Proxying downloads

[Martin Marcher]

Hello,

more a recipe question. I'm working on a proxy that will download a
file for a client. The thing that doesn't yield problems is:

Alice (Client)
Bob (Client)
Sam (Server)

1 Alice asks Sam for "foobar.iso"
2 Sam can't find "foobar.iso" in "cachedir"
3 Sam requests "foobar.iso" from the uplink
4 Sam now saves each chunk received to "cachedir/foobar.iso"
5 At the same time Sam forwards each chunk to Alice.

But I can't figure out how I would solve the following:

1 Alice asks Sam for "foobar.iso"
2 Sam can't find "foobar.iso" in "cachedir"
3 Sam requests "foobar.iso" from uplink
4 Sam saves and forwards to Alice
5 At about 30 % of the download Bob asks Sam for "foobar.iso"
6 How do I serve Bob now?

Now because the internal link is _a lot_ faster than the uplink Bob
will probably reach the end of (the local) "foobar.iso" before Sam has
received "foobar.iso" in total from uplink. So Bob will end up with a
incomplete file...

How do I solve that. The already downloaded data should of course be
served internally.

The solutions I think of are
 * Some kind of subscriber list for the file in question
  * That is serve internally and if the state of "foobar.iso" is in
progress switch to receiving chunk directly from Sam as it comes down
the link
  * How would I realize this switch from internal serving to pass thru
of chunks?

 * Send an acknowledge (lie to the client that we have this file in
the cache) wait until it's finished and then serve the file from the
internal cache)
  * This could lead to timeouts for very large files, at least I think so

 * Forget about all of it and just pass thru from uplink, with a new
request, as long as files are in progress. This would in the worst
case download the file n times where n is the number of clients.
  * I guess that's the easiest one but also the least desirable solution.

I hope I explained my problem somehow understandable.

any hints are welcome
thanks
martin

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