2013-10-17 00:29:35 +00:00
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---
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layout: post
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title: Generations
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categories:
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- crypticio
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---
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2013-10-09 03:15:23 +00:00
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A simple file distribution strategy for very large scale, high-availability
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file-services.
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# The problem
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Working at a shop where we have millions of different files, any of which could
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be arbitrarily chosen to serve to a file at any given time. These files are
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uploaded by users of the app and retrieved by others.
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Scaling such a system is no easy task. The chosen solution involves shuffling
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files around on a nearly constant basis, making sure that files which are more
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"popular" are on fast drives, while at the same time making sure that no drives
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are at capicty and at the same time that all files, even newly uploaded ones,
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are stored redundantly.
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The problem with this solution is one of coordination. At any given moment the
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app needs to be able to "find" a file so it can give the client a link to
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download the file from one of the servers that it's on. Full-filling this simple
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requirement means that all datastores/caches where information about where a
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file lives need to be up-to-date at all times, and even then there are
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race-conditions and network failures to contend with, while at all times the
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requirements of the app evolve and change.
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# A simpler solution
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Let's say you want all files which get uploaded to be replicated in triplicate
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in some capacity. You buy three identical hard-disks, and put each on a separate
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server. As files get uploaded by clients, each file gets put on each drive
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immediately. When the drives are filled (which should be at around the same
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time), you stop uploading to them.
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That was generation 0.
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You buy three more drives, and start putting all files on them instead. This is
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going to be generation 1. Repeat until you run out of money.
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That's it.
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## That's it?
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It seems simple and obvious, and maybe it's the standard thing which is done,
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but as far as I can tell no-one has written about it (though I'm probably not
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searching for the right thing, let me know if this is the case!).
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## Advantages
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* It's so simple to implement, you could probably do it in a day if you're
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starting a project from scratch
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* By definition of the scheme all files are replicated in multiple places.
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* Minimal information about where a file "is" needs to be stored. When a file is
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uploaded all that's needed is to know what generation it is in, and then what
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nodes/drives are in that generation.
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* Drives don't need to "know" about each other. What I mean by this is that
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whatever is running as the receive point for file-uploads on each drive doesn't
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have to coordinate with its siblings running on the other drives in the
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generation. In fact it doesn't need to coordinate with anyone. You could
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literally rsync files onto your drives if you wanted to. I would recommend using
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[marlin][0] though :)
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* Scaling is easy. When you run out of space you can simply start a new
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generation. If you don't like playing that close to the chest there's nothing to
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say you can't have two generations active at the same time.
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* Upgrading is easy. As long as a generation is not marked-for-upload, you can
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easily copy all files in the generation into a new set of bigger, badder drives,
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add those drives into the generation in your code, remove the old ones, then
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mark the generation as uploadable again.
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* Distribution is easy. You just copy a generation's files onto a new drive in
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Europe or wherever you're getting an uptick in traffic from and you're good to
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go.
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* Management is easy. It's trivial to find out how many times a file has been
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replicated, or how many countries it's in, or what hardware it's being served
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from (given you have easy access to information about specific drives).
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## Caveats
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The big caveat here is that this is just an idea. It has NOT been tested in
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production. But we have enough faith in it that we're going to give it a shot at
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cryptic.io. I'll keep this page updated.
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The second caveat is that this scheme does not inherently support caching. If a
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file suddenly becomes super popular the world over your hard-disks might not be
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able to keep up, and it's probably not feasible to have an FIO drive in *every*
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generation. I think that [groupcache][1] may be the answer to this problem,
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assuming your files are reasonably small, but again I haven't tested it yet.
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[0]: https://github.com/cryptic-io/marlin
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[1]: https://github.com/golang/groupcache
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