Builtin Topics

Concept

Built-in Topics are a special kind of topics that are used by RTI Connext DDS applications to discover each other. These Topics are handled automatically by the middleware, but in certain scenarios it is useful to access to them. For instance, these Topics allow us to access some relevant information about Connext DDS entities, such as:

  • The publications and susbcriptions available
  • The TypeCode of publications
  • The QoS requested/offered by DataReaders and DataWriters.
  • Access to application specific data in discovery (e.g. USER_DATA)

Example Description

This example shows how USER_DATA QoS can be used for security during discovery phase. For this purpose, we check USER_DATA fields for discovered Participants and DataReaders. If the Participant USER_DATA matches an authorization string, we allow all DataReaders from that Participant read access. Otherwise, we check the USER_DATA from individual DataReaders to see if it is authorized.

On the publisher side, we show how to attach listeners to Builtin Topics. Each Participant has a builtin Subscriber with three Topics. These subscriptions receive data when the Participant gets information about another Participant, DataReader or DataWriter.

This exposes two points:

  1. Even though the builtin Topics correspond to a hierarchy of Participants -> DataWriters/DataReaders, there is no hierarchy of listener callbacks. If, as in this example, we listen for Participant and DataReader information, both listeners will get the on_data_available() callbacks. If we want to track relationships between discovered participants and DataReaders/DataWriters, we may do so by keys.
  2. The callbacks for Participant discovery are guaranteed to fire before the callbacks for readers or writers that are children of that Participant.

On the subscriber side, we show how to set the user_data QoS fields for participants and data readers. To do this, we first get the relevant default QoS, then add our data as an DDS_OctetSeq. These Octets are opaque to DDS, and will not undergo any conversions during transmission.

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