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

[Konstantin Osipov <kostja@tarantool.org>]

* 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.

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. 

> 
>   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.
> 
> Needed for #2129
> ---
>  src/box/vinyl.c | 128 +++++++++++++++++++++++++++-----------------------------
>  1 file changed, 62 insertions(+), 66 deletions(-)
> 

-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov