Re: Global snapshots

On Mon, 13 Jul 2020 at 20:18, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jul 10, 2020 at 8:46 AM Masahiko Sawada
> <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
> >
> > On Wed, 8 Jul 2020 at 21:35, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > >
> > > Cool. While studying, if you can try to think whether this approach is
> > > different from the global coordinator based approach then it would be
> > > great. Here is my initial thought apart from other reasons the global
> > > coordinator based design can help us to do the global transaction
> > > management and snapshots. It can allocate xids for each transaction
> > > and then collect the list of running xacts (or CSN) from each node and
> > > then prepare a global snapshot that can be used to perform any
> > > transaction. OTOH, in the design proposed in this patch, we don't need any
> > > coordinator to manage transactions and snapshots because each node's
> > > current CSN will be sufficient for snapshot and visibility as
> > > explained above.
> >
> > Yeah, my thought is the same as you. Since both approaches have strong
> > points and weak points I cannot mention which is a better approach,
> > but that 2PC patch would go well together with the design proposed in
> > this patch.
> >
>
> I also think with some modifications we might be able to integrate
> your 2PC patch with the patches proposed here. However, if we decide
> not to pursue this approach then it is uncertain whether your proposed
> patch can be further enhanced for global visibility.

Yes. I think even if we decide not to pursue this approach it's not
the reason for not pursuing the 2PC patch. if so we would need to
consider the design of 2PC patch again so it generically resolves the
atomic commit problem.

> Does it make
> sense to dig the design of this approach a bit further so that we can
> be somewhat more sure that pursuing your 2PC patch would be a good
> idea and we can, in fact, enhance it later for global visibility?

Agreed.

> AFAICS, Andrey has mentioned couple of problems with this approach
> [1], the details of which I am also not sure at this stage but if we
> can dig those it would be really great.
>
> > > Now, sure this assumes that there is no clock skew
> > > on different nodes or somehow we take care of the same (Note that in
> > > the proposed patch the CSN is a timestamp.).
> >
> > As far as I read Clock-SI paper, we take care of the clock skew by
> > putting some waits on the transaction start and reading tuples on the
> > remote node.
> >
>
> Oh, but I am not sure if this patch is able to solve that, and if so, how?

I'm not sure the details but, as far as I read the patch I guess the
transaction will sleep at GlobalSnapshotSync() when the received
global csn is greater than the local global csn.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
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