Re: question regarding copyData containers

Hello,

thank you for your feedback.

I agree that modifying the COPY subprotocols is hard to do because it would
have an impact on the client ecosystem.

My understanding (which seems to be confirmed by what Tom Lane said) is
that the server discards the framing and
manages to make sense of the underlying data.

> the expectation is that clients can send CopyData messages that are
> split up however they choose; the message boundaries needn't correspond
> to any semantic boundaries in the data stream.

So I thought that a client could decide to have the same behavior and could
start parsing the payload of a copyData message without assembling it first.
It works perfectly with COPY TO but I hit a roadblock on copyBoth during
logical replication with test_decoding because the subprotocol doesn't have
any framing.

> Right now all 'w' messages should be contained in one CopyData/'d' that
> doesn't contain anything but the XLogData/'w'.

The current format of the XLogData/'w' message is
w lsn lsn time byten

and even if it is maybe too late now I was wondering why it was not decided
to be
w lsn lsn time n byten

because it seems to me that the missing n ties the XLogData to the copyData
framing.

>The input data exists in a linear
>buffer already, so you're not going to reduce peak memory usage by
>sending smaller CopyData chunks.

That is very surprising to me. Do you mean that on the server in COPY TO
mode, a full row is prepared in a linear buffer in memory before
beeing sent as a copyData/d'
I found the code around
https://github.com/postgres/postgres/blob/master/src/backend/commands/copy.c#L2153
and
indeed the whole row seems to be buffered in memory.

Good thing or bad thing, users tend to use bigger fields (text, jsonb,
bytea) and that can be very memory hungry.
Do you know a case in postgres (other than large_objects I suppose) where
the server can flush data from a field without buffering it in memory ?

And then as you noted, there is the multiplexing of events. a very long
copyData makes the communication impossible between the client and the
server during the transfer.

I briefly looked at
https://github.com/postgres/postgres/blob/master/src/backend/replication/walsender.c
and
I found

/*
* Maximum data payload in a WAL data message. Must be >= XLOG_BLCKSZ.
*
* We don't have a good idea of what a good value would be; there's some
* overhead per message in both walsender and walreceiver, but on the other
* hand sending large batches makes walsender less responsive to signals
* because signals are checked only between messages. 128kB (with
* default 8k blocks) seems like a reasonable guess for now.
*/
#define MAX_SEND_SIZE (XLOG_BLCKSZ * 16)
so I thought that the maximum copyData/d' I would receive during logical
replication was MAX_SEND_SIZE but it seems that this is not used for
logical decoding.
the whole output of the output plugin seem to be prepared in memory so for
an insert like

insert into mytable (col) values (repeat('-', pow(2, 27)::int)

a 128MB linear buffer will be created on the server and sent as 1 copyData
over many network chunks.

So I understand that in the long term copyData framing should not carry any
semantic to be able to keep messages small enough to allow multiplexing but
that there are many steps to climb before that.

Would it make sense one day in some way to try and do streaming at the
sub-field level ? I guess that is a huge undertaking since most of the
field unit interfaces are probably based on a buffer/field one-to-one
mapping.

Greetings,
Jérôme

On Thu, Jun 4, 2020 at 12:08 AM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2020-06-03 19:28:12 +0200, Jerome Wagner wrote:
> > I have been working on a node.js streaming client for different COPY
> > scenarios.
> > usually, during CopyOut, clients tend to buffer network chunks until they
> > have gathered a full copyData message and pass that to the user.
> >
> > In some cases, this can lead to very large copyData messages. when there
> > are very long text fields or bytea fields it will require a lot of memory
> > to be handled (up to 1GB I think in the worst case scenario)
> >
> > In COPY TO, I managed to relax that requirement, considering that
> copyData
> > is simply a transparent container. For each network chunk, the relevent
> > message content is forwarded which makes for 64KB chunks at most.
>
> Uhm.
>
>
> > We loose the semantics of the "row" that copyData has according to the
> > documentation
> > https://www.postgresql.org/docs/10/protocol-flow.html#PROTOCOL-COPY
> > >The backend sends a CopyOutResponse message to the frontend, followed by
> > zero or more >CopyData messages (**always one per row**), followed by
> > CopyDone
> >
> > but it is not a problem because the raw bytes are still parsable (rows +
> > fields) in text mode (tsv) and in binary mode)
>
> This seems like an extremely bad idea to me. Are we really going to ask
> clients to incur the overhead (both in complexity and runtime) to parse
> incoming data just to detect row boundaries? Given the number of
> options there are for COPY, that's a seriously complicated task.
>
> I think that's a completely no-go.
>
>
> Leaving error handling aside (see para below), what does this actually
> get you? Either your client cares about getting a row in one sequential
> chunk, or it doesn't. If it doesn't care, then there's no need to
> allocate a buffer that can contain the whole 'd' message. You can just
> hand the clients the chunks incrementally. If it does, then you need to
> reassemble either way (or worse, you force to reimplement the client to
> reimplement that).
>
> I assume what you're trying to get at is being able to send CopyData
> messages before an entire row is assembled? And you want to send
> separate CopyData messages to allow for error handling? I think that's
> a quite worthwhile goal, but I don't think it can sensibly solved by
> just removing protocol level framing of row boundaries. And that will
> mean evolving the protocol in a non-compatible way.
>
>
> > Now I started working on copyBoth and logical decoding scenarios. In this
> > case, the server send series of copyData. 1 copyData containing 1
> message :
> >
> > at the network chunk level, in the case of large fields, we can observe
> >
> > in: CopyData Int32 XLogData Int64 Int64 Int64 Byten1
> > in: Byten2
> > in: CopyData Int32 XLogData Int64 Int64 Int64 Byten3
> > in: CopyData Int32 XLogData Int64 Int64 Int64 Byten4
> >
> > out: XLogData Int64 Int64 Int64 Byten1
> > out: Byten2
> > out: XLogData Int64 Int64 Int64 Byten3
> > out: XLogData Int64 Int64 Int64 Byten4
>
> > but at the XLogData level, the protocol is not self-describing its
> length,
>
> > so there is no real way of knowing where the first XLogData ends apart
> from
> > - knowing the length of the first copyData (4 + 1 + 3*8 + n1 + n2)
> > - knowing the internals of the output plugin and benefit from a plugin
> > that self-describe its span
> > when a network chunks contains several copyDatas
> > in: CopyData Int32 XLogData Int64 Int64 Int64 Byten1 CopyData Int32
> > XLogData Int64 Int64 Int64 Byten2
> > we have
> > out: XLogData Int64 Int64 Int64 Byten1 XLogData Int64 Int64 Int64
> Byten2
>
> Right now all 'w' messages should be contained in one CopyData/'d' that
> doesn't contain anything but the XLogData/'w'.
>
> Do you just mean that if we'd change the server side code to split 'w'
> messages across multiple 'd' messages, then we couldn't make much sense
> of the data anymore? If so, then I don't really see a problem. Unless
> you do a much larger change, what'd be the point in allowing to split
> 'w' across multiple 'd' chunks? The input data exists in a linear
> buffer already, so you're not going to reduce peak memory usage by
> sending smaller CopyData chunks.
>
> Sure, we could evolve the logical decoding interface to output to be
> able to send data in a much more incremental way than, typically,
> per-row basis. But I think that'd quite substantially increase
> complexity. And the message framing seems to be the easier part of such
> a change.
>
> Greetings,
>
> Andres Freund
>