2. Library Overview¶
Fast DDS (formerly Fast RTPS) is an efficient and high-performance implementation of the DDS specification, a data-centric communications middleware (DCPS) for distributed application software. This section reviews the architecture, operation and key features of Fast DDS.
The architecture of Fast DDS is shown in the figure below, where a layer model with the following different environments can be seen.
Application layer. The user application that makes use of the Fast DDS API for the implementation of communications in distributed systems.
Fast DDS layer. Robust implementation of the DDS communications middleware. It allows the deployment of one or more DDS domains in which DomainParticipants within the same domain exchange messages by publishing/subscribing under a domain topic.
RTPS layer. Implementation of the Real-Time Publish-Subscribe (RTPS) protocol for interoperability with DDS applications. This layer acts an abstraction layer of the transport layer.
Transport Layer. Fast DDS can be used over various transport protocols such as unreliable transport protocols (UDP), reliable transport protocols (TCP), or shared memory transport protocols (SHM).
2.1.1. DDS Layer¶
Several key elements for communication are defined in the DDS layer of Fast DDS. The user will create these elements in their application, thus incorporating DDS application elements and creating a data-centric communication system. Fast DDS, following the DDS specification, defines these elements involved in communication as Entities. A DDS Entity is any object that supports Quality of Service configuration (QoS), and the implements listener.
QoS. The mechanism by which the behavior of each of the entities is defined.
Listener. The mechanism by which the entities are notified of the possible events that arise during the application’s execution.
Below are listed the DDS Entities together with their description and functionality. For a more detailed explanation of each entity, their QoS, and their listeners, please refer to DDS Layer section.
Domain. A positive integer which identifies the DDS domain. Each DomainParticipant will have an assigned DDS domain, so that DomainParticipants in the same domain can communicate, as well as isolate communications between DDS domains. This value must be given by the application developer when creating the DomainParticipants.
DomainParticipant. Object containing other DDS entities such as publishers, subscribers, topics and multitopics. It is the entity that allows the creation of the previous entities it contains, as well as the configuration of their behavior.
Publisher. The Publisher publishes data under a topic using a DataWriter, which reads the data from the transport. It is the entity that creates and configures the DataWriter entities it contains, and may contain one or more of them.
DataWriter. It is the entity in charge of publishing messages. The user must provide a Topic when creating this entity which will be the Topic under which the data will be published. Publication is done by writing the data-objects as a change in the DataWriterHistory.
DataWriterHistory. This is a list of changes to the data-objects. When the DataWriter proceeds to publish data under a specific Topic, it actually creates a change in this data. It is this change that is registered in the History. These changes are then sent to the DataReader that subscribes to that specific topic.
Subscriber. The Subscriber subscribes to a topic using a DataReader, which reads the data from the transport. It is the entity that creates and configures the DataReader entities it contains, and may contain one or more DataReader entities.
DataReader. It is the entity that subscribes to the topics for the reception of publications. The user must provide a subscription Topic when creating this entity. A DataReader receives the messages as changes in its HistoryDataReader.
DataReaderHistory. It contains the changes in the data-objects that the DataReader receives as a result of subscribing to a certain Topic.
Topic. Entity that binds Publishers’ DataWriters with Subscribers’ DataReaders.
2.1.2. RTPS layer¶
As mentioned above, the RTPS protocol in Fast DDS allows the abstraction of DDS application entities from the transport layer. According to the graph shown above, the RTPS layer has four main Entities.
RTPSDomain. It is the extension of the DDS domain to the RTPS protocol.
RTPSParticipant. Entity containing other RTPS entities. It allows the configuration and creation of the entities it contains.
RTPSWriter. The source of the messages. It reads the changes written in the DataWriterHistory and transmits them to all the RTPSReaders to which it has previously matched.
RTPSReader. Receiving entity of the messages. It writes the changes reported by the RTPSWriter into the DataReaderHistory.
For a more detailed explanation of each entity, their attributes, and their listeners, please refer to RTPS Layer section.
2.1.3. Transport layer¶
Fast DDS supports the implementation of applications over various transport protocols. Those are UDPv4, UDPv6, TCPv4, TCPv6 and Shared Memory Transport (SHM). By default, a DomainParticipant implements a UDPv4 and a SHM transport protocol. The configuration of all supported transport protocols is detailed in the Transport Layer section.
2.2. Programming and execution model¶
Fast DDS is concurrent and event-based. The following explains the multithreading model that governs the operation of Fast DDS as well as the possible events.
2.2.1. Concurrency and multithreading¶
Fast DDS implements a concurrent multithreading system. Each DomainParticipant spawns a set of threads to take care of background tasks such as logging, message reception, and asynchronous communication. This should not impact the way you use the library, i.e. the Fast DDS API is thread safe, so you can fearlessly call any methods on the same DomainParticipant from different threads. However, this multithreading implementation must be taken into account when external functions access to resources that are modified by threads running internally in the library. An example of this is the modified resources in the entity listener callbacks. The following is a brief overview of how Fast DDS multithreading schedule work:
Main thread: Managed by the application.
Event thread: Each DomainParticipant owns one of these. It processes periodic and triggered time events.
Asynchronous writer thread: This thread manages asynchronous writes for all DomainParticipants. Even for synchronous writers, some forms of communication must be initiated in the background.
Reception threads: DomainParticipants spawn a thread for each reception channel, where the concept of a channel depends on the transport layer (e.g. a UDP port).
2.2.2. Event-driven architecture¶
There is a time-event system that enables Fast DDS to respond to certain conditions, as well as schedule periodic
Few of them are visible to the user since most are related to DDS and RTPS metadata.
However, the user can define in their application periodic time-events by inheriting from the
Fast DDS has some added features that can be implemented and configured by the user in their application. These are outlined below.
2.3.1. Discovery Protocols¶
The discovery protocols define the mechanisms by which DataWriters publishing under a given Topic, and DataReaders subscribing to that same Topic are matched, so that they can start sharing data. This applies at any point in the communication process. Fast DDS provides the following discovery mechanisms:
Simple Discovery. This is the default discovery mechanism, which is defined in the RTPS standard and provides compatibility with other DDS implementations Here the DomainParticipants are discovered individually at an early stage to subsequently match the DataWriter and DataReader they implement.
Discovery Server. This discovery mechanism uses a centralized discovery architecture, where servers act as a hubs for discovery meta traffic.
Static Discovery. This implements the discovery of DomainParticipants to each other but it is possible to skip the discovery of the entities contained in each DomainParticipant (DataReader/DataWriter) if these entities are known in advance by the remote DomainParticipants.
Manual Discovery. This mechanism is only compatible with the RTPS layer. It allows the user to manually match and unmatch RTPSParticipants, RTPSWriters, and RTPSReaders using whatever external meta-information channel of its choice.
The detailed explanation and configuration of all the discovery protocols implemented in Fast DDS can be seen in the Discovery section.
Fast DDS can be configured to provide secure communications by implementing pluggable security at three levels:
Authentication of remote DomainParticipants. The DDS:Auth:PKI-DH plugin provides authentication using a trusted Certificate Authority (CA) and ECDSA Digital Signature Algorithms to perform the mutual authentication. It also establishes a shared secret using Elliptic Curve Diffie-Hellman (ECDH) Key Agreement protocol.
Access control of entities. The DDS:Access:Permissions plugin provides access control to DomainParticipants at the DDS Domain and Topic level.
Encryption of data. The DDS:Crypto:AES-GCM-GMAC plugin provides authenticated encryption using Advanced Encryption Standard (AES) in Galois Counter Mode (AES-GCM).
More information about security configuration in Fast DDS is available in the Security section.
Fast DDS provides an extensible Logging system.
Log class is the entry point of the Logging system.
It exposes three macro definitions to ease its usage:
Moreover, it allows the definition of new categories, in addition to those already available
It provides filtering by category using regular expressions, as well as control of the verbosity of the Logging system.
Details of the possible Logging system configurations can be found in the Logging section.
2.3.4. XML profiles configuration¶
Fast DDS offers the possibility to make changes in its default settings by using XML profile configuration files. Thus, the behavior of the DDS Entities can be modified without the need for the user to implement any program source code or re-build an existing application.
The user has XML tags for each of the API functionalities.
Therefore, it is possible to build and configure DomainParticipant profiles through the
<participant> tag, or
the DataWriter and DataReader profiles with the
<data_reader> tags respectively.
For a better understanding of how to write and use these XML profiles configuration files you can continue reading the XML profiles section.
2.3.5. Environment variables¶
Environment variables are those variables that are defined outside the scope of the program, through operating system functionalities. Fast DDS relies on environment variables so that the user can easily customize the default settings of DDS applications. Please, refer to the Environment variables section for a complete list and description of the environment variables affecting Fast DDS.