Re: Perform streaming logical transactions by background workers and parallel apply

On Wed, Dec 7, 2022 at 8:28 AM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> Besides, I fixed a bug where there could still be messages left in memory
> queue and the PA has started to apply spooled message.
>

Few comments on the recent changes in the patch:
========================================
1. It seems you need to set FS_SERIALIZE_DONE in
stream_prepare/commit/abort. They are still directly setting the state
as READY. Am, I missing something or you forgot to change it?

2.
case TRANS_PARALLEL_APPLY:
pa_stream_abort(&abort_data);

+ /*
+ * Reset the stream_fd after aborting the toplevel transaction in
+ * case the parallel apply worker is applying spooled messages
+ */
+ if (toplevel_xact)
+ stream_fd = NULL;

I think we can keep the handling of stream file the same in
abort/commit/prepare code path.

3. It is already pointed out by Peter that it is better to add some
comments in pa_spooled_messages() function that we won't be
immediately able to apply changes after the lock is released, it will
be done in the next cycle.

4. Shall we rename FS_SERIALIZE as FS_SERIALIZE_IN_PROGRESS? That will
appear consistent with FS_SERIALIZE_DONE.

5. Comment improvements:
diff --git a/src/backend/replication/logical/worker.c
b/src/backend/replication/logical/worker.c
index b26d587ae4..921d973863 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -1934,8 +1934,7 @@ apply_handle_stream_abort(StringInfo s)
}

/*
- * Check if the passed fileno and offset are the last fileno and position of
- * the fileset, and report an ERROR if not.
+ * Ensure that the passed location is fileset's end.
*/
static void
ensure_last_message(FileSet *stream_fileset, TransactionId xid, int fileno,
@@ -2084,9 +2083,9 @@ apply_spooled_messages(FileSet *stream_fileset,
TransactionId xid,
nchanges++;

/*
- * Break the loop if stream_fd is set to NULL which
means the parallel
- * apply worker has finished applying the transaction.
The parallel
- * apply worker should have closed the file before committing.
+ * It is possible the file has been closed because we
have processed
+ * some transaction end message like stream_commit in
which case that
+ * must be the last message.
*/

--
With Regards,
Amit Kapila.