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httpnetwork-programmingtcpcomet

HTTP Response before Request

My question might sound stupid, but I just wanted to be sure:

  • Is it possible to send an HTTP response before having the request for that resource?

Say for example you have an HTML page index.html that only shows a picture called img.jpg. Now, if your server knows that a visitor will request the HTML file and then the jpg image every time:

Would it be possible for the server to send the image just after the HTML file to save time?

I know that HTTP is a synchronous protocol, so in theory it should not work, but I just wanted someone to confirm it (or not).

Solution

  • If someone requests /index.html and you send two responses (one for /index.html and the other for /img.jpg), how do you know the recipient will get the two responses and know what to do with them before the second request goes in?

    The problem is not really with the sending. The problem is with the receiver possibly getting unexpected data.

    One other issue is that you're denying the client the ability to use HTTP caching tools like If-Modified-Since and If-None-Match (i.e. the client might not want /img.jpg to be sent because it already has a cached copy).

    That said, you can approximate the server-push benefits by using Comet techniques. But that is much more involved than simply anticipating incoming HTTP requests.