Re: Transactions involving multiple postgres foreign servers

On Mon, Oct 30, 2017 at 5:48 PM, Ashutosh Bapat
<ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> On Thu, Oct 26, 2017 at 7:41 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>
>> Because I don't want to break the current user semantics. that is,
>> currently it's guaranteed that the subsequent reads can see the
>> committed result of previous writes even if the previous transactions
>> were distributed transactions. And it's ensured by writer side. If we
>> can make the reader side ensure it, the backend process don't need to
>> wait for the resolver process.
>>
>> The waiting backend process are released by resolver process after the
>> resolver process tried to resolve foreign transactions. Even if
>> resolver process failed to either connect to foreign server or to
>> resolve foreign transaction the backend process will be released and
>> the foreign transactions are leaved as dangling transaction in that
>> case, which are processed later. Also if resolver process takes a long
>> time to resolve foreign transactions for whatever reason the user can
>> cancel it by Ctl-c anytime.
>>
>
> So, there's no guarantee that the next command issued from the
> connection *will* see the committed data, since the foreign
> transaction might not have committed because of a network glitch
> (say). If we go this route of making backends wait for resolver to
> resolve the foreign transaction, we will have add complexity to make
> sure that the waiting backends are woken up in problematic events like
> crash of the resolver process OR if the resolver process hangs in a
> connection to a foreign server etc. I am not sure that the complexity
> is worth the half-guarantee.
>

Hmm, maybe I was wrong. I now think that the waiting backends can be
woken up only in following cases;
- The resolver process succeeded to resolve all foreign transactions.
- The user did the cancel (e.g. ctl-c).
- The resolver process failed to resolve foreign transaction for a
reason of there is no such prepared transaction on foreign server.

In other cases the resolver process should not release the waiters.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center