Re: [HACKERS] some review comments on logical rep code

At Wed, 19 Apr 2017 10:59:00 +0200, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote in
<3ef9c831-0508-51a9-5ded-c2e31e958a9f@2ndquadrant.com>
> On 19/04/17 10:45, Kyotaro HORIGUCHI wrote:
> > At Wed, 19 Apr 2017 17:43:17 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote
in<20170419.174317.114509231.horiguchi.kyotaro@lab.ntt.co.jp>
 
> >> At Wed, 19 Apr 2017 10:33:29 +0200, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote in
<ed73a706-9e15-f137-2d55-f05361f2de9a@2ndquadrant.com>
> >> Some other process may modify it then go to there. With a kind of
> >> priority inversion, the process may modify the data on the memory
> >> *before* we modify it. Of course this is rather unrealistic,
> >> though.
> > 
> > A bit short.
> > 
> > Some other process may modify it *after* we read it then go to
> > there. With a kind of priority inversion, the process may modify
> > the data on the memory *before* we modify it. Of course this is
> > rather unrealistic, though.
> > 
> 
> Yeah but I think that's risk anyway in MVCC read committed database, the
> only way to protect against that would be to hold lock over the catalogs
> access which was what we wanted to get rid of :)

Agreed, or, I'm not sure about the real risks.

> BTW the whole sync coordination is explicitly written in a way that
> unless you mess with catalogs manually only the tablesync process should
> do UPDATEs to that catalog (with the exception of s->r state switch
> which can happen in apply and has no effect on sync because both states
> mean that synchronization is done, only makes apply stop tracking the
> table individually).

Hmm. Inhibiting retrospective updates of the on-memory data would
save from some kind of priority inversions but such kind of
manual operation is similar to overwriting of pg_control and I
think is not worth bothering about:p

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center