Re: Logical Replication of sequences

On Thu, 1 Aug 2024 at 04:25, Peter Smith <smithpb2250@gmail.com> wrote:
>
> Hi Vignesh,
>
> I noticed that when replicating sequences (using the latest patches
> 0730_2*)  the subscriber-side checks the *existence* of the sequence,
> but apparently it is not checking other sequence attributes.
>
> For example, consider:
>
> Publisher: "CREATE SEQUENCE s1 START 1 INCREMENT 2;" should be a
> sequence of only odd numbers.
> Subscriber: "CREATE SEQUENCE s1 START 2 INCREMENT 2;" should be a
> sequence of only even numbers.
>
> Because the names match, currently the patch allows replication of the
> s1 sequence. I think that might lead to unexpected results on the
> subscriber. IMO it might be safer to report ERROR unless the sequences
> match properly (i.e. not just a name check).
>
> Below is a demonstration the problem:
>
> ==========
> Publisher:
> ==========
>
> (publisher sequence is odd numbers)
>
> test_pub=# create sequence s1 start 1 increment 2;
> CREATE SEQUENCE
> test_pub=# select * from nextval('s1');
>  nextval
> ---------
>        1
> (1 row)
>
> test_pub=# select * from nextval('s1');
>  nextval
> ---------
>        3
> (1 row)
>
> test_pub=# select * from nextval('s1');
>  nextval
> ---------
>        5
> (1 row)
>
> test_pub=# CREATE PUBLICATION pub1 FOR ALL SEQUENCES;
> CREATE PUBLICATION
> test_pub=#
>
> ==========
> Subscriber:
> ==========
>
> (subscriber sequence is even numbers)
>
> test_sub=# create sequence s1 start 2 increment 2;
> CREATE SEQUENCE
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>        2
> (1 row)
>
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>        4
> (1 row)
>
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>        6
> (1 row)
>
> test_sub=# CREATE SUBSCRIPTION sub1 CONNECTION 'dbname=test_pub'
> PUBLICATION pub1;
> 2024-08-01 08:43:04.198 AEST [24325] WARNING:  subscriptions created
> by regression test cases should have names starting with "regress_"
> WARNING:  subscriptions created by regression test cases should have
> names starting with "regress_"
> NOTICE:  created replication slot "sub1" on publisher
> CREATE SUBSCRIPTION
> test_sub=# 2024-08-01 08:43:04.294 AEST [26240] LOG:  logical
> replication apply worker for subscription "sub1" has started
> 2024-08-01 08:43:04.309 AEST [26244] LOG:  logical replication
> sequence synchronization worker for subscription "sub1" has started
> 2024-08-01 08:43:04.323 AEST [26244] LOG:  logical replication
> synchronization for subscription "sub1", sequence "s1" has finished
> 2024-08-01 08:43:04.323 AEST [26244] LOG:  logical replication
> sequence synchronization worker for subscription "sub1" has finished
>
> (after the CREATE SUBSCRIPTION we are getting replicated odd values
> from the publisher, even though the subscriber side sequence was
> supposed to be even numbers)
>
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>        7
> (1 row)
>
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>        9
> (1 row)
>
> test_sub=# SELECT * FROM nextval('s1');
>  nextval
> ---------
>       11
> (1 row)
>
> (Looking at the description you would expect odd values for this
> sequence to be impossible)
>
> test_sub=# \dS+ s1
>                              Sequence "public.s1"
>   Type  | Start | Minimum |       Maximum       | Increment | Cycles? | Cache
> --------+-------+---------+---------------------+-----------+---------+-------
>  bigint |     2 |       1 | 9223372036854775807 |         2 | no      |     1

Even if we check the sequence definition during the CREATE
SUBSCRIPTION/ALTER SUBSCRIPTION ... REFRESH PUBLICATION or ALTER
SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES commands, there's still
a chance that the sequence definition might change after the command
has been executed. Currently, there's no mechanism to lock a sequence,
and we also permit replication of table data even if the table
structures differ, such as mismatched data types like int and
smallint. I have modified it to log a warning to inform users that the
sequence options on the publisher and subscriber are not the same and
advise them to ensure that the sequence definitions are consistent
between both.
The v20240805 version patch attached at [1] has the changes for the same.
[1] - https://www.postgresql.org/message-id/CALDaNm1Y_ot-jFRfmtwDuwmFrgSSYHjVuy28RspSopTtwzXy8w%40mail.gmail.com

Regards,
Vignesh