[lttng-dev] babeltrace2 python api related question

[Philippe Proulx]

On Sun, Aug 14, 2022 at 4:48 AM zenan <dantefu9001 at 163.com> wrote:
>
> In command line, I use “babeltrace2 net://somehost/some-session” to read live lttng session trace, and I want to use python bindings to do the same, is it possible?
>
> here is my code:

Almost there.

Some `src.ctf.lttng-live` component and message iterator methods may
return a "try again" status code (I don't remember exactly under which
circumstances). This propagates to bt_graph_run() [1] (in C) eventually
which itself returns `BT_GRAPH_RUN_STATUS_AGAIN`. The Python
bt2.Graph.run() method translates this status code to throwing
`bt2.TryAgain`.

Just catch those `bt2.TryAgain`, wait a bit, and call `graph.run()`
again:

    # Run the trace processing graph.
    while True:
        try:
            graph.run()
        except bt2.TryAgain:
            time.sleep(.1)

Works for me.

If you ever need to access the messages themselves instead of forwarding
them to a `sink.text.pretty` component, then use a
`bt2.TraceCollectionMessageIterator` instance [2].

Hope it helps,

Phil

[1]: https://babeltrace.org/docs/v2.0/libbabeltrace2/group__api-graph.html#gad2e1c1ab20d1400af1b552e70b3d567c
[2]: https://babeltrace.org/docs/v2.0/python/bt2/examples.html#iterate-trace-events

>
> import bt2
> import sys
>
> # Create an empty graph.
> graph = bt2.Graph()
>
> # Add a `source.text.dmesg` component.
> #
> # graph.add_component() returns the created and added component.
> #
> # Such a component reads Linux kernel ring buffer messages (see
> # `dmesg(1)`) from the standard input and creates corresponding event
> # messages. See `babeltrace2-source.text.dmesg(7)`.
> #
> # `my source` is the unique name of this component within `graph`.
>
> # the original source code here in the example is:
> # comp_cls = bt2.find_plugin('text').source_component_classes['dmesg']
> # src_comp = graph.add_component(comp_cls,'my source')
> comp_cls = bt2.find_plugin('ctf').source_component_classes['lttng-live']
> src_comp = graph.add_component(comp_cls, 'my source', params = {
> 'inputs': [sys.argv[1]],
> })
>
> # Add a `sink.text.pretty` component.
> #
> # Such a component pretty-prints event messages on the standard output
> # (one message per line). See `babeltrace2-sink.text.pretty(7)`.
> #
> # The `babeltrace2 convert` CLI command uses a `sink.text.pretty`
> # sink component by default.
> comp_cls = bt2.find_plugin('text').sink_component_classes['pretty']
> sink_comp = graph.add_component(comp_cls, 'my sink')
>
> # Connect the `out` output port of the `source.text.dmesg` component
> # to the `in` input port of the `sink.text.pretty` component.
> graph.connect_ports(src_comp.output_ports['out'], sink_comp.input_ports['in'])
>
> # Run the trace processing graph.
> graph.run()
>
>
>
>
>
> 发件人: Philippe Proulx
> 发送时间: 2022年8月10日 22:08
> 收件人: zenan
> 抄送: lttng-dev at lists.lttng.org
> 主题: Re: [lttng-dev] babeltrace2 python api related question
>
>
>
> On Wed, Aug 10, 2022 at 9:54 AM zenan via lttng-dev
>
> <lttng-dev at lists.lttng.org> wrote:
>
> >
>
> >
>
> >
>
> > Hi there,
>
> >
>
> > I’m trying to use babeltrace2 python bindings to track live lttng trace, as shown in some examples in the documents, I tried to use ctf.lttng-live( ctf.fs in the documents) plugin as source, and use text.pretty as sink, but it didn’t work out as I expected.  So does the api support tracking live lttng trace and print them out yet?
>
> >
>
> >
>
>
>
> What did you try exactly?
>
>
>
> I need command lines and/or some code (keep it minimal) to help you.
>
>
>
> Philippe
>
>
>
> >
>
> >
>
> >
>
> > Yours sincerely
>
> >
>
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