Re: Global snapshots

On 2020-09-08 05:49, Fujii Masao wrote:
> On 2020/09/05 3:31, Alexey Kondratov wrote:
>> 
>> Attached is a patch, which implements a plain 2PC in the postgres_fdw 
>> and adds a GUC 'postgres_fdw.use_twophase'. Also it solves these 
>> errors handling issues above and tries to add proper comments 
>> everywhere. I think, that 0003 should be rebased on the top of it, or 
>> it could be a first patch in the set, since it may be used 
>> independently. What do you think?
> 
> Thanks for the patch!
> 
> Sawada-san was proposing another 2PC patch at [1]. Do you have any 
> thoughts
> about pros and cons between your patch and Sawada-san's?
> 
> [1]
> https://www.postgresql.org/message-id/CA+fd4k4z6_B1ETEvQamwQhu4RX7XsrN5ORL7OhJ4B5B6sW-RgQ@mail.gmail.com

Thank you for the link!

After a quick look on the Sawada-san's patch set I think that there are 
two major differences:

1. There is a built-in foreign xacts resolver in the [1], which should 
be much more convenient from the end-user perspective. It involves huge 
in-core changes and additional complexity that is of course worth of.

However, it's still not clear for me that it is possible to resolve all 
foreign prepared xacts on the Postgres' own side with a 100% guarantee. 
Imagine a situation when the coordinator node is actually a HA cluster 
group (primary + sync + async replica) and it failed just after PREPARE 
stage of after local COMMIT. In that case all foreign xacts will be left 
in the prepared state. After failover process complete synchronous 
replica will become a new primary. Would it have all required info to 
properly resolve orphan prepared xacts?

Probably, this situation is handled properly in the [1], but I've not 
yet finished a thorough reading of the patch set, though it has a great 
doc!

On the other hand, previous 0003 and my proposed patch rely on either 
manual resolution of hung prepared xacts or usage of external 
monitor/resolver. This approach is much simpler from the in-core 
perspective, but doesn't look as complete as [1] though.

2. In the patch from this thread all 2PC logic sit in the postgres_fdw, 
while [1] tries to put it into the generic fdw core, which also feels 
like a more general and architecturally correct way. However, how many 
from the currently available dozens of various FDWs are capable to 
perform 2PC? And how many of them are maintained well enough to adopt 
this new API? This is not an argument against [1] actually, since 
postgres_fdw is known to be the most advanced FDW and an early adopter 
of new feature, just a little doubt about a usefulness of this 
preliminary generalisation.

Anyway, I think that [1] is a great work and really hope to find more 
time to investigate it deeper later this year.


Regards
-- 
Alexey Kondratov

Postgres Professional https://www.postgrespro.com
Russian Postgres Company