This question has been asked a number of times, but in a slightly different ways. Here are a few references that are worth a read:
- What are Long-Polling, Websockets, Server-Sent Events (SSE) and Comet?
- Using comet with PHP?
- Apache with Comet Support
- Server Scalability - HTML 5 websockets vs Comet
- How to implement event listening in PHP
In summary: if you are looking at building your solution using PHP on Apache then holding open persistent connections (HTTP long-polling or streaming) is going to use up resources very quickly (is highly inefficient). So, you would be better using a hosted solution (*disclaimer - I work for a hosted solution).
HTTP-Long polling and HTTP Streaming are solutions which have been superseded by Server-Sent Events and WebSockets. So, where possible (where the web client provides support) you should use one of these solutions before falling back to an HTTP-based solution. A good realtime web technology will automatically handle this for you.
Since your diagram shows you are subscribing to multiple data streams you should also consider a Publish/Subscribe solution that naturally fits with this. Again, a good realtime web tech solution will provide you with this.
Also see the realtime web technology guide.