Re: [Tarantool-patches] [PATCH 3/6] iproto: Add negotiation phase

[Konstantin Osipov <kostja.osipov@gmail.com>]

* Alexander Turenko <alexander.turenko@tarantool.org> [20/03/27 11:14]:

> > > Why have it at all and not look at server version, which is part
> > > of the greeting already?
> > > 
> > The cause is backward compatibility.
> > For example: a client application may expect an error as a string (IPROTO_OK
> > case) and instead of which it will receive an error as an “object”. A
> > greeting is sent only from the server side to the client, but the server
> > must know what format should be used to send errors (what format does the
> > client expect).
> 
> (After the discussion with Leonid and Vlad.)
> 
> The new idea appears: don't do any sessions setting / negotiation, but
> extend IPROTO_OK response in the backward compatible way. (It is a bit
> wild, I understood.)
> 
> Send everything inside IPROTO_BODY as usual: with errors as strings. Add
> the new field into the response, say, IPROTO_ERROR_META. It is mp_map,
> where keys are offset / path to strings, which were errors. Values are
> maps, which will contain all necessary data to create full-featured
> error: error code, backtrace, etc.
> 
> We possibly can even reuse the format from the stacked disgnostics RFC.
> 
> We can even strip error messages from this map to reduce traffic.
> 
> It will be tricky to collect this metainfo during serialization and it
> will be tricky to replace strings with errors during deserialization.
> But doable.
> 
> NB: Whether this mechanism can be reusable for other similar cases? It
> looks too heavy in coding to do it just for errors.
> 

It's a good idea to extend existing packets in backward compatible
manner if it can be done in a meaningful way and without breaking
old clients.

All tarantool clients should ignore unknown map keys.

-- 
Konstantin Osipov, Moscow, Russia