Re: AtEOXact_ApplyLauncher() and subtransactions

On 15 June 2018 at 09:46, Peter Eisentraut
<peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> On 6/5/18 07:02, Amit Khandekar wrote:
>> I haven't written a patch for it, but I think we should have a
>> separate on_commit_stop_workers for eachyou get subtransaction. At
>> subtransaction commit, we replace the on_commit_stop_workers list of
>> the parent subtransaction with the one from the committed
>> subtransaction; and on abort, discard the list of the current
>> subtransaction. So have a stack of the lists.
>
> Is there maybe a more general mechanism we could attach this to? Maybe
> resource owners?

You mean using something like ResourceOwnerRelease() ? We need to
merge the on_commit_stop_workers list into the parent transaction's
list. So we can't release the whole list on commit.

The way I am implementing this can be seen in attached
apply_launcher_subtrans_WIP.patch. (check launcher.c changes). I
haven't started testing it yet though.
on_commit_stop_workers denotes a list of subscriptions, and each
element also contains a list of relations for that subscription.
This list is built by ALTER-SUBSCRIPTION commands that have run in the
current (sub)transaction.

At sub-transaction start, the on_commit_stop_workers is pushed into a
stack. Then on_commit_stop_workers starts with empty list. This list
is then populated by ALTER-SUBCSRIPTION commands of the current
sub-transaction.

At sub-transaction commit, this list is merged into the list of parent
subtransaction, the parent transaction stack element is popped out,
and the merged list becomes the new on_commit_stop_workers.
So say, parent has sub1(tab1, tab2), sub2(tab2, tab3), where sub means
subscription.
and current on_commit_workers has sub2(tab4) and sub3(tab1)
At commit, for subscription sub2, we should replace the outer
sub2(tab2, tab3) with the newer sub2(tab4).
So, the merged list will have :
sub1(tab1, tab2), sub2(tab4), sub3(tab1)

At sub-transaction abort, the on_commit_stop_workers is discarded. The
parent transaction worker list is assigned back to
on_commit_stop_workers.

>
>> Subscription mysub is set to synchronise tables tx1 and tx2 :
>> CREATE SUBSCRIPTION mysub ... PUBLICATION pubx;
>>
>> Now suppose the subscription is altered to synchronise ty1 and ty2,
>> and then again altered back to synchronise tx1 and tx2 in the same
>> transaction.
>> begin;
>> ALTER SUBSCRIPTION mysub set publication puby;
>> ALTER SUBSCRIPTION mysub set publication pubx;
>> commit;
>>
>> Here, workers for tx1 and tx2 are added to on_commit_stop_workers
>> after the publication is set to puby. And then workers for ty1 and ty2
>> are further added to that list after the 2nd ALTER command where
>> publication is set to pubx, because the earlier ALTER command has
>> already changed the catalogs to denote that ty1 and ty2 are being
>> synchronised. Effectively, at commit, all the workers are targetted to
>> be stopped, when actually at commit time there is no final change in
>> the tables to be synchronised.
>
> I'm not so bothered about this scenario. When you drop and then
> recreate a table in the same transaction, that doesn't mean you keep the
> data that was previously in the table. If you want to *undo* a change,
> you need to do rollback, not commit further changes that try to recreate
> the previous state.

May be the example I gave is not practical. But a user can as well do
something like :

CREATE SUBSCRIPTION mysub ... PUBLICATION pub1;
...
begin;
ALTER SUBSCRIPTION mysub set publication pub2;
....
ALTER SUBSCRIPTION mysub set publication pub3;
commit;
So pub3 is yet another publication. So here the old one is not set back again.

Or may there can be ALTER SUBSCRIPTION REFRESH.

I believe on_commit_stop_workers was designed keeping in mind that all
actions are to be done only at commit; we should not immediately stop
workers. So it implies that the workers it determines in the end of
commit, also should be accurate. I have anyways included the changes
for this in the same attached patch (changes are in
subscriptioncmds.c)

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company