Thread: [HACKERS] DROP SUBSCRIPTION and ROLLBACK
Hi all, While testing logical replciation I found that if the transaction issued DROP SUBSCRIPTION rollbacks then the logical repliation stops and the subscription can never be removed later. The document says that the replication worker associated with the subscription will not stop until after the transaction that issued this command has committed but it doesn't work. The cause of this is that DropSubscription stops the apply worker and drops corresponding replication slot on publisher side without waiting for commit or rollback. The launcher process launches the apply worker again but the launched worker will fail to start logical replication because corresponding replication slot is already removed. And the orphan subscription can not be removed later. I think the logical replication should not stop and the corresponding replication slot and replication origin should not be removed until the transaction commits. The solution for this I came up with is that the launcher process stops the apply worker after DROP SUBSCRIPTION is committed rather than DropSubscription does. And the apply worker drops replication slot and replication origin before exits. Attached draft patch fixes this issue. Please give me feedback. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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On Tue, Feb 7, 2017 at 9:10 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> Hi all,
>
> While testing logical replciation I found that if the transaction
> issued DROP SUBSCRIPTION rollbacks then the logical repliation stops
> and the subscription can never be removed later. The document says
> that the replication worker associated with the subscription will not
> stop until after the transaction that issued this command has
> committed but it doesn't work.
Yeah, this is a bug.
ISTM that CREATE SUBSCRIPTION also has the similar issue. It creates
the replication slot on the publisher side before the transaction has been
committed. Even if the transaction is rollbacked, that replication slot is
not removed. That is, in a transaction block, we should not connect to
the publisher. Instead, the launcher or worker should do.
> The cause of this is that DropSubscription stops the apply worker and
> drops corresponding replication slot on publisher side without waiting
> for commit or rollback. The launcher process launches the apply worker
> again but the launched worker will fail to start logical replication
> because corresponding replication slot is already removed. And the
> orphan subscription can not be removed later.
>
> I think the logical replication should not stop and the corresponding
> replication slot and replication origin should not be removed until
> the transaction commits.
Yes.
> The solution for this I came up with is that the launcher process
> stops the apply worker after DROP SUBSCRIPTION is committed rather
> than DropSubscription does. And the apply worker drops replication
> slot and replication origin before exits. Attached draft patch fixes
> this issue.
>
> Please give me feedback.
The patch failed to apply to HEAD.
+ worker = logicalrep_worker_find(subid);
+ if (worker) {
- heap_close(rel, NoLock);
- return;
+ if (stmt->drop_slot)
+ worker->drop_slot = true;
+ worker->need_to_stop = true;
"drop_slot" and "need_to_stop" seem to be set to true even if the transaction
is rollbacked. This would cause the same problem that you're trying to fix.
I think that we should make the launcher periodically checks pg_subscription
and stop the worker if there is no its corresponding subscription. Then,
if necessary, the worker should remove its replication slot from the publisher.
Regards,
--
Fujii Masao
On 07/02/17 13:10, Masahiko Sawada wrote: > Hi all, > > While testing logical replciation I found that if the transaction > issued DROP SUBSCRIPTION rollbacks then the logical repliation stops > and the subscription can never be removed later. The document says > that the replication worker associated with the subscription will not > stop until after the transaction that issued this command has > committed but it doesn't work. Ah then the docs are wrong and should be fixed. Maybe we should not allow DROP SUBSCRIPTION inside transaction similarly to CREATE INDEX CONCURRENTLY. > The cause of this is that DropSubscription stops the apply worker and > drops corresponding replication slot on publisher side without waiting > for commit or rollback. The launcher process launches the apply worker > again but the launched worker will fail to start logical replication > because corresponding replication slot is already removed. And the > orphan subscription can not be removed later. > > I think the logical replication should not stop and the corresponding > replication slot and replication origin should not be removed until > the transaction commits. > > The solution for this I came up with is that the launcher process > stops the apply worker after DROP SUBSCRIPTION is committed rather > than DropSubscription does. And the apply worker drops replication > slot and replication origin before exits. Attached draft patch fixes > this issue. > I don't think we can allow the slot drop to be postponed. There is too many failure scenarios where we would leave the remote slot in the database and that's not acceptable IMHO. 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? -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
On 07/02/17 16:26, Petr Jelinek wrote: > On 07/02/17 13:10, Masahiko Sawada wrote: >> I think the logical replication should not stop and the corresponding >> replication slot and replication origin should not be removed until >> the transaction commits. >> >> The solution for this I came up with is that the launcher process >> stops the apply worker after DROP SUBSCRIPTION is committed rather >> than DropSubscription does. And the apply worker drops replication >> slot and replication origin before exits. Attached draft patch fixes >> this issue. >> > > I don't think we can allow the slot drop to be postponed. There is too > many failure scenarios where we would leave the remote slot in the > database and that's not acceptable IMHO. > > 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? > Not to mention that slot creation/drop is not transactional by itself so even if there was some way to tie remote transaction to local transaction (like say 2pc), it would still not work with ROLLBACK. -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
On Wed, Feb 8, 2017 at 12:26 AM, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote: > On 07/02/17 13:10, Masahiko Sawada wrote: >> Hi all, >> >> While testing logical replciation I found that if the transaction >> issued DROP SUBSCRIPTION rollbacks then the logical repliation stops >> and the subscription can never be removed later. The document says >> that the replication worker associated with the subscription will not >> stop until after the transaction that issued this command has >> committed but it doesn't work. > > Ah then the docs are wrong and should be fixed. Maybe we should not > allow DROP SUBSCRIPTION inside transaction similarly to CREATE INDEX > CONCURRENTLY. > >> The cause of this is that DropSubscription stops the apply worker and >> drops corresponding replication slot on publisher side without waiting >> for commit or rollback. The launcher process launches the apply worker >> again but the launched worker will fail to start logical replication >> because corresponding replication slot is already removed. And the >> orphan subscription can not be removed later. >> >> I think the logical replication should not stop and the corresponding >> replication slot and replication origin should not be removed until >> the transaction commits. >> >> The solution for this I came up with is that the launcher process >> stops the apply worker after DROP SUBSCRIPTION is committed rather >> than DropSubscription does. And the apply worker drops replication >> slot and replication origin before exits. Attached draft patch fixes >> this issue. >> > > I don't think we can allow the slot drop to be postponed. There is too > many failure scenarios where we would leave the remote slot in the > database and that's not acceptable IMHO. > > 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. Also DROP SUBSCRIPTION should call CommitTransactionCommand() just after removing the entry from pg_subscription, then connect to the publisher and remove the replication slot. Regards, -- Fujii Masao
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. > 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. -- Michael
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. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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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? -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
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. Thought? Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
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. Note that we do give users options to not create and not drop slots if they wish so we should really treat slot related failures as command errors. I don't want subscription setup to be 20 step where you have to deal with various things on multiple servers when it can be just plain simple one command on each side in many cases. -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
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. > I basically agree this option, but why do we need to change CREATE SUBSCRIPTION as well? Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
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. Regards, -- Fujii Masao
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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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On 15/02/17 06:43, Masahiko Sawada wrote: > 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. Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote: > On 15/02/17 06:43, Masahiko Sawada wrote: >> 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. > > Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. > Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached fixed version patch. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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On Thu, Feb 16, 2017 at 11:40 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: > On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek > <petr.jelinek@2ndquadrant.com> wrote: >> On 15/02/17 06:43, Masahiko Sawada wrote: >>> 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. >> >> Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. >> > > Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached > fixed version patch. We should disallow CREATE/DROP SUBSCRIPTION inside a user transaction block only when CREATE/DROP SLOT option is set? + /* + * We cannot run CREATE SUBSCRIPTION inside a user transaction + * block. + */ + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION"); I think that more comments about why the command is disallowed inside a user transaction block are necessary. For example, ---------------------- Disallow CREATE SUBSCRIPTION [CREATE SLOT] inside a user transaction block. When CREATE SLOT is set, this command creates the replication slot on the remote server. This operation is not transactional. So, if the transaction is rollbacked, the created replication slot unexpectedly remains while there is no corresponding entry in pg_subscription. To reduce the possibility of this problem, we allow CREATE SLOT option only outside a user transaction block. XXX Note that this restriction cannot completely prevent "orphan" replication slots. The transaction of CREATE SUBSCRIPTION can still fail after creating the replication slot on the remote server, though it's basically less likely to happen. ---------------------- + * We cannot run DROP SUBSCRIPTION inside a user transaction + * block. + */ + PreventTransactionChain(isTopLevel, "DROP SUBSCRIPTION"); Same as above. Regards, -- Fujii Masao
On Tue, Feb 21, 2017 at 3:42 AM, Fujii Masao <masao.fujii@gmail.com> wrote: > On Thu, Feb 16, 2017 at 11:40 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >> On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek >> <petr.jelinek@2ndquadrant.com> wrote: >>> On 15/02/17 06:43, Masahiko Sawada wrote: >>>> 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. >>> >>> Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. >>> >> >> Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached >> fixed version patch. > > We should disallow CREATE/DROP SUBSCRIPTION inside a user transaction > block only when CREATE/DROP SLOT option is set? > > + /* > + * We cannot run CREATE SUBSCRIPTION inside a user transaction > + * block. > + */ > + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION"); > > I think that more comments about why the command is disallowed inside > a user transaction block are necessary. For example, I agree with you. > > ---------------------- > Disallow CREATE SUBSCRIPTION [CREATE SLOT] inside a user transaction block. > > When CREATE SLOT is set, this command creates the replication slot on > the remote server. This operation is not transactional. So, if the transaction > is rollbacked, the created replication slot unexpectedly remains while > there is no corresponding entry in pg_subscription. To reduce the possibility > of this problem, we allow CREATE SLOT option only outside a user transaction > block. > > XXX Note that this restriction cannot completely prevent "orphan" replication > slots. The transaction of CREATE SUBSCRIPTION can still fail after creating > the replication slot on the remote server, though it's basically less likely > to happen. > ---------------------- > > + * We cannot run DROP SUBSCRIPTION inside a user transaction > + * block. > + */ > + PreventTransactionChain(isTopLevel, "DROP SUBSCRIPTION"); > > Same as above. While writing regression test for this issue, I found an another bug of DROP SUBSCRIPTION; DROP SUBSCRIPTION with DROP SLOT option fails to parse because DROP is a keyword, not IDENT. Attached 000 patch fixes it, and 001 patches fixes the original issue on this thread. Please review these. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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On Tue, Feb 21, 2017 at 7:52 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: > On Tue, Feb 21, 2017 at 3:42 AM, Fujii Masao <masao.fujii@gmail.com> wrote: >> On Thu, Feb 16, 2017 at 11:40 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >>> On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek >>> <petr.jelinek@2ndquadrant.com> wrote: >>>> On 15/02/17 06:43, Masahiko Sawada wrote: >>>>> 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. >>>> >>>> Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. >>>> >>> >>> Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached >>> fixed version patch. >> >> We should disallow CREATE/DROP SUBSCRIPTION inside a user transaction >> block only when CREATE/DROP SLOT option is set? >> >> + /* >> + * We cannot run CREATE SUBSCRIPTION inside a user transaction >> + * block. >> + */ >> + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION"); >> >> I think that more comments about why the command is disallowed inside >> a user transaction block are necessary. For example, > > I agree with you. > >> >> ---------------------- >> Disallow CREATE SUBSCRIPTION [CREATE SLOT] inside a user transaction block. >> >> When CREATE SLOT is set, this command creates the replication slot on >> the remote server. This operation is not transactional. So, if the transaction >> is rollbacked, the created replication slot unexpectedly remains while >> there is no corresponding entry in pg_subscription. To reduce the possibility >> of this problem, we allow CREATE SLOT option only outside a user transaction >> block. >> >> XXX Note that this restriction cannot completely prevent "orphan" replication >> slots. The transaction of CREATE SUBSCRIPTION can still fail after creating >> the replication slot on the remote server, though it's basically less likely >> to happen. >> ---------------------- >> >> + * We cannot run DROP SUBSCRIPTION inside a user transaction >> + * block. >> + */ >> + PreventTransactionChain(isTopLevel, "DROP SUBSCRIPTION"); >> >> Same as above. > > While writing regression test for this issue, I found an another bug > of DROP SUBSCRIPTION; DROP SUBSCRIPTION with DROP SLOT option fails to > parse because DROP is a keyword, not IDENT. Good catch! > Attached 000 patch fixes it Or we should change the syntax of DROP SUBSCRIPTION as follows, and handle the options in the same way as the options like "CREATE SLOT" in CREATE/ALTER SUBSCRIPTION? In CREATE/ALTER commands, the options are specified with WITH clause. But only DROP command doesn't accept WITH clause. This looks inconsistent. ---------------------- DROP SUBSCRIPTION [ IF EXISTS ] name [ WITH (option [, ... ]) ] where option can be: | DROP SLOT | NODROP SLOT ---------------------- Regards, -- Fujii Masao
On Wed, Feb 22, 2017 at 1:52 AM, Fujii Masao <masao.fujii@gmail.com> wrote: > On Tue, Feb 21, 2017 at 7:52 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >> On Tue, Feb 21, 2017 at 3:42 AM, Fujii Masao <masao.fujii@gmail.com> wrote: >>> On Thu, Feb 16, 2017 at 11:40 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >>>> On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek >>>> <petr.jelinek@2ndquadrant.com> wrote: >>>>> On 15/02/17 06:43, Masahiko Sawada wrote: >>>>>> 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. >>>>> >>>>> Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. >>>>> >>>> >>>> Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached >>>> fixed version patch. >>> >>> We should disallow CREATE/DROP SUBSCRIPTION inside a user transaction >>> block only when CREATE/DROP SLOT option is set? >>> >>> + /* >>> + * We cannot run CREATE SUBSCRIPTION inside a user transaction >>> + * block. >>> + */ >>> + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION"); >>> >>> I think that more comments about why the command is disallowed inside >>> a user transaction block are necessary. For example, >> >> I agree with you. >> >>> >>> ---------------------- >>> Disallow CREATE SUBSCRIPTION [CREATE SLOT] inside a user transaction block. >>> >>> When CREATE SLOT is set, this command creates the replication slot on >>> the remote server. This operation is not transactional. So, if the transaction >>> is rollbacked, the created replication slot unexpectedly remains while >>> there is no corresponding entry in pg_subscription. To reduce the possibility >>> of this problem, we allow CREATE SLOT option only outside a user transaction >>> block. >>> >>> XXX Note that this restriction cannot completely prevent "orphan" replication >>> slots. The transaction of CREATE SUBSCRIPTION can still fail after creating >>> the replication slot on the remote server, though it's basically less likely >>> to happen. >>> ---------------------- >>> >>> + * We cannot run DROP SUBSCRIPTION inside a user transaction >>> + * block. >>> + */ >>> + PreventTransactionChain(isTopLevel, "DROP SUBSCRIPTION"); >>> >>> Same as above. >> >> While writing regression test for this issue, I found an another bug >> of DROP SUBSCRIPTION; DROP SUBSCRIPTION with DROP SLOT option fails to >> parse because DROP is a keyword, not IDENT. > > Good catch! > >> Attached 000 patch fixes it > > Or we should change the syntax of DROP SUBSCRIPTION as follows, and > handle the options in the same way as the options like "CREATE SLOT" in > CREATE/ALTER SUBSCRIPTION? In CREATE/ALTER commands, the options > are specified with WITH clause. But only DROP command doesn't accept > WITH clause. This looks inconsistent. +1 for adding WITH clause to DROP SUBSCRIPTION option. That way we can check conflicting or redundant options easier. Will update patches. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
On Wed, Feb 22, 2017 at 2:17 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: > On Wed, Feb 22, 2017 at 1:52 AM, Fujii Masao <masao.fujii@gmail.com> wrote: >> On Tue, Feb 21, 2017 at 7:52 PM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >>> On Tue, Feb 21, 2017 at 3:42 AM, Fujii Masao <masao.fujii@gmail.com> wrote: >>>> On Thu, Feb 16, 2017 at 11:40 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote: >>>>> On Thu, Feb 16, 2017 at 7:52 AM, Petr Jelinek >>>>> <petr.jelinek@2ndquadrant.com> wrote: >>>>>> On 15/02/17 06:43, Masahiko Sawada wrote: >>>>>>> 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. >>>>>> >>>>>> Why? ALTER SUBSCRIPTION does not create/drop anything on remote server ever. >>>>>> >>>>> >>>>> Hmm you're right. ALTER SUBSCRIPTION can support transaction. Attached >>>>> fixed version patch. >>>> >>>> We should disallow CREATE/DROP SUBSCRIPTION inside a user transaction >>>> block only when CREATE/DROP SLOT option is set? >>>> >>>> + /* >>>> + * We cannot run CREATE SUBSCRIPTION inside a user transaction >>>> + * block. >>>> + */ >>>> + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION"); >>>> >>>> I think that more comments about why the command is disallowed inside >>>> a user transaction block are necessary. For example, >>> >>> I agree with you. >>> >>>> >>>> ---------------------- >>>> Disallow CREATE SUBSCRIPTION [CREATE SLOT] inside a user transaction block. >>>> >>>> When CREATE SLOT is set, this command creates the replication slot on >>>> the remote server. This operation is not transactional. So, if the transaction >>>> is rollbacked, the created replication slot unexpectedly remains while >>>> there is no corresponding entry in pg_subscription. To reduce the possibility >>>> of this problem, we allow CREATE SLOT option only outside a user transaction >>>> block. >>>> >>>> XXX Note that this restriction cannot completely prevent "orphan" replication >>>> slots. The transaction of CREATE SUBSCRIPTION can still fail after creating >>>> the replication slot on the remote server, though it's basically less likely >>>> to happen. >>>> ---------------------- >>>> >>>> + * We cannot run DROP SUBSCRIPTION inside a user transaction >>>> + * block. >>>> + */ >>>> + PreventTransactionChain(isTopLevel, "DROP SUBSCRIPTION"); >>>> >>>> Same as above. >>> >>> While writing regression test for this issue, I found an another bug >>> of DROP SUBSCRIPTION; DROP SUBSCRIPTION with DROP SLOT option fails to >>> parse because DROP is a keyword, not IDENT. >> >> Good catch! >> >>> Attached 000 patch fixes it >> >> Or we should change the syntax of DROP SUBSCRIPTION as follows, and >> handle the options in the same way as the options like "CREATE SLOT" in >> CREATE/ALTER SUBSCRIPTION? In CREATE/ALTER commands, the options >> are specified with WITH clause. But only DROP command doesn't accept >> WITH clause. This looks inconsistent. > > +1 for adding WITH clause to DROP SUBSCRIPTION option. That way we can > check conflicting or redundant options easier. Will update patches. Attached updated version patches. Please review these. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
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On 23/02/17 08:24, Masahiko Sawada wrote: > Attached updated version patches. Please review these. > This version looks good to me, I'd only change the > + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION CREATE SLOT"); to "CREATE SUBSCRIPTION ... CREATE SLOT" as that's afaik how we do it for other commands (and same with DROP). -- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
On 3/3/17 13:58, Petr Jelinek wrote: > On 23/02/17 08:24, Masahiko Sawada wrote: >> Attached updated version patches. Please review these. >> > > This version looks good to me, I'd only change the > >> + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION CREATE SLOT"); > > to "CREATE SUBSCRIPTION ... CREATE SLOT" as that's afaik how we do it > for other commands (and same with DROP). I have committed fixes for these issues. I didn't like the syntax change in DROP SUBSCRIPTION, so I have just fixed the parsing of the existing syntax. We can discuss syntax changes separately. The second patch I have committed after some editing. I think it was generated on top of the existing data copy patch, so it was a bit of a mess. -- Peter Eisentraut http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
On Sat, Mar 4, 2017 at 1:46 PM, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote: > On 3/3/17 13:58, Petr Jelinek wrote: >> On 23/02/17 08:24, Masahiko Sawada wrote: >>> Attached updated version patches. Please review these. >>> >> >> This version looks good to me, I'd only change the >> >>> + PreventTransactionChain(isTopLevel, "CREATE SUBSCRIPTION CREATE SLOT"); >> >> to "CREATE SUBSCRIPTION ... CREATE SLOT" as that's afaik how we do it >> for other commands (and same with DROP). > > I have committed fixes for these issues. Thanks! > > I didn't like the syntax change in DROP SUBSCRIPTION, so I have just > fixed the parsing of the existing syntax. We can discuss syntax changes > separately. Understood. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center