6.3.6. Parameter Server¶

Table of Contents

Initialization
- Options
- Low memory mode
Callback
Add a parameter
Delete a parameter
Parameters description
Memory requirements
Cleaning up

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 on Understanding parameters.

Ready to use code related to this concepts can be found on micro-ROS demos repository parameter_server example.

6.3.6.1. Initialization¶

A micro-ROS parameter server can be initiated using the RCLC parameter server API:

// 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;
}

6.3.6.1.1. Options¶

A parameter server can be configured at configuration time, the following options can be adjusted:

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;
}

6.3.6.1.2. Low memory mode¶

There is a low memory mode that 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 operations.
List parameter request has no prefixes enabled nor depth.
Parameter description strings not allowed, rclc_add_parameter_description is disabled.

Note

Using low memory mode in a STM32F4 with 7 parameters with RCLC_PARAMETER_MAX_STRING_LENGTH = 50 and notify_changed_over_dds = true the memory usage drops from 11.7 kB to 4.1 kB.

6.3.6.2. 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);

6.3.6.3. 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);

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

Note

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.

6.3.6.4. 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.

6.3.6.5. Parameters description¶

Parameter description: Adds a description of a parameter and its constraints, which will be returned on a describe parameter request:
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 that can be added to int and double parameters, returning these 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);
Note

This constrains will not be applied by the parameter server, leaving values filtering to the user callback.
Read-only parameters: 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);

6.3.6.6. 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:

Note

Check micro-ROS memory management for entity creating tutorial for more information.

# 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_EXECUTOR_PARAMETER_SERVER_HANDLES 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_EXECUTOR_PARAMETER_SERVER_HANDLES, &allocator);

6.3.6.7. 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.