For publishing content from a git-annex repository, it would be useful to be able to export a tree of files to a special remote, using the filenames and content from the tree.

(See also export and dumb, unsafe, human-readable backend)

configuring a special remote for tree export

If a special remote already has files stored in it, switching it to be a tree export would result in a mix of files named by key and by filename. That's not desirable. So, the user should set up a new special remote when they want to export a tree. (It would also be possible to drop all content from an existing special remote and reuse it, but there does not seem much benefit in doing so.)

Add a new initremote configuration exporttree=true, that cannot be changed by enableremote:

git annex initremote myexport type=... exporttree=true

It does not make sense to encrypt an export, so exporttree=true requires (and can even imply) encryption=false.

Note that the particular tree to export is not specified yet. This is because the tree that is exported to a special remote may change.

exporting a treeish

To export a treeish, the user can run:

git annex export $treeish --to myexport

That does all necessary uploads etc to make the special remote contain the tree of files. The treeish can be a tag, a branch, or a tree.

Users may sometimes want to export multiple treeishes to a single special remote. For example, exporting several tags. This interface could be complicated to support that, putting the treeishes in subdirectories on the special remote etc. But that's not necessary, because the user can use git commands to graft trees together into a larger tree, and export that larger tree.

If an export is interrupted, running it again should resume where it left off.

It would also be nice to have a way to say, "I want to export the master branch", and have git-annex sync and the assistant automatically update the export. This could be done by recording the treeish in eg, refs/remotes/myexport/HEAD. git-annex export could do this by default (if the user doesn't want the export to track the branch, they could instead export a tree or a tag).

updating an export

The user can at any time re-run git-annex export with a new treeish to change what's exported. While some use cases for git annex export involve publishing datasets that are intended to remain immutable, other use cases include eg, making a tree of files available to a computer that can't run git-annex, and in such use cases, the tree needs to be able to be updated.

To efficiently update an export, git-annex can diff the tree that was exported with the new tree. The naive approach is to upload new and modified files and remove deleted files.

Note that a file may have been partially uploaded to an export, and then the export updated to a tree without that file. So, need to try to delete all removed files, even if location tracking does not say that the special remote contains them.

With rename detection, if the special remote supports moving files, more efficient updates can be done. It gets complicated; consider two files that swap names.

If the special remote supports copying files, that would also make some updates more efficient.

resuming exports

Resuming an interrupted export needs to work well.

There are two cases here:

  1. Some of the files in the tree have been uploaded; others have not.
  2. A file has been partially uploaded.

These two cases need to be disentangled somehow in order to handle them. One way is to use the location log as follows:

  • Before a file is uploaded, look up what key is currently exported using that filename. If there is one, update the location log, saying it's not present in the special remote.
  • Upload the file.
  • Update the location log for the newly exported key.

Note that this method does not allow resuming a partial upload by appending to a file, because we don't know if the file actually started to be uploaded, or if the file instead still has the old key's content. Instead, the whole file needs to be re-uploaded.

Alternative: Keep an index file that's the current state of the export. See comment #4 of export. Not sure if that works? Perhaps it would be overkill if it's only used to support resuming partial uploads.

changes to special remote interface

This needs some additional methods added to special remotes, and to the external special remote protocol.

    Requests the transfer of a File on local disk to or from a given Name on the special remote.
    The Name will be in the form of a relative path, and may contain path separators, whitespace, and other special characters.
    The Key is provided in case the special remote wants to use eg SETURIPRESENT.
    The remote responds with either TRANSFER-SUCCESS or TRANSFER-FAILURE, and a remote where exports do not make sense may always fail.
  • CHECKPRESENTEXPORT Key Name Requests the remote to check if a Name is present in it.
  • REMOVEEXPORT Key Name Requests the remote to remove content stored by TRANSFEREXPORT.
    The Key is provided in case the remote wants to use eg SETURIMISSING. The remote responds with either REMOVE-SUCCESS or REMOVE-FAILURE.
  • RENAMEEXPORT Key OldName NewName Requests the remote rename a file stored on it from OldName to NewName.
    The Key is provided in case the remote wants to use eg SETURIMISSING and SETURIPRESENT.
    The remote responds with RENAMEEXPORT-SUCCESS, RENAMEEXPORT-FAILURE, or withRENAMEEXPORT-UNSUPPORTED` if an efficient rename cannot be done.

To support old external special remote programs that have not been updated to support exports, git-annex will need to handle an ERROR response when using any of the above.

location tracking

Does a copy of a file exported to a special remote count as a copy of a file as far as ?numcopies goes? Should git-annex get download a file from an export? Or should exporting not update location tracking?

The problem is that special remotes with exports are not key/value stores. The content of a file can change, and if multiple repositories can export a special remote, they can be out of sync about what files are exported to it.

To avoid such problems, when updating an exported file on a special remote, the key could be recorded there too. But, this would have to be done atomically, and checked atomically when downloading the file. Special remotes lack atomicity guarantees for file storage, let alone for file retrieval.

Possible solution: Make exporttree=true cause the special remote to be untrusted, and rely on annex.verify to catch cases where the content of a file on a special remote has changed. This would work well enough except for when the WORM or URL backend is used. So, prevent the user from exporting such keys. Also, force verification on for such special remotes, don't let it be turned off.

recording exported filenames in git-annex branch

In order to download the content of a key from a file exported to a special remote, the filename that was exported needs to somehow be recorded in the git-annex branch. How to do this? The filename could be included in the location tracking log or a related log file, or the exported tree could be grafted into the git-annex branch (under eg, exported/uuid/). Which way uses less space in the git repository?

Grafting in the exported tree records the necessary data, but the file-to-key map needs to be reversed to support downloading from an export. It would be too expensive to traverse the tree each time to hunt for a key; instead would need a database that gets populated once by traversing the tree.

On the other hand, for updating what's exported, having access to the old exported tree seems perfect, because it and the new tree can be diffed to find what changes need to be made to the special remote.

If the filenames are stored in the location tracking log, the exported tree could be reconstructed, but it would take O(N) queries to git, where N is the total number of keys git-annex knows about; updating exports of small subsets of large repositories would be expensive. So grafting in the exported tree seems the better approach.

export conflicts

What if different repositories can access the same special remote, and different trees get exported to it concurrently?

This would be very hard to untangle, because it's hard to know what content was exported to a file last, and thus what content the file actually has. The location log's timestamps might give a hint, but clocks vary too much to trust it.

Also, if the exported tree is grafted in to the git-annex branch, there would be a merge conflict. Union merging would scramble the exported tree, so even if a smart merge is added, old versions of git-annex would corrupt the exported tree.

To avoid that problem, add a log file exported/uuid.log that lists the sha1 of the exported tree and the uuid of the repository that exported it. To avoid the exported tree being GCed, do graft it in to the git-annex branch, but follow that with a commit that removes the tree again, and only update refs/heads/git-annex after making both commits.

If exported/uuid.log contains multiple active exports, there was an export conflict. Short of downloading the whole export to checksum it, or deleting the whole export, what can be done to resolve it?

In this case, git-annex knows both exported trees. Have the user provide a tree that resolves the conflict as they desire (it could be the same as one of the exported trees, or some merge of them). Then diff each exported tree in turn against the resolving tree. If a file differs, re-export that file. In some cases this will do unncessary re-uploads, but it's reasonably efficient.

The documentation should suggest strongly only exporting to a given special remote from a single repository, or having some other rule that avoids export conflicts.