[PATCH 6/6] Introduce blackhole engine

[Konstantin Osipov]

* Vladimir Davydov <vdavydov.dev at gmail.com> [18/07/20 20:45]:
> Blackhole is a very simple engine that allows to create spaces that may
> written to, but not read from. It only supports INSERT/REPLACE requests.
> It doesn't support any indexes hence SELECT is impossible. It does check
> space format though and supports on_replace and before_replace triggers.

Allowing DELETE requests could be practical in the future.
> 
> The whole purpose of this new engine is writing arbitrary rows to WAL
> without storing them anywhere. In particular, we need this engine to
> write deferred DELETEs generated for vinyl spaces to WAL.

This is OK to push.
> 
> Needed for #2129
> 
> @TarantoolBot document
> Title: Document blackhole engine
> Spaces created with blackhole engine may be written to, but not read
> from (they don't even support space.create_index command). Operations
> done on a blackhole space are written to WAL and hence get replicated,
> but not stored anywhere else.

The docbot comment is insufficient. Please describe the effect of
adding triggers to this engine. It should be OK to mix this
engine with others in transaction - please could write a follow
up which allows this? 

Please document how it could be used for logical replication.

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