4.7.5. INTRA Transport

The built-in intra participant transport (INTRA) is a transport that bypasses RTPS and reduces the number of data-copies from three to one for data published by a DataWriter to a DataReader within the same participant. When a sample is published, it is copied directly to the data reader’s cache (if there is space). This transport is used for communication between DataReaders and DataWriters created within the same participant by default.

Please refer to Threading Model for important details regarding application constraints when using this transport.

4.7.5.1. Registering the INTRA Transport

The built-in INTRA transport is a RTI Connext DDS Micro component that is automatically registered when the DDS_DomainParticipantFactory_get_instance() method is called. By default, data published by a DataWriter is sent to all DataReaders within the same participant using the INTRA transport.

In order to prevent the INTRA transport from being used it is necessary to remove it as a transport and a user-data transport. The following code shows how to only use the builtin UDP transport for user-data.

struct DDS_DomainParticipantQos dp_qos =
                            DDS_DomainParticipantQos_INITIALIZER;

REDA_StringSeq_set_maximum(&dp_qos.transports.enabled_transports,1);
REDA_StringSeq_set_length(&dp_qos.transports.enabled_transports,1);
*REDA_StringSeq_get_reference(&dp_qos.transports.enabled_transports,0) =
                                    REDA_String_dup(NETIO_DEFAULT_UDP_NAME);

/* Use only unicast for user-data traffic. */
REDA_StringSeq_set_maximum(&dp_qos.user_traffic.enabled_transports,1);
REDA_StringSeq_set_length(&dp_qos.user_traffic.enabled_transports,1);
*REDA_StringSeq_get_reference(&dp_qos.user_traffic.enabled_transports,0) =
                                    REDA_String_dup("_udp://");

Note that the INTRA transport is never used for discovery traffic internally. It is not possible to disable matching of DataReaders and DataWriters within the same participant.

4.7.5.2. Reliability and Durability

Because a sample sent over INTRA bypasses the RTPS reliability and DDS durability queue, the Reliability and Durability Qos policies are not supported by the INTRA transport. However, by creating all the DataReaders before the DataWriters durability is not required.

4.7.5.3. Threading Model

The INTRA transport does not create any threads. Instead, a DataReader receives data over the INTRA transport in the context of the DataWriter’s send thread.

This model has two important limitations:

• Because a DataReader’s on_data_available() listener is called in the context of the DataWriter’s send thread, a DataReader may potentially process data at a different priority than intended (the DataWriter’s). While it is generally not recommended to process data in a DataReader’s on_data_available() callback, it is particularly important to not do so when using the INTRA transport. Instead, use a DDS WaitSet or a similar construct to wake up a separate thread to process data.
• Because a DataReader’s on_data_available() listener is called in the context of the DataWriter’s send thread, any method called in the on_data_available() callback is done in the context of the DataWriter’s stack. Calling a DataWriter write() in the callback could result in an infinite call stack. Thus, it is recommended not to call in this listener any Connext DDS Micro APIs that write data.