Re: Force streaming every change in logical decoding

On Tue, Dec 20, 2022 at 2:46 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> Dear hackers,
>
> > We have discussed three different ways to provide GUC for these
> > features. (1) Have separate GUCs like force_server_stream_mode,
> > force_server_serialize_mode, force_client_serialize_mode (we can use
> > different names for these) for each of these; (2) Have two sets of
> > GUCs for server and client. We can have logical_decoding_mode with
> > values as 'stream' and 'serialize' for the server and then
> > logical_apply_serialize = true/false for the client. (3) Have one GUC
> > like logical_replication_mode with values as 'server_stream',
> > 'server_serialize', 'client_serialize'.
>
> I also agreed for adding new GUC parameters (and I have already done partially
> in parallel apply[1]), and basically options 2 made sense for me. But is it OK
> that we can choose "serialize" mode even if subscribers require streaming?
>
> Currently the reorder buffer transactions are serialized on publisher only when
> the there are no streamable transaction. So what happen if the
> logical_decoding_mode = "serialize" but streaming option streaming is on? If we
> break the first one and serialize changes on publisher anyway, it may be not
> suitable for testing the normal operation.
>

I think the change will be streamed as soon as the next change is
processed even if we serialize based on this option. See
ReorderBufferProcessPartialChange. However, I see your point that when
the streaming option is given, the value 'serialize' for this GUC may
not make much sense.

> Therefore, I came up with the variant of (2): logical_decoding_mode can be
> "normal" or "immediate".
>
> "normal" is a default value, which is same as current HEAD. Changes are streamed
> or serialized when the buffered size exceeds logical_decoding_work_mem.
>
> When users set to "immediate", the walsenders starts to stream or serialize all
> changes. The choice is depends on the subscription option.
>

The other possibility to achieve what you are saying is that we allow
a minimum value of logical_decoding_work_mem as 0 which would mean
stream or serialize each change depending on whether the streaming
option is enabled. I think we normally don't allow a minimum value
below a certain threshold for other *_work_mem parameters (like
maintenance_work_mem, work_mem), so we have followed the same here.
And, I think it makes sense from the user's perspective because below
a certain threshold it will just add overhead by either writing small
changes to the disk or by sending those over the network. However, it
can be quite useful for testing/debugging. So, not sure, if we should
restrict setting logical_decoding_work_mem below a certain threshold.
What do you think?

-- 
With Regards,
Amit Kapila.