Re: The plan for FDW-based sharding

On Sat, Feb 27, 2016 at 2:29 AM, Konstantin Knizhnik
<k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
>> How do you prevent clock skew from causing serialization anomalies?
>
> If node receives message from "feature" it just needs to wait until this
> future arrive.
> Practically we just "adjust" system time in this case, moving it forward
> (certainly system time is not actually changed, we just set correction value
> which need to be added to system time).
> This approach was discussed in the article:
> http://research.microsoft.com/en-us/people/samehe/clocksi.srds2013.pdf
> I hope, in this article algorithm is explained much better than I can do
> here.

Hmm, the approach in that article is very interesting, but it sounds
different than what you are describing - they do not, AFAICT, have
anything like a "correction value".

> There are well know limitation of this pg_tsdtm which we will try to
> address in future.

How well known are those limitations? Are they documented somewhere?
Or are they only well-known to you?

> What we want is to include XTM API in PostgreSQL to be able to continue our
> experiments with different transaction managers and implementing multimaster
> on top of it (our first practical goal) without affecting PostgreSQL core.
>
> If XTM patch will be included in 9.6, then we can propose our multimaster as
> PostgreSQL extension and everybody can use it.
> Otherwise we have to propose our own fork of Postgres which significantly
> complicates using and maintaining it.

Well I still think what I said before is valid. If the code is good,
let it be a core submission. If it's not ready yet, submit it to core
when it is. If it can't be made good, forget it.

>> This seems rather defeatist. If the code is good and reliable, why
>> should it not be committed to core?
>
> Two reasons:
> 1. There is no ideal implementation of DTM which will fit all possible needs
> and be efficient for all clusters.

Hmm, what is the reasoning behind that statement? I mean, it is
certainly true that there are some places where we have decided that
one-size-fits-all is not the right approach. Indexing, for example.
But there are many other places where we have not chosen to make
things pluggable, and that I don't think it should be taken for
granted that plugability is always an advantage.

I fear that building a DTM that is fully reliable and also
well-performing is going to be really hard, and I think it would be
far better to have one such DTM that is 100% reliable than two or more
implementations each of which are 99% reliable.

> 2. Even if such implementation exists, still the right way of it integration
> is Postgres should use kind of TM API.

Sure, APIs are generally good, but that doesn't mean *this* API is good.

> I hope that everybody will agree that doing it in this way:
>
> #ifdef PGXC
> /* In Postgres-XC, stop timestamp has to follow the timeline of GTM
> */
> xlrec.xact_time = xactStopTimestamp + GTMdeltaTimestamp;
> #else
> xlrec.xact_time = xactStopTimestamp;
> #endif

PGXC chose that style in order to simplify merging. I wouldn't have
picked the same thing, but I don't know why it deserves scorn.

> or in this way:
>
> xlrec.xact_time = xactUseGTM ? xactStopTimestamp + GTMdeltaTimestamp
> : xactStopTimestamp;
>
> is very very bad idea.

I don't know why that is such a bad idea. It's a heck of a lot faster
than insisting on calling some out-of-line function. It might be a
bad idea, but I think we need to decide that, not assume it.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company