of A, I think that "committed locally" message can confuse user.
>> Because although messages show committed but the transaction is
>> "ABORTED".
>>
>> I think "committed" message means that "ABORT" is committed locally.
>> But is there a possibility of misunderstanding?
>
> No, you're right. I'll fix it in the next version patch.
>
> I think synchronous replication also has the same problem. It says
> "the transaction has already committed" but it's not true when
> executing ROLLBACK PREPARED.

Yes. Also the same message is logged when executing PREPARE TRANSACTION.
Maybe it should be changed to "the transaction has already prepared".

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION