176 lines
8.6 KiB
Markdown
176 lines
8.6 KiB
Markdown
When [[balanced_preferred_content]] is used, there may be many repositories
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in a location -- either a server or a cluster -- and getting any given file
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may need to access any of them. Configuring remotes for each repository
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adds a lot of complexity, both in setting up access controls on each
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server, and for the user.
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Particularly on the user side, when ssh is used they may have to deal with
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many different ssh host keys, as well as adding new remotes or removing
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existing remotes to keep up with changes are made on the server side.
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A proxy would avoid this complexity. It also allows limiting network
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ingress to a single point.
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Ideally a proxy would look like any other git-annex remote. All the files
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stored anywhere in the cluster would be available to retrieve from the
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proxy. When a file is sent to the proxy, it would store it somewhere in the
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cluster.
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Currently the closest git-annex can get to implementing such a proxy is a
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transfer repository that wants all content that is not yet stored in the
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cluster. This allows incoming transfers to be accepted and distributed to
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nodes of the cluster. To get data back out of the cluster, there has to be
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some communication that it is preferred content (eg, setting metadata),
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then after some delay for it to be copied back to the transfer repository,
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it becomes available for the client to download it. And once it knows the
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client has its copy, it can be removed from the transfer repository.
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That is quite slow, and rather clumsy. And it risks the transfer repository
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filling up with data that has been requested by clients that have not yet
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picked it up, or with incoming transfers that have not yet reached the
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cluster.
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A proxy would not hold the content of files itself. It would be a clone of
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the git repository though, probably. Uploads and downloads would stream
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through the proxy. The git-annex [[P2P_protocol]] could be relayed in this way.
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## UUID discovery
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A significant difficulty in implementing a proxy is that each git-annex
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remote has a single UUID. But the remote that points at the proxy can't
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just have the UUID of the proxy's repository, git-annex needs to know that
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the proxy's remote can be used to access repositories with every UUID in
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the cluster.
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### UUID discovery via P2P protocol extension
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Could the P2P protocol be extended to let the proxy communicate the UUIDs
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of all the repositories behind it?
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Once the client git-annex knows the set of UUIDs behind the proxy, it could
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eg instantiate a remote object per UUID, each of which accesses the proxy, but
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with a different UUID.
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But, git-annx usually only does UUID discovery the first time a ssh remote
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is accessed. So it would need to discover at that point that the remote is
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a proxy. Then it could do UUID discovery each time git-annex starts up.
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But that adds significant overhead, git-annex would be making a connection
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to the proxy in situations where it is not going to use it.
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### UUID discovery via git-annex branch
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Could the proxy's set of UUIDs instead be recorded somewhere in the
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git-annex branch?
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With this approach, git-annex would know as soon as it sees the proxy's
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UUID that this is a proxy for this other set of UUIDS. (Unless its
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git-annex branch is not up-to-date.)
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One difficulty with this is that, when the git-annex branch is not up to
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date with changes from the proxy, git-annex may try to access repositories
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that are no longer available behind the proxy. That failure would be
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handled the same as any other currently unavailable repository. Also
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git-annex would not use the full set of repositories, so might not be able
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to store data when eg, all the repositories that is knows about are full.
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Just getting the git-annex back in sync should recover from either
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situation.
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## user interface
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What to name the instantiated remotes? Probably the best that could
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be done is to use the proxy's own remote names as suffixes on the client.
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Eg, the proxy's "node1" remote is "proxy-node1".
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But the user probably doesn't want to pick which node to send content to.
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They don't necessarily know anything about the nodes. Ideally the user
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would `git-annex copy --to proxy` or `git-annex push` and let it pick
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which instantiated remote(s) to send to.
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To make `git-annex copy --to proxy` work, `storeKey` could be changed to
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allow returning a UUID (or UUIDs) where the content was actually stored.
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That would also allow a single upload to the proxy to fan out and be stored
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in multiple nodes. The proxy would use preferred content to pick which of
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its nodes to store on.
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Instantiated remotes would still be needed for `git-annex get` and similar
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to work.
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To make `git-annex copy --from proxy` work, the proxy would need to pick
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a node and stream content from it. That's doable, but how to handle a case
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where a node gets corrupted? The best it could do is mark that node as no
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longer containing the content (as if a fsck failed) and try another one
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next time. This complication might not be necessary. Consider that
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while `git-annex copy --to foo` followed later by `git-annex copy --from foo`
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will usually work, it doesn't work when eg first copying to a transfer
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remote, which then sends the content elsewhere and drops its copy.
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What about dropping? `git-annex drop --from proxy` could be made to work,
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by having `removeKey` return a list of UUIDs that the content was dropped
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from. What should that do if it's able to drop from some nodes but not
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others? Perhaps it would need to be able to return a list of UUIDs that
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content was dropped from but still indicate it overall failed to drop.
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(Note that it's entirely possible that dropping from one node of the proxy
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involves lockContent on another node of the proxy in order to satisfy
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numcopies.)
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A command like `git-annex push` would see all the instantiated remotes and
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would pick one to send content to. Seems like the proxy might choose to
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`storeKey` the content on other node(s) than the requested one. Which would
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be fine. But, `git-annex push` would still do considerable extra work in
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interating over all the instantiated remotes. So it might be better to make
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such commands not operate on instantiated remotes for sending content but
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only on the proxy.
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Commands like `git-annex push` and `git-annex pull`
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should also skip the instantiated remotes when pushing or pulling the git
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repo, because that would be extra work that accomplishes nothing.
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## streaming to special remotes
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As well as being an intermediary to git-annex repositories, the proxy could
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provide access to other special remotes. That could be an object store like
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S3, which might be internal to the cluster or not. When using a cloud
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service like S3, only the proxy needs to know the access credentials.
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Currently git-annex does not support streaming content to special remotes.
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The remote interface operates on object files stored on disk. See
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[[todo/transitive_transfers]] for discussion of that problem. If proxies
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get implemented, that problem should be revisited.
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## speed
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A passthrough proxy should be as fast as possible so as not to add overhead
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to a file retrieve, store, or checkpresent. This probably means that
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it keeps TCP connections open to each host in the cluster. It might use a
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protocol with less overhead than ssh.
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In the case of checkpresent, it would be possible for the proxy to not
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communicate with the cluster to check that the data is still present on it.
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As long as all access is intermediated via the proxy, its git-annex branch
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could be relied on to always be correct, in theory. Proving that theory,
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making sure to account for all possible race conditions and other scenarios,
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would be necessary for such an optimisation.
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Another way the proxy could speed things up is to cache some subset of
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content. Eg, analize what files are typically requested, and store another
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copy of those on the proxy. Perhaps prioritize storing smaller files, where
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latency tends to swamp transfer speed.
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## encryption
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When the proxy is in front of a special remote that uses encryption, where
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does the encryption happen? It could either happen on the client before
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sending to the proxy, or the proxy could do the encryption since it
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communicates with the special remote. For security, doing the encryption on
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the client seems like the best choice by far.
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But, git-annex's git remotes don't currently ever do encryption. And
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special remotes don't communicate via the P2P protocol with a git remote.
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So none of git-annex's existing remote implementations would be able to handle
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this case. Something will need to be changed in the remote
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implementation for this.
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(Chunking has the same problem.)
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There's potentially a layering problem here, because exactly how encryption
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(or chunking) works can vary depending on the type of special remote.
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