1.1. What is DDS?

The Data Distribution Service (DDS) is a data-centric communication protocol used for distributed software application communications. It describes the communications Application Programming Interfaces (APIs) and Communication Semantics that enable communication between data providers and data consumers.

Since it is a Data-Centric Publish Subscribe (DCPS) model, three key application entities are defined in its implementation: publication entities, which define the information-generating objects and their properties; subscription entities, which define the information-consuming objects and their properties; and configuration entities that define the types of information that are transmitted as topics, and create the publisher and subscriber with its Quality of Service (QoS) properties, ensuring the correct performance of the above entities.

DDS uses QoS to define the behavioral characteristics of DDS Entities. QoS are comprised of individual QoS policies (objects of type deriving from QoSPolicy). These are described in Policy.

1.1.1. The DCPS conceptual model

In the DCPS model, four basic elements are defined for the development of a system of communicating applications.

  • Publisher. It is the DCPS entity in charge of the creation and configuration of the DataWriters it implements. The DataWriter is the entity in charge of the actual publication of the messages. Each one will have an assigned Topic under which the messages are published. See Publisher for further details.

  • Subscriber. It is the DCPS Entity in charge of receiving the data published under the topics to which it subscribes. It serves one or more DataReader objects, which are responsible for communicating the availability of new data to the application. See Subscriber for further details.

  • Topic. It is the entity that binds publications and subscriptions. It is unique within a DDS domain. Through the TopicDescription, it allows the uniformity of data types of publications and subscriptions. See Topic for further details.

  • Domain. This is the concept used to link all publishers and subscribers, belonging to one or more applications, which exchange data under different topics. These individual applications that participate in a domain are called DomainParticipant. The DDS Domain is identified by a domain ID. The DomainParticipant defines the domain ID to specify the DDS domain to which it belongs. Two DomainParticipants with different IDs are not aware of each other’s presence in the network. Hence, several communication channels can be created. This is applied in scenarios where several DDS applications are involved, with their respective DomainParticipants communicating with each other, but these applications must not interfere. The DomainParticipant acts as a container for other DCPS Entities, acts as a factory for Publisher, Subscriber and Topic Entities, and provides administrative services in the domain. See Domain for further details.

These elements are shown in the figure below.

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DCPS model entities in the DDS Domain.

1.2. What is RTPS?

The Real-Time Publish Subscribe (RTPS) protocol, developed to support DDS applications, is a publication-subscription communication middleware over best-effort transports such as UDP/IP. Furthermore, Fast DDS provides support for TCP and Shared Memory (SHM) transports.

It is designed to support both unicast and multicast communications.

At the top of RTPS, inherited from DDS, the Domain can be found, which defines a separate plane of communication. Several domains can coexist at the same time independently. A domain contains any number of RTPSParticipants, that is, elements capable of sending and receiving data. To do this, the RTPSParticipants use their Endpoints:

  • RTPSWriter: Endpoint able to send data.

  • RTPSReader: Endpoint able to receive data.

A RTPSParticipant can have any number of writer and reader endpoints.

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RTPS high-level architecture

Communication revolves around Topics, which define and label the data being exchanged. The topics do not belong to a specific participant. The participant, through the RTPSWriters, makes changes in the data published under a topic, and through the RTPSReaders receives the data associated with the topics to which it subscribes. The communication unit is called Change, which represents an update in the data that is written under a Topic. RTPSReaders/RTPSWriters register these changes on their History, a data structure that serves as a cache for recent changes.

In the default configuration of eProsima Fast DDS, when you publish a change through a RTPSWriter endpoint, the following steps happen behind the scenes:

  1. The change is added to the RTPSWriter’s history cache.

  2. The RTPSWriter sends the change to any RTPSReaders it knows about.

  3. After receiving data, RTPSReaders update their history cache with the new change.

However, FastDDS supports numerous configurations that allow you to change the behavior of RTPSWriters/RTPSReaders. A modification in the default configuration of the RTPS entities implies a change in the data exchange flow between RTPSWriters and RTPSReaders. Moreover, by choosing Quality of Service (QoS) policies, you can affect how these history caches are managed in several ways, but the communication loop remains the same. You can continue reading section RTPS Layer to learn more about the implementation of the RTPS protocol in Fast DDS.