Custom uptime to Stdout

In this demo, we will go through a new use case. You want to create your own measurement or use an existing measurement provided by dc_measurements but not exactly, you want to add a field and also modify the JSON schema. That is what we will do here: create a new plugin, inside dc_demos, another package, create a plugin and load it

We are going to take the uptime measurement, change it slightly and collect the data.

To test it, run:

ros2 launch dc_demos uptime_custom_stdout.launch.py

JSON schema

Located in dc_demos/plugins/measurements/json/uptime_custom.json:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "Uptime Custom",
  "description": "Time the system has been up. Intentionally failing to demonstrate customization and callback",
  "properties": {
    "time": {
      "description": "Time the system has been up",
      "type": "integer",
      "maximum": 0
    }
  },
  "type": "object"
}

It is almost the same as the standard uptime, but for the sake of the example, we will set the maximum value to 0, which will certainly make the validation fail!

CPP code

First, we create a hpp file in dc_demos/include/dc_demos/plugins/measurements:

#ifndef DC_DEMOS__PLUGINS__MEASUREMENTS__UPTIME_CUSTOM_HPP_
#define DC_DEMOS__PLUGINS__MEASUREMENTS__UPTIME_CUSTOM_HPP_

#include <nlohmann/json-schema.hpp>
#include <nlohmann/json.hpp>

#include "dc_measurements/measurement.hpp"
#include "dc_measurements/plugins/measurements/uptime.hpp"

namespace dc_demos
{
using json = nlohmann::json;

class UptimeCustom : public dc_measurements::Uptime
{
protected:
  void onFailedValidation(json data_json) override;
};

}  // namespace dc_demos

#endif  // DC_DEMOS__PLUGINS__MEASUREMENTS__UPTIME_CUSTOM_HPP_

We create a new class UptimeCustom, which inherits from dc_measurements::Uptime.

The method onFailedValidation is not mandatory but it is here to show it is possible to trigger a custom function when the validation fails.

Then the cpp code, currently located in dc_demos/plugins/measurements/uptime_custom.cpp:

#include "dc_demos/plugins/measurements/uptime_custom.hpp"

namespace dc_demos
{

void UptimeCustom::onFailedValidation(json data_json)
{
  (void)data_json;
  RCLCPP_INFO(logger_, "Callback! Validation failed for uptime custom");
}

}  // namespace dc_demos

#include "pluginlib/class_list_macros.hpp"
PLUGINLIB_EXPORT_CLASS(dc_demos::UptimeCustom, dc_core::Measurement)

We include the uptime_custom file header. Then, define the onFailedValidation function (triggered when validation fails).

The schema is picked up without any code: a Measurement validates against plugins/measurements/json/<plugin type>.json from the package registering it, so dc_demos/UptimeCustom reads dc_demos/plugins/measurements/json/uptime_custom.json. Set json_schema_path to point elsewhere, or enable_validator: false to turn validation off.

Plugin file

Create the xml file, here will be measurement_plugin.xml for us, at the source of the package. It defines the plugins of the package.

<class_libraries>
    <library path="dc_uptime_custom_measurement">
        <class name="dc_demos/UptimeCustom" type="dc_demos::UptimeCustom" base_class_type="dc_core::Measurement">
            <description>
                dc_measurement_uptime_custom
            </description>
        </class>
    </library>
</class_libraries>

CMakeLists.txt

Now that you have all files set up, you can add the build process to the CMakeLists.txt:

# Measurement plugins
add_library(dc_uptime_custom_measurement SHARED
  plugins/measurements/uptime_custom.cpp
)
list(APPEND dc_measurement_plugin_libs dc_uptime_custom_measurement)

foreach(measurement_plugin ${dc_measurement_plugin_libs})
  ament_target_dependencies(${measurement_plugin} ${dependencies})
  target_link_libraries(
    ${measurement_plugin}
    nlohmann_json::nlohmann_json
    nlohmann_json_schema_validator
  )
  target_compile_definitions(${measurement_plugin} PRIVATE BT_PLUGIN_EXPORT)
endforeach()

pluginlib_export_plugin_description_file(dc_core measurement_plugin.xml)

install(FILES measurement_plugin.xml
  DESTINATION share/${PROJECT_NAME}
)

install(DIRECTORY plugins/measurements/
  DESTINATION share/${PROJECT_NAME}/plugins/measurements/
)

It creates the library, installs and exports it.

Console output

In the measurement server log, the plugin is detected properly

[component_container_isolated-1] [INFO] [1788506243.714587305] [measurement_server]: Creating measurement plugin uptime_custom: Type dc_demos/UptimeCustom, Group key: uptime, Polling interval: 5000, Debug: 0, Validator enabled: 1, Schema path: , Tags: [], Init collect: 1, Init Max measurement: 0, Include measurement name: 1, Include measurement plugin name: 0, Remote keys: , Remote prefixes: , Nest: 0, Flatten: 0, Include measurement plugin name: 0, Max measurement on condition: 0, If all condition: , If any condition: , If none condition: , Gate condition: , Buffer duration sec: 0, Post roll duration sec: 0, Cooldown sec: 0, Max flush rate hz: 0, Flush topic: /dc/flush
[component_container_isolated-1] [INFO] [1788506243.724443694] [measurement_server]: Done configuring uptime_custom
[component_container_isolated-1] [INFO] [1788506243.724997657] [measurement_server]: Looking for schema at /root/ws/install/dc_demos/share/dc_demos/plugins/measurements/json/uptime_custom.json
[component_container_isolated-1] [INFO] [1788506243.725045689] [measurement_server]: schema: {"$schema":"http://json-schema.org/draft-07/schema#","description":"Time the system has been up. Intentionally failing to demonstrate customization and callback","properties":{"time":{"description":"Time the system has been up","maximum":0,"type":"integer"}},"title":"Uptime Custom","type":"object"}

Then, it fails as expected:

[component_container_isolated-1] [ERROR] [1788506243.726002578] [measurement_server]: Validation failed: At /time of 1638628 - instance exceeds maximum of 0
[component_container_isolated-1] data={"time":1638628}
[component_container_isolated-1] [INFO] [1788506243.726050184] [measurement_server]: Callback! Validation failed for uptime custom
[component_container_isolated-1] [ERROR] [1788506248.725121430] [measurement_server]: Validation failed: At /time of 1638633 - instance exceeds maximum of 0
[component_container_isolated-1] data={"time":1638633}
[component_container_isolated-1] [INFO] [1788506248.725329122] [measurement_server]: Callback! Validation failed for uptime custom

Even though every Record fails validation, dc_bridge still ships the raw data to the console Destination — validation failure only triggers onFailedValidation, it doesn't drop the Record:

[dc_bridge-2] {"date":1788506243.7255764,"flattened":false,"host":"127.0.0.1","name":"uptime_custom","nested":false,"source_type":"fluent","tag":"dc.measurement.uptime_custom","time":1638628,"timestamp":"2026-09-04T07:17:23.725576334Z"}