Hello Ruoxiang!

Thank you for your question. It's true that there are no Python bindings examples specific to lttng live (we've provided a brief example below). Unfortunately, the python bindings documentation is currently incomplete, but information about using lttng live can be pieced together using the various documentation links below (see Useful Links section).

Some adjustments typically needed when using lttng live and the Python bindings are:

- When referring to a trace source, use a URL (e.g. net://localhost/host/luna/my-session) rather than a file path (e.g. /path/to/trace)

- When querying, use the source.ctf.lttng-live component class (rather than the file system class: source.ctf.fs) - source.ctf.lttng-live docs https://babeltrace.org/docs/v2.0/man7/babeltrace2-source.ctf.lttng-live.7/

Hope this helps!

Best,

Erica

----

Useful links

- Python bindings docs (Installation, Examples) - https://babeltrace.org/docs/v2.0/python/bt2/index.html

- LTTng live, General information - https://lttng.org/docs/v2.13/#doc-lttng-live (e.g. How to express a live trace source: net://localhost/host/HOSTNAME/my-session)

- source.ctf.lttng-live component - https://babeltrace.org/docs/v2.0/man7/babeltrace2-source.ctf.lttng-live.7/ (C API doc not Python, but can be used to adapt the source.ctf.fs examples by comparing)

----

Quick Example: Trace reading with python bindings and lttng live

Note: This is a local example where the tracing and reading with babeltrace are happening on the same machine (luna).

1. REQUIREMENTS

Make sure babeltrace is installed with the python plugins and python bindings: https://babeltrace.org/docs/v2.0/python/bt2/installation.html

2. PROCEDURE

- Start root session daemon: $sudo lttng-sessiond --daemonize

- Create live session: $lttng create my-session --live

- Enable events: $lttng enable-event --kernel sched_switch,sched_process_fork

- Start tracing: $lttng start

- Run python script: (see below) $python3 lttng-live.py

3. PYTHON SCRIPT

File name: lttng-live.py

File contents:

import bt2

import time

msg_iter = bt2.TraceCollectionMessageIterator('net://localhost/host/luna/my-session') # The hostname (i.e. machine name) is 'luna'

while True:

try:

for msg in msg_iter:

if type(msg) is bt2._EventMessageConst:

print(msg.event.name)

except bt2.TryAgain:

print('Try again. The iterator has no events to provide right now.')

time.sleep(0.5)

Reading trace data using the bt2.TraceCollectionMessageIterator and lttng-live: When using the message iterator with live, the iterator never ends by default and can be polled for trace data infinitely (hence the while loop in the example). When there is available data, the iterator will return it. However, there will not always be data available to consume. When this is the case, the iterator returns a "Try again" exception which must be caught in order to continue polling.

4. OUTPUT SNIPPET (Example)

erica@luna:~$ python3 lttng-live-example.py

Try again. The iterator has no events to provide right now.

sched_switch

sched_process_fork

sched_switch

[...]