This is an appendix to the external special remote protocol.

Some special remotes interface to a key/value datastore using keys that are eg hashes, and it won't make sense for them to implement any of this.

When a special remote interfaces with something that looks like a directory of files, the simple export interface can be implemented to allow the git-annex-export command to be used to store file trees on the special remote.

If other tools can write files to the special remote too, the import/export interface can be implemented. This allows for both git-annex-import and git-annex-export to be used with the special remote.

simple export interface

git-annex will use this when the special remote is initialized with exporttree=yes but without importtree=yes and indicates that it supports exports.

These are requests git-annex sends to the special remote program. Once the special remote has finished performing a request, it should send one of the listed replies. Or, if it does not support a request, it can reply with UNSUPPORTED-REQUEST.

  • EXPORTSUPPORTED
    Used to check if a special remote supports exports. Note that this request may be made before or after PREPARE.
    • EXPORTSUPPORTED-SUCCESS
      Indicates that it makes sense to export to this special remote.
    • EXPORTSUPPORTED-FAILURE
      Indicates that it does not make sense to export to this special remote.
  • 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.
    No response is made to this message.
  • 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.
    While the transfer is running, the remote can send any number of PROGRESS messages. Once the transfer is complete, it finishes by sending one of these replies:
    • TRANSFER-SUCCESS STORE|RETRIEVE Key
      Indicates the transfer completed successfully.
    • TRANSFER-FAILURE STORE|RETRIEVE Key ErrorMsg
      Indicates the transfer failed.
  • CHECKPRESENTEXPORT Key
    Requests the remote to check if the previously provided Name is present in it.
    • CHECKPRESENT-SUCCESS Key
      Indicates that a content has been positively verified to be present in the remote.
    • CHECKPRESENT-FAILURE Key
      Indicates that a contents has been positively verified to not be present in the remote.
    • CHECKPRESENT-UNKNOWN Key ErrorMsg
      Indicates that it is not currently possible to verify if content is present in the remote. (Perhaps the remote cannot be contacted.)
  • REMOVEEXPORT Key
    Requests the remote to remove content stored by TRANSFEREXPORT with the previously provided Name.
    • REMOVE-SUCCESS Key
      Indicates the content has been removed from the remote. May be returned when the content was already not present.
    • REMOVE-FAILURE Key ErrorMsg
      Indicates that the content was unable to be removed from the remote.
  • REMOVEEXPORTDIRECTORY Directory
    Requests the remote remove an exported directory.
    If the remote does not use directories, or REMOVEEXPORT cleans up directories that are empty, this does not need to be implemented.
    The directory will be in the form of a relative path, and may contain path separators, whitespace, and other special characters.
    Typically the directory will be empty, but it could possibly contain files or other directories, and it's ok to remove those, but not required to do so.
    • REMOVEEXPORTDIRECTORY-SUCCESS
      Indicates that a REMOVEEXPORTDIRECTORY was done successfully.
    • REMOVEEXPORTDIRECTORY-FAILURE
      Indicates that a REMOVEEXPORTDIRECTORY failed for whatever reason. Should not be returned if the directory did not exist.
  • RENAMEEXPORT Key NewName
    Requests the remote rename a file stored on it from the previously provided Name to the NewName. Remotes that support exports but not renaming do not need to implement this.
    • RENAMEEXPORT-SUCCESS Key
      Indicates that a RENAMEEXPORT was done successfully.
    • RENAMEEXPORT-FAILURE Key
      Indicates that a RENAMEEXPORT failed for whatever reason.

import/export interface

(This part is a draft, not implemented yet.)

git-annex will use this interface when the special remote is initialized with both exporttree=yes and importtree=yes and indicates that it supports both imports and exports.

content identifiers

The special remote needs to have some way to identify a particular version of a file. This is called a ContentIdentifier. A good ContentIdentifier needs to:

  • Be stable, so when a file has not changed, its content identifier remains the same.
  • Change when a file is modified.
  • Be as unique as possible, but not necessarily fully unique. A hash of the content would be ideal. A (size, mtime, inode) tuple is as good a content identifier as git uses in its index.
  • Be reasonably short, since it will be stored in the git-annex branch.

It's up to the implementor of a external special remote program what to use for their ContentIdentifier, but not meeting those criteria will leas to unhappy users, and it's better not to implement this interface if you can't do it well.

protocol messages

These are requests that git-annex sends to the special remote program. Once the special remote has finished performing a request, it should send one of the listed replies. Or, if it does not support a request, it can reply with UNSUPPORTED-REQUEST.

  • EXPORTSUPPORTED
    Used to check if a special remote supports exports. Note that this request may be made before or after PREPARE.
    • EXPORTSUPPORTED-SUCCESS
      Indicates that it makes sense to use this special remote as an export.
    • EXPORTSUPPORTED-FAILURE
      Indicates that it does not make sense to use this special remote as an export.
  • IMPORTSUPPORTED
    Used to check if a special remote supports imports. Note that this request may be made before or after PREPARE.
    • IMPORTSUPPORTED-SUCCESS
      Indicates that it makes sense to import from this special remote.
    • IMPORTSUPPORTED-FAILURE
      Indicates that it does not make sense to import from this special remote.
  • LISTIMPORTABLECONTENTS
    Used to get a list of all the files that are stored in the special remote. A block of responses can be made to this, which must always end with END.
    • CONTENT Size Name
      An file stored in the special remote. The Size is its size in bytes. The Name is the name of the file on the remote, in the form of a relative path, and may contain path separators, whitespace, and other special characters.
      Always followed by CONTENTIDENTIFIER.
    • CONTENTIDENTIFIER ContentIdentifier
      Provide the ContentIdentifier for the previous CONTENT.
    • HISTORY
      When a special remote stores historical versions of files, this can be used to list those versions. It opens a new block of responses. This can be repeated any number of times (indicating a branching history), and histories can also be nested multiple levels deep.
    • END
      Indicates the end of a block of responses.
  • LOCATION Name
    Comes before each of the following requests, specifying the name of the file on the remote. It will be in the form of a relative path, and may contain path separators, whitespace, and other special characters.
    No response is made to this message.
  • EXPECTED ContentIdentifier
    Comes before each of the following requests, specifying the ContentIdentifier that is expected to be present on the remote.
  • NOTHINGEXPECTED
    If no ContentIdentifier is expected to be present, this is sent rather than EXPECTED.
  • RETRIEVEEXPORTEXPECTED File
    Retrieves the content of a file from the special remote to the File on local disk. Must take care to only retrieve content that has the ContentIdentifier specified by by EXPECTED.
    While the transfer is running, the remote can send any number of PROGRESS messages. Once the transfer is complete, it finishes by sending one of these replies:
    • RETRIEVE-SUCCESS Indicates that the retrieve was successful.
    • RETRIEVE-FAILURE ErrorMsg
      Indicates the retrieve failed.
  • STOREEXPORTEXPECTED Key File
    Stores the content of File on the local disk to the previously provided Name on the remote. If the Name already exists on the remote, must take care to only overwrite it when it has the ContentIdentifier that was previously provided by EXPECTED. If some other content has been written to the Name, it should not overwrite it.
    While the transfer is running, the remote can send any number of PROGRESS messages. Once the transfer is complete, it finishes by sending one of these replies:
    • STORE-SUCCESS Key ContentIdentifier Indicates that the store was successful. The ContentIdentifier must be for the content that was stored.
    • STORE-FAILURE Key ErrorMsg
      Indicates the store failed.
  • CHECKPRESENTEXPORTEXPECTED Key
    Requests the remote to check if the previously provided Name is present in it, and still has the ContentIdentifier that was previously provided by EXPECTED.
    • CHECKPRESENT-SUCCESS Key
      Indicates that a content has been positively verified to be present in the remote.
    • CHECKPRESENT-FAILURE Key
      Indicates that a content has been positively verified to not be present in the remote, or does not have the expected ContentIdentifier.
    • CHECKPRESENT-UNKNOWN Key ErrorMsg
      Indicates that it is not currently possible to verify if content is present in the remote. (Perhaps the remote cannot be contacted.)
  • REMOVEEXPORTEXPECTED Key
    Requests the remote to remove the content at the previously provided Name, but only when it matches the ContentIdentifier that was provided by EXPECTED.
    • REMOVE-SUCCESS Key
      Indicates the content has been removed from the remote. May be returned when the content was already not present.
    • REMOVE-FAILURE Key ErrorMsg
      Indicates that the content was unable to be removed from the remote.
  • REMOVEEXPORTDIRECTORYWHENEMPTY Directory
    Requests the remote remove an exported directory, so long as it's empty.
    If the remote does not use directories, or REMOVEEXPORTEXPECTED cleans up directories that are empty, this does not need to be implemented.
    The directory will be in the form of a relative path, and may contain path separators, whitespace, and other special characters.
    • REMOVEEXPORTDIRECTORY-SUCCESS
      Indicates that the directory was removed, or was not empty.
    • REMOVEEXPORTDIRECTORY-FAILURE
      Indicates that the directory was empty, but could not be removed for some reason.

protocol example

The protocol starts off with VERSION etc as usual, and then git-annex asks the special remote if it supports export and import, which it does:

VERSION 1
EXTENSIONS INFO
EXTENSIONS
PREPARE
EXPORTSUPPORTED
EXPORTSUPPORTED-SUCCESS
IMPORTSUPPORTED
IMPORTSUPPORTED-SUCCESS

git-annex asks for a list of files stored in the special remote:

LISTIMPORTABLECONTENTS

Which in the simple case looks like this:

CONTENT 100 foo
CONTENTIDENTIFIER 100 48511528411921470
CONTENT 200 bar
CONTENTIDENTIFIER 200 48511528411963410
END

Or it could look like this, for a special remote that supports versions, and has two branches of history, with one branch having several different versions of the file "foo", and the second branch having a single version. Here the blocks have been indented to aid understanding; the real protocol does not include indenation.

  CONTENT 100 foo
  CONTENTIDENTIFIER 100 48511528411921470
  CONTENT 200 bar
  CONTENTIDENTIFIER 200 48511528411963410
  HISTORY
    CONTENT 99 foo
    CONTENTIDENTIFIER 99 2113620116963530
    HISTORY
      CONTENT 1 foo
      CONTENTIDENTIFIER 1 2110338579019192
    END
  END
  HISTORY
    CONTENT 88 foo
    CONTENTIDENTIFIER 88 2104982727272727
  END
END

(Note that "bar" is a new file; it was not present in any of the older versions.)

Next git-annex asks for the content of a file to be retrieved.

LOCATION foo
EXPECTED 100 48511528411921470  
RETRIEVEEXPORTEXPECTED tmpfile

But, the file "foo" has been modified and so the ContentIdentifier no longer matches.

RETRIEVE-FAILURE content has changed