[masswerk]

Downloading is an async operation of unknown duration, which is best represented by a spinner. Processing, on the other hand, should be deterministic. I'd say, have a spinner and a progress bar (dimmed, while the spinner is active, then switch to a dimmed, but filled spinner and an active progress bar.)

[atoav]

Imagine a train network operator who says: we cannot say how long a trip will take with 100% accuracy, therefore we are not going to tell you at all. And evwn more evil: on the trains they blind the windows and rob you of every clue that would tell you where you are and how fast you are going.

This would be pretty uncomfortable.

What train riders (and users) are interested in is how long a thing should take in principle as well as clues about whether they are on track. When reality differs from that it is ok, as long as the ETA is somewhat in the ballpark area of the actual time of arrival.

[masswerk]

On the other hand, time tables should provide actual data in linear time (not some "apparent time" – "You'll arrive in 'just fine' at your destination."). I don't know the extent of the download, but, if not not substantial, it should just add a small offset prior to the main operation. Otherwise, if you're downloading a huge database, you can draw estimates from progress and display a sub-progress of this first task. (There are APIs for this).

In the now gone days of usability, it was considered good practice to annotate the progress bar of a complex task by displaying textual information regarding the sub-task and the progress made. This could be considered here as well. (E.g., "Loading data, estimate: 3.4 secs.")

[ancarda]

Why use a spinner for downloading? Don't you know how many bytes you need to fetch and how many you have received so far?

wget has a progress bar, and it works perfectly fine.

EDIT: Though, if the server does not have a Content-Length header, you will get an indeterminate progress bar (a kind of spinner, I suppose) in web browsers, if that's the kind of thing you mean?

[_jal]

Downloads can occupy some fuzzy middle ground between unknown and deterministic.

The normal case is that bandwidth throughput can be estimated well enough to make humans happy. But a broken network throws that out the window and will do its best to find ways to make your progress bar behave weirdly.

I can't remember what it was, but there was a Mac app a very long time ago (System 7 era, in the 90s) - maybe a news reader? - that would eventually hide the progress bar and show a message about bad networking when that happened.

[masswerk]

There are browser APIs, as well. However, network performance may vary. Personally, I would add textual information, like an estimate of the time required.

[Fargren]

I'm pretty sure in most systems I use, processing time is not deterministic and it depends on what else the system is doing at the time and it's temperature, among other factor of which the progress bar won't be aware.

A progress bar will usually present the "expected" duration of several operations, [almost] none of which have a deterministic duration, or a deterministic ratio from one to the other. The best we can do is put heuristics in place to estimate how long the operations will take. We have some choices, such as estimating the total time or estimating each sub-operation separately. We can even use AI for the estimation, if we want to get fancy. But the problem is not tractable, and we'd rather given estimates where we can that just use spinners everywhere.

[elcomet]

And yet users prefer progress bars. You know the average duration of a data transfer and can show a progress bar even if it's not very accurate.