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Comment by tlb

5 years ago

The question I had is answered by this line of code:

  xhr.setRequestHeader("Range", "bytes=" + from + "-" + to);

I am a little surprised you can just do that. In https://github.com/phiresky/sql.js-httpvfs/blob/master/src/l...

23 comments

tlb

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dathinab  5 years ago

Range headers are a pretty standard tools to e.g. continue interrupted downloads and similar.

Any well designed system, especially if it has static sources and is server cached should support it.

Surprisingly many web-frameworks don't support it out of the box, or don't support it well.

Either way gh-pages are static content and probably with some server side regional caches, so I'm not surprised it works.

  • hinkley  5 years ago

    Range headers are also how DownloadThemAll works.

    If you're pulling a single TCP stream across a crowded network, you get maybe 1/40th of the available bandwidth. If you do four range requests instead, you might see >3/40th of the bandwidth.

    This place we were contracting at, the managers were widely rumored to stream sports games at their desks, and our release cycle happened to fall on a game day. My poor coworker was downloading our installer every week, and the ETA was over 40 minutes. "Are you using DTA?" "No, what's that?" <fiddle fiddle> ETA: 12 minutes.

    12 minute pauses in the middle of a manual process are a lot easier to stomach than 40 minutes. Especially if something goes wrong and you have to do it twice.

fanf2  5 years ago

One of the heaviest users of range requests is (or was) the Adobe Acrobat PDF plugin.

  • scottlamb  5 years ago

    Also .mp4 files. The format is designed for seekability, and browsers take advantage of this.

    • sp332  5 years ago

      You can even point VLC at a .iso file on a web server, and seek around in it.

    • fy20  5 years ago

      Progressive JPEGs work well for this too, so you could have the same file used for a tiny thumbnail and large preview and full sized photo by sending different range requests. However you need to know how many bytes to request.

      I'm surprised this isn't used on mobile browsers to lower data usage. I'm sure with a little research you could figure out what a good mapping from pixel size to byte size should be to give good enough results.

      2 replies →

  • 0x0  5 years ago

    I remember putting together a sloppy http range implementation that initially only supported single ranges, it had quite the explosive effect on adobe reader when it didn't get the expected response to its multi-range requests :)

crazygringo  5 years ago

Not all webservers support/enable it, so YMMV.

But as long as you're dealing with a known server that does, then gravy!

  • stabbles  5 years ago

    nginx enables it by default. Another ingenious use of range request is zsync, it allows you to diff compressed binaries on a remote with local ones, so that you only have to download what has changed on an update. AppImage uses this

  • altfredd  5 years ago

    > Not all webservers support/enable it

    Could you provide an example of server that does not?

    AFAIK, Range is supported by all major CDNs, so not supporting it in web server would be a death knell for it's real-world adoption.

    • crazygringo  5 years ago

      I can't think of a specific one, but a decent proportion (maybe a quarter?) of HTTP downloads I attempt don't support resuming a partial download, therefore don't support Range. (I.e. resuming always starts from the beginning.)

      I would assume this is often because the site in question isn't using Apache etc. to serve a file directly, but is either essentially proxying it to some custom-built file serving service, or a script that processes/authenticates the file in some way, and they just never bothered to implement Range.

    • enneff  5 years ago

      Depends on what’s being served. Any decent static file server should support it, but if the content is at all dynamically produced then the authors would have to think to implement it and rarely do.