How to write a socket server VI


Once you’ve got a socket server running locally on a web-hosting machine, you need to expose it to the outside world. Luckily this is quite easy using PHP. Sockets are well-supported and already heavily used to communicate with MySQL.

Take the source from the previous article, and build the server part on the machine you’ll be hosting the service on. You’ll want this service to run even if you’re logged off, so for now use the command

nohup ./wcserver /tmp/myservicesocket &

This will keep the server process running even if you exit the current terminal window. On a production system you’ll actually want to start the server when the system reboots, but this approach is simpler for testing purposes.

The next hurdle to overcome is making sure that your Apache httpd process, which runs PHP, has the right permissions to access that socket file. This will depend on the user setup on your machine, but typically you’ll have a special apache user account that you’ll need to add to the file access list. For testing purposes you can always grant everyone on the machine access to the socket file, though it would be preferable for security reasons to be a bit more picky in production. Run this command to grant everyone permission to access it:

chmod a+rw /tmp/myservicesocket

Now the server is running and available, you need to write an equivalent to the command-line client in PHP. Here’s the source and I’ll go over the details below.

$fp = stream_socket_client("unix:///tmp/myservicesocket", 
$errno, $errstr, 30);

PHP takes care of a lot of the socket setup code for you. The only bit I found tricky was specifying a local file socket, it turns out you do that using the special ‘unix’ protocol specification in the URL, followed by the file path.

    fwrite($fp, "Some Message\n");
    while (!feof($fp)) {
        echo fgets($fp, 1024);

The connect call returns a file handle that you can use with the standard file access functions. As you can see, it looks pretty similar to the C version of the same code. You can see the results running on one of my servers here.

The example service isn’t doing very much so far. What’s really exciting about this approach is that it offers a completely language-independent way for any interesting computational service to be integrated into the standard LAMP stack. A lot of my work involves processor-heavy work on hard problems like laying out large graphs and statistical analysis. This should let me move them from the client to become online services.

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