Re: Synchronizing slots from primary to standby

On Wed, Dec 27, 2023 at 11:36 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Tue, Dec 26, 2023 at 9:27 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > I would like to revisit the current dependency of slotsync worker on
> > dbname used in 002 patch. Currently we accept dbname in
> > primary_conninfo and thus the user has to make sure to provide one (by
> > manually altering it) even in case of a conf file auto-generated by
> > "pg_basebackup -R".
> > Thus I would like to discuss if there are better ways to do it.
> > Complete background is as follow:
> >
> > We need dbname for 2 purposes:
> >
> > 1) to connect to remote db in order to run SELECT queries to fetch the
> > info needed by slotsync worker.
> > 2) to make connection in slot-sync worker itself in order to be able
> > to use libpq APIs for 1)
> >
> > We run 3 kind of select queries in slot-sync worker currently:
> >
> > a) To fetch all failover slots (logical slots) info at once in
> > synchronize_slots().
> > b) To fetch a particular slot info during
> > wait_for_primary_slot_catchup() logic (logical slot).
> > c) To validate primary slot (physical one) and also to distinguish
> > between standby and cascading standby by running pg_is_in_recovery().
> >
> >  1) One approach to avoid dependency on dbname is using commands
> > instead of SELECT.  This will need implementing LIST_SLOTS command for
> > a), and for b) we can use LIST_SLOTS and fetch everything (even though
> > it is not needed) or have LIST_SLOTS with a filter on slot-name or
> > extend READ_REPLICATION_SLOT,  and for c) we can have some other
> > command to get pg_is_in_recovery() info. But, I feel by relying on
> > commands we will be making the extension of the slot-sync feature
> > difficult. In future, if there is some more requirement to fetch any
> > other info,
> > then there too we have to implement a command. I am not sure if it is
> > good and extensible approach.
> >
> > 2) Another way to avoid asking for a dbname in primary_conninfo is to
> > use the default dbname internally. This brings us to two questions:
> > 'How' and 'Which default db'?
> >
> > 2.1) To answer 'How':
> > Using default dbname is simpler for the purpose of slot-sync worker
> > having its own db-connection, but is a little tricky for the purpose
> > of connection to remote_db. This is because we have to inject this
> > dbname internally in our connection-info.
> >
> > 2.1.1) Say we use primary_conninfo (i.e. original one w/o dbname),
> > then currently it could have 2 formats:
> >
> > a) The simple "=" format for key-value pairs, example:
> > 'user=replication host=127.0.0.1 port=5433 dbname=postgres'.
> > b) URI format, example:
> > postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp
> >
> > We can distinguish between the 2 formats using 'uri_prefix_length' but
> > injecting the dbname part will be messy specially for URI format.  If
> > we want to do it w/o injecting and only by changing libpq interfaces
> > to accept dbname separately apart from conninfo, then there is no
> > current simpler way available. It will need a good amount of changes
> > in libpq.
> >
> > 2.1.2) Another way is to not rely on primary_conninfo directly but
> > rely on 'WalRcv->conninfo' in order to connect to remote_db. This is
> > because the latter is never URI format, it is some parsed format and
> > appending may work. As an example, primary_conninfo =
> > 'postgresql://replication@localhost:5433', WalRcv->conninfo loaded
> > internally is:
> > "user=replication passfile=/home/shveta/.pgpass channel_binding=prefer
> > dbname=replication host=localhost port=5433
> > fallback_application_name=walreceiver sslmode=prefer sslcompression=0
> > sslcertmode=allow sslsni=1 ssl_min_protocol_version=TLSv1.2
> > gssencmode=disable krbsrvname=postgres gssdelegation=0
> > target_session_attrs=any load_balance_hosts=disable", '\000'
> >
> > So we can try appending our default dbname to this. But all the
> > defaults loaded in WalRcv->conninfo need some careful analysis to
> > figure out if they work for slot-sync worker case.
> >
> > 2.2) Now coming to 'Which default db':
> >
> > 2.2.1) If we use 'template1' as default db, it may block 'create db'
> > operations on primary for the time when the slot-sync worker is
> > connected to remote using this dbname. Example:
> >
> > postgres=# create database newdb1;
> > ERROR:  source database "template1" is being accessed by other users
> > DETAIL:  There is 1 other session using the database.
> >
> > 2.2.2) If we use 'postgres' as default db, there are chances that it
> > can be dropped as unlike 'template1', it is allowed to be dropped by
> > user, and if slotsync worker is connected to it, user may see:
> > newdb1=# drop database postgres;
> > ERROR:  database "postgres" is being accessed by other users
> > DETAIL:  There is 1 other session using the database.
> >
> > But once the slot-sync worker or standby goes down, user can always
> > drop this and next time slot-sync worker may not be able to come up.
> >
>
> Other random ideas for discussion are:
>
> 3) The slotsync worker uses primary_conninfo but also uses a new GUC
> parameter, say slot_sync_dbname, to specify the database to connect.
> The slot_sync_dbname overwrites the dbname if primary_conninfo also
> specifies it. If both don't have a dbname, raise an error.
>

Would the users prefer to provide a value for a separate GUC instead
of changing primary_conninfo? It is possible that we can have some
users prefer to use one GUC and others prefer a separate GUC but we
should add a new GUC if we are sure that is what users would prefer.
Also, even if have to consider this option, I think we can easily
later add a new GUC to provide a dbname in addition to having the
provision of giving it in primary_conninfo.

Also, I think having a separate GUC for dbanme has some complexity in
terms of appending the dbname to primary_conninfo as pointed out by
Shveta.

> 4) The slotsync worker uses a new GUC parameter, say
> slot_sync_conninfo, to specify the connection string to the primary
> aside from primary_conninfo. And pg_basebackup -R generates
> slot_sync_conninfo as well if required (new option required).
>

Yeah, this is worth considering but won't slot_sync_conninfo be mostly
a duplicate of primary_conninfo apart from dbname? I am not sure if
the benefit outweighs the disadvantage of having mostly similar
information in two GUCs.

--
With Regards,
Amit Kapila.