Data Distribution Service
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Data Distribution Service for Real-time Systems (DDS) is a specification of a publish/subscribe middleware for distributed systems created in response to the need to augment CORBA with a data-centric publish-subscribe specification. A few proprietary DDS solutions have been available for several years, until 2004 when the two major DDS vendors, the American Real-Time Innovations and the French Thales Group teamed up to create the DDS specifications that have been approved by the Object Management Group
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[edit] Version History
- DDS 1.3 — Working version, OMG members only.
- DDS 1.2 (January 1, 2007)
- DDS 1.1 (December 4, 2005)
- DDS 1.0
The DDS specification describes two levels of interfaces:
- A lower DCPS (Data-Centric Publish-Subscribe) level that is targeted towards the efficient delivery of the proper information to the proper recipients.
- An optional higher DLRL (Data Local Reconstruction Layer) level, which allows for a simple integration of DDS into the application layer.
[edit] Architecture
[edit] DDS elements
- DDS Publisher
- A DDS object that is responsible for data distribution. A publisher may publish data of different types.
- DDS DataWriter
- A DDS object that an application must use to communicate to a publisher the existence and value of data-objects of a given type.
- DDS Subscriber
- A DDS object that is responsible for receiving published data and making it available to the receiving applications. A subscriber may receive and dispatch data of different types.
- DDS DataReader
- A DDS object, attached to a subscriber, that is responsible accessing received data and making it available to an application.
[edit] DDS model
DDS is networking middleware that simplifies complex network programming. It implements a publish/subscribe model for sending and receiving data, events, and commands among the nodes. Nodes that are producing information (publishers) create "topics" (e.g., temperature, location, pressure) and publish "samples." DDS takes care of delivering the sample to all subscribers that declare an interest in that topic.
DDS handles all the transfer chores: message addressing, data marshaling and demarshalling (so subscribers can be on different platforms than the publisher), delivery, flow control, retries, etc. Any node can be a publisher, subscriber, or both simultaneously.
The DDS publish-subscribe model virtually eliminates complex network programming for distributed applications.
DDS supports mechanisms that go beyond the basic publish-subscribe model. The key benefit is that applications that use DDS for their communications are entirely decoupled. Very little design time has to be spent on how to handle their mutual interactions. In particular, the applications never need information about the other participating applications, including their existence or locations. DDS automatically handles all aspects of message delivery, without requiring any intervention from the user applications, including:
- determining who should receive the messages,
- where recipients are located,
- what happens if messages cannot be delivered.
This is made possible by the fact that DDS allows the user to specify Quality of Service (QoS) parameters as a way to configure automatic-discovery mechanisms and specify the behavior used when sending and receiving messages. The mechanisms are configured up-front and require no further effort on the user's part. By exchanging messages in a completely anonymous manner, DDS greatly simplifies distributed application design and encourages modular, well-structured programs.
DDS also automatically handles hot-swapping redundant publishers if the primary fails. Subscribers always get the sample with the highest priority whose data is still valid (that is, whose publisher-specified validity period has not expired). It automatically switches back to the primary when it recovers, too. DDS is available from different vendors with C, C++ and Java APIs.
