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Find Closest Download Mirror


This essay does not describe an existing computer program, just one that should exist. This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have prepared to help you is right here.

This project outline is not like the artificial, tidy little problems you are spoon-fed in school, when all the facts you need are included, nothing extraneous is mentioned, the answer is fully specified, along with hints to nudge you toward a single expected canonical solution. This project is much more like the real world of messy problems where it is up to you to fully the define the end point, or a series of ever more difficult versions of this project and research the information yourself to solve them.

Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; or give you any additional materials. I have too many other projects of my own.

Though I am a programmer by profession, I don’t do people’s homework for them. That just robs them of an education.

You have my full permission to implement this project in any way you please and to keep all the profits from your endeavour.

Please do not email me about this project without reading the disclaimer above.

The Problem

When you go to download a program in your browser, it often presents you with a list of twenty or so mirror sites you could download it from. Which should you choose? This program automatically selects the best one and downloads the file for you.

Factors Considered

How would software decide which download site was best?

How it Works

The user copies the webpage containing the list of mirrors to the clipboard and pastes it into the downloader program. You just click Download with material in your clipboard. The downloader parses the page to extract useful information. Often the countries of the various mirrors are available. You know the user’s country from the system property.

Perhaps you can guess an approximate location of the mirror from DNS (Domain Name Service) lookup or the IP.

This gives you a short list of candidates. You then try downloading say 20K of the file from each mirror and see which responds fastest. (If the file is short, you just pick a random mirror. It is just a waste of time optimising.) This will pick a close, lightly loaded mirror. This will also exclude mirrors that are down.

Extract the filename from the webpage also. Then download the file to the predesignated target directory in the background, showing a progress bar if the user is curious.

Getting Fancy

Make the downloader multi-thread so that it can queue up requests and do several at once. It determines by experiment how many at once is optimal. If it gets a disconnect or the system crashes, it does its best to pick up where it left off, perhaps completely restarting the downloads’ in progress. It may even restart the download from a different mirror.

Building It In

If some large mirror site like SourceForge or Apache Jakarta could be persuaded to adopt a server protocol for determining optimum mirror, then the entire download process could be completely automated with an Applet. It could even pick up with an alternate mirror in midlight if one failed. From there the use of the protocol could spread to become the standard for all mirrored downloads.

You might find the HTTP methods useful in implementing this project.

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