Introduction

DDS-XRCE protocol

Micro XRCE-DDS implements DDS-XRCE protocol specified in the DDS for eXtremely Resource Constrained Environments proposal submitted to the Object Management Group (OMG) consortium. This protocol allows to communicate resource constrained devices with a larger DDS (Data Distribution Service) network. This communication is done using a client-server architecture, where the server (Agent) acts as an intermediary between clients and DDS Global Data Space.

DDS-XRCE protocol defines a wire protocol between XRCE Agents and XRCE Clients. The messages exchanged revolve around operations and their responses. XRCE Clients ask the XRCE Agents to run operations and the XRCE Agents reply with the result of those requested operations. Making use of those operations, XRCE Clients are able to create the DDS entities hierarchy necessary to publish and/or receive data from DDS. DDS entities are created and stored on the XRCE Agent side so the XRCE Clients can reuse them at will.

Micro XRCE-DDS implements the DDS-XRCE protocol using a Micro XRCE-DDS Agent as server and providing a C API for developing XRCE Clients applications. Micro XRCE-DDS Agent uses Fast RTPS to reach the DDS Global Data Space.

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Fast RTPS

eProsima Fast RTPS is a C++ implementation of the RTPS (Real-Time Publish-Subscribe) protocol, which provides publisher-subscriber communications over unreliable transports such as UDP, as defined and maintained by the OMG consortium. XRCE-DDS is also the wire interoperability protocol defined for the DDS standard, again by the OMG.

For deeper information please refer to the official documentation: eProsima Fast RTPS

Operations and entities

Micro XRCE-DDS communication between XRCE Client and XRCE Agent is based upon Operations and responses. Clients request operations to the Agent which will generate responses with the result of these operations. Clients may process the responses to know how an operations request has ended in the Agent.

Micro XRCE-DDS Client can request a variety of operations to the Micro XRCE-DDS Agent:

  • Create and delete sessions.
  • Create and delete entities.
  • Write and read Data.

First of all, the Agent must know about any Client. This is done by sending a Create session operation from the Client to the Agent, which will register the Client. If you do not register the session, all the operations sending to the Agent will be refused. Once registered, the Client can request operations to the Agent. The Client can create and query entities using operations. The communication with DDS is handled by Agent using Entities.

Create entity Operation is the request you use to create entities in the Agent. Each one of these entities corresponds with a Fast RTPS object. The entities you can create are:

  • Participants.
  • Topics.
  • Publisher.
  • Subscriber.
  • DataWriter.
  • DataReader.

For sending and receiving data from/to DDS, Client has access to the DataWriter and DataReader entities. These entities handle the write/read operations. For sending and receiving any topic the Write Data and Read Data operations must be used.

If you want to remove any entity from the Agent you can use Delete entity operations. Also, if you want to log out a Client session from the Agent, you can use the Delete session operation.

Topic Type

The data sent by the Client to the DDS Global Data Space uses the same principles as in Fast RTPS. The Interface Definition Language (IDL) is used to define the type and must be known by the Client. Having the type defined as IDL we provide the Micro XRCE-DDS Gen tool. This tool is able to generate a compatible type that the Micro XRCE-DDS Client can use to send and receive. The type should match the one used on the DDS Side.