Re: [Tarantool-patches] [PATCH v2 2/2] wal: reorder tx rows so that a tx ends on a global row

[Serge Petrenko <sergepetrenko@tarantool.org>]

29.05.2020 01:54, Vladislav Shpilevoy пишет:
> Thanks   for   the   patch,
> But   I   have  a  comment!
> It   is   a   nice  crutch,
> Yet we need one more moment.
>
> The patch basically sacrifices transaction rows order
> and WAL correctness for the sake of replication. It
> does not look right. Why can't we leave WAL as is, and
> tweak all these things in relay? It looks really wrong
> to change statements order. Especially taking into
> account this is needed *only* for replication. For example,
> consider FKs.
>
> A local space has a FOREIGN KEY reference to a global
> space. To make it work, we need to insert into the global
> space first, and then into the local space. When you change
> the order, the local insert goes first, and violates the
> foreign key. So if we will check FKs on recovery (when we
> will have FKs in box), this patch will break them.
>
> Alternative to relay - append a dummy NOP statement in the
> end of the transaction, which would be global. But this is
> also a crutch. I think the TSNs figuring out should be done
> in relay. It could keep track of the current transaction,
> change TSNs and is_commit when necessary.

Hi! Thanks for the review!

I understand this is a crutch, but it's the best solution I could come
up with.  Appending dummy  NOPs will increase instance LSN by one,
which  also looks  ugly. The correct solution is, indeed, to collect  a tx
in relay and mangle with it in any means  we need before sending, however,
I faced some problems with this approach. See more in v1 of this patchset
(letter [PATCH 2/2] replication: make relay send txs in batches).

By the way, your txn_limbo implementation assigns the tx an lsn of the last
tx row, which doesn't work in case the last row is a local one. So, 
looks like
we either need to reorder the rows or insert a NOP at the tx end. Or just
assign the tx an lsn of the last global row.

Still, it's up to you which solution you find a better one, and I'll
implement it.

-- 
Serge Petrenko