DROP SUBSCRIPTION and ROLLBACK

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

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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

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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

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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

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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

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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

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

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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

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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

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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

--
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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

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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

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

--
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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

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

--
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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

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

--
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To make changes to your subscription:
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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

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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