Re: Re: Is anything preventing us from allowing write to foreign tables from standby?

On Wed, Oct 18, 2017 at 5:44 AM, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
> On 18 October 2017 at 02:01, Alexander Korotkov
> <a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>> On Wed, Sep 6, 2017 at 4:42 PM, Alexander Korotkov
>> <a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>>>
>>> We're currently blocking writing queries on standby if even they are
>>> modifying contents of foreign tables. But do we have serious reasons for
>>> that?
>>> Keeping in the mind FDW-sharding, making FDW-tables writable from standby
>>> would be good to prevent single-master bottleneck.
>>> I wrote simple patch enabling writing to foreign tables from standbys. It
>>> works from the first glance for me.
>>
>>
>> No interest yet, but no objections too :-)
>> I'm going to add this to next commitfest.
>
> Superficially at least, it sounds like a good idea.
>
> We should only need a virtual xid when we're working with foreign
> tables since we don't do any local heap changes.
>
> How's it work with savepoints?
>

In a nearby thread, we are discussing about atomic commit of
transactions involving foreign transactions. For maintaining
consistency, atomicity of transactions writing to foreign server, we
will need to create local transactions. Will that be possible on
standby? Obviously, we could add a restriction that no consistency and
atomic commit is guaranteed when foreign servers are written from a
standby. I am not sure how easy would be to impose such a restriction
and whether such a restriction would be practical.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company