This is a fairly detailed design proposal for using git-annex to build [[!toc ]] ## sharding to scale The IA contains some 24 million Items. git repositories do not scale well in the 1-10 million file range, and very badly above that. Storing individual IA Items would strain git's scalability badly. Solution: Create multiple git repositories, and split the Items amoung them. * Needs a map from an Item to its repository. (Could be stored in a database, or whatever.) * If each git repository holds 10 thousand items, that's 2400 repositories, which is not an unmanagable number. (For comparison, git.debian.org has 18500 repositories.) (100 thousand items would be the higher end, for 240 repositories.) * The IA is ~20 Petabytes large. Each shard would thus be around 8 Terabytes. (Item sizes will vary a lot, so there's the potential to get a shard that's unusually small or large. This could be dealt with when assigning Items to the shards, to balance sizes out.) * The Items in each shard are then distributed out to the clients who have been assigned that shard. Clients will store varying amounts of data, but probably under 1 Terabyte per client. And we want redundancy (LOCKSS) -- say at least 3 copies. So, estimate around 25-100 clients need to be assigned to each shard to get it backed up. * Add new shards as the IA continues to grow. Question: How many files are in IA across all Items? It might be better to use $item/$file rather than $item.tar as the unit that's stored in the git-annex repository. This would need more shards. ## the IA git repository We're building a pyramid of git-annex repositories, and at the tip of this is a single git repository, which represents the entire Internet Archive. This IA git repository contains no files. But, git-annex in each of the ~2400 shards knows about it, and by default every Item in every shard is recorded as having a copy present in the IA git repository. If the IA lost an Item somehow, this would be reflected by updating the git-annex location tracking to say the IA git repository no longer contains the item. Creating this repository is simple: git init ia.git cd ia.git git annex init "The Internet Archive" git annex trust . ## creating the shards Each shard starts as a clone of the IA git repository. Items are added to the shard, either all at once, or perhaps on-demand. To add an Item to the shard: 1. Create a (reproducible checksum) tarball of all the files in the Item (probably excluding "derived" files). 2. Checksum the tarball and derive a git-annex key, and add it to the git repository. The symlink can have a name corresponding to the Item name. (Eg "LauraPoitrasCitizenfour.tar" for ) The easy way is to write the tarball to disk in the shard's git repo, and "git annex add", but it's also possible to do this without ever storing the tarball on disk. (The tarball would then be reconstructed on the fly each time a client requests to download it.) 4. Update git-annex location tracking to indicate that this item is present in the Internet Archive. If $iauuid is the UUID of the IA git repository, the command is: `setpresentkey $key $iauuid 1` (This command needs git-annex 5.20141231) 5. git commit ## adding a client When a client registers to participate: 1. Generate a UUID, which is assigned to this client, and send it to the client, and assign that UUID to a particular shard. 2. Send the client an appropriate auth token (eg, a locked down ssh private key) to let them access the shard's git repository (or all the shards). 3. Client clones its assigned shard git repository, runs `git annex init reinit $UUID`, and enables direct mode. Note that a client could be assigned to multiple shards, rather than just one. Probably good to keep a pool of empty shards that have clients waiting for new Items to be added. Note that direct mode seems like a good idea because it lets the user easily delete files to free up space. ## distributing Items 1. Client runs `git annex sync --content`, which downloads as many Items from the IA as will fit in their disk's free space (leaving some configurable amount free in reserve by configuring annex.diskreserve) 2. Note that [[numcopies|copies]] and [[preferred_content]] settings can be used to make clients only want to download an Item if it's not yet reached the desired number of copies. Lots of flexability here in git-annex. 3. git-annex will push back to the server an updated git-annex branch, which will record when it has successfully stored an Item. ## bad actors Clients can misbehave in probably many ways. The best defense for many misbehaviors is to distribute Items to enough different clients that we can trust some of them. The main git-annex specific misbehavior is that a client could try to push garbage information back to the origin repository on the server. To guard against this, the server will reject all pushes of branches other than the git-annex branch, which is the only one clients need to modify. Check pushes of the git-annex branch. There are only a few files that clients can legitimately modify, and the modifications will always involve that client's UUID, not some other client's UUID. Reject anything shady. These checks can be done in a git `update` hook. Rough estimate is that such a hook would be a couple hundred lines of code. ## verification We want a lightweight verification process, to verify that a client still has the data. This can be done using `git annex fsck`, which can be configured to eg, check each file only once per month. git-annex will need a modification here. Currently, a successful fsck does not leave any trace in the git-annex branch that it happened. But we want the server to track when a client is not fscking (the user probably dropped out). The modification is simple; just have a successful fsck update the timestamp in the fscked file's location log. It will probably take just a few hours to code. With that change, the server can check for files that not enough clients have verified they have recently, and distribute them to more clients. Note that bad actors can lie about this verification; it's not a proof they still have the file. But, a bad actor could prove they have a file, and refuse to give it back if the IA needed to restore the backup, too. ## fire drill If we really want to test how well the system is working, we need a fire drill. 1. Pick some Items that we'll assume the IA has lost in some disaster. 2. Look up the shard the Item belongs to. 3. Get the git-annex key of the Item, and tell git-annex it's been lost from the IA, by running in its shard: `setpresentkey $key $iauuid 0` 4. The next time a client runs `git annex sync --content`, it will notice that the IA repo doesn't have the Item anymore. The client will then send the Item back to the origin repo. 5. To guard against bad actors, that restored Item should be checked with `git annex fsck`. If its checksum is good, it can be re-injected back into the IA. (Or, the fire drill was successful.) (Remember to turn off the fire alarm by running `setpresentkey $key $iauuid 1`) ## shard servers A server at the IA (or otherwise with a fast pipe) is needed to serve one or more shards. Let's consider what this server needs to have on it: * git and git-annex * ssh server * rsync * The git repository for the shard. Probably a few hundred mb? * The git update hook to filter out bad pushes. * Some way to get the content of a given Item from the IA when a client wants to download it. This probably means generating the $item.tar file and buffering it to disk for a while. * So, enough disk to buffer a reasonable number of items. * Some way to learn when a new user has registered to access a shard, so their ssh key is given access. ## other optional nice stuff The user running a client can delete some or all of their files at any time, to free up disk space. The next time `git-annex sync` runs on the client, it'll notice and let the server know, and other clients will then take over storing it. (Or if the git-annex assistant is run on the client, it would inform the server immediately.) The user is also free to move Items around (within the git repository directory), unpack Items to examine their contents, etc. This doesn't affect anyone else. Offline storage is supported. As long as the user can spin it up from time to time to run `git annex fsck`. More advanced users might have multiple repositories on different disks. Each has their own UUID, and they could move Items around between them as desired; this would be communicated back to the origin repository automatically. Shards could have themes, and users could request to be part of the shard that includes Software, or Grateful Dead, etc. This might encourage users to devote more resources. Or, rather than doing a lucky dip and getting one or a couple shards, a user could clone em all, and pick just which Items to store. The contents of Items sometimes changes. This can be reflected by updating an Item's file in the git repository. Clients will then download the new version of the Item. Items sometimes go dark; this could be reflected by deleting the item from the repository. It's up to the clients what they do with the content of such Items. ## other potential gotchas If any single Item is very large (eg, 10 terabytes), there may not be any clients that can handle it. This could be dealt with by splitting up the item into smaller files. Word is there is a single 2 tb item, and a few more around 100 gb, so this is probably not a concern. A client could add other files to its local repo, and git-annex branch pushes would include junk data about those files. It should probably be filtered out by the git update hook (rejecting the whole push because of this seems excessive). There may be a thundering herd problem, where many clients end up downloading the same Item at the same time, and more copies than neecessary result. The next `git annex sync --content` in some of the redundant clients will notice this and drop that Item, and presumably download some other Item. However, it might be good to rate limit the number of concurrent downloads of a given item, to prevent this and perhaps other issues. This could be done by a wrapper around git-annex shell or perhaps a git-annex modification.