2.3.5. XML Participant¶

This type of Participant refers to a DomainParticipant that uses QoS profiles loaded from XML files to be configured.

Fast DDS supports XML to fully configure a DomainParticipant. Using XML configuration, users have whole access to the full configuration of a DDS DomainParticipant. Check the following documentation for further information on how to configure a DDS DomainParticipant with XML. For further information regarding how to load XML configuration files to the DDS Router, check the Load XML Configuration section.

Note

This kind of Participant is meant for advanced users as XML profiles will overwrite the default internal settings of the DDS Router.

2.3.5.1. Use case¶

Use this Participant to fully configure a DomainParticipant, its discovery methods, transport options, DDS QoS, etc. The main use case for this Participant is using DDS Security (see Security), which requires XML configuration from the user’s side.

Warning

This Participant kind does not support RPC. Thus services and actions of ROS 2 will not work correctly.

2.3.5.2. Kind aliases¶

xml
XML

2.3.5.3. Configuration¶

The XML Participant allows setting a profile name for the internal DomainParticipant of the DDS Router. Such profile name will be used as the QoS profile when creating the internal DomainParticipant.

2.3.5.3.1. Create a Fast DDS XML Participant profile¶

The whole DomainParticipant configuration settings must be configured via XML, DDS Router will not configure any attribute or QoS for it. To configure the profile, check the Profile configuration section.

However, there are specific QoS that will affect the performance of the DDS Router and that are advisable for the user to set them. Notice that not setting such QoS will not affect the correct functionality of the application, but may affect its performance.

ignore_local_endpoints avoid local matching for this participant’s endpoints:

<participant profile_name="ignore_local_endpoints_domainparticipant_xml_profile">
    <rtps>
        <propertiesPolicy>
            <properties>
                <property>
                    <name>fastdds.ignore_local_endpoints</name>
                    <value>true</value>
                </property>
            </properties>
        </propertiesPolicy>
    </rtps>
</participant>

2.3.5.3.2. Endpoint profiles¶

When the XML Participant creates a DataWriter or DataReader for a topic, it looks for a loaded XML data_writer or data_reader profile to configure that endpoint. By default, the DDS Router looks for a profile whose name matches the topic name. If a matching profile is found, the endpoint is configured using that profile’s QoS, giving the user control over fields such as history, memory policy, transport, etc. If no matching profile exists, the endpoint falls back to default QoS with values derived from the YAML configuration and from discovery.

Note

Endpoint profiles are only applied by XML Participants. The endpoints of any other Participant kind ignore them, even if a profile whose name matches the topic name is loaded.

Note

Certain QoS are always enforced by the DDS Router regardless of the XML profile: deadline on DataWriters (set to the minimum value so it matches any DataReader), autodispose_unregistered_instances on DataWriters (set to false to preserve dispose/unregister forwarding semantics), and expects_inline_qos on DataReaders for keyed topics.

The following example loads a data_writer and a data_reader profile named my_topic that will be automatically applied when creating endpoints for a topic of that name:

<?xml version="1.0" encoding="UTF-8" ?>
<profiles xmlns="http://www.eprosima.com">
    <data_writer profile_name="my_topic">
        <historyMemoryPolicy>DYNAMIC</historyMemoryPolicy>
    </data_writer>
    <data_reader profile_name="my_topic">
        <historyMemoryPolicy>DYNAMIC</historyMemoryPolicy>
    </data_reader>
</profiles>

Selecting a profile explicitly¶

Instead of relying on the topic name, a specific profile can be selected for a topic with the endpoint-profile-name tag of the Topic QoS. When set, the DDS Router looks up the data_writer and data_reader profiles with that name instead of the topic name:

topics:
  - name: "rt/chatter"
    qos:
      endpoint-profile-name: "chatter_profile"
    participants:
      - xml_participant

Like any other Topic QoS, endpoint-profile-name can be set in the Manual Topics, the Participant Topic QoS and the Specs Topic QoS, with the same precedence among them. Since XML profiles are loaded for the whole DDS Router process, this is the way to apply different profiles to the same topic in different XML Participants. If no profile with that name is loaded, the DDS Router does not look for a profile named after the topic; the endpoint falls back to default QoS instead.

Overriding profile QoS from the YAML configuration¶

When a matching XML profile is applied, the following Topic QoS override the corresponding values from the profile, but only if they are explicitly set in the YAML configuration (in the Manual Topics, the Participant Topic QoS or the Specs Topic QoS): durability, reliability, ownership and history-depth. Every other field keeps the value from the XML profile.

QoS values that the DDS Router learns from remote endpoints during discovery never override the XML profile. Fields that are set neither in the XML profile nor in the YAML configuration therefore take the Fast DDS default values (e.g. KEEP_LAST history with depth 1), instead of being adapted to the discovered endpoints.

Warning

Setting history-depth in the Specs Topic QoS overrides the history of every matching XML profile, even when it is set to its default value of 5000. Likewise, if the remote DataWriters use EXCLUSIVE_OWNERSHIP_QOS, set ownership either in the XML profile or in the YAML configuration, otherwise the DataReaders of the XML Participant will not match them.

2.3.5.3.3. Repeater¶

This Participant allows a tag repeater to be used as a Repeater server. Please refer to section Repeater DDS Router for more information.

2.3.5.4. Configuration Example¶

Configure a XML Participant that gets all of its QoS from XML profile named custom_participant_configuration. This XML profile must be previously loaded. Use Fast DDS or DDS Router support to load XML configuration files as explained in this section.

- name: xml_participant                       # Participant Name = xml_participant

  kind: xml

  profile: custom_participant_configuration   # Configure participant with this profile