Re: Perform streaming logical transactions by background workers and parallel apply - Mailing list pgsql-hackers

On Thu, Oct 6, 2022 at 10:38 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, Sep 30, 2022 at 1:56 PM Peter Smith <smithpb2250@gmail.com> wrote:
> >
> > Here are my review comments for the v35-0001 patch:
> >
> > ======
> >
> > 3. GENERAL
> >
> > (this comment was written after I wrote all the other ones below so
> > there might be some unintended overlaps...)
> >
> > I found the mixed use of the same member names having different
> > meanings to be quite confusing.
> >
> > e.g.1
> > PGOutputData 'streaming' is now a single char internal representation
> > the subscription parameter streaming mode ('f','t','p')
> > - bool streaming;
> > + char streaming;
> >
> > e.g.2
> > WalRcvStreamOptions 'streaming' is a C string version of the
> > subscription streaming mode ("on", "parallel")
> > - bool streaming; /* Streaming of large transactions */
> > + char    *streaming; /* Streaming of large transactions */
> >
> > e.g.3
> > SubOpts 'streaming' is again like the first example - a single char
> > for the mode.
> > - bool streaming;
> > + char streaming;
> >
> >
> > IMO everything would become much simpler if you did:
> >
> > 3a.
> > Rename "char streaming;" -> "char streaming_mode;"
> >
> > 3b.
> > Re-designed the "char *streaming;" code to also use the single char
> > notation, then also call that member 'streaming_mode'. Then everything
> > will be consistent.
> >
>
> Won't this impact the previous version publisher which already uses
> on/off? We may need to maintain multiple values which would be
> confusing.
>

I only meant that the *internal* struct member names mentioned could
change - not anything exposed as user-visible parameter names or
column names etc. Or were you referring to it as causing unnecessary
troubles for back-patching? Anyway, the main point of this review
comment was #3b. Unless I am mistaken, there is no reason why that one
cannot be changed to use 'char' instead of 'char *', for consistency
across all the same named members.

> >
> > 9. - parallel_apply_can_start
> >
> > +/*
> > + * Returns true, if it is allowed to start a parallel apply worker, false,
> > + * otherwise.
> > + */
> > +static bool
> > +parallel_apply_can_start(TransactionId xid)
> >
> > (The commas are strange)
> >
> > SUGGESTION
> > Returns true if it is OK to start a parallel apply worker, false otherwise.
> >
>
> +1 for this.
> >
> > 28. - logicalrep_worker_detach
> >
> > + /* Stop the parallel apply workers. */
> > + if (am_leader_apply_worker())
> > + {
> >
> > Should that comment rather say like below?
> >
> > /* If this is the leader apply worker then stop all of its parallel
> > apply workers. */
> >
>
> I think this would be just saying what is apparent from the code, so
> not sure if it is an improvement.
>
> >
> > 38. - apply_handle_commit_prepared
> >
> > + *
> > + * Note that we don't need to wait here if the transaction was prepared in a
> > + * parallel apply worker. Because we have already waited for the prepare to
> > + * finish in apply_handle_stream_prepare() which will ensure all the operations
> > + * in that transaction have happened in the subscriber and no concurrent
> > + * transaction can create deadlock or transaction dependency issues.
> >   */
> >  static void
> >  apply_handle_commit_prepared(StringInfo s)
> >
> > "worker. Because" -> "worker because"
> >
>
> I think this will make this line too long. Can we think of breaking it
> in some way?

OK, how about below:

Note that we don't need to wait here if the transaction was prepared
in a parallel apply worker. In that case, we have already waited for
the prepare to finish in apply_handle_stream_prepare() which will
ensure all the operations in that transaction have happened in the
subscriber, so no concurrent transaction can cause deadlock or
transaction dependency issues.

>
> >
> > 43.
> >
> >   /*
> > - * Initialize the worker's stream_fileset if we haven't yet. This will be
> > - * used for the entire duration of the worker so create it in a permanent
> > - * context. We create this on the very first streaming message from any
> > - * transaction and then use it for this and other streaming transactions.
> > - * Now, we could create a fileset at the start of the worker as well but
> > - * then we won't be sure that it will ever be used.
> > + * For the first stream start, check if there is any free parallel apply
> > + * worker we can use to process this transaction.
> >   */
> > - if (MyLogicalRepWorker->stream_fileset == NULL)
> > + if (first_segment)
> > + parallel_apply_start_worker(stream_xid);
> >
> > This comment update seems misleading. The
> > parallel_apply_start_worker() isn't just checking if there is a free
> > worker. All that free worker logic stuff is *inside* the
> > parallel_apply_start_worker() function, so maybe no need to mention
> > about it here at the caller.
> >
>
> It will be good to have some comments here instead of completely removing it.
>
> >
> > 39. - apply_handle_stream_abort
> >
> > + /* We receive abort information only when we can apply in parallel. */
> > + if (MyLogicalRepWorker->parallel_apply)
> > + read_abort_info = true;
> >
> > 44a.
> > SUGGESTION
> > We receive abort information only when the publisher can support parallel apply.
> >
>
> The existing comment seems better to me in this case.
>
> >
> > 55. - LogicalRepWorker
> >
> > + /* Indicates whether apply can be performed parallelly. */
> > + bool parallel_apply;
> > +
> >
> > 55a.
> > "parallelly" - ?? is there a better way to phrase this? IMO that is an
> > uncommon word.
> >
>
> How about ".. can be performed in parallel."?
>
> > ~
> >
> > 55b.
> > IMO this member name should be named slightly different to give a
> > better feel for what it really means.
> >
> > Maybe something like one of:
> > "parallel_apply_ok"
> > "parallel_apply_enabled"
> > "use_parallel_apply"
> > etc?
> >
>
> The extra word doesn't seem to be useful here.
>
> > 58.
> >
> > --- fail - streaming must be boolean
> > +-- fail - streaming must be boolean or 'parallel'
> >  CREATE SUBSCRIPTION regress_testsub CONNECTION
> > 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect =
> > false, streaming = foo);
> >
> > I think there are tests already for explicitly create/set the
> > subscription parameter streaming = on/off/parallel
> >
> > But what about when there is no value explicitly specified? Shouldn't
> > there also be tests like below to check that *implied* boolean true
> > still works for this enum?
> >
> > CREATE SUBSCRIPTION ... WITH (streaming)
> > ALTER SUBSCRIPTION ... SET (streaming)
> >
>
> I think before adding new tests for this, please check if we have any
> similar tests for other boolean options.

IMO this one is a bit different because it's not really a boolean
option anymore - it's a kind of a hybrid boolean/enum. That's why I
thought this ought to be tested regardless if there are existing tests
for the (normal) boolean options.

------
Kind Regards,
Peter Smith.
Fujitsu Australia