This is a small push-server example for Sinatra. I did this mostly for me in order to understand how such a server could be implemented. There are certainly more robust ways to achieve this. Ask Google.

You need to have several gems, install them as follows:

sudo gem install eventmachine thin sinatra json uuid

After this, you can start the web server like this:

ruby app.rb

Point your browser to the right URL (look at your terminal output). An example:

http://localhost:4567

Open up another browser using the same URL and start drawing. Both browsers should display the same drawing!

When a Javascript client connects to the webserver, the server keeps the connection open by pretending that the data hasn't fully loaded yet. As soon as another Javascript client sends the server a message, the server will append this to the sent data. It's kind of like sending an image line for line.

Because the messages are not separate from each other, but sent as one big document, a pointer has to be maintained in order to know which data has already been interpreted and which is new.

• Carlo Jörges, started it all.
• Peter Gassner peter@naehrstoff.ch, cleaned up the code and put it into a neat package.