Scalability

I have a general question about scalability.  The FAQ states:
  
"Unlike mod_pubsub, Pushlets can only maintain a few simultaneous
connections, and require Java programming."

Looking at something like Tomcat, it's easy to see that Pushlets are
using a thread per-request, and since the request never returns,  each
client is tying up one thread.  Obviously this isn't a requirement of
HTTP, it's just how it happens to be implemented.  Normally the server
is expected to handle the request quickly and return - then the thread
goes back into some pool.

My question is: what is mod-pubsub doing differently?  Is it working
at the socket level then, maintaining a list of open connections per
subscription and only writing out to them from some service thread
when an event occurs on that subscription?

In this case I suppose you're only limited by the number of sockets
availble to the OS.

Sorry if this is a basic question, but I can't find anywhere that
spells this out.

Thanks


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