Re: bt2 python - accessing component instances added to graph

[Rocky Dunlap via lttng-dev <lttng-dev@lists.lttng.org>]

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(Adding the list back in - I meant to reply-all before...)

Simon,

Yes, the obj solution should work just fine - I was wondering about that
parameter and the approach makes sense to me.  Thanks!

Rocky

On Mon, Mar 30, 2020 at 2:38 PM Simon Marchi <simark@simark.ca> wrote:

> On 2020-03-30 4:24 p.m., Rocky Dunlap wrote:
> > Simon,
> >
> > In my case, after the graph completes, I was trying to access the sink
> instances in order to collect some statistics collected inside the instance
> during the graph run.  I guess then the question is how are the graph
> components supposed to communicate with the outside world in general, such
> as filling in some Python data structures with info from a trace for
> post-processing or further analysis in Python?  Or is the expectation that
> sink components will retain no state at all after the graph completes?
> >
> > Rocky
>
> Hi Rocky,
>
> Was it your intention to not do a reply-all?  I think this would be good
> info for all to read.  If not,
> please do a reply-all and I'll replay again what I wrote below on the list.
>
> Of course the sinks are expected to produce some result outside of their
> own little internal state.  For
> sinks that produce some trace files, they'll typically write on the
> filsystem (like sink.ctf.fs).
>
> But if you want to send some results to another Python object/data
> structure, the `obj` parameter of
> `add_component` is pretty much meant for that.  You can pass in a
> reference to a Python object, and
> the component receives it in its `__init__`.  We thought that this could
> be useful, for example, if
> the sink needs to receive a connection handle to a database, for example.
> That handle can be created
> outside of the sink and passed it when instantiating the component.  Think
> of it like some void* user
> data in C.
>
> In your case, you could create whatever object you need to collect your
> statistics/results, and pass
> a reference to it.
>
> Here's an example in the tests:
>
>
> https://github.com/efficios/babeltrace/blob/8b305066676fc7aa433e8eb668f9de8802008025/tests/bindings/python/bt2/test_graph.py#L106-L120
>
> Does that help?
>
> Simon
>

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