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

[lvasiliev <lvasiliev@tarantool.org>]

On 24.03.2020 23:02, Konstantin Osipov wrote:
> * Leonid Vasiliev <lvasiliev@tarantool.org> [20/03/24 16:02]:
>> The negotiation phase has been added to IPROTO
>>
>> For possibility to have a custom parameters of session the negotiation
>> phase has been added. This is necessary to enable the transmission of
>> an error in different formats(depending on the choice of the client).
>>
>> @TarantoolBot document
>> Title: IPROTO: The negatiation phase
>> For backward compatibility of the data transmission format,
>> the negotiation phase has been added to IPROTO.
>> A new key (IPROTO_NEGOTIATION) has been added to IPROTO command codes.
>> NEGOTIATION BODY: CODE = 0x0E
>> +==========================+
>> |                          |
>> |  NEGOTIATION PARAMETERS  |
>> |                          |
>> +==========================+
>>             MP_MAP
>> Session negotiation parameters are a map with keys like ERROR_FORMAT_VERSION ...
>> The response is a map with all stated negotiation parameters.
>> So, for work with the new format of errors, it is necessary to perform the negotiation phase,
>> otherwise errors will be transmitted in the old format (by default).
> 
> Why not make it a key in IPROTO_AUTH, and require a separate
> round-trip?
Hi. Because it's not a part of AAA.
> 
> 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).