git-annex/doc/design/exporting_trees_to_special_remotes.mdwn

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 [[todo/export]] and [[todo/dumb, unsafe, human-readable_backend]])

[[!toc ]]

## 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=yes`, that cannot be
changed by `enableremote`:

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

It does not make sense to encrypt an export, so exporttree=yes requires
encryption=none.

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.

If a file's content is not present, it won't be exported. Re-running the
same export later should export files whose content has become present.
(This likely means a second pass, and needs location tracking to track
which files are in the export.)

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.

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.

## changes to special remote interface

This needs some additional methods added to special remotes, and to
the [[external_special_remote_protocol]].

Here's the changes to the latter:

* `EXPORTSUPPORTED`  
  Used to check if a special remote supports exports. The remote
  responds with either `EXPORTSUPPORTED-SUCCESS` or
  `EXPORTSUPPORTED-FAILURE`
* `EXPORT Name`  
  Comes immediately before each of the following requests, 
  specifying the name of the exported file. It will be in the form
  of a relative path, and may contain path separators, whitespace,
  and other special characters.
* `TRANSFEREXPORT STORE|RETRIEVE Key File`  
  Requests the transfer of a File on local disk to or from the previously 
  provided Name on the special remote.  
  Note that it's important that, while a file is being stored,
  CHECKPRESENTEXPORT not indicate it's present until all the data has
  been transferred.  
  The remote responds with either `TRANSFER-SUCCESS` or
  `TRANSFER-FAILURE`, and a remote where exports do not make sense
  may always fail.
* `CHECKPRESENTEXPORT Key`  
  Requests the remote to check if the previously provided Name is present
  in it.  
  The remote responds with `CHECKPRESENT-SUCCESS`, `CHECKPRESENT-FAILURE`,
  or `CHECKPRESENT-UNKNOWN`.
* `REMOVEEXPORT Key`  
  Requests the remote to remove content stored by `TRANSFEREXPORT`
  with the previously provided Name.  
  The remote responds with either `REMOVE-SUCCESS` or
  `REMOVE-FAILURE`.
* `RENAMEEXPORT Key NewName`  
  Requests the remote rename a file stored on it from the previously
  provided Name to the NewName.  
  The remote responds with `RENAMEEXPORT-SUCCESS`,
  `RENAMEEXPORT-FAILURE`, or with `RENAMEEXPORT-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

Since not all the files in an exported treeish may have content
present when the export is done, location tracking will be needed so that
getting the files and exporting again transfers their content.

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?

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. 

Possible solution: Make exporttree=yes 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.

The same file contents may be in a treeish multiple times under different
filenames. That complicates using location tracking. One file may have been
exported and the other not, and location tracking says that the content
is present in the export.

## 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 `export.log` that contains the uuid
of the remote that was exported to, and the sha1 of the exported tree.
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 `export.log` contains multiple active exports of different trees,
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 or an entirely new tree).
The UI to do this can just be another `git annex export $tree --to remote`.
To resolve, diff each exported tree in turn against the resolving tree
and delete all files that differ.

## when to update export.log for efficient resuming of exports

When should `export.log` be updated? Possibilities:

* Before performing any work, to set the goal.
* After the export is fully successful, to record the current state.
* After some mid-point.

Lots of things could go wrong during an export. A file might fail to be
transferred or only part of it be transferred; a file's content might not
be present to transfer at all. The export could be interrupted part way.
Updating the export.log at the right point in time is important to handle
these cases efficiently.

If the export.log is updated first, then it's only a goal and does not tell
us what's been done already.

If the export.log is updated only after complete success, then the common
case of some files not having content locally present will prevent it from
being updated. When we resume, we again don't know what's been done
already.

If the export.log is updated after deleting any files from the
remote that are not the same in the new treeish as in the old treeish,
and as long as TRANSFEREXPORT STORE is atomic, then when resuming we can
trust CHECKPRESENTEXPORT to only find files that have the correct content
for the current treeish. (Unless a conflicting export was made from
elsewhere, but in that case, the conflict resolution will have to fix up
later.)

Efficient resuming can then first check if the location log says the
export contains the content. (If not, transfer a copy.) If the location
log says the export contains the content, use CHECKPRESENTEXPORT to see if
the file exists, and if not transfer a copy. The CHECKPRESENTEXPORT check
deals with the case where the treeish has two files with the same content.
If we have a key-to-files map for the export, then we can skip the 
CHECKPRESENTEXPORT check when there's only one file using a key. So,
resuming can be quite efficient.