Re: Logical replication from PG v13 and below to PG v14 (devel version) is not working. - Mailing list pgsql-hackers

On Thu, Sep 24, 2020 at 5:31 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Thu, Sep 24, 2020 at 5:11 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > On Thu, Sep 24, 2020 at 4:45 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > >
> > > Have you checked what will function walrcv_server_version() will
> > > return? I was thinking that if we know that subscriber is connected to
> > > Publisher version < 14 then we can send the right value.
> >
> > But, suppose we know the publisher version is < 14 and user has set
> > streaming on while  creating subscriber then still we will send the
> > right version?
> >
>
> Yeah we can send the version depending on which server we are talking to?

Ok

> >  I think tablesync we are forming a query so we are
> > forming as per the publisher's version.  But here we are sending which
> > protocol version we are expecting for the data transfer so I feel it
> > should be LOGICALREP_PROTO_VERSION_NUM if we expect non-streaming
> > transfer and LOGICALREP_PROTO_STREAM_VERSION_NUM if we expect the
> > streaming transfer.
> >
>
> I am not sure if this is the right strategy. See
> libpqrcv_startstreaming, even if the user asked for streaming unless
> the server supports it we don't send the streaming option to the user.
> Another thing is this check will become more complicated if we need
> another feature like decode prepared to send different version or even
> if it is the same version, we might need additional checks. Why do you
> want to send a streaming protocol version when we know the server
> doesn't support it, lets behave similar to what we do in
> libpqrcv_startstreaming.

Okay, so even if the streaming is enabled we are sending the streaming
on to the server versions which are >= 14 which make sense.  But I
still doubt that it is right thing to send the protocol version based
on the server version.   For example suppose in future PG20 we change
the streaming protocol version to 3,  that mean from PG14 to PG20 we
may not support the streaming transmission but we still be able to
support the normal transamission.  Now if streaming is off then
ideally we should send the LOGICALREP_PROTO_VERSION_NUM and that is
still supporetd by the publisher but if we send the
LOGICALREP_PROTO_STREAM_VERSION_NUM then it will error out because the
streaming protocol changed in the latest subscriber and the same is
not supported by the older publisher (14).  So now if we want non
streaming transmission from 14 to 20 and that should be allowed but if
based on the server version we always send the
LOGICALREP_PROTO_STREAM_VERSION_NUM then that will be a problem
because our streaming version has changed.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com