Re: Transactions involving multiple postgres foreign servers

Hi,

On 2016/10/04 13:26, Ashutosh Bapat wrote:
>>>
>>> Why always rollback any dangling transaction? There can be a case that
>>> a foreign server has a dangling transaction which needs to be
>>> committed because the portions of that transaction on the other shards
>>> are committed.
>>
>> Right, we can heuristically make a decision whether we do COMMIT or
>> ABORT on local server.
>> For example, if COMMIT PREPARED succeeded on at least one foreign
>> server, the local server return OK to client and the other dangling
>> transactions should be committed later.
>> We can find out that we should do either commit or abort the dangling
>> transaction by checking CLOG.
>
> Heuristics can not become the default behavior. A user should be given
> an option to choose a heuristic, and he should be aware of the
> pitfalls when using this heuristic. I guess, first, we need to get a
> solution which ensures that the transaction gets committed on all the
> servers or is rolled back on all the foreign servers involved. AFAIR,
> my patch did that. Once we have that kind of solution, we can think
> about heuristics.

I wonder if Sawada-san is referring to some sort of quorum-based (atomic)
commitment protocol [1, 2], although I agree that that would be an
advanced technique for handling the limitations such as blocking nature of
the basic two-phase commit protocol in case of communication failures,
IOW, meant for better availability rather than correctness.

Thanks,
Amit

[1]
https://en.wikipedia.org/wiki/Quorum_(distributed_computing)#Quorum-based_voting_in_commit_protocols

[2] http://hub.hku.hk/bitstream/10722/158032/1/Content.pdf