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

On Friday, November 4, 2022 7:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Fri, Nov 4, 2022 at 1:36 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Nov 3, 2022 at 6:36 PM houzj.fnst@fujitsu.com
> > <houzj.fnst@fujitsu.com> wrote:
> > >
> > > Thanks for the analysis and summary !
> > >
> > > I tried to implement the above idea and here is the patch set.
> > >
> >
> > Few comments on v42-0001
> > ===========================
> >

Thanks for the comments.

> Few more comments on v42-0001
> ===============================
> 1. In parallel_apply_send_data(), it seems winfo->serialize_changes
> and switching_to_serialize are set to indicate that we have changed
> parallel to serialize mode. Isn't using just the
> switching_to_serialize sufficient? Also, it would be better to name
> switching_to_serialize as parallel_to_serialize or something like
> that.

I slightly change the logic to let serialize the message directly when timeout
instead of invoking apply_dispatch again so that we don't need the
switching_to_serialize.

> 
> 2. In parallel_apply_send_data(), the patch has already initialized
> the fileset, and then again in apply_handle_stream_start(), it will do
> the same if we fail while sending stream_start message to the parallel
> worker. It seems we don't need to initialize fileset again for
> TRANS_LEADER_PARTIAL_SERIALIZE state in apply_handle_stream_start()
> unless I am missing something.

Fixed.

> 3.
> apply_handle_stream_start(StringInfo s)
> {
> ...
> + if (!first_segment)
> + {
> + /*
> + * Unlock the shared object lock so that parallel apply worker
> + * can continue to receive and apply changes.
> + */
> + parallel_apply_unlock(winfo->shared->stream_lock_id);
> ...
> }
> 
> Can we have an assert before this unlock call that the lock must be
> held? Similarly, if there are other places then we can have assert
> there as well.

It seems we don't have a standard API can be used without a transaction.
Maybe we can use the list ParallelApplyLockids to check that ?

> 4. It is not very clear to me how maintaining ParallelApplyLockids
> list is helpful.

I will think about this and remove this in next version list if possible.

> 
> 5.
> /*
> + * Handle STREAM START message when the transaction was spilled to disk.
> + *
> + * Inintialize fileset if not yet and open the file.
> + */
> +void
> +serialize_stream_start(TransactionId xid, bool first_segment)
> +{
> + /*
> + * Start a transaction on stream start,
> 
> This function's name and comments seem to indicate that it is to
> handle stream_start message. Is that really the case? It is being
> called from parallel_apply_send_data() which made me think it can be
> used from other places as well.

Adjusted the comment.

Here is the new version patch set which addressed comments as of last Friday.
I also added some comments for the newly introduced codes in this version.

And thanks a lot for the comments that Sawada-san, Peter and Kuroda-san posted today.
I will handle them in next version soon.

Best regards,
Hou zj