[lttng-dev] relayd with many consumer daemons

[Jonathan Rajotte-Julien]

Hi Kuba,

On Tue, Feb 27, 2018 at 03:34:59PM +0000, Jakub Witczak wrote:
> Hi,
> 
> I would like to consult on setup where relayd is serving multiple consumer daemons at the same time.

On the same host? On multiple hosts?

In both cases, it is supported.

It is recommended to have unique session id across multiples sessiond/consumer on the same host but not required.

> 
> I assume above setup is nothing new and is not a problem as long as each of consumer daemons is having unique tracing session (please confirm).

As for multiple hosts, relayd also differentiate based on the hostname.

>From lttng-relayd man page:

    By default, the relay daemon writes the traces to:

    $LTTNG_HOME/lttng-traces/HOSTNAME/SESSION/DOMAIN

    with:

    HOSTNAME
        Remote hostname.

    SESSION
        Full session name.

    DOMAIN
        Tracing domain.

    You can override the default output directory prefix
    ($LTTNG_HOME/lttng-traces) with the --output option. The other parts
    depend on the remote configuration.

For example, user on host DATA1 create and start a tracing a session (live/streaming) with the name MySession.
Relayd will create its internal state with the hostname, session name, time.
Hence, trace data will be written somewhere like this:
    $LTTNG_HOME/lttng-traces/DATA1/MySession-TIMESTAMP/DOMAIN

At the same time on host DATA2, a user create and start a tracing session
(live/streaming) with the name MySession.
Trace data will be written somewhere like this:
    $LTTNG_HOME/lttng-traces/DATA2/MySession-TIMESTAMP/DOMAIN

> 
> But what if I would like to have more consumer daemons collecting data from *single session* - is it possible to aggregate data in relayd?

Note that Babeltrace can read and merge data from multiple sources:
    babeltrace $LTTNG_HOME/lttng-traces/DATA1/MySession-TIMESTAMP $LTTNG_HOME/lttng-traces/DATA2/MySession-TIMESTAMP/DOMAIN<Paste>

Not sure what you mean by "consumer daemons collecting data from *single session*".

Do you mean having multiple consumer "writing" to the same session on lttng-relayd side?
What would motivate this design choice, is a limitation you encountered?

Is it not sufficient that multiple traces can be merged using babeltrace?

Cheers

-- 
Jonathan Rajotte-Julien
EfficiOS