eProsima Fast DDS Documentation

eProsima

eProsima Fast DDS is a C++ implementation of the DDS (Data Distribution Service) Specification, a protocol defined by the Object Management Group (OMG). The eProsima Fast DDS library provides both an Application Programming Interface (API) and a communication protocol that deploy a Data-Centric Publisher-Subscriber (DCPS) model, with the purpose of establishing efficient and reliable information distribution among Real-Time Systems. eProsima Fast DDS is predictable, scalable, flexible, and efficient in resource handling. For meeting these requirements, it makes use of typed interfaces and hinges on a many-to-many distributed network paradigm that neatly allows separation of the publisher and subscriber sides of the communication. eProsima Fast DDS comprises:

  • The DDS API implementation.

  • Fast DDS-Gen, a generation tool for bridging typed interfaces with the middleware implementation.

  • The underlying RTPS wire protocol implementation.

For all the above, eProsima Fast DDS has been chosen as the default middleware supported by the Robot Operating System 2 (ROS 2).

DDS API

The communication model adopted by DDS is a many-to-many unidirectional data exchange where the applications that produce the data publish it to the local caches of subscribers belonging to applications that consume the data. The information flow is regulated by Quality of Service (QoS) policies established between the entities in charge of the data exchange.

As a data-centric model, DDS builds on the concept of a “global data space” accessible to all interested applications. Applications that want to contribute information declare their intent to become publishers, whereas applications that want to access portions of the data space declare their intent to become subscribers. Each time a publisher posts new data into this space, the middleware propagates the information to all interested subscribers.

The communication happens across domains, i. e. isolated abstract planes that link all the distributed applications able to communicate with each other. Only entities belonging to a same domain can interact, and the matching between entities subscribing to data and entities publishing them is mediated by topics. Topics are unambiguous identifiers that associate a name, which is unique in the domain, to a data type and a set of attached data-specific QoS.

DDS entities are modeled either as classes or typed interfaces. The latter imply a more efficient resource handling as knowledge of the data type prior to the execution allows allocating memory in advance rather than dynamically.

_images/DDS_concept.svg

Conceptual diagram of how information flows within DDS domains. Only entities belonging to the same domain can discover each other through matching topics, and consequently exchange data between publishers and subscribers.

Fast DDS-Gen

Relying on interfaces implies the need for a generation tool that translates type descriptions into appropriate implementations that fill the gap between the interfaces and the middleware. This task is carried out by a dedicated generation tool, Fast DDS-Gen, a Java application that generates source code using the data types defined in an Interface Definition Language (IDL) file.

RTPS Wire Protocol

The protocol used by eProsima Fast DDS to exchange messages over standard networks is the Real-Time Publish-Subscribe protocol (RTPS), an interoperability wire protocol for DDS defined and maintained by the OMG consortium. This protocol provides publisher-subscriber communications over transports such as TCP/UDP/IP, and guarantees compatibility among different DDS implementations.

Given its publish-subscribe roots and its specification designed for meeting the same requirements addressed by the DDS application domain, the RTPS protocol maps to many DDS concepts and is therefore a natural choice for DDS implementations. All the RTPS core entities are associated with an RTPS domain, which represents an isolated communication plane where endpoints match. The entities specified in the RTPS protocol are in one-to-one correspondence with the DDS entities, thus allowing the communication to occur.

Main Features

  • Two API Layers. eProsima Fast DDS comprises a high-level DDS compliant layer focused on usability and a lower-level RTPS compliant layer that provides finer access to the RTPS protocol.

  • Real-Time behaviour. eProsima Fast DDS can be configured to offer real-time features, guaranteeing responses within specified time constrains.

  • Built-in Discovery Service. eProsima Fast DDS is based on the dynamical discovery of existing publishers and subscribers, and performs this task continuously without the need to contacting or setting any servers. However, a Client-Server discovery as well as other discovery paradigms can also be configured.

  • Sync and Async publication modes. eProsima Fast DDS supports both synchronous and asynchronous data publication.

  • Best effort and reliable communication. eProsima Fast DDS supports an optional reliable communication paradigm over Best Effort communications protocols such as UDP. Furthermore, another way of setting a reliable communication is to use our TCP transport.

  • Transport layers. eProsima Fast DDS implements an architecture of pluggable transports. The current version implements five transports: UDPv4, UDPv6, TCPv4, TCPv6 and SHM (shared memory).

  • Security. eProsima Fast DDS can be configured to provide secure communications. For this purpose, it implements pluggable security at three levels: authentication of remote participants, access control of entities and encryption of data.

  • Throughput controllers. We support user-configurable throughput controllers, that can be used to limit the amount of data to be sent under certain conditions.

  • Plug-and-play Connectivity. New applications and services are automatically discovered, and can join and leave the network at any time without the need for reconfiguration.

  • Scalability and Flexibility. DDS builds on the concept of a global data space. The middleware is in charge of propagating the information between publishers and subscribers. This guarantees that the distributed network is adaptable to reconfigurations and scalable to a large number of entities.

  • Application Portability. The DDS specification includes a platform specific mapping to IDL, allowing an application using DDS to switch among DDS implementations with only a re-compile.

  • Extensibility. eProsima Fast DDS allows the protocol to be extended and enhanced with new services without breaking backwards compatibility and interoperability.

  • Configurability and Modularity. eProsima Fast DDS provides an intuitive way to be configured, either through code or XML profiles. Modularity allows simple devices to implement a subset of the protocol and still participate in the network.

  • High performance. eProsima Fast DDS uses a static low-level serialization library, Fast CDR, a C++ library that serializes according to the standard CDR serialization mechanism defined in the RTPS Specification (see the Data Encapsulation chapter as a reference).

  • Easy to use. The project comes with an out-of-the-box example, the DDSHelloWorld (see Getting Started) that puts into communication a publisher and a subscriber, showcasing how eProsima Fast DDS is deployed. Additionally, the interactive demo ShapesDemo is available for the user to dive into the DDS world. The DDS and the RTPS layers are thoroughly explained in the DDS Layer and RTPS Layer sections.

  • Low resources consumption. eProsima Fast DDS:

    • Allows to preallocate resources, to minimize dynamic resource allocation.

    • Avoids the use of unbounded resources.

    • Minimizes the need to copy data.

  • Multi-platform. The OS dependencies are treated as pluggable modules. The user can easily implement his platform modules to eProsima Fast DDS library in his specific platform. By default, the project can run over Linux, Windows and MacOS.

  • Free and Open Source. The Fast DDS library, the underneath RTPS library, the generator tool, the internal dependencies (such as eProsima Fast CDR) and the external ones (such as the foonathan library) are free and open source.

Contacts and Commercial support

Find more about us at eProsima’s webpage.

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Structure of the documentation

This documentation is organized into the sections below.

Fast DDS