Diagnostic Topic Monitor

Offers diagnostic tools which monitor topics for health, such as frequency and age of messages. The output can be further processed with the diagnostic_aggregator for cleaner observation.

Monitors

There are two monitors currently available, which can be used as components or standalone nodes:

• topic_frequency_monitor – this supports any kind of topic and checks for frequency based on receive time
• topic_age_monitor – this supports only topics with a Header, and it checks that message age is within a given range

It is totally possible to run both of these at the same time, for different or the same topics, to check for both frequency and max message age at the same time.

Usage

You typically add the monitor(s) you want to a launch file in your package and configure the parameters to your liking. However, the nodes and components are ROS 2 life-cycle based, which means you need to configure and activate them manually.

See the launch tests in the test folder for examples of how start the standalone monitor nodes, and example configs in test/config.

TIP: If you like the brevity and don’t mind the added dependency, you can use the nav2_lifecycle_manager with parameter autostart set to true and target your monitor nodes to autostart them.

Example launch snippet:

# Include this node in your LaunchConfiguration
autostart_monitors_node = LifecycleNode(
        package="nav2_lifecycle_manager",
        executable="lifecycle_manager",
        name="autostart_monitors",
        parameters=[
            {
              "autostart": True,
              "node_names": [<your monitor nodes>],
              "bond_timeout": 0.0,
            }
        ],
    )

For this, you need to have the nav2_lifecycle_manager package available (binary or source code installation), and ideally add it as an exec_depend in your package.

Limiting which topics to monitor

By default, the following internal topic names will be ignored by the monitors, as they are usually irrelevant:

• ^/rosout$
• .*/parameter_events
• ^/diagnostics$
• .*/transition_event

ROS2 parameter monitor_configured_only for both monitors:

If false (the default), the topic monitor will monitor everything but the internal topics mentioned above. To instead only monitor those topics which have been explicitly configured, set this to “true”

Influencing the output

ROS2 parameter diag_prefix for both the monitors:

This string will be prefixed to the name of the diagnostic for use within the aggregator.

topic_frequency_monitor

Without any parameters, this node (component also available) will subscribe to all normal topics in a system and minimally monitor that there is activity on them (defined as “at least one message per second”). For more specific configuration, the following parameters are available.

Frequency Monitoring

Usually, we want to make sure that the frequency of a topic is within certain bounds.

For example, to monitor that the frequency of the “talker” topic is about 25Hz and the frequency of the “cmd_vel” topic is about 10Hz, use the following three parameters and pass them lists in the same order. This means, the second element in “min_values” applies to the topic given by the second element of the “topics” parameter.

topics: [ "talker", "cmd_vel" ]
min_values: [ 23.0, 8.0 ]
max_values: [ 26.5, 11.0 ]

Please note that the frequencies will be computed purely based on arrival timing, the header is currently ignored.

topic_age_monitor

Without any parameters, this node (component also available) will subscribe to all normal topics in a system and minimally monitor that there is activity on them (defined as “at least one message per second”). For more specific configuration, the following parameters are available.

Message Age Monitoring

Usually, we want to make sure that message is not much older than expected (allowing for some jitter due to network transmission), but also not newer than expected (which would indicate issues with timestamp or system time between machines).

Since this relies on a sender-side timestamp, this monitor only works with message that include a Header.

NOTE: You can configure it for any topic and it will interpret the data in there as a timestamp, but unless it really is a header, the results will be rather random ;-)

Configuration is similar to the frequency monitor, but giving a range for delays instead:

topics: [ "talker", "cmd_vel" ]
min_values: [ 80, 8.0 ]
max_values: [ 120, 11.0 ]
monitor_configured_only: True

It is highly prudent to only monitor configured topics in the age monitor. If you set it to monitor all, and you have topics without header fields, you will get a lot of warning messages!!

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