Re: [RFC PATCH 08/23] vinyl: check key uniqueness before modifying tx write set

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Tue, Jul 31, 2018 at 11:34:10PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev@gmail.com> [18/07/08 22:52]:
> 
> > Currently, we handle INSERT/REPLACE/UPDATE requests by iterating over
> > all space indexes starting from the primary and inserting the
> > corresponding statements to tx write set, checking key uniqueness if
> > necessary. This means that by the time we write a REPLACE to the write
> > set of a secondary index, it has already been written to the primary
> > index write set. This is OK, and vy_tx_prepare() relies on that to
> > implement the common memory level. However, this also means that when we
> > check uniqueness of a secondary index, the new REPLACE can be found via
> > the primary index. This is OK now, because all indexes are fully
> > independent, but it isn't going to fly after #2129 is implemented. The
> > problem is in order to check if a tuple is present in a secondary index,
> > we will have to look up the corresponding full tuple in the primary
> > index. To illustrate the problem, consider the following situation:
> 
> I don't understand how this patch works. You need to put the key
> into transaction write set first, and yield second. If there is a
> change which happens during yield made on behalf of the unique
> check, it must be able to see the keys your transaction is reading
> and abort it.

It will. We abort transactions on conflicts in the read set.
Write set has nothing to do with transaction conflict resolution.

> 
> Besides, I don't understand how the order of checks is making any
> difference in your example - until a transaction is committed it
> is not present in the common memory level anyway. 

It isn't, but it will see its own write set, which is inconsistent
between the primary and secondary indexes.

> 
> > 
> >   Primary index covers field 1.
> >   Secondary index covers field 2.
> > 
> >   Committed statements:
> > 
> >     REPLACE{1, 10, lsn=1} - present in both indexes
> >     DELETE{1, lsn=2} - present only in the primary index
> > 
> >   Transaction:
> > 
> >     REPLACE{1, 10}
> > 
> > When we check uniqueness of the secondary index, we find committed
> > statement REPLACE{1, 10, lsn=1}, then look up the corresponding full
> > tuple in the primary index and find REPLACE{1, 10}. Since the two tuples
> > match, we mistakenly assume that there's a conflict.
> > 
> > To avoid a situation like that, let's check uniqueness before modifying
> > the write set of any index.