91 lines
4.2 KiB
Text
91 lines
4.2 KiB
Text
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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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## discovering UUIDS
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A significant difficulty in implementing a proxy for the P2P protocol is
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that each git-annex remote has a single UUID. But the remote that points at
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the proxy can't just have the UUID of the proxy's repository, git-annex
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needs to know that the remote can be used to access repositories with every
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UUID in the cluster.
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----
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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 can
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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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----
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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.) And then it can instantiate a UUID for
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each remote.
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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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## 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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