Re: [PATCH 6/6] Replace schema lock with fine-grained locking

[Konstantin Osipov <kostja@tarantool.org>]

* Vladimir Davydov <vdavydov.dev@gmail.com> [19/07/03 23:00]:
> TBO I don't think you follow. It isn't a lock, actually. It's just a
> flag saying the space is busy building an index. If someone tries to do
> something with a space that is busy, they will fail. This is consistent
> with vinyl tx manager behavior. No deadlock is possible by design.

It's a kind of locking called optimistic locking. Any lock can be acquired in
optimistic mode (_trylock()) and in pessimistic mode. Once you
have normal locks you can trylock them ad nausea, the only
question is whether users will like it. Imagine I have a DDL
script which I use to update/upgrade my data on production. Do I really
want to add re-tries to this DDL script now that I know it worked
in my test environment? Most users don't. 

With transactional data dictionary multiple metadata locks are
around the corner. 

Imagine this:

box.begin()
box.space.foo:rename('tmp')
box.space.bar:rename('foo')
box.space.foo:rename('bar')
box.commit()

the same can will soon be possible to do in SQL as well. 

all these locks will have to stay around till commit, so leave
through WAL yield.

In other words, we either need to switch the data dictionary to
MVCC or to use locks, or better yet, have both mechanisms
available so that we can use whichever we need when we need it.


-- 
Konstantin Osipov, Moscow, Russia