git-annex/doc/design/p2p_protocol_over_http.mdwn

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## introduction

The [[P2P protocol]] is a custom protocol that git-annex speaks over a ssh
connection (mostly). This is a translation of that protocol to HTTP.

[[git-annex-p2phttp]] serves this protocol.

To indicate that an url uses this protocol, use 
`annex+http` or `annex+https` as the url scheme. Such an url uses
port 9417 by default, although another port can be specified. 
The last part of the path of such an url is always the repository uuid.
For example, `annex+http://example.com/git-annex/$uuid` or 
`annex+http://example.com:80/git-annex/$uuid`

## base64 encoding of keys, uuids, and filenames

A git-annex key can contain text in any encoding. So can a filename,
and it's even possible, though unlikely, that the UUID of a git-annex
repository might.

But this protocol requires that UTF-8 be used throughout, except 
where bodies use `Content-Type: application/octet-stream`.

So this protocol allows using 
[base64url](https://datatracker.ietf.org/doc/html/rfc4648#section-5)
encoding for such values. Any key, filename, or UUID wrapped in square
brackets is a base64url encoded value. 
For example, "[Zm9v]" is the same as "foo".

A filename like "[foo]" will need to itself be encoded that way: "[W2Zvb10=]"

## authentication

Some requests need authentication. Which requests do depends on the
configuration of the HTTP server. When a request needs authentication,
it will fail with 401 Unauthorized.

Authentication is done using HTTP basic auth. The realm to use when
authenticating is "git-annex". The charset is UTF-8.

When authentication is successful but does not allow a request to be
performed, it will fail with 403 Forbidden.

Note that HTTP basic auth is not encrypted so is only secure when used
over HTTPS.

## protocol version

Requests are versioned. The versions correspond to
P2P protocol versions. The version is part of the request path,
eg "v3"

If the server does not support a particular protocol version, the
request will fail with a 404, and the client should fall
back to an earlier protocol version.

## common request parameters

Every request supports this parameter, and unless documented
otherwise, it is required to be included.

* `clientuuid`  

  The value is the UUID of the git-annex repository of the client.

Any request may also optionally include these parameters:

* `bypass`

  The value is the UUID of a cluster gateway, which the server should avoid
  connecting to when serving a cluster. This is the equivilant of the
  `BYPASS` message in the [[P2P_Protocol]].

  This parameter can be given multiple times to list several cluster
  gateway UUIDs.

  This parameter is only available for v2 and above.

[Internally, git-annex can use these common parameters, plus the protocol
version, and remote UUID, to create a P2P session. The P2P session is
driven through the AUTH, VERSION, and BYPASS messages, leaving the session
ready to service requests.]

## requests

### GET /git-annex/$uuid/key/$key

This is a simple, unversioned interface to get the content of a key
from a repository.

It is not part of the P2P protocol per se, but is provided to let
other clients than git-annex easily download the content of keys from the
http server.

When the key is not present on the server, it will respond
with 404 Not Found.

### GET /git-annex/$uuid/v3/key/$key

Get the content of a key from the repository with the specified uuid.

Example:

    > GET /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/key/SHA1--foo&associatedfile=bar&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < X-git-annex-data-length: 3
    < Content-Type: application/octet-stream
    < 
    < foo

All parameters are optional, including the common parameters, and these:

* `associatedfile`

  The name of a file in the git repository, for informational purposes
  only.

* `offset`

  Number of bytes to skip sending from the beginning of the file. 

Request headers are currently ignored, so eg Range requests are
not supported. (This would be possible to implement, up to a point.)

The body of the request is empty.

The server's response will have a `Content-Type` header of
`application/octet-stream`.

The server's response will have a `X-git-annex-data-length` 
header that indicates the number of bytes of content that are expected to
be sent. Note that there is no Content-Length header.

The body of the response is the content of the key.

If the length of the body is different than what the the
X-git-annex-data-length header indicated, then the data is invalid and
should not be used. This can happen when eg, the data was being sent from
an unlocked annexed file, which got modified while it was being sent.

When the content is not present, the server will respond with 
422 Unprocessable Content.

### GET /git-annex/$uuid/v2/key/$key

Identical to v3.

### GET /git-annex/$uuid/v1/key/$key

Identical to v3.

### GET /git-annex/$uuid/v0/key/$key

Same as v3, except the X-git-annex-data-length header is not used.
Additional checking client-side will be required to validate the data.

### POST /git-annex/$uuid/v3/checkpresent

Checks if a key is currently present on the server.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/checkpresent?key=SHA1--foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"present": true}

There is one required additional parameter, `key`.

The body of the request is empty.

The server responds with a JSON object with a "present" field that is true
if the key is present, or false if it is not present.

### POST /git-annex/$uuid/v2/checkpresent

Identical to v3.

### POST /git-annex/$uuid/v1/checkpresent

Identical to v3.

### POST /git-annex/$uuid/v0/checkpresent

Identical to v3.

### POST /git-annex/$uuid/v3/lockcontent

Locks the content of a key on the server, preventing it from being removed.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/lockcontent?key=SHA1--foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"locked": true, "lockid": "foo"}

There is one required additional parameter, `key`.

The server will reply with `{"locked": true}` if it was able
to lock the key, or `{"locked": false}` if it was not.

The key will remain locked for 10 minutes. But, usually `keeplocked`
is used to control the lifetime of the lock, using the "lockid"
parameter from the server's reply. (See below.)

### POST /git-annex/$uuid/v2/lockcontent

Identical to v3.

### POST /git-annex/$uuid/v1/lockcontent

Identical to v3.

### POST /git-annex/$uuid/v0/lockcontent

Identical to v3.

### POST /git-annex/$uuid/v3/keeplocked

Controls the lifetime of a lock on a key that was earlier obtained
with `lockcontent`.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/keeplocked?lockid=foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    > Connection: Keep-Alive
    > Keep-Alive: timeout=1200
    [some time later]
    > {"unlock": true}
    < {"locked": false}

There is one required additional parameter, `lockid`.

This uses long polling. So it's important to use 
Connection and Keep-Alive headers.

This keeps an active lock from expiring until the client sends
`{"unlock": true}`, and then it immediately unlocks it.

The client can send `{"unlock": false}` any number of times first.
This has no effect, but may be useful to keep the connection alive.

This must be called within ten minutes of `lockcontent`, otherwise
the lock will have already expired when this runs. Note that this
does not indicate if the lock expired, it always returns 
`{"locked": false}`.

If the connection is closed before the client sends `{"unlock": true},
or even if the web server gets shut down, the content will remain
locked for 10 minutes from the time it was first locked.

Note that the common parameters bypass and clientuuid, while
accepted, have no effect.

### POST /git-annex/$uuid/v2/keeplocked

Identical to v3.

### POST /git-annex/$uuid/v1/keeplocked

Identical to v3.

### POST /git-annex/$uuid/v0/keeplocked

Identical to v3.

### POST /git-annex/$uuid/v3/remove

Remove a key's content from the server.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/remove?key=SHA1--foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"removed": true}

There is one required additional parameter, `key`.

The body of the request is empty.

The server responds with a JSON object with a "removed" field that is true
if the key was removed (or was not present on the server), 
or false if the key was not able to be removed.

The JSON object can have an additional field "plusuuids" that is a list of
UUIDs of other repositories that the content was removed from.

### POST /git-annex/$uuid/v2/remove

Identical to v3.

### POST /git-annex/$uuid/v1/remove

Same as v3, except the JSON will not include "plusuuids".

### POST /git-annex/$uuid/v0/remove

Identical to v1.

## POST /git-annex/$uuid/v3/remove-before

Remove a key's content from the server, but only before a specified time.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/remove-before?timestamp=4949292929&key=SHA1--foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"removed": true}

This is the same as the `remove` request, but with an additional parameter,
`timestamp`.

If the server's monotonic clock is past the specified timestamp, the
removal will fail and the server will respond with: `{"removed": false}`

This is used to avoid removing content after a point in 
time where it is no longer locked in other repostitories.

## POST /git-annex/$uuid/v3/gettimestamp

Gets the current timestamp from the server.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/gettimestamp?clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"timestamp": 59459392}

The body of the request is empty.

The server responds with JSON object with a timestmap field that has the
current value of its monotonic clock, as a number of seconds.

Important: If multiple servers are serving this protocol for the same
repository, they MUST all use the same monotonic clock.

### POST /git-annex/$uuid/v3/put

Store content on the server.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/put?key=SHA1--foo&associatedfile=bar&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    > Content-Type: application/octet-stream
    > X-git-annex-data-length: 3
    > 
    > foo
    < {"stored": true}

There is one required additional parameter, `key`.

There are are also these optional parameters:

* `associatedfile`

  The name of a file in the git repository, for informational purposes
  only.

* `offset`

  Number of bytes that have been omitted from the beginning of the file. 
  Usually this will be determined by making a `putoffset` request.

The `Content-Type` header should be `application/octet-stream`.

The `X-git-annex-data-length` must be included. It indicates the number
of bytes of content that are expected to be sent.
Note that there is no need to send a Content-Length header.

If the length of the body is different than what the the
X-git-annex-data-length header indicated, then the data is invalid and
should not be used. This can happen when eg, the data was being sent from
an unlocked annexed file, which got modified while it was being sent.

The server responds with a JSON object with a field "stored"
that is true if it received the data and stored the content.

The JSON object can have an additional field "plusuuids" that is a list of
UUIDs of other repositories that the content was stored to.

### POST /git-annex/$uuid/v2/put

Identical to v3.

### POST /git-annex/$uuid/v1/put

Same as v3, except the JSON will not include "plusuuids".

### POST /git-annex/$uuid/v0/put

Same as v1, except additional checking is done to validate the data.

### POST /git-annex/$uuid/v3/putoffset

Asks the server what `offset` can be used in a `put` of a key.

This should usually be used right before sending a `put` request.
The offset may not be valid after some point in time, which could result in
the `put` request failing.

Example:

    > POST /git-annex/ecf6d4ca-07e8-11ef-8990-9b8c1f696bf6/v3/putoffset?key=SHA1--foo&clientuuid=79a5a1f4-07e8-11ef-873d-97f93ca91925 HTTP/1.1
    < {"offset": 10}

There is one required additional parameter, `key`.

The body of the request is empty.

The server responds with a JSON object with an "offset" field that 
is the largest allowable offset.

If the server already has the content of the key, it will respond instead
with a JSON object with an "alreadyhave" field that is set to true. This JSON
object may also have a field "plusuuids" that lists 
the UUIDs of other repositories where the content is stored, in addition to
the serveruuid.

[Implementation note: This will be implemented by sending `PUT` and
returning the `PUT-FROM` offset. To avoid leaving the P2P protocol stuck
part way through a `PUT`, a synthetic empty `DATA` followed by `INVALID`
will be used to get the P2P protocol back into a state where it will accept
any request.]

### POST /git-annex/$uuid/v2/putoffset

Identical to v3.

### POST /git-annex/$uuid/v1/putoffset

Same as v3, except the JSON will not include "plusuuids".

## parts of P2P protocol that are not supported over HTTP

`NOTIFYCHANGE` is not supported, but it would be possible to extend
this HTTP protocol to support it.

`CONNECT` is not supported, and due to the bi-directional message passing
nature of it, it cannot easily be done over HTTP (would need websockets).
It should not be necessary anyway, because the git repository itself can be
accessed over HTTP.