Reddit launches MSPaint MMO

When a user clicks on the canvas to place a tile, that click is transmitted to the server along with an x,y location. The server decides whether the user is authorized to place the tile.

The canvas exists on the server as an N-by-M array of values. If the user is authorized to place the tile, this array is modified at the specified x,y location to the value the user selected.

When the page is first loaded, the server transmits that NxM array of values to your browser. The browser then builds a canvas element which is essentially a "viewer" of that NxM array of values.

Presumably the page leverages caching, so that when you reload the page, the server only re-transmits the parts of the NxM array that have actually changed since the last time you visited.

Throw in some DoS protection and some realtime updates and you're done.

It's a neat technical achievement with interesting results. Unfortunately HN doesn't seem to be picking up the story.