Re: [Tarantool-patches] [PATCH] limbo: introduce request processing hooks

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

On Mon, Jul 12, 2021 at 10:03:32AM +0200, Vladislav Shpilevoy wrote:
> > 
> > Now imagine the following: we validated the incoming packet and
> > remember its term in promote_term_map, then we start writting
> > this packet into our WAL and write procedure failed.
> 
> That is the core problem of your entire approach - why do you imagine
> we update promote_term_map before writing to WAL? We do not do that.

We don't do that now, and this is all this patch about - we must not
write the PROMOTE from split-brained node into our wal file at all,
for exactly this reason I splitted processing into phases: "filter" and
"application".

> And you should not do that. Before WAL write there should no be any
> changes. **Zero changes before WAL write**. Before you write to WAL,
> you can only validate requests. Using `const struct txn_limbo *`. With
> zero changes.

If we write screwed PROMOTEs request into our WAL then this patch is not
needed at all.