freeDDS

This work has moved to https://github.com/ros2/ros2_embedded_nuttx

freeDDS is an open source Data Distribution Service (DDS) minimal implementation for embedded devices that should run on bare-metal.

Milestones

This section gives an overview of the milestones needed to complete freeDDS. It also provides an effort to approximate the time needs for each one of the tasks:

• [x] Quick overview/understand fo the OMG DDS standart [1 week]
  • [ ] Read/understand deeply the OMG DDS standart [2 weeks]
• [ ] Implement a basic skelethon for the DDS classes [1 week]
• [ ] Use the POSIX network stack to create a simple UDP/IP publisher/subscriber example [ <1 week ]
• [ ] Read/understand the OMG RTPS protocol [2 weeks ]
• [ ] Implement the RTPS layer for freeDDS [ 3 weeks]
  • [ ] Assess the RT aspects of the overall DDS implementation [ 2 weeks]
• [ ] Evaluate the overall implementation (wireshark) and do interfacing tests with other DDSs [ 2 weeks]
• [ ] ROS client nano library client API [ 2 weeks]
• [ ] ROS 2 Desktop <-> ROS 2 Embedded tests [ 4 weeks]

Total estimated 20 weeks.

Code structure

The code has been designed according to the DDS specification and it is structured in 5 modules ("Data Distribution Service for Real-time Systems, v1.2", pg. 24):

• The Infrastructure Module (src/infrastructure) defines the abstract classes and the interfaces that are refined by the other modules. It also provides support for the two interaction styles (notification- and wait- based) with the middleware.
• The Domain Module (src/domain) contains the DomainParticipant class that acts as an entry-point of the Service and acts as a factory for many of the classes. The DomainParticipant also acts as a container for the other objects that make up the Service.
• The Topic-Definition Module (src/topic) contains the Topic, ContentFilteredTopic, and MultiTopic classes, the TopicListener interface, and more generally, all that is needed by the application to define Topic objects and attach QoS policies to them.
• The Publication Module (src/publication) contains the Publisher and DataWriter classes as well as the PublisherListener and DataWriterListener interfaces, and more generally, all that is needed on the publication side.
• The Subscription Module (src/subscription) contains the Subscriber, DataReader, ReadCondition, and QueryCondition classes, as well as the SubscriberListener and DataReaderListener interfaces, and more generally, all that is needed on the subscription side.

Design considerations:

• The code has been programmed in ANSI C - C11 (link to a draft).
• Each one of the abstract concepts mentioned in the DDS standard has been coded using a pseudo-OOP mechanism containing pseudo-attributes and pseudo-methods. E.g., the header of the Topic interface:

typedef struct Topic {
  /* pseudo-attributes */
  char *name;
  QoSPolicy* qos;
  DomainParticipant* domain_participant;

} Topic;

  /* pseudo-methods */
Topic * new_Topic(const char *);
void delete_Topic(Topic *);
void set_qos_Topic(struct Topic *, QoSPolicy*);
QoSPolicy* get_qos_Topic(struct Topic *); 

• In order to make the implementation suitable to deep-embedded systems with low RAM characteristics, the code passes memory pointers instead of making local copies and dynamic memory will be only used in an intial allocation phase.