What is CometD?
What is Socket.IO?
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I use Socket.IO because using HTTP requests for a real-time multiplayer game just blows! Even with websockets, I had to scrunch the data being transmitted down to a bare minimum, and do some cheap compression tricks so that I can send data in JSON format. Otherwise, I would have to resort to sending binary data. I may end up doing that anyway when the time comes that I need to scale.
How do I use it? Each client opens a socket connection at startup. The server keeps track of these connections, and sends each client the visible portion of the Playfield repeatedly. The clients render this information, while sending requests and commands to the server (join,turn,fire,thrust,bomb,viewport change,etc.) in response to the player's actions. The server uses that to make adjustments to the player's ship on the Playfield.
Where we have browser support (recent Chrome, Firefox, and Safari), we make a WebSocket connection so that the server can push changes made by other people down to browsers listening on the appropriate channels. We use a modified version* of the Socket.io client and server libraries that allows us to keep many thousands of open WebSockets on each of our servers at very little cost in terms of CPU or memory usage. So when anything happens to a board you’re watching, that action is published to our server processes and propagated to your watching browser with very minimal latency, usually well under a second.
Socket.IO has a decent community footprint, including integrations with popular JS frameworks, and has fallbacks to maintain an app's services if websockets are not available for some reason. Websockets are an important factor in most of the web-facing apps I build, to provide asynchronous two-way communication between the app and whatever server or data source it is connected to.
Another one that we're not using, yet. But have realtime data updates within our applications and the central API will be a great bit of functionality that gives our clients more control and keep them informed of changes and updates in their stores, in real time.