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

On Monday, November 7, 2022 9:19 PM houzj.fnst@fujitsu.com <houzj.fnst@fujitsu.com> wrote:
> 
> 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.
>

Sorry, I posted the wrong patch for V43 which lack some changes.
Attach the correct patch set here.

Best regards,
Hou zj