Wish: git annex get [files] -jN should run up to N downloads of files concurrently.

This can already be done by just starting up N separate git-annex processes all trying to get the same files. They'll coordinate themselves to avoid downloading the same file twice.

But, the output of concurrent git annex get's in a single teminal is a mess.

It would be nice to have something similar to docker's output when fetching layers of an image. Something like:

get foo1 ok
get foo2 ok
get foo3 -> 5% 100 KiB/s
get foo4 -> 3% 90 KiB/s
get foo5 -> 20% 1 MiB/s

Where the bottom N lines are progress displays for the downloads that are currently in progress. When a download finishes, it can scroll up the screen with "ok".

get foo1 ok
get foo2 ok
get foo5 ok
get foo3 -> 5% 100 KiB/s
get foo4 -> 3% 90 KiB/s
get foo6 -> 0% 110 Kib/S

This display could perhaps be generalized for other concurrent actions. For example, drop:

drop foo1 ok
drop foo2 failed
  Not enough copies ...
drop foo3 -> (checking r1...)
drop foo4 -> (checking r2...)

But, do get first.

Pain points:

  1. Currently, git-annex lets tools like rsync and wget display their own progress. This makes sense for the single-file at a time get, because rsync can display better output than just a percentage. (This is especially the case with aria2c for torrents, which displays seeder/leecher info in addition to percentage.)

    But in multi-get mode, the progress display would be simplified. git-annex can already get percent done information, either as reported by individiual backends, or by falling back to polling the file as it's downloaded.

  2. The mechanics of updating the screen for a multi-line progress output require some terminal handling code. Using eg, curses, in a mode that doesn't take over the whole screen display, but just moves the cursor up to the line for the progress that needs updating and redraws that line. Doing this portably is probably going to be a pain, especially I have no idea if it can be done on Windows.

    An alternative would be a display more like apt uses for concurrent downloads, all on one line:

    get foo1 ok get foo2 ok get [foo3 -> 5% 100 KiB/s] [foo4 -> 3% 90 KiB/s] [foo5 -> 20% 1 MiB/s]

    The problem with that is it has to avoid scrolling off the right side, so it probably has to truncate the line. Since filenames are often longer than "fooN", it probably has to elipsise the filename. This approach is just not as flexible or nice in general.

See also: parallel possibilities

I am looking at using the ascii-progress library for this. It has nice support for multiple progress bars, and is portable. I have filed 7 issues on it, around 4 of which need to get fixed before it's suitable for git-annex to use.. --Joey

git annex get -JN works now, but lacks any progress display. Waiting on some updates to ascii-progress. --Joey

Wrote concurrent-output; done --Joey