2.6. Create DomainParticipant, Topic, and Type

A DomainParticipantFactory creates DomainParticipants, and a DomainParticipant itself is the factory for creating Publishers, Subscribers, and Topics.

When creating a DomainParticipant, you may need to customize DomainParticipantQos, notably for:

• Resource limits. Default resource limits are set at minimum values.

• Initial peers.

• Discovery. The name of the registered discovery component (“dpde” or “dpse”) must be assigned to DiscoveryQosPolicy’s name. Please note that in Connext Cert, only the DPSE discovery plugin is supported.

• Participant Name. Every DomainParticipant is given the same default name. Must be unique when using DPSE discovery.

Example code:

• Create a DomainParticipant with configured DomainParticipantQos:

  DDS_DomainParticipant *participant = NULL;
  struct DDS_DomainParticipantQos dp_qos =
                            DDS_DomainParticipantQos_INITIALIZER;
  
  /* DDS domain of DomainParticipant */
  DDS_Long domain_id = 0;
  
  /* Name of your registered Discovery component. The name must match the
   * name the discovery plugin was registered with.
   */
  if (!RT_ComponentFactoryId_set_name(&dp_qos.discovery.discovery.name,
                                      "dpde"))
  {
       /* failure */
  }
  
  /* Initial peers: use only default multicast peer
   */
  DDS_StringSeq_set_maximum(&dp_qos.discovery.initial_peers,1);
  DDS_StringSeq_set_length(&dp_qos.discovery.initial_peers,1);
  *DDS_StringSeq_get_reference(&dp_qos.discovery.initial_peers,0) =
      DDS_String_dup("_udp://239.255.0.1");
  
  /* Configure discovery to use the UDP transport and listen on all allowed
   * interfaces and the multicast address 239.255.0.1
   */
  DDS_StringSeq_set_maximum(&dp_qos.discovery.enabled_transports,2);
  DDS_StringSeq_set_length(&dp_qos.discovery.enabled_transports,2);
  *DDS_StringSeq_get_reference(&dp_qos.discovery.enabled_transports,0) =
          DDS_String_dup("_udp://");
  *DDS_StringSeq_get_reference(&dp_qos.discovery.enabled_transports,1) =
          DDS_String_dup("_udp://239.255.0.1");
  
  /* Configure user-data to use UDP transport and receive on all allowed
   * interfaces
   */
  DDS_StringSeq_set_maximum(&dp_qos.user_traffic.enabled_transports,1);
  DDS_StringSeq_set_length(&dp_qos.user_traffic.enabled_transports,1);
  *DDS_StringSeq_get_reference(&dp_qos.user_traffic.enabled_transports,0) =
          DDS_String_dup("_udp://");
  
  /* Resource limits to discover up to 4 remote participants */
  dp_qos.resource_limits.local_topic_allocation = 1;
  dp_qos.resource_limits.local_type_allocation = 1;
  dp_qos.resource_limits.local_reader_allocation = 1;
  dp_qos.resource_limits.local_writer_allocation = 1;
  dp_qos.resource_limits.remote_participant_allocation = 4;
  dp_qos.resource_limits.remote_reader_allocation = 4;
  dp_qos.resource_limits.remote_writer_allocation = 4;
  
  /* Participant name */
  strcpy(dp_qos.participant_name.name, "Participant_1");
  
  participant =
       DDS_DomainParticipantFactory_create_participant(factory,
                                                       domain_id,
                                                       &dp_qos,
                                                       NULL,
                                                       DDS_STATUS_MASK_NONE);
   if (participant == NULL)
   {
       /* failure */
   }
  

2.6.1. Register Type

Your data types that have been generated from IDL need to be registered with the DomainParticipants that will be using them. Each registered type must have a unique name.

DDS_ReturnCode_t retcode;

retcode = DDS_DomainParticipant_register_type(participant,
                                              "HelloWorld",
                                              HelloWorldTypePlugin_get());
if (retcode != DDS_RETCODE_OK)
{
    /* failure */
}

2.6.2. Create Topic of Registered Type

DDS Topics encapsulate the types being communicated, and you can create Topics for your type once your type is registered.

A topic is given a name at creation (e.g. “Example HelloWorld”). The type associated with the Topic is specified with its registered name.

DDS_Topic *topic = NULL;

topic = DDS_DomainParticipant_create_topic(participant,
                                           "Example HelloWorld",
                                           "HelloWorld",
                                           &DDS_TOPIC_QOS_DEFAULT,
                                           NULL,
                                           DDS_STATUS_MASK_NONE);

if (topic == NULL)
{
    /* failure */
}

2.6.3. DPSE Discovery: Assert Remote Participant

DPSE Discovery relies on the application to specify the remote DomainParticipants that its local DomainParticipants are allowed to discover. Your application must call a DPSE API for each remote participant to be discovered. The API takes as input the name of the remote participant.

/* Allow discovery of remote participant with name Participant_2 */
retcode = DPSE_RemoteParticipant_assert(participant, "Participant_2");
if (retcode != DDS_RETCODE_OK)
{
    /* failure */
}

For more information, see DDS Domains.