Re: Allowing pg_recvlogical to create temporary replication slots

On Sat, Nov 16, 2024 at 12:49:05PM +0100, Torsten Förtsch wrote:
> Thanks. Here is an updated version of the patch including documentation.

+ <para>
+ If used together with <option>--if-not-exists</option> and if a
+ permanent slot by the requested name exists, that slot is used instead
+ of creating a new one. That permanent slot is then not automatically
+ removed when the connection is dropped.
+ </para>

I'm wondering if this is the best behavior to have. Wouldn't it be
simpler to just error out if combining --temporary-slot and
--if-not-exists altogether? For example, this part of the
documentation is not true if --if-not-exists bumps on an existing
temporary slot, no?

Another thing that is confusing is the fact of always requiring
--create-slot when using the new option. Here is a simpler idea:
if using --temporary-slot, imply that --create-slot is set then use a
slot name based on the PID of the remote connection, like
pg_basebackup (see call of PQbackendPID() in pg_basebackup.c). If
--create-slot and --temporary-slot are both specified, error.

My point is, do we actually need to care about the name of the slot
for monitoring purposes as we know this is a slot that's going to be
thrown away on disconnect? Probably not, so we could just use a
pg_recvlogical_%u.

Enforcing a permanent slot if --slot is set may not be the best thing
to do, so I'd make things simpler and issue an error if attempting to
use --slot and --temporary-slot in this case. This makes for less
cases to think about when creating the slot once the connection to the
remote is done.

+ the slot will be recreated first. If at that time another slot by the
+ same name exists, creation will fail.

And concurrency issues like this one are very unlikely going to happen
once you do that.
--
Michael