Re: [PATCH] Reuse Workers and Replication Slots during Logical Replication

On Wed, Feb 22, 2023 at 8:04 AM Melih Mutlu <m(dot)melihmutlu(at)gmail(dot)com> wrote:
>
> Hi Wang,
>
> Thanks for reviewing.
> Please see updated patches. [1]

This is cool! Thanks for working on this.
I had a chance to review your patchset and I had some thoughts and
questions.

I notice that you've added a new user-facing option to make a snapshot.
I think functionality to independently make a snapshot for use elsewhere
has been discussed in the past for the implementation of different
features (e.g. [1] pg_dump but they ended up using replication slots for
this I think?), but I'm not quite sure I understand all the implications
for providing a user-visible create snapshot command. Where can it be
used? When can the snapshot be used? In your patch's case, you know that
you can use the snapshot you are creating, but I just wonder if any
restrictions or caveats need be taken for its general use.

For the worker reuse portion of the code, could it be a separate patch
in the set? It could be independently committable and would be easier to
review (separate from repl slot reuse).

Given table sync worker reuse, I think it is worth considering a more
explicit structure for the table sync worker code now -- i.e. having a
TableSyncWorkerMain() function. Though they still do the
LogicalRepApplyLoop(), much of what else they do is different than the
apply leader.

Apply worker leader does:

ApplyWorkerMain()
walrcv_startstreaming()
LogicalRepApplyLoop()
launch table sync workers
walrcv_endstreaming()
proc_exit()

Table Sync workers master:

ApplyWorkerMain()
start_table_sync()
walrcv_create_slot()
copy_table()
walrcv_startstreaming()
start_apply()
LogicalRepApplyLoop()
walrcv_endstreaming()
proc_exit()

Now table sync workers need to loop back and do start_table_sync() again
for their new table.
You have done this in ApplyWorkerMain(). But I think that this could be
a separate main function since their main loop is effectively totally
different now than an apply worker leader.

Something like:

TableSyncWorkerMain()
TableSyncWorkerLoop()
start_table_sync()
walrcv_startstreaming()
LogicalRepApplyLoop()
walrcv_endstreaming()
wait_for_new_rel_assignment()
proc_exit()

You mainly have this structure, but it is a bit hidden and some of the
shared functions that previously may have made sense for table sync
worker and apply workers to share don't really make sense to share
anymore.

The main thing that table sync workers and apply workers share is the
logic in LogicalRepApplyLoop() (table sync workers use when they do
catchup), so perhaps we should make the other code separate?

Also on the topic of worker reuse, I was wondering if having workers
find their own next table assignment (as you have done in
process_syncing_tables_for_sync()) makes sense.

The way the whole system would work now (with your patch applied), as I
understand it, the apply leader would loop through the subscription rel
states and launch workers up to max_sync_workers_per_subscription for
every candidate table needing sync. The apply leader will continue to do
this, even though none of those workers would exit unless they die
unexpectedly. So, once it reaches max_sync_workers_per_subscription, it
won't launch any more workers.

When one of these sync workers is finished with a table (it is synced
and caught up), it will search through the subscription rel states
itself looking for a candidate table to work on.

It seems it would be common for workers to be looking through the
subscription rel states at the same time, and I don't really see how you
prevent races in who is claiming a relation to work on. Though you take
a shared lock on the LogicalRepWorkerLock, what if in between
logicalrep_worker_find() and updating my MyLogicalRepWorker->relid,
someone else also updates their relid to that relid. I don't think you
can update LogicalRepWorker->relid with only a shared lock.

I wonder if it is not better to have the apply leader, in
process_syncing_tables_for_apply(), first check for an existing worker
for the rel, then check for an available worker without an assignment,
then launch a worker?

Workers could then sleep after finishing their assignment and wait for
the leader to give them a new assignment.

Given an exclusive lock on LogicalRepWorkerLock, it may be okay for
workers to find their own table assignments from the subscriptionrel --
and perhaps this will be much more efficient from a CPU perspective. It
feels just a bit weird to have the code doing that buried in
process_syncing_tables_for_sync(). It seems like it should at least
return out to a main table sync worker loop in which workers loop
through finding a table and assigning it to themselves, syncing the
table, and catching the table up.

- Melanie

[1] https://www.postgresql.org/message-id/flat/CA%2BU5nMLRjGtpskUkYSzZOEYZ_8OMc02k%2BO6FDi4una3mB4rS1w%40mail.gmail.com#45692f75a1e79d4ce2d4f6a0e3ccb853