[Tarantool-patches] [RFC v3 0/3] relay: provide downstream lag information

[Vladislav Shpilevoy]

Hi! Thanks for the patchset!

> Please take a look on applier notifications structure and naming. Actually
> I don't really like `downstream.lag` name either because this is not a counterpart
> for `upstream.lag` as far as I understand but rather packet traverse so maybe
> `dowstream.wal-lag` would be more suitable? Also in idle cycles downstream.lag

For me 'downstream.lag' looks good. `wal-lag` looks bad, because you will need
to take this name into quotes to use it in Lua instead of indexing via '.'.

But another option would be 'downstream.latency'.

> is not changed which might confuse the readers because `upstream.lag` does.

This is indeed not good. Should be fixed somehow. If there are no rows to ACK,
it could be dropped to 0, for example. Also you could use relay heartbeat timestamps
on the applier side to report just network latency (since there are no rows to
write to WAL).