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Babeltrace 2: The command-line interface (CLI)


Babeltrace 2 complex graph

Back in the Montréal's cold winter of 2015, Jérémie Galarneau asked Philippe Proulx if he could lend a hand with adapting Babeltrace 1's TSDL-to-declarations visitor to a new TSDL-to-CTF IR visitor for an upcoming revision of the project (we weren't even sure we'd be bumping the major version at the time).

“Sure.” said Philippe.

This was five years ago and we just released Babeltrace 2 earlier this year.

The project's agenda mutated a lot during the past years. From a vague “new plugin-based trace converter with filters using the CTF writer API” to what we have now, many design ideas were actually coded and then abandoned in favor of better, often simpler and cleaner alternatives. Our awareness of Babeltrace 2's real goals also crystallized as time passed.

We won't say we're proud that Babeltrace 2 gestated for so long. But in every negative there's positive: we're proud of what we technically achieved and how it lays the groundwork for future features and even other EfficiOS projects.

Today's post is about Babeltrace 2's CLI, more precisely:

  1. The run command is versatile and powerful, but not magic.
  2. The convert command is merely a special case of the run command.
  3. The convert command's stream intersection mode supports any source component class.
  4. The convert command automatically discovers source components.

Quick overview of Babeltrace 2's CLI

babeltrace2 is Babeltrace 2's command-line interface. It is command-based, like git and lttng, where the first non-option argument can be a command name.

There are five commands:

Workhorse of the CLI with lots of dedicated options which strictly performs a trace conversion job. This command is backward compatible with babeltrace, the CLI tool of Babeltrace 1.
Prints the description and help text of a plugin and of its component classes.
Lists the available plugins and their component classes.
Performs a query operation on a component class and prints the results.
Creates and runs a trace processing graph for more complex trace processing tasks.

Make sure to have a look at babeltrace-intro(7) to learn more about the project and its core concepts.

The run command is versatile and powerful, but not magic

With only the --component, --params, and --connect options of the run command, you can do a lot. This is because you use those options to create a custom trace processing graph topology.

The three options above are position-dependent: --component instantiates a component class as the current component and the following --params options, until the next --component, append to its initialization parameters. So this creates three components:

--component=ctf:source.ctf.fs --params='inputs=["my-trace"]' \
--component=muxer:filter.utils.muxer --component=pretty:sink.text.pretty

As you can see, initialization parameters are optional: the filter.utils.muxer and sink.text.pretty components do not require any.

You can place --connect options anywhere on the run command line to specify connection rules, but they are still position-dependent between them. This is because of how the connection rules work: if an output port does not match a given connection rule, then the run command tries the next one. This means you can create fallbacks by putting more specific connection rules first, for example:

--connect='ctf1.*sys23*:flt1' --connect='ctf2.*sys23*:flt2' --connect='ctf*:outg'

This connects:

  • All the output ports of the ctf1 component containing sys23 to the flt1 component.
  • All the output ports of the ctf2 component containing sys23 to the flt2 component.
  • All the remaining output ports of any component starting with ctf to the outg component.

You don't need to specify a single *: A:B is equivalent to A.*:B.*, connecting any output port of component A to the first available input port (by position) of component B.

With the run command, you must specifiy each component to create and how they will be connected within the trace processing graph. In other words, there's no magic: if the command cannot connect an output port, it prints an error. If you need message muxing, you need to explicitly instantiate a filter.utils.muxer component class and connect it. This strictness makes the run command predictable and therefore more easy to debug.

A brief comparison with ffmpeg

ffmpeg and ffplay are other tools which create a processing graph from the command line (a media processing graph in their case).

The FFmpeg to Babeltrace 2 concept mapping is:

Trace processing graph
Input "file"
Source component
Output "file"
Sink component
Filter component
Input and output pads
Input and output ports

With ffmpeg, you specify the filters to use and how they form a “filterchain” (how they are connected together) with the -filter_complex option's argument, a single string which also sets the initialization parameters for each filter. For example:

hello.mkv -i zoom.mp4 \
-filter_complex '[1:v][2:a]filterA=param1=23[out]' \
-map '[out]' out.avi

See the ffmpeg-filters(1) man page to explore the ffmpeg filter syntax.

The equivalent babeltrace2 run command line (using the short options) would be something like:

-c hello:source.mkv.mkv -p inputs='["hello.mkv"]' \
-c zoom:source.mp4.mp4 -p inputs='["zoom.mp4"]' \
-c fa:filter.A.A -p param1=23 \
-c out:sink.avi.avi -p path=out.avi \
-x hello.video:fa.in0 -x zoom.audio:fa.in1 \
-x fa:out
Babeltrace 2 trace processing graph equivalent to FFmpeg filtergraph.
Babeltrace 2 trace processing graph equivalent to FFmpeg filtergraph.

While the babeltrace2 run command line is more verbose, we tend to prefer it because:

  • It's more obvious what's instantiated and what's connected to what.

  • Each component has a unique name.

  • Initialization parameters are strongly typed, they are not just strings.

    For example, with -p param1=23, component fa receives a map containing a signed integer entry (param1).

The convert command is merely a special case of the run command

That's right: except for the stream intersection feature (--stream-intersection option), everything you can do with the convert command, you can also do with the run command.

In fact, internally, the convert command only cooks a run command line and then executes the run command. You can get a convert command line's equivalent run command line with the --run-args option.

Here's an example:

babeltrace2 my-ctf-trace

This very simple convert command line (convert is the default command, so you don't need to specify it) converts a CTF trace to a pretty, plain text form which is the text format of Babeltrace 1's babeltrace tool.

To achieve this conversion task, the command line above:

  • Instantiates a source.ctf.fs component class to read a CTF trace located on the file system.

    Such a component creates one output port per data stream.

  • Instantiates a filter.utils.muxer component class to mux (merge) the multiple CTF data stream messages into a single sequence of messages (multiple input ports to a single output port).

  • Instantiates a sink.text.pretty to pretty-print the consumed messages to the standard output.

  • Connects the components above.

The equivalent run command line is:

babeltrace2 my-ctf-trace --run-args
--component auto-disc-source-ctf-fs:source.ctf.fs --params 'trace-name="my-ctf-trace"' \
--params 'inputs=["/path/to/my-ctf-trace"]' \
--component pretty:sink.text.pretty \
--component muxer:filter.utils.muxer \
--connect auto-disc-source-ctf-fs:muxer --connect muxer:pretty
Simple Babeltrace 2 conversion graph.
Simple Babeltrace 2 conversion graph.

In other words, you can pass those exact arguments to babeltrace2 run and get the same results.

As you can see, the convert command automatically:

  • Finds which source component class to use for the my-ctf-trace argument thanks to the automatic source component discovery feature.

    This is the --component auto-disc-source-ctf-fs:source.ctf.fs and --params 'inputs=["/path/to/my-ctf-trace"]' parts.

  • Adds a muxer filter component.

    This is the --component muxer:filter.utils.muxer part.

  • Adds the default pretty-printing sink component.

    This is the --component pretty:sink.text.pretty part.

  • Sets the unique names of the components: auto-disc-source-ctf-fs, muxer, and pretty.

  • Connects all the output ports of the source.ctf.fs component to the input ports of the muxer component (--connect auto-disc-source-ctf-fs:muxer) and the output port of the muxer component to the input port of the pretty-printing sink component (--connect muxer:pretty).

Let's try adding the --debug-info option to augment event messages with debugging information (when available):

babeltrace2 my-ctf-trace --debug-info --run-args
--component auto-disc-source-ctf-fs:source.ctf.fs --params 'trace-name="my-ctf-trace"' \
--params 'inputs=["/path/to/my-ctf-trace"]' \
--component pretty:sink.text.pretty \
--component muxer:filter.utils.muxer \
--component debug-info:filter.lttng-utils.debug-info \
--connect auto-disc-source-ctf-fs:muxer --connect muxer:debug-info \
--connect debug-info:pretty
Babeltrace 2 conversion graph with a debugging information filter component.
Babeltrace 2 conversion graph with a debugging information filter component.

Using this option adds a filter.lttng-utils.debug-info component to the conversion chain (after the muxer component and before the pretty-printing sink component). Francis Deslauriers wrote most of the current filter.lttng-utils.debug-info component class. Antoine Busque introduced the debugging information resolving code in Babeltrace 1 and we reused it for this component class.

Let's use the --debug-info-full-path option instead to get absolute, instead of relative, source file paths in debugging information fields:

babeltrace2 my-ctf-trace --debug-info-full-path --run-args
--component auto-disc-source-ctf-fs:source.ctf.fs --params 'trace-name="my-ctf-trace"' \
--params 'inputs=["/path/to/my-ctf-trace"]' \
--component pretty:sink.text.pretty \
--component muxer:filter.utils.muxer \
--component debug-info:filter.lttng-utils.debug-info --params full-path=yes \
--connect auto-disc-source-ctf-fs:muxer --connect muxer:debug-info \
--connect debug-info:pretty

The output is similar to the previous example, but now the debug-info filter component is initialized with its full-path parameter set to true.

Finally, let's use another sink component instead of sink.text.pretty:

babeltrace2 my-ctf-trace --debug-info-full-path --component=sink.text.details --run-args
--component sink.text.details:sink.text.details \
--component auto-disc-source-ctf-fs:source.ctf.fs --params 'trace-name="my-ctf-trace"' \
--params 'inputs=["/path/to/my-ctf-trace"]' \
--component muxer:filter.utils.muxer \
--component debug-info:filter.lttng-utils.debug-info --params full-path=yes \
--connect auto-disc-source-ctf-fs:muxer \
--connect muxer:debug-info \
--connect 'debug-info:sink\.text\.details'
Babeltrace 2 conversion graph with a debugging information filter and a detail sink component.
Babeltrace 2 conversion graph with a debugging information filter and a detail sink component.

The convert command's stream intersection mode supports any source component class

The convert command's --stream-intersection option enables the stream intersection mode.

The stream intersection mode makes the command discard, for a given trace, messages which occur outside the common time range of all the trace's streams.

Babeltrace 1's babeltrace tool knows the CTF format specifically to implement its stream intersection mode limiting the functionality to CTF sources.

In Babeltrace 2, we designed the stream intersection mode so that it can work with any source component class, not just source.ctf.fs. This is in part possible due to the notion of query. Here's what happens behind the scenes:

  1. For each source component to add, the convert command queries the babeltrace.trace-infos object from its class.

    The parameters of this query operation are the exact same parameters that a source component of the same class would receive at initialization time. For example, for a source.ctf.fs component class, the query operation parameters contain the inputs array which contains the paths of the CTF traces to open.

    The results of this query operation contain, for each trace, the beginning and end times of each stream. How the queried source component class obtains those times exactly is implementation-defined. The source.ctf.fs component class uses packet indexes, just like in Babeltrace 1.

    The results also contain, for each stream, the name of its dedicated output port, should a source component be created and initialized with the same parameters.

    Here's a babeltrace.trace-infos query example, my-ctf-trace is a single trace made of four streams:

    babeltrace2 query source.ctf.fs babeltrace.trace-infos \
          port-name: da82bf1c-bb28-4347-804d-dc8f26817aec | 0 | 0
            begin: 1509556764947050643
            end: 1509557102182770973
          port-name: da82bf1c-bb28-4347-804d-dc8f26817aec | 0 | 1
            begin: 1509556764956409290
            end: 1509557102182770490
          port-name: da82bf1c-bb28-4347-804d-dc8f26817aec | 0 | 2
            begin: 1509556764965620172
            end: 1509557102182769632
          port-name: da82bf1c-bb28-4347-804d-dc8f26817aec | 0 | 3
            begin: 1509556764975082100
            end: 1509557102181554525
  2. For each trace amongst all the source components, the command computes the time range of its stream intersection.

  3. For each source component output port, the command inserts a filter.utils.trimmer component with its time range set to the appropriate stream intersection time range.

    The command connects all trimmer filter components to a single filter.utils.muxer component as usual.

Here's a command-line example and a diagram showing the resulting conversion graph:

babeltrace2 my-ctf-trace some-other-trace --stream-intersection
Babeltrace 2's stream intersection mode conversion graph.
Babeltrace 2's stream intersection mode conversion graph.

The convert command automatically discovers source components

When you run

babeltrace /path/to/trace

the babeltrace command assumes that /path/to/trace is a path to a CTF trace.

Because Babeltrace 2 is all about plugins offering many trace format encoders and decoders, we thought that this CTF-by-default strategy was a bit too selective.

For Babeltrace 2, Simon Marchi did a wonderful job developing an automatic source component discovery feature for both the CLI and the Python bindings (for bt2.TraceCollectionMessageIterator).

babeltrace2's convert command uses this feature when you pass any non-option argument, for example:

babeltrace2 /path/to/trace

Unlike the babeltrace CLI, babeltrace2 convert does not assume that /path/to/trace is a CTF trace path. Instead, the convert command loads all the plugins it can find, and for each source component class of a given plugin, it queries its babeltrace.support-info object.

The babeltrace.support-info object contains a weight which represents how much the queried source component class is confident that one of its instances can handle the given input. A weight of zero means it cannot handle the input.

The source.ctf.fs component class returns a non-zero weight when it can find a metadata file in the given directory and perform other quick CTF trace validations.

The convert command then chooses the source component class having the greatest weight to handle the given input.

The automatic source component discovery system has other features which we won't detail here such as automatic directory recursion and input grouping.

New to Babeltrace 2, you can pass an LTTng live URL as a non-option argument for the convert command to add a source.ctf.lttng-live component:

babeltrace2 net://somehost/host/target/session

This is possible because the source.ctf.lttng-live component class handles babeltrace.support-info queries,

With the original babeltrace tool, you need to explicitly give the LTTng live input format with -i lttng-live.

You can always use the --run-args option to see the equivalent run command line:

babeltrace2 net://somehost/host/target/session --run-args
--component lttng-live:source.ctf.lttng-live \
--params 'inputs=["net://some-host/host/target/session"]' \
--params 'session-not-found-action="end"' \
--component pretty:sink.text.pretty \
--component muxer:filter.utils.muxer \
--connect lttng-live:muxer --connect muxer:pretty

To fully support the automatic source component discovery feature in the babeltrace2 convert command, we had to stop using popt and create our own little argument parser. This is because you can use the --params and --log-level options after a non-option argument to append initialization parameters and set the logging level of all the source components summoned by the argument, for example:

babeltrace2 /path/to/trace --params=lorem=ipsum \
            /path/to/other-trace --params=factor=23.18 --log-level=INFO

popt, getopt(), and the GLib command-line parser did not satisfy all our needs to support this and other command-line parsing musts.


As the items of this post show, the Babeltrace 2 CLI is an important upgrade from its Babeltrace 1 counterpart for the end user, but also at the implementation level.

The babeltrace2 convert command offers options and features to convert one or more traces to a given format without its user having to understand many Babeltrace 2 trace processing graph concepts.

For example, you can set the convert command in stream intersection mode, and not only for CTF inputs, with the --stream-intersection mode.

The convert command can also automatically discover which component classes to instantiate for one or more non-option arguments.

That being said, internally, the convert command strictly remains a layer over run, the general-purpose trace processing graph building and running command.

We want your feedback

We're always looking for more testers, and for more people to read the documentation and try things out. The babeltrace-intro(7) man page introduces the Babeltrace 2 concepts and parts. You can clone the project’s repository and build it to get the man pages. If you want to share feedback, please join us on the lttng-dev mailing list or on OFTC’s #lttng IRC channel. We'd love to hear from you.