Communication between git-annex and a program implementing an external special remote uses this protocol.

  1. starting the program
  2. protocol overview
  3. example session
  4. git-annex request messages and replies
  5. special remote messages
  6. general messages
  7. protocol versions
  8. extensions
  9. signals
  10. long running network connections
  11. claiming custom uri schemes for use with git-annex addurl
  12. readonly mode for http downloads
  13. TODO

starting the program

The external special remote program has a name like git-annex-remote-$bar. When git annex initremote foo type=external externaltype=$bar is run, git-annex finds the appropriate program in PATH.

The program is started by git-annex when it needs to access the special remote, and may be left running for a long period of time. This allows it to perform expensive setup tasks, etc. Note that git-annex may choose to start multiple instances of the program (eg, when multiple git-annex commands are run concurrently in a repository).

protocol overview

Communication is via stdin and stdout. Therefore, the external special remote must avoid doing any prompting, or outputting anything like eg, progress to stdout. (Such stuff can be sent to stderr instead.)

The protocol is line based. Messages are sent in either direction, from git-annex to the special remote, and from the special remote to git-annex.

In order to avoid confusing interactions, one or the other has control at any given time, and is responsible for sending requests, while the other only sends replies to the requests.

Each protocol line starts with a command, which is followed by the command's parameters (a fixed number per command), each separated by a single space. The last parameter may contain spaces. Parameters may be empty, but the separating spaces are still required in that case.

example session

The special remote is responsible for sending the first message, indicating the version of the protocol it is using.

VERSION 2

Recent versions of git-annex respond with a message indicating protocol extensions that it supports. Older versions of git-annex do not send this message.

EXTENSIONS INFO ASYNC GETGITREMOTENAME UNAVAILABLERESPONSE

The special remote can respond to that with its own EXTENSIONS message, listing any extensions it wants to use. (It's also fine to reply with UNSUPPORTED-REQUEST.)

EXTENSIONS

Next, git-annex will generally send a message telling the special remote to start up. (Or it might send an INITREMOTE or EXPORTSUPPORTED or LISTCONFIGS, or perhaps other things in the future, so don't hardcode this order.)

PREPARE

The special remote can now ask git-annex for its configuration, as needed, and check that it's valid. git-annex responds with the configuration values

GETCONFIG directory
VALUE /media/usbdrive/repo
GETCONFIG automount
VALUE true

Once the special remote is satisfied with its configuration and is ready to go, it tells git-annex that it's done with the PREPARE step:

PREPARE-SUCCESS

Now git-annex will make a request. Let's suppose it wants to store a key.

TRANSFER STORE somekey tmpfile

The special remote can then start reading the tmpfile and storing it. While it's doing that, the special remote can send messages back to git-annex to indicate what it's doing, or ask for other information. It will typically send progress messages, indicating how many bytes have been sent:

PROGRESS 10240
PROGRESS 20480

Once the key has been stored, the special remote tells git-annex the result:

TRANSFER-SUCCESS STORE somekey

Now git-annex will send its next request.

Once git-annex is done with the special remote, it will close its stdin. The special remote program can then exit.

git-annex request messages and replies

These are messages git-annex sends to the special remote program.

Once the special remote has finished performing the request, it should send one of the listed replies.

The following requests must all be supported by the special remote.

  • INITREMOTE
    Requests the remote to initialize itself. This is where any one-time setup tasks can be done, for example creating an Amazon S3 bucket.
    Note: This may be run repeatedly over time, as a remote is initialized in different repositories, or as the configuration of a remote is changed. (Both git annex initremote and git-annex enableremote run this.) So any one-time setup tasks should be done idempotently.
    • INITREMOTE-SUCCESS
      Indicates the INITREMOTE succeeded and the remote is ready to use.
    • INITREMOTE-FAILURE ErrorMsg
      Indicates that INITREMOTE failed.
  • PREPARE
    Tells the remote that it's time to prepare itself to be used.
    Only a few requests for details about the remote can come before this (EXTENSIONS, INITREMOTE, EXPORTSUPPORTED and LISTCONFIGS, but others may be added later).
    • PREPARE-SUCCESS
      Sent as a response to PREPARE once the special remote is ready for use.
    • PREPARE-FAILURE ErrorMsg
      Sent as a response to PREPARE if the special remote cannot be used.
  • TRANSFER STORE|RETRIEVE Key File
    Requests the transfer of a key. This is the main thing a special remote does. For STORE, the File contains the content to upload; for RETRIEVE the File is where to store the content you download.
    When retrieving, the File may already exist, if its retieval was interrupted before. That lets the remote resume downloading, if it's able to.
    Note that the File should not influence the filename used on the remote; that filename should be based on the Key.
    Note that in some cases, the File's name may include whitespace or other special characters.
    While the transfer is running, the remote can send any number of PROGRESS messages to indicate its progress. It can also send any of the other special remote messages. Once the transfer is done, 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.
  • CHECKPRESENT Key
    Requests the remote to check if a key is present in it.
    It's important that, while a key is being transferred to a remote, CHECKPRESENT not indicate it's present in the remote until all the data has been sent.
    • CHECKPRESENT-SUCCESS Key
      Indicates that a key has been positively verified to be present in the remote.
    • CHECKPRESENT-FAILURE Key
      Indicates that a key 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 the key is present in the remote. (Perhaps the remote cannot be contacted.)
  • REMOVE Key
    Requests the remote to remove a key's contents.
    • REMOVE-SUCCESS Key
      Indicates the key has been removed from the remote. May be returned if the remote didn't have the key at the point removal was requested.
    • REMOVE-FAILURE Key ErrorMsg
      Indicates that the key was unable to be removed from the remote.

Special remotes can optionally support tree exports and imports, which makes the git-annex-export and git-annex-import commands work with them. See the export and import appendix for additional requests that git-annex will make when using special remotes in this way.

The following requests can optionally be supported. If not supported, the special remote can reply with UNSUPPORTED-REQUEST.

  • EXTENSIONS List
    Sent to indicate protocol extensions which git-annex is capable of using. The list is a space-delimited list of protocol extension keywords. The remote can reply to this with its own EXTENSIONS list. See the section on extensions below for details.
    • EXTENSIONS List
      Sent in response to a EXTENSIONS request, to indicate the protocol extensions that the special remote is using.
  • LISTCONFIGS
    Requests the remote to return a list of settings it uses (with GETCONFIG and SETCONFIG). Providing a list makes git annex initremote work better, because it can check the user's input, and can also display a list of settings with descriptions. Note that the user is not required to provided all the settings listed here. A block of responses can be made to this, which must always end with CONFIGEND.
    (Do not include config like "encryption" that are common to all external special remotes. Also avoid including a config named "versioning" unless using it as desribed in the export and import appendix.)
    • CONFIG Name Description
      Indicates the name and description of a config setting. The description should be reasonably short. Example: "CONFIG directory store data here"
    • CONFIGEND
      Indicates the end of the response block.
  • GETCOST
    Requests the remote to return a use cost. Higher costs are more expensive. (See Config/Cost.hs for some standard costs.)
    • COST Int
      Indicates the cost of the remote.
  • GETORDERED
    Asks the remote if it will always write files in order when performing a TRANSFER RETRIEVE. Writing in order lets a proxy stream content from the remote. When this is not implemented, git-annex assumes the remote may write parts of the file out of order.
    • ORDERED Indicates that files are written in order.
    • UNORDERED Indicates that files are not written in order.
  • GETAVAILABILITY
    Asks the remote if it is locally or globally available. (Ie stored in the cloud vs on a local disk.)
    If the remote replies with UNSUPPORTED-REQUEST, its availability is assumed to be global. So, only remotes that are only reachable locally need to worry about implementing this.
    This is queried at remote startup, so should avoid doing anything that can take long to run or is expensive. Checking if a directory where the remote stores files is currently mounted is the kind of thing it makes sense to do here.
    • AVAILABILITY GLOBAL|LOCAL
      Indicates if the remote is globally or only locally available.
    • AVAILABILITY UNAVAILABLE
      Indicates that the remote is not currently available.
      This will prevent some git-annex commands like git-annex sync from trying to use the remote.
      Older versions of git-annex do not support this response, so avoid sending it unless the UNAVAILABLERESPONSE extension is enabled.

  • ORDERED

  • CLAIMURL Url
    Asks the remote if it wishes to claim responsibility for downloading an url.

    • CLAIMURL-SUCCESS
      Indicates that the CLAIMURL url will be handled by this remote.
    • CLAIMURL-FAILURE
      Indicates that the CLAIMURL url wil not be handled by this remote.
  • CHECKURL Url
    Asks the remote to check if the url's content can currently be downloaded (without downloading it).
    • CHECKURL-CONTENTS Size|UNKNOWN Filename
      Indicates that the requested url has been verified to exist.
      The Size is the size in bytes, or use "UNKNOWN" if the size could not be determined.
      The Filename can be empty (in which case a default is used), or can specify a filename that is suggested to be used for this url.
    • CHECKURL-MULTI Url1 Size1|UNKNOWN Filename1 Url2 Size2|UNKNOWN Filename2 ...
      Indicates that the requested url has been verified to exist, and contains multiple files, which can each be accessed using their own url. Each triplet of url, size, and filename should be listed, one after the other. Note that since a list is returned, neither the Url nor the Filename can contain spaces.
    • CHECKURL-FAILURE ErrorMsg
      Indicates that the requested url could not be accessed.
  • WHEREIS Key
    Asks the remote to provide additional information about ways to access the content of a key stored in it, such as eg, public urls. This will be displayed to the user by eg, git annex whereis. Note that users expect git annex whereis to run fast, without eg, network access.
    • WHEREIS-SUCCESS String
      Indicates a location of a key. Typically an url, the string can be anything that it makes sense to display to the user about content stored in the special remote.
    • WHEREIS-FAILURE
      Indicates that no location is known for a key. This is not needed when SETURIPRESENT is used, since such uris are automatically displayed by git annex whereis.
  • GETINFO
    Requests the remote to send some information describing its configuration, for display by git annex info. A block of responses can be made to this, which must always end with INFOEND.
    • INFOFIELD Name
      Gives the name of an info field. The name can be anything you want to be displayed to the user. Must be immediately followed by INFOVALUE.
    • INFOVALUE Value
      Gives the value of an info field.
    • INFOEND
      Indicates the end of the response block.

More optional requests may be added, without changing the protocol version, so if an unknown request is seen, don't crash, just reply with UNSUPPORTED-REQUEST.

special remote messages

These messages may be sent by the special remote at any time that it's handling a request.

  • VERSION Int
    Supported protocol version. Current version is 2. Must be sent first thing at startup, as until it sees this git-annex does not know how to talk with the special remote program!
    (git-annex does not send a reply to this message, but may give up if it doesn't support the necessary protocol version.)
  • PROGRESS Int
    Indicates the current progress of the transfer. The Int is the number of bytes from the beginning of the file that have been transferred.
    May be repeated any number of times during the transfer process, but it's wasteful to update the progress too frequently. Bear in mind that this is used both to display a progress meter for the user, and for annex.stalldetection. So, sending an update on each 1% of the file may not be frequent enough, as it could appear to be a stall when transferring a large file.
    This is highly recommended for STORE. (It is optional but good for RETRIEVE; git-annex will fall back to tracking the size of the file as it grows.)
    (git-annex does not send a reply to this message.)
  • DIRHASH Key
    Gets a two level hash associated with a Key. Something like "aB/Cd/". This is always the same for any given Key, so can be used for eg, creating hash directory structures to store Keys in. This is the same directory hash that git-annex uses inside .git/annex/objects/
    (git-annex replies with VALUE followed by the value.)
  • DIRHASH-LOWER Key
    Gets a two level hash associated with a Key, using only lower-case. Something like "abc/def/". This is always the same for any given Key, so can be used for eg, creating hash directory structures to store Keys in. This is the same directory hash that is used by eg, the directory special remote.
    (git-annex replies with VALUE followed by the value.)
  • SETCONFIG Setting Value
    Sets one of the special remote's configuration settings.
    Normally this is sent during INITREMOTE, which allows these settings to be stored in the git-annex branch, so will be available if the same special remote is used elsewhere. (If sent after INITREMOTE, the changed configuration will only be available while the remote is running.)
    See also GETGITREMOTENAME for a way to access git configuration of the remote.
    (git-annex does not send a reply to this message.)
  • GETCONFIG Setting
    Gets one of the special remote's configuration settings, which can have been passed by the user when running git annex initremote, or can have been set by a previous SETCONFIG. Can be run at any time.
    It's recommended that special remotes that use this implement LISTCONFIGS.
    (git-annex replies with VALUE followed by the value. If the setting is not set, the value will be empty.)
  • SETCREDS Setting User Password
    When some form of user and password is needed to access a special remote, this can be used to securely store them for later use. (Like SETCONFIG, this is normally sent only during INITREMOTE.)
    The Setting indicates which value in a remote's configuration can be used to store the creds.
    Note that creds are normally only stored in the remote's configuration when it's surely safe to do so; when gpg encryption is used, in which case the creds will be encrypted using it. If creds are not stored in the configuration, they'll only be stored in a local file.
    (embedcreds can be set to yes by the user or by SETCONFIG to force the creds to be stored in the remote's configuration).
    (git-annex does not send a reply to this message.)
  • GETCREDS Setting
    Gets any creds that were previously stored in the remote's configuration or a file. (git-annex replies with "CREDS User Password". If no creds are found, User and Password are both empty.)
  • GETUUID
    Queries for the UUID of the special remote being used.
    (git-annex replies with VALUE followed by the UUID.)
  • GETGITDIR
    Queries for the path to the git directory of the repository that is using the external special remote. (git-annex replies with VALUE followed by the path.)
  • GETGITREMOTENAME
    Gets the name of the git remote that represents this special remote. This can be used, for example, to look up git configuration belonging to that git remote. This name will often be the same as what is passed to git-annex initremote and enableremote, but it is possible for git remotes to be renamed, and this will provide the remote's current name.
    (git-annex replies with VALUE followed by the name.)
    This message is a protocol extension; it's only safe to send it to git-annex after it sent an EXTENSIONS that included GETGITREMOTENAME.
  • SETWANTED PreferredContentExpression
    Can be used to set the preferred content of a repository. Normally this is not configured by a special remote, but it may make sense in some situations to hint at the kind of content that should be stored in the special remote. Note that if a unparsable expression is set, git-annex will ignore it.
    (git-annex does not send a reply to this message.)
  • GETWANTED
    Gets the current preferred content setting of the repository. (git-annex replies with VALUE followed by the preferred content expression.)
  • SETSTATE Key Value
    Can be used to store some form of state for a Key. The state stored can be anything this remote needs to store, in any format. It is stored in the git-annex branch. Note that this means that if multiple repositories are using the same special remote, and store different state, whichever one stored the state last will win. Also, it's best to avoid storing much state, since this will bloat the git-annex branch. Most remotes will not need to store any state.
    (git-annex does not send a reply to this message.)
  • GETSTATE Key
    Gets any state that has been stored for the key.
    (git-annex replies with VALUE followed by the state.)
  • SETURLPRESENT Key Url
    Records an URL where the Key can be downloaded from.
    Note that this does not make git-annex think that the url is present on the web special remote.
    Keep in mind that this stores the url in the git-annex branch. This can result in bloat to the branch if the url is large and/or does not delta pack well with other information (such as the names of keys) already stored in the branch.
    (git-annex does not send a reply to this message.)
  • SETURLMISSING Key Url
    Records that the key can no longer be downloaded from the specified URL.
    (git-annex does not send a reply to this message.)
  • SETURIPRESENT Key Uri
    Records an URI where the Key can be downloaded from. Use with uris that cannot be downloaded with http. (git-annex does not send a reply to this message.)
  • SETURIMISSING Key Uri
    Records that the key can no longer be downloaded from the specified URI.
    (git-annex does not send a reply to this message.)
  • GETURLS Key Prefix
    Gets the recorded urls where a Key can be downloaded from. Only urls that start with the Prefix will be returned. The Prefix may be empty to get all urls. (git-annex replies one or more times with VALUE for each url. The final VALUE has an empty value, indicating the end of the url list.)
  • DEBUG message
    Tells git-annex to display the message if --debug is enabled.
    (git-annex does not send a reply to this message.)
  • INFO message Tells git-annex to display the message to the user.
    When git-annex is in --json mode, the message will be emitted immediately in its own json object, with an "info" field.
    This message is a protocol extension; it's only safe to send it to git-annex after it sent an EXTENSIONS that included INFO.
    (git-annex does not send a reply to this message.)

general messages

These messages can be sent at any time by either git-annex or the special remote.

  • ERROR ErrorMsg
    Generic error. Can be sent at any time if things get too messed up to continue. When possible, use a more specific reply from the list above.
    The special remote program should exit after sending this, as git-annex will not talk to it any further. If the program receives an ERROR from git-annex, it can exit with its own ERROR.

protocol versions

Currently git-annex supports VERSION 1 and VERSION 2. The two protocol versions are actually identical.

Old versions of git-annex that supported only VERSION 1 had a bug in their implementation of the part of the protocol documented in the export and import appendix. The bug could result in ontent being exported to the wrong file. External special remotes that implement that should use VERSION 2 to avoid talking to the buggy old version of git-annex.

extensions

These protocol extensions are currently supported.

  • INFO
    This allows using the INFO message.
  • ASYNC
    This lets multiple actions be performed at the same time by a single external special remote program, rather than starting multiple programs. See the async appendix for details.
  • GETGITREMOTENAME
    This allows using the GETGITREMOTENAME message.
  • UNAVAILABLERESPONSE
    This allows the AVAILABILITY UNAVAILABLE response to be used in reply to GETAVAILABILITY.

signals

The external special remote program should not block SIGINT, or SIGTERM. Doing so may cause git-annex to hang waiting on it to exit. Of course it's ok to catch those signals and do some necessary cleanup before exiting.

long running network connections

Since an external special remote is started only when git-annex needs to access the remote, and then left running, it's ok to open a network connection in the PREPARE stage, and continue to use that network connection as requests are made.

If you're unable to open a network connection, or the connection closes, perhaps because the network is down, it's ok to fail to perform any requests. Or you can try to reconnect when a new request is made.

Note that the external special remote program may be left running for quite a long time, especially when the git-annex assistant is using it. The assistant will detect when the system connects to a network, and will start a new process the next time it needs to use a remote.

claiming custom uri schemes for use with git-annex addurl

If a special remote has its own uri scheme, or some other way to identify a particular url as being content that is stored in the special remote, and can be downloaded by it, it can implement CLAIMURL and CHECKURL. This lets git-annex addurl be used with such urls.

For example, the ipfs special remote implements CLAIMURL and CHECKURL for "ipfs:ADDRESS" uris. And the bittorrent special remote implements them for http urls ending in ".torrent".

When a special remote has claimed an url, commands like git-annex addurl will use TRANSFER RETRIEVE to request it download the content of a key. To find out what url to download, the special remote can use GETURLS to find out what urls are recorded for the key.

For example, the ipfs special remote sends "GETURLS $KEY ipfs:", in order to get only the "ipfs:" uris.

The special remote can also use SETURIPRESENT or SETURLPRESENT, eg after transferring content to the remote it might know the uri or url that can be used to download it. And SETURIMISSING or SETURLMISSING can be used after removing content from the remote. This information can then be looked up using GETURLS. But it's not necessary to do this in order to simply claim an url, because git-annex addurl takes care of it.

For example, the ipfs special remote sends "SETURIPRESENT $KEY ipfs:ADDRESS" after storing each key in ipfs. It can later look up that uri when downloading the key, and the ipfs uri is also displayed by git-annex whereis.

readonly mode for http downloads

Some storage services allow downloading the content of a file using a regular http connection, with no authentication. An external special remote for such a storage service can support a readonly mode of operation.

It works like this:

  • When a key's content is stored on the remote, use SETURLPRESENT to tell git-annex the public url from which it can be downloaded.
  • When a key's content is removed from the remote, use SETURLMISSING.

  • Document that this external special remote can be used in readonly mode.

    The user doesn't even need to install your external special remote program to use such a remote! All they need to do is run: git annex enableremote $remotename readonly=true

  • The readonly=true parameter makes git-annex download content from the urls recorded earlier by SETURLPRESENT.

TODO

  • When storing encrypted files stream the file up/down the pipe, rather than using a temp file. Will probably involve less space and disk IO, and makes the progress display better, since the encryption can happen concurrently with the transfer. Also, no need to use PROGRESS in this scenario, since git-annex can see how much data it has sent/received from the remote. However, \n and probably \0 need to be escaped somehow in the file data, which adds complication.
  • uuid discovery during INITREMOTE.
  • Hook into webapp. Needs a way to provide some kind of prompt to the user in the webapp, etc.