http pull / push

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For push:
server sends Content-type: multipart/x-mixed-replace and then sends each part whenever it wants.
The browser will keep waiting for parts and present each part as a different page.
-in ASP do response.flush for each part and "sleep" until next part?
-is CGI ok since they will stay alive for so long?
-how is the server to cope with many of this such request, what is better separate processes (CGI, out of process ASP) or in-server-process scripts (pooled ASP)
-what about apache, is CGI or mod-python better
-what about third parties like webware?

How does someone do incremental updates of only one part of the page?
Maybe via JavaScript window.XMLHttpRequest and document.write or element.innerHTML ?
Maybe the MIME part is some javascript in a hidden (1 pixel) frame that updates the rest of the page (frames)?

It kind of starts to smell already, but I should give it a try and eventualy look for more info, for now i think I'll stay with pull via javascript XMLHttpRequest or HTML <meta refresh>.