C++ HTTP API Design

Question

I have a massive library written in C++ that handles a lot of mathematical operations, using C++ makes it fast and in all honesty, I don't want to re-write the whole functionality.

I am thinking about opening this library up to other, web languages (Such as PHP) and will pass the data between the two using JSON.

The issue, I have not had much experience designing/developing an API in C++ and from thinking about the implementation I have come up with this:

The API can be accessed from a HTTP URL: http://localhost/api/ and pass in different commands into it. For example: getDeter .. Which I know how I will do this using PHP.

The problem that I am having difficult understanding is how will visiting http://localhost/api work in C++? Is there another approach to doing this:

• Compiling the C++ into a output file main.out
• Have a function in PHP that calls a command ./main.out [command] [arg]
• Return the response

I did not want to do this approach as I don't think that it is very clean, but I can't seem to think of an alternative to solving this problem.

P.S. I don't want to really open up the core C++ code and make changes. I would like to do the following (In C++)

function calculateDeter(std::vector<double> input)
{
    // call library function 
    // call another library function 

    // return json data

} 

Answer 1

The technique you are describing is (very roughly) similar to CGI, a way for web servers to call external code of any kind. PHP itself is usually run as a CGI application. If you were to implement it this way, a good place to start would be to look at the FCGI documentation. In short, you could use the FCGI library to write an interface for a web server to call your C++ code.

Another approach would be to embed an HTTP server directly into your application. This is a little simpler, since it removes the need for a separate web server, but leaves you to deal with issues like scalability and security that a full-fledged web server will include out the box. If you went down this route you might use a library like libevent.