Parameter server

ROS 2 parameters allow the user to create variables on a node and manipulate/read them with different ROS 2 commands. Further information about ROS 2 parameters can be found here

Ready to use code examples related to this tutorial can be found in rclc/rclc_examples/src/example_parameter_server.c.

Initialization

  • Default initialization: 
      // Parameter server object
  rclc_parameter_server_t param_server;

  // Initialize parameter server with default configuration
  rcl_ret_t rc = rclc_parameter_server_init_default(&param_server, &node);

  if (RCL_RET_OK != rc) {
      ... // Handle error
      return -1;
  }

  • Initialization with custom options:

    The following options can be configured:

    • notify_changed_over_dds: Publish parameter events to other ROS 2 nodes as well.
    • max_params: Maximum number of parameters allowed on the rclc_parameter_server_t object.
    • allow_undeclared_parameters: Allows creation of parameters from external parameter clients. A new parameter will be created if a set operation is requested on a non-existing parameter.

    • low_mem_mode: Reduces the memory used by the parameter server, functionality constrains are applied.

      // Parameter server object
rclc_parameter_server_t param_server;

// Initialize parameter server options
const rclc_parameter_options_t options = {
    .notify_changed_over_dds = true,
    .max_params = 4,
    .allow_undeclared_parameters = true,
    .low_mem_mode = false; };

// Initialize parameter server with configured options
rcl_ret_t rc = rclc_parameter_server_init_with_option(&param_server, &node, &options);

if (RCL_RET_OK != rc) {
  ... // Handle error
  return -1;
}

  • Low memory mode:

    This mode ports the parameter functionality to memory constrained devices. The following constrains are applied:

    • Request size limited to one parameter on Set, Get, Get types and Describe services.
    • List parameter request has no prefixes enabled nor depth.
    • Parameter description strings not allowed, rclc_add_parameter_description is disabled.

    Memory benchmark results on STM32F4 for 7 parameters with RCLC_PARAMETER_MAX_STRING_LENGTH = 50 and notify_changed_over_dds = true:

    • Full mode: 11736 B
    • Low memory mode: 4160 B

Memory requirements

The parameter server uses five services and an optional publisher. These need to be taken into account on the rmw-microxrcedds package memory configuration:

# colcon.meta example with memory requirements to use a parameter server
{
    "names": {
        "rmw_microxrcedds": {
            "cmake-args": [
                "-DRMW_UXRCE_MAX_NODES=1",
                "-DRMW_UXRCE_MAX_PUBLISHERS=1",
                "-DRMW_UXRCE_MAX_SUBSCRIPTIONS=0",
                "-DRMW_UXRCE_MAX_SERVICES=5",
                "-DRMW_UXRCE_MAX_CLIENTS=0"
            ]
        }
    }
}

At runtime, the variable RCLC_PARAMETER_EXECUTOR_HANDLES_NUMBER defines the necessary number of handles required by a parameter server for the rclc Executor:

// Executor init example with the minimum RCLC executor handles required
rclc_executor_t executor = rclc_executor_get_zero_initialized_executor();
rc = rclc_executor_init(
    &executor, &support.context,
    RCLC_PARAMETER_EXECUTOR_HANDLES_NUMBER, &allocator);

Humble: the variable is RCLC_PARAMETER_EXECUTOR_HANDLES_NUMBER has been renamed to RCLC_EXECUTOR_PARAMETER_SERVER_HANDLES.

Add a parameter

The micro-ROS parameter server supports the following parameter types:

  • Boolean: 
      const char * parameter_name = "parameter_bool";
  bool param_value = true;

  // Add parameter to the server
  rcl_ret_t rc = rclc_add_parameter(&param_server, parameter_name, RCLC_PARAMETER_BOOL);

  // Set parameter value (Triggers `on_parameter_changed` callback, if defined)
  rc = rclc_parameter_set_bool(&param_server, parameter_name, param_value);

  // Get parameter value and store it in "param_value"
  rc = rclc_parameter_get_bool(&param_server, "param1", &param_value);

  if (RCL_RET_OK != rc) {
      ... // Handle error
      return -1;
  }
  • Integer: 
      const char * parameter_name = "parameter_int";
  int param_value = 100;

  // Add parameter to the server
  rcl_ret_t rc = rclc_add_parameter(&param_server, parameter_name, RCLC_PARAMETER_INT);

  // Set parameter value
  rc = rclc_parameter_set_int(&param_server, parameter_name, param_value);

  // Get parameter value on param_value
  rc = rclc_parameter_get_int(&param_server, parameter_name, &param_value);
  • Double: 
      const char * parameter_name = "parameter_double";
  double param_value = 0.15;

  // Add parameter to the server
  rcl_ret_t rc = rclc_add_parameter(&param_server, parameter_name, RCLC_PARAMETER_DOUBLE);

  // Set parameter value
  rc = rclc_parameter_set_double(&param_server, parameter_name, param_value);

  // Get parameter value on param_value
  rc = rclc_parameter_get_double(&param_server, parameter_name, &param_value);

Parameters can also be created by external clients if the allow_undeclared_parameters flag is set. The client just needs to set a value on a non-existing parameter. Then this parameter will be created if the server has still capacity and the callback allows the operation.

Max name size is controlled by the compile-time option RCLC_PARAMETER_MAX_STRING_LENGTH, default value is 50.

Delete a parameter

Parameters can be deleted by both, the parameter server and external clients:

rclc_delete_parameter(&param_server, "param2");

For external delete requests, the server callback will be executed, allowing the node to reject the operation.

Parameters description

  • Parameter description Adds a description of a parameter and its constrains, which will be returned on a describe parameter requests: 
      rclc_add_parameter_description(&param_server, "param2", "Second parameter", "Only even numbers");

    The maximum string size is controlled by the compilation time option RCLC_PARAMETER_MAX_STRING_LENGTH, default value is 50.

  • Parameter constraints Informative numeric constraints can be added to int and double parameters, returning this values on describe parameter requests:
    • from_value: Start value for valid values, inclusive.
    • to_value: End value for valid values, inclusive.

    • step: Size of valid steps between the from and to bound.

        int64_t int_from = 0;
  int64_t int_to = 20;
  uint64_t int_step = 2;
  rclc_add_parameter_constraint_integer(&param_server, "param2", int_from, int_to, int_step);

  double double_from = -0.5;
  double double_to = 0.5;
  double double_step = 0.01;
  rclc_add_parameter_constraint_double(&param_server, "param3", double_from, double_to, double_step);

    This constrains will not be applied by the parameter server, leaving values filtering to the user callback.

  • Read-only parameters: The new API offers a read-only flag. This flag blocks parameter changes from external clients, but allows changes on the server side: 
      bool read_only = true;
  rclc_set_parameter_read_only(&param_server, "param3", read_only);

Callback

When adding the parameter server to the executor, a callback could to be configured. This callback would then be executed on the following events:

  • Parameter value change: Internal and external parameter set on existing parameters.
  • Parameter creation: External parameter set on unexisting parameter if allow_undeclared_parameters is set.
  • Parameter delete: External parameter delete on existing parameter.
  • The user can allow or reject this operations using the bool return value.

Callback parameters:

  • old_param: Parameter actual value, NULL for new parameter request.
  • new_param: Parameter new value, NULL for parameter removal request.
  • context: User context, configured on rclc_executor_add_parameter_server_with_context.
bool on_parameter_changed(const Parameter * old_param, const Parameter * new_param, void * context)
{
  (void) context;

  if (old_param == NULL && new_param == NULL) {
    printf("Callback error, both parameters are NULL\n");
    return false;
  }

  if (old_param == NULL) {
    printf("Creating new parameter %s\n", new_param->name.data);
  } else if (new_param == NULL) {
    printf("Deleting parameter %s\n", old_param->name.data);
  } else {
    printf("Parameter %s modified.", old_param->name.data);
    switch (old_param->value.type) {
      case RCLC_PARAMETER_BOOL:
        printf(
          " Old value: %d, New value: %d (bool)", old_param->value.bool_value,
          new_param->value.bool_value);
        break;
      case RCLC_PARAMETER_INT:
        printf(
          " Old value: %ld, New value: %ld (int)", old_param->value.integer_value,
          new_param->value.integer_value);
        break;
      case RCLC_PARAMETER_DOUBLE:
        printf(
          " Old value: %f, New value: %f (double)", old_param->value.double_value,
          new_param->value.double_value);
        break;
      default:
        break;
    }
    printf("\n");
  }

  return true;
}

Parameters modifications are disabled while the callback on_parameter_changed is executed, causing the following methods to return RCLC_PARAMETER_DISABLED_ON_CALLBACK if they are invoked:

  • rclc_add_parameter
  • rclc_delete_parameter
  • rclc_parameter_set_bool
  • rclc_parameter_set_int
  • rclc_parameter_set_double
  • rclc_set_parameter_read_only
  • rclc_add_parameter_constraint_double
  • rclc_add_parameter_constraint_integer

Once the parameter server and the executor are initialized, the parameter server must be added to the executor in order to accept parameter commands from ROS 2:

// Add parameter server to the executor including defined callback
rc = rclc_executor_add_parameter_server(&executor, &param_server, on_parameter_changed);

Note that this callback is optional as its just an event information for the user. To use the parameter server without a callback:

// Add parameter server to the executor without a callback
rc = rclc_executor_add_parameter_server(&executor, &param_server, NULL);

Configuration of the callback context:

// Add parameter server to the executor including defined callback and a context
rc = rclc_executor_add_parameter_server_with_context(&executor, &param_server, on_parameter_changed, &context);

Cleaning up

To destroy an initialized parameter server:

// Delete parameter server
rclc_parameter_server_fini(&param_server, &node);

This will delete any automatically created infrastructure on the agent (if possible) and deallocate used memory on the parameter server side.