Document differences between memtx and vinyl transaction managers

[locker]

Related dev. issue(s): tarantool/tarantool#1944
Product: Tarantool
Since: always
Audience/target: users
Root document: https://www.tarantool.io/en/doc/latest/book/box/engines/memtx_vinyl_diff/
SME: @ locker @ alyapunov @ Mons

Details

There's a documentation page that enumerates differences between memtx and vinyl engines. We need to mention that starting from 2.6.1 memtx supports MVCC (aka yields in transactions) provided the box.cfg.memtx_use_mvcc_engine is set. Vinyl has always supported MVCC.

We also need to describe differences between memtx and vinyl implementations of MVCC. There's the only difference known so far: how tracking non-existing/deleted keys is implemented. Let's assume that one transaction reads a range of keys using select and yields. During the yield, another transaction deletes a key that doesn't exist in the index from the range using delete. The delete would abort the first transaction in vinyl, but wouldn't abort it in memtx. In particular, if a transaction reads a single key that doesn't exist in the index, then deleting the key in another transaction would abort it in vinyl, but not in memtx.

Example:

local net = require('net.box')

box.cfg{
    listen = 3301,
    log_level = 'warn',
    memtx_use_mvcc_engine = true,
}
box.once('init', function()
    box.schema.user.grant('guest', 'super')
    box.schema.create_space('memtx', {engine = 'memtx'})
    box.space.memtx:create_index('primary')
    box.schema.create_space('vinyl', {engine = 'vinyl'})
    box.space.vinyl:create_index('primary')
end)

box.space.memtx:truncate()
box.space.vinyl:truncate()

local conn = net.connect('localhost:3301')
local stream = conn:new_stream()

stream:begin()
stream.space.memtx:get{1}
stream.space.memtx:replace{2}
box.space.memtx:delete{1}
local ok, err = pcall(stream.commit, stream)
print(string.format("memtx: %s", ok and 'Success' or tostring(err)))

stream:begin()
stream.space.vinyl:get{1}
stream.space.vinyl:replace{2}
box.space.vinyl:delete{1}
local ok, err = pcall(stream.commit, stream)
print(string.format("vinyl: %s", ok and 'Success' or tostring(err)))

os.exit(0)

Output:

memtx: Success
vinyl: Transaction has been aborted by conflict

The issue is closely related to another memtx vs vinyl difference: in memtx delete returns the deleted tuple while in vinyl it doesn't, because it may not know if a tuple was actually deleted - it just writes a "delete" statement to the index without reading the key.

Definition of done

• Added MVCC / yields in transactions entry to the memtx vs vinyl comparison table.
• Described differences between memtx and vinyl transaction managers.

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