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