babeltrace2-intro(7)

NAME

babeltrace2-intro — Introduction to Babeltrace 2

DESCRIPTION

This manual page is an introduction to the Babeltrace 2 project.

The WHAT IS BABELTRACE 2? section describes the parts of the project and shows the major changes from Babeltrace 1 to Babeltrace 2 while the BABELTRACE 2 CONCEPTS section defines the core concepts of Babeltrace 2.

The TRACE PROCESSING GRAPH REPRESENTATION section shows how some concepts are visually represented in other Babeltrace 2 manual pages.

WHAT IS BABELTRACE 2?

Babeltrace 2 is an open-source software project of which the purpose is to process or convert traces.

The Babeltrace 2 project includes the following parts:

Babeltrace 2 library (libbabeltrace2)

A shared library with a C API.

With libbabeltrace2, you can programmatically create plugins and component classes, build and run trace processing graphs, and more (see the BABELTRACE 2 CONCEPTS section for more details about those concepts).

All the other Babeltrace 2 parts rely on this library.

babeltrace2 command-line program

A command-line interface which uses libbabeltrace2 to load plugins, create a trace processing graph, create components, connect their ports correctly, and run the graph.

You can also use babeltrace2 to list the available plugins or to query an object from a component class.

See babeltrace2(1).

Babeltrace 2 Python bindings

A Python 3 package (bt2) which offers a Pythonic interface of libbabeltrace2.

You can perform the same operations which are available in libbabeltrace2 with the Python bindings, but more conveniently and with less code. However, the Python bindings are less performant than libbabeltrace2.

Babeltrace 2 project’s plugins

The Babeltrace 2 plugins shipped with the project.

Those plugins are not special in that they only rely on libbabeltrace2 and you don’t need them to use libbabeltrace2, babeltrace2(1), or the Python bindings. However, the project’s plugins provide many widely used trace format encoders/decoders as well as common trace processing graph utilities.

The Babeltrace 2 project’s plugins are:

ctf

Common Trace Format (CTF) input/output, including the LTTng live source.

See babeltrace2-plugin-ctf(7).

lttng-utils

Graph utilities specific to LTTng traces.

See babeltrace2-plugin-lttng-utils(7).

text

Plain text input/output.

See babeltrace2-plugin-text(7).

utils

Common graph utilities (muxer, trimmer, counter, dummy sink).

See babeltrace2-plugin-utils(7).

Changes since Babeltrace 1

This manual page is an introduction to Babeltrace 2, a rewrite of Babeltrace 1 with a focus on extensibility, flexibility, and interoperability.

Babeltrace 1 exists since 2010.

You can install both projects on the same file system as there are no file name conflicts.

The major improvements brought by Babeltrace 2 are:

General

Full plugin support: any user can distribute a Babeltrace 2 plugin and, as long as libbabeltrace2 finds it, any application linked to libbabeltrace2 can load it and use it.

Plugins are not just trace format encoders and decoders: they package source, filter, and sink component classes so that you can connect specialized, reusable components together in a trace processing graph to create a customized trace conversion or analysis device.

This modular strategy is much like how the FFmpeg, GStreamer, and DirectShow projects approach media stream processing.
All the parts of the Babeltrace 2 project run on the major operating systems, including Windows and macOS.
Some component classes, such as sink.text.pretty (similar to the text output format of babeltrace(1)) and sink.text.details, can write color codes to the standard output when it’s connected to a color-enabled terminal.

The Babeltrace 2 log, printed to the standard output, can also be colorized.

Command-line interface

Whereas you can convert traces from one format to another with Babeltrace 1’s CLI tool, babeltrace(1), you can also execute a custom trace manipulation task with babeltrace2(1) thanks to the babeltrace2-run(1) command.
The babeltrace2-convert(1) command features an automatic source component discovery algorithm to find the best suited components to create for a given non-option argument (file or directory path, or custom string like an LTTng live URL).

For example:
```
$ babeltrace2 /path/to/ctf/trace
```
```
$ babeltrace2 net://localhost/host/myhost/my-session
```

CTF input/output

The source.ctf.fs component class, which is more or less the equivalent of Babeltrace 1’s ctf input format, has features not found in Babeltrace 1:
- The component handles many trace quirks which are the results of known tracer bugs and corner cases (LTTng-UST, LTTng-modules, and barectf), making it possible to decode malformed packets.
- The component merges CTF traces sharing the same UUID into a single, logical trace.
  
  This feature supports LTTng 2.11’s tracing session rotation trace chunks.
With a sink.ctf.fs component, you can create CTF traces on the file system.

With babeltrace2(1), you can use the --output-format=ctf and --output options to create an implicit sink.ctf.fs component.

For example:
```
$ babeltrace2 /path/to/input/trace \
              --output-format=ctf --output=trace-dir
```

LTTng live input

The babeltrace(1) command exits successfully when it cannot find an LTTng live (--input-format=lttng-live option) tracing session.

The session-not-found-action initialization parameter controls what a source.ctf.lttng-live message iterator does when it cannot find the remote tracing session.

If the action is end, the message iterator does like babeltrace(1) and simply ends successfully.

If the action is continue (the default), the message iterator never ends: it keeps on trying until the tracing session exists, indeed subscribing to the session.

Library

libbabeltrace2 shares nothing with libbabeltrace.

The Babeltrace 2 library C API has features such as:
- A single header file.
- Function precondition and postcondition checking.
- Object-oriented model with shared and unique objects.
- Strict C typing and const correctness.
- User-extensible classes.
- Rich, thread-safe error reporting.
- Per-component and per-subsystem logging levels.
- Trace intermediate representation (IR) objects to make the API trace-format-agnostic.
- A versioned protocol for message interchange between components to enable forward and backward compatibility.
You can build the library in developer mode to enable an extensive set of function precondition and postcondition checks.

The developer mode can help detect programming errors early when you develop a Babeltrace 2 plugin or an application using libbabeltrace2.

See the project’s README for build-time requirements and detailed build instructions.

BABELTRACE 2 CONCEPTS

This section defines the main concepts of the Babeltrace 2 project.

These concepts translate into types and functions in libbabeltrace2 and its Python bindings, but also as command-line actions and options in the babeltrace2 program. The other Babeltrace 2 manual pages assume that you are familiar with the following definitions.

Some Babeltrace 2 concepts are interdependent: it is normal to jump from one definition to another to understand the big picture.

Component class

A reusable class which you can instantiate as one or more components within a trace processing graph.

There are three types of component classes used to create the three types of components: source, filter, and sink.

A component class implements methods, one of which is an initialization method, or constructor, to create a component. You pass initialization parameters to this method to customize the created component. For example, the initialization method of the source.ctf.fs component class accepts a mandatory inputs parameter which is an array of file system path(s) to the CTF trace(s). It also accepts an optional clock-class-offset-ns parameter which is an offset, in nanoseconds, to add to all the clock classes (descriptors of stream clocks) found in the traces’s metadata.

A component class can have a description and a help text.

Component

A node within a trace processing graph.

There are three types of components:

Source component

An input component which produces messages.

Examples: CTF files input, log file input, LTTng live input, random event generator.

Filter component

An intermediate component which can transform the messages it consumes, augment them, sort them, discard them, or create new ones.

Examples: filter which removes messages based on an expression, filter which adds debugging information to selected events, message muxer, trace trimmer.

Sink component

An output component which consumes messages and usually writes them to one or more formatted files.

Examples: log file output, CTF files output, pretty-printed plain text output.

Components are connected together within a trace processing graph through their ports. Source components have output ports, sink components have input ports, and filter components have both.

A component is the instance of a component class. The terms component and component class instance are equivalent.

Within a trace processing graph, each component has a unique name. This is not the name of its component class, but an instance name. If human is a component class name, than Nancy and John could be component names.

Once a graph is configured (the first time it runs), you cannot add components to it for the remaining graph’s lifetime.

Port

A connection point, on a component, from which are sent or where are received messages when the trace processing graph runs.

An output port is from where messages are sent. An input port is where messages are received. Source components have output ports, sink components have input ports, and filter components have both.

You can only connect an output port to a single input port.

All ports do not need to be connected.

A filter or sink component receiving messages from its input ports is said to consume messages.

The link between an output port and input port is a connection.

Once a graph is configured (the first time it runs), you cannot connect ports for the remaining graph’s lifetime.

Connection

The link between an output port and an input port through which messages flow when a trace processing graph runs.

Message iterator

An iterator on an input port of which the returned elements are messages.

A component or another message iterator can create many message iterators on a single input port, before or while the trace processing graph runs.

Message

The element of a message iterator.

Messages flow from output ports to input ports.

A source component message iterator produces messages, while a sink component consumes them. A filter component message iterator can both consume and produce messages.

The main types of messages are:

Event

A trace event record within a packet or within a stream.

Packet beginning

The beginning of a packet within a stream.

A packet is a conceptual container of events.

Packet end

The end of a packet within a stream.

Stream beginning

The beginning of a stream.

A stream is a conceptual container of packets and/or events.

Usually, a given source component’s output port sends packet and event messages which belong to a single stream, but it’s not required.

Stream end

The end of a stream.

Discarded events

A count of discarded events within a given time interval for a given stream.

Discarded packets

A count of discarded packets within a given time interval for a given stream.

Trace processing graph

A filter graph where nodes are components and messages flow from output ports to input ports.

You can build a trace processing graph with libbabeltrace2, with the Babeltrace 2 Python bindings, or with the babeltrace2-run(1) and babeltrace2-convert(1) CLI commands.

When a trace processing graph runs, the sink components consume messages from their input ports, making all the graph’s message iterators work one message at a time to perform the trace conversion or analysis duty.

Plugin

A container, or package, of component classes as a shared library or Python module.

Each component class within a plugin has a type (source, filter, or sink) and a name. The type and name pair is unique within a given plugin.

libbabeltrace2 can load a plugin (.so, .dll, or .py file) at run time: the result is a plugin object in which you can find a specific component class and instantiate it within a trace processing graph as a component.

The babeltrace2 program uses the COMP-CLS-TYPE.PLUGIN-NAME.COMP-CLS-NAME format to identify a specific component class within a specific plugin. COMP-CLS-TYPE is either source (or src), filter (or flt), or sink.

You can list the available Babeltrace 2 plugins with the babeltrace2-list-plugins(1) command.

Query

An operation with which you can get a named object from a component class, possibly with custom query parameters.

The plain text metadata stream of a CTF trace and the available LTTng live sessions of a given LTTng relay daemon are examples of query objects.

You can use libbabeltrace2, the Babeltrace 2 Python bindings, or the babeltrace2-query(1) CLI command to query a component class’s object.

TRACE PROCESSING GRAPH REPRESENTATION

In the Babeltrace 2 manual pages, a component is represented with a box. The box has the component class type, plugin name, and component class name at the top. Just below, between square brackets, is its component name within the trace processing graph. Each port is represented with an @ symbol on the border(s) of the component box with its name inside the box. Output ports are on the box’s right border while input ports are on the box’s left border.

For example, here’s a source component box:

+------------+
| src.ctf.fs |
|  [my-src]  |
|            |
|    stream0 @
|    stream1 @
|    stream2 @
+------------+

This one is an instance of the source.ctf.fs component class named my-src. It has three output ports named stream0, stream1, and stream2.

A trace processing graph is represented with multiple component boxes connected together. The connections are arrows from output ports to input ports.

For example, here’s a simple conversion graph:

+------------+    +-----------------+    +------------------+
| src.ctf.fs |    | flt.utils.muxer |    | sink.text.pretty |
|    [ctf]   |    |     [muxer]     |    |      [text]      |
|            |    |                 |    |                  |
|    stream0 @--->@ in0         out @--->@ in               |
|    stream1 @--->@ in1             |    +------------------+
|    stream2 @--->@ in2             |
+------------+    @ in3             |
                  +-----------------+

Note that input port in3 of component muxer is not connected in this example.

Sometimes, we symbolically represent other resources which are consumed from or produced by components. In this case, arrows are used, but they do not go to or from port symbols (@), except for messages. For example, in the graph above, the ctf source component consumes a CTF trace and the text sink component prints plain text to the terminal, so here’s a more complete diagram:

CTF trace
  |
  |   +------------+    +-----------------+    +------------------+
  |   | src.ctf.fs |    | flt.utils.muxer |    | sink.text.pretty |
  '-->|    [ctf]   |    |     [muxer]     |    |      [text]      |
      |            |    |                 |    |                  |
      |    stream0 @--->@ in0         out @--->@ in               |
      |    stream1 @--->@ in1             |    +-----+------------+
      |    stream2 @--->@ in2             |          |
      +------------+    @ in3             |          '--> Terminal
                        +-----------------+

Here’s another example of a more complex graph which splits a specific stream using some criteria:

+------------+    +-----------------+    +------------------+
| src.ctf.fs |    | flt.utils.muxer |    | sink.text.pretty |
|  [ctf-in]  |    |     [muxer]     |    |      [text]      |
|            |    |                 |    |                  |
|    stream0 @--->@ in0         out @--->@ in               |
|    stream1 @--->@ in1             |    +------------------+
|    stream2 @-.  @ in2             |
+------------+ |  +-----------------+      +-------------+
               |                           | sink.ctf.fs |
               |                           |  [ctf-out0] |
               |  +-------------------+    |             |
               |  | flt.some.splitter | .->@ in          |
               |  |     [splitter]    | |  +-------------+
               |  |                   | |
               '->@ in              A @-'  +-------------+
                  |                 B @-.  | sink.ctf.fs |
                  +-------------------+ |  |  [ctf-out1] |
                                        |  |             |
                                        '->@ in          |
                                           +-------------+