Re: DROP SUBSCRIPTION and ROLLBACK

On Mon, Feb 13, 2017 at 4:05 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> On Sat, Feb 11, 2017 at 8:18 PM, Petr Jelinek
> <petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>> On 10/02/17 19:55, Masahiko Sawada wrote:
>>> On Thu, Feb 9, 2017 at 12:44 AM, Petr Jelinek
>>> <petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>>>> On 08/02/17 07:40, Masahiko Sawada wrote:
>>>>> On Wed, Feb 8, 2017 at 9:01 AM, Michael Paquier
>>>>> <michael(dot)paquier(at)gmail(dot)com> wrote:
>>>>>> On Wed, Feb 8, 2017 at 1:30 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>>>>>>> On Wed, Feb 8, 2017 at 12:26 AM, Petr Jelinek
>>>>>>> <petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>>>>>>>> For example what happens if apply crashes during the DROP
>>>>>>>> SUBSCRIPTION/COMMIT and is not started because the delete from catalog
>>>>>>>> is now visible so the subscription is no longer there?
>>>>>>>
>>>>>>> Another idea is to treat DROP SUBSCRIPTION in the same way as VACUUM, i.e.,
>>>>>>> make it emit an error if it's executed within user's transaction block.
>>>>>>
>>>>>> It seems to me that this is exactly Petr's point: using
>>>>>> PreventTransactionChain() to prevent things to happen.
>>>>>
>>>>> Agreed. It's better to prevent to be executed inside user transaction
>>>>> block. And I understood there is too many failure scenarios we need to
>>>>> handle.
>>>>>
>>>>>>> Also DROP SUBSCRIPTION should call CommitTransactionCommand() just
>>>>>>> after removing the entry from pg_subscription, then connect to the publisher
>>>>>>> and remove the replication slot.
>>>>>>
>>>>>> For consistency that may be important.
>>>>>
>>>>> Agreed.
>>>>>
>>>>> Attached patch, please give me feedback.
>>>>>
>>>>
>>>> This looks good (and similar to what initial patch had btw). Works fine
>>>> for me as well.
>>>>
>>>> Remaining issue is, what to do about CREATE SUBSCRIPTION then, there are
>>>> similar failure scenarios there, should we prevent it from running
>>>> inside transaction as well?
>>>>
>>>
>>> Hmm, after thought I suspect current discussing approach. For
>>> example, please image the case where CRAETE SUBSCRIPTION creates
>>> subscription successfully but fails to create replication slot for
>>> whatever reason, and then DROP SUBSCRIPTION drops the subscription but
>>> dropping replication slot is failed. In such case, CREAET SUBSCRIPTION
>>> and DROP SUBSCRIPTION return ERROR but the subscription is created and
>>> dropped successfully. I think that this behaviour confuse the user.
>>>
>>> I think we should just prevent calling DROP SUBSCRIPTION in user's
>>> transaction block. Or I guess that it could be better to separate the
>>> starting/stopping logical replication from subscription management.
>>>
>>
>> We need to stop the replication worker(s) in order to be able to drop
>> the slot. There is no such issue with startup of the worker as that one
>> is launched by launcher after the transaction has committed.
>>
>> IMO best option is to just don't allow DROP/CREATE SUBSCRIPTION inside a
>> transaction block and don't do any commits inside of those (so that
>> there are no rollbacks, which solves your initial issue I believe). That
>> way failure to create/drop slot will result in subscription not being
>> created/dropped which is what we want.

On second thought, +1.

> I basically agree this option, but why do we need to change CREATE
> SUBSCRIPTION as well?

Because the window between the creation of replication slot and the transaction
commit of CREATE SUBSCRIPTION should be short. Otherwise, if any error happens
during that window, the replication slot unexpectedly remains while there is no
corresponding subscription. Of course, even If we prevent CREATE SUBSCRIPTION
from being executed within user's transaction block, there is still such
window. But we can reduce the possibility of that problem.

Regards,

--
Fujii Masao