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

[[!toc]]

## 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 1

Once it knows the version, git-annex will generally 
send a message telling the special remote to start up.
(Or it might send a INITREMOTE, 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.

	PREPARE-SUCCESS

Now git-annex will tell the special remote what to do. Let's suppose
it wants to store a key. 

	TRANSFER STORE somekey tmpfile

The special remote can continue sending messages to git-annex during this
transfer. 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

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

These are messages git-annex sends to the special remote program.
None of these messages require an immediate reply. The special
remote can send any messages it likes while handling the requests.

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

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

* `INITREMOTE`  
  Request that the remote 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.
* `PREPARE`  
  Tells the special remote it's time to prepare itself to be used.  
  Only INITREMOTE can come before this.
* `TRANSFER STORE|RETRIEVE Key File`  
  Requests the transfer of a key. For Send, the File is the file to upload;
  for Receive the File is where to store the download.  
  Note that the File should not influence the filename used on the remote.
  The filename will not contain any whitespace.  
  Multiple transfers might be requested by git-annex, but it's fine for the 
  program to serialize them and only do one at a time.
* `CHECKPRESENT Key`  
  Requests the remote check if a key is present in it.
* `REMOVE Key`  
  Requests the remote remove a key's contents.

The following requests can optionally be supported. If not handled,
replying with `UNSUPPORTED-REQUEST` is acceptable.

* `GETCOST`  
  Requests the remote return a use cost. Higher costs are more expensive.
  (See Config/Cost.hs for some standard costs.)
* `GETAVAILABILITY`
  Requests the remote send back an `AVAILABILITY` reply.
  If the remote replies with `UNSUPPORTED-REQUEST`, its availability
  is asssumed to be global. So, only remotes that are only reachable
  locally need to worry about implementing this.

More optional requests may be added, without changing the protocol version,
so if an unknown request is seen, reply with `UNSUPPORTED-REQUEST`.

## special remote replies

These should be sent only in response to the git-annex request messages.
They do not have to be sent immediately after the request; the special
remote can send its own requests (listed in the next section below)
while it's handling a request.

* `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-SUCCESS STORE|RETRIEVE Key`  
  Indicates the transfer completed successfully.
* `TRANSFER-FAILURE STORE|RETRIEVE Key ErrorMsg`  
  Indicates the transfer failed.
* `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-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.
* `COST Int`  
  Indicates the cost of the remote.
* `AVAILABILITY GLOBAL|LOCAL`
  Indicates if the remote is globally or only locally available.
  (Ie stored in the cloud vs on a local disk.)
* `INITREMOTE-SUCCESS`  
  Indicates the INITREMOTE succeeded and the remote is ready to use.
* `INITREMOTE-FAILURE ErrorMsg`  
  Indicates that INITREMOTE failed.
* `UNSUPPORTED-REQUEST`  
  Indicates that the special remote does not know how to handle a request.

## special remote messages

These messages may be sent by the special remote at any time that it's
in control.

* `VERSION Int`  
  Supported protocol version. Current version is 1. 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!
* `PROGRESS Int`  
  Indicates the current progress of the transfer (in bytes). May be repeated
  any number of times during the transfer process, but it's wasteful to
  update the progress until at least another 1% of the file has been sent.
  This is highly recommended for STORE. (It is optional but good for RETRIEVE.)  
  (git-annex does not send a reply to this message.)
* `DIRHASH Key`  
  Gets a two level hash associated with a Key. 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.  
  (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.)
* `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.  
  (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).
* `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.)
* `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.
* `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.
* `GETSTATE Key`  
  Gets any state that has been stored for the key.  
  (git-annex replies with VALUE followed by the state.)
* `DEBUG message`
  Tells git-annex to display the message if --debug is enabled.

## 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.

## 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.

## 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.
* Support for splitting files into chunks.
* Support for getting and setting the list of urls that can be associated
  with a key.
* Hook into webapp. Needs a way to provide some kind of prompt to the user
  in the webapp, etc.
