cluon-javascript-cpp

This sub-project contains the C++ part of the JavaScript--C++ communication demo.

Table of Contents

• Description
• Dependencies
• Usage
• Building
• License

Description

The purpose of this sample program is demonstrate how to design a simple message using the ODVD message specification from libcluon to send a message from a C++ program via websockets to JavaScript. In a JavaScript application, this message is received, unpacked, and responded to.

The first step is define the message; here, we use the file example.odvd in this folder on the folder js. Apparently, these message files must match - otherwise, the encoding and decoding of the messages will fail.

The next step is to transform this message specification into C++ using cluon-msc - the message specification compiler from libcluon. The following steps are contained in the CMakeLists.txt file; they are described here for the sake of completeness.

This compiler is contained in the single-file, header-only distribution. To compile the message compiler from the header-only library, we set the compiler definition HAVE_CLUON_MSC. As the GNU compiler does not allow to create executables from a given .hpp file, we create a symbolic link to the single-file, header-only cluon-complete-v0.0.104.hpp file:

ln -sf cluon-complete-v0.0.104.hpp cluon-complete.cpp

Now, we compile the executable for the message compiler using a C++14-compliant compiler. The cluon-msc tool depends on pthreads on Linux.

g++ -DHAVE_CLUON_MSC -std=c++14 -pthread -o cluon-msc cluon-complete.cpp

Next, we create the self-contained, C++14-compliant header file from our message specification file example.odvd:

./cluon-msc --cpp --out=example.hpp example.odvd

Now, we can create a simple C++ program, that is using an OD4Session to exchange the messages that we just defined. Therefore, we simply create an instance of class OD4Session that we pass a conference identifier (CID) that must be identical to all microservices that are supposed to send and receive messages with each other:

cluon::OD4Session od4{111};

In this case, all microservices belonging to CID 111 can see each other and communicate with each other. Next, we define the a lambda that is called whenever our program is receiving an Envelope of the desired type example::Pong. The body of this lambda is simply unpacking the Envelope and printing the content to stdout. The lambda is registered at the OD4Session for the message identifier of example::Pong.

auto pong = [](cluon::data::Envelope &&env){
    example::Pong p = cluon::extractMessage<example::Pong>(std::move(env));
    std::cout << "Received: '" << p.text() << "', " << p.number() << std::endl;
};
od4.dataTrigger(example::Pong::ID(), pong);

Next, we add a time-triggered stimulus to trigger some computation at the JavaScript application. Therefore, we define another lambda that we pass the reference to the OD4Session so that we can access send(...) method. Furthermore, we add a simple counter variable so that we can send some more meaningful data. Finally, the time-triggered lambda needs to return true as long as it wants to be kept executed; if it returns false, its time-triggered execution would end.

uint32_t counter{0};
auto ping = [&od4, &counter](){
    example::Ping p;
    p.number(counter).text("Hello World: " + std::to_string(counter++));
    od4.send(p);
    return true;
};

const float RUN_AT_TWO_HERTZ{2.0f};
od4.timeTrigger(RUN_AT_TWO_HERTZ, ping); // Won't return until the lambda returns false.

Dependencies

No dependencies! You just need a C++14-compliant compiler to compile this project as it ships its dependencies as part of the source distribution:

• libcluon -

Usage

This microservice is provided via Docker's public registry for:

• 
• 

Running the C++ demo program to send data in time-triggered mode:

docker run --rm -ti --net=host chrberger/cluon-javascript-cpp-amd64:latest ping-pong --cid=111

You can check whether this program works properly by using cluon-livefeed:

docker run --rm -ti --init --net=host -v $PWD:/opt chrberger/cluon-livefeed-multi:v0.0.89 --cid=111 --odvd=/opt/example.odvd

Building

To natively build this demo program, simply compile it using a C++14-compliant compiler:

1. Create and change to an out-of-source build folder:

mkdir build && cd build

2. Run cmake to create the build files:

cmake ..

3. Build the C++ application:

make

License

• This project is released under the terms of the MIT License.