prioritizing torrent download sequences using libtorrent

Question

Suppose I have 2+ clients (developed by me) ALL using libtorrent ( http://www.rasterbar.com/products/libtorrent/manual.html#queuing )

Can I prioritize download of a file from other clients effectively so that they download the file's pieces/chunks (whatever is torrent terminology here) from beginning of the file towards its end and not quite in random order?

(of course I'm allowing some "multiplexing" / "intertwining" pieces for reasons of availability and performance, but the goal here is to download as linearly and quickly from the start of the file towards the end as possible)

The goal I'm thinking about here is obviously previewing the file quickly. How to do this most effectively using libtorrent / possibly other C++ torrent library?

(I'm not quite interested in torrent implementations using non-binary languages, like Java or Python - I need machine code for reasons of performance and security, so, C, C++ or possibly D would all fit the bill)

Answer 1

You can certainly prioritize pieces and files with torrent_handle::prioritize_pieces() and torrent_handle::prioritize_files(). See the documentation.

This won't be enough to download in-order though. To do that, you can enable sequential download with torrent_handle::set_sequential_download(). This will issue new piece requests in-order. Keep in mind that the time a request take to be satisfied varies a lot depending on which peer you talk to. Making the requests in-order does not necessarily mean receiving the pieces in order.

There is another mechanism to attempt to do that. torrent_handle::set_piece_deadline() is used to set a target completion time for a piece. Such pieces are considered time-critical pieces, and they are ordered by their deadline and the fastest peers are used to request blocks from those pieces, attempting to download them in deadline-order.

Now, I also get the impression that you want two separate clients (presumably running on different machines) to coordinate which pieces they download. Is that right? It's not entirely clear what you're asking about, but there's no simple way of asking libtorrent to do that.

You could write a plugin for libtorrent that implements a new extension message for these clients to chat and coordinate, which could de-select certain pieces the other client is downloading by setting their priority to 0.