[lttng-dev] UST app and lttng-tools compatibility

[dgoulet@efficios.com (David Goulet)]

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Mathieu Desnoyers:
> * Mathieu Desnoyers (mathieu.desnoyers at efficios.com) wrote:
>> * Francis Giraldeau (francis.giraldeau at gmail.com) wrote:
> [...]
>>> Could the registration process block until the sessiond returns
>>> some status?
>> 
>> The session is not "returning" anything to the application. The 
>> application is registering to the sessiond, and all the sessiond
>> can do is to keep the socket alive or close it.
>> 
>> Even if we added a "command" that the sessiond could use to tell
>> the application it has a wrong version, that would not be 2.0
>> material, and the application wouldn't know about it. Moreover,
>> given we have changed the communication protocol, not sure we
>> would like to have this extra command as entirely fixed for now,
>> as it would be part of the communication protocol (which is not
>> the case for the initial app registration, with is part of a
>> lower level protocol).
>> 
>>> I understand that in commercial setup, the application should 
>>> not be prevented to start and run normally if tracing is not
>>> available or misconfigured.
>> 
>> By default, we only block the application for up to 3 seconds at 
>> registration. I think that if we start the application with 
>> LTTNG_UST_REGISTER_TIMEOUT=-1, the application will, in this
>> case, block forever (see man lttng-ust).
> 
> Let me take this last part back. That was a pre-morning-coffee 
> statement. ;) The application will receive a "registration done"
> message when the version is found to be incompatible. Therefore, it
> will never wait for the incompatible sessiond.
> 
> One thing we could do to make transition smoother between future 
> versions would be to add a new command to let the sessiond send its
> own version info to the application. In this command's handler,
> the application could show a warning on stderr. This could not
> apply to 2.0, but we could do it starting from 2.1.
> 
> Thoughts ?

Why sending version infos instead of a "incompatible" command ?

David

> 
> Thanks,
> 
> Mathieu
> 
> 
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