P2P Communication

This section illustrates the peer-to-peer communication mode offered by eProsima Micro XRCE-DDS. It is organized as follows:

• Introduction

• Publish/Subscribe P2P Example

Introduction

The peer-to-peer (P2P) mode allows direct communication between applications without DDS, where by application is to be understood the combination of an Agent and one or more Clients.

In this communication mode, the Agent uses the CedMiddleware. The Agent is in charge of discovering other External Agents and create an Internal Client for each one of them. Each Internal Client connects to an External Agent and subscribes to the set of Topics managed by its own Agent. Thus, a cloud of interconnected Agents is created.

Some consideration shall be taken into account in order to use the P2P communication:

1. Only the create_<entity>_by_ref() functions shall be used.

2. The Topic’s reference represents the name of the Topic.

3. The DataWriter’s and DataReader’s references need to match the Topic’s reference.

4. Publishers and Subscribers have no role.

5. Agents use the CedMiddleware and the Discovery mechanism.

Publish/Subscribe P2P Example

This kind of behaviour can be probed by putting in communication a Publisher P2P application with a Subscriber P2P application.

Agent with CedMiddleware

First of all, install the Agent as explained in the Installing the Agent standalone section. On Linux, this would be:

$ git clone https://github.com/eProsima/Micro-XRCE-DDS-Agent.git
$ cd Micro-XRCE-DDS-Agent
$ mkdir build && cd build
$ cmake ..
$ make
$ sudo make install

After having installed the Agent system-wide, launch it with the ced option activated:

$ ./MicroXRCEAgent udp4 -p <port> -m ced -d

Client P2P publisher app

Let’s now install the Client locally, and with the -DUCLIENT_BUILD_EXAMPLES=ON flag enabled, so as to activate the compilation of the examples. On Linux, this implies running the following:

$ git clone https://github.com/eProsima/Micro-XRCE-DDS-Client.git
$ cd Micro-XRCE-DDS-Client
$ mkdir build && cd build
$ cmake .. -DUCLIENT_BUILD_EXAMPLES=ON
$ make

At this point, it’s possible to launch the PublishHelloWorldClientP2P executable located in the folder Micro-XRCE-DDS-Client/build/examples/PublishHelloWorldP2P, which’ll make the Client publish in the DDS World the HelloWorld topic (take a look at the IDL defining this topic in the file Micro-XRCE-DDS-Client/examples/PublishHelloWorldP2P/HelloWorld.idl).

$ examples/PublishHelloWorld2P2/PublishHelloWorldClientP2P 127.0.0.1 2019

The source code of the PublishHelloWorldClientP2P can be found in Micro-XRCE-DDS-Client/examples/PublishHelloWorldP2P/main.c.

Client P2P subscriber app

After having executed the publisher app, we can launch the SubscribeHelloWorldClientP2P excutable, which is located in the folder Micro-XRCE-DDS-Client/build/examples/SubscribeHelloWorldP2P, which’ll make this Client subscribe to the same HelloWorld topic from the DDS World.

$ examples/SubscriberHelloWorldP2P/SubscribeHelloWorldClientP2P 127.0.0.1 2019

The source code of the SubscribeHelloWorldClientP2P can be found in Micro-XRCE-DDS-Client/examples/SubscribeHelloWorldP2P/main.c.

At this point, the subscriber will receive the topics that are being sent by the publisher.