Re: [HACKERS] DROP SUBSCRIPTION and ROLLBACK

On Tue, Feb 14, 2017 at 1:13 AM, Fujii Masao <masao.fujii@gmail.com> wrote:
> On Mon, Feb 13, 2017 at 4:05 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>> On Sat, Feb 11, 2017 at 8:18 PM, Petr Jelinek
>> <petr.jelinek@2ndquadrant.com> wrote:
>>> On 10/02/17 19:55, Masahiko Sawada wrote:
>>>> On Thu, Feb 9, 2017 at 12:44 AM, Petr Jelinek
>>>> <petr.jelinek@2ndquadrant.com> wrote:
>>>>> On 08/02/17 07:40, Masahiko Sawada wrote:
>>>>>> On Wed, Feb 8, 2017 at 9:01 AM, Michael Paquier
>>>>>> <michael.paquier@gmail.com> wrote:
>>>>>>> On Wed, Feb 8, 2017 at 1:30 AM, Fujii Masao <masao.fujii@gmail.com> wrote:
>>>>>>>> On Wed, Feb 8, 2017 at 12:26 AM, Petr Jelinek
>>>>>>>> <petr.jelinek@2ndquadrant.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.

Thank you for the explanation. I understood and agree.

I think we should disallow to call ALTER SUBSCRIPTION inside a user's
transaction block as well.
Attached patch changes these three DDLs so that they cannot be called
inside a user's transaction block.

Regards,

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

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