How does Google Docs autosave work?

Question

Okay, I know it sounds generic. But I mean on an AJAX level. I've tried using Firebug to track the NET connections and posts and it's a mystery. Does anyone know how they do the instant autosave constantly without DESTROYING the network / browser?

Answer 1

My guess (and this is only a guess) is that google uses a PUSH service. This seems like the most viable option given their chat client (which is also integrated within the window) also uses this to delivery "real time" messages with minimal latency.

I'm betting they have a whole setup that manages everything connection related and send flags to trigger specific elements. You won't see connection trigers because the initial page visit establishes the connection then just hangs on the entire duration you have the page open. e.g.

1. You visit the page
   • The browser established a connection to [example]api.docs.google.com[/example] and remains open
2. The client-side code then sends various commands and receives an assortment of responses.
3. These commands are sent back and forth until you either:
   • Lose the connection (timeout, etc.) in which case it's re-established
   • The browser window is closed

Example of, how I see, a typical communication:

SERVER:                              CLIENT:
-------                              -------
                                     DOC_FETCH mydocument.doc
DOC_CONTENT mydocument.doc 15616 ...      
                                     DOC_AUTOSAVE mydocument.doc 24335 ...
                                     IM collaboratorName Hi Joe!
IM_OK collaboratorName OK
AUTOSAVE_OK mydocument.doc OK

Where the DOC_FETCH command is saying I want the data. The server replies with the corresponding DOC_CONTENT <docname> <length> <contents>. Then the client triggers DOC_AUTOSAVE <docname> <length> <content>. Given the number of potential simultaneous requests, I would bet they keep the "context" in the requests/responses so after something is sent it can be matched up. In this example, it knows the IM_OK matches the second request (IM), and the AUTOSAVE_OK matches the first request (AUTOSAVE)--Something like how AOL's IM protocol works.

Again, this is only a guess.

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To prove this, use something like ethereal and see if you can see the information transferring in the background.