Application Service API
=======================

The Matrix client-server API and server-server APIs provide the means to
implement a consistent self-contained federated messaging fabric. However, they
provide limited means of implementing custom server-side behaviour in Matrix
(e.g. gateways, filters, extensible hooks etc). The Application Service API
defines a standard API to allow such extensible functionality to be implemented
irrespective of the underlying homeserver implementation.

.. TODO-spec
  Add in Client-Server services? Overview of bots? Seems weird to be in the spec
  given it is VERY implementation specific.

Passive Application Services
----------------------------
"Passive" application services can only observe events from a given home server,
and inject events into a room they are participating in.
They cannot prevent events from being sent, nor can they modify the content of
the event being sent. In order to observe events from a homeserver, the
homeserver needs to be configured to pass certain types of traffic to the
application service.  This is achieved by manually configuring the homeserver
with information about the AS.

.. NOTE::
  Previously, application services could register with a homeserver via HTTP
  APIs. This was removed as it was seen as a security risk. A compromised
  application service could re-register for a global ``*`` regex and sniff
  *all* traffic on the homeserver. To protect against this, application
  services now have to register via configuration files which are linked to
  the homeserver configuration file. The addition of configuration files
  allows homeserver admins to sanity check the registration for suspicious
  regex strings.

.. TODO
  Removing the API entirely is probably a mistake - having a standard cross-HS
  way of doing this stops ASes being coupled to particular HS implementations.
  A better solution would be to somehow mandate that the API done to avoid
  abuse.

An example HS configuration required to pass traffic to the AS is:

.. code-block:: yaml

    url: <base url of AS>
    as_token: <token AS will add to requests to HS>
    hs_token: <token HS will add to requests to AS>
    sender_localpart: <localpart of AS user>
    namespaces:
      users:  # Namespaces of users which should be delegated to the AS
        - exclusive: <bool>
          regex: <regex>
        - ...
      aliases: []  # Namespaces of room aliases which should be delegated to the AS
      rooms: [] # Namespaces of room ids which should be delegated to the AS

.. WARNING::
  If the homeserver in question has multiple application services, each
  ``as_token`` MUST be unique per application service as this token is used to
  identify the application service. The homeserver MUST enforce this.

- An application service can state whether they should be the only ones who 
  can manage a specified namespace. This is referred to as an "exclusive" 
  namespace. An exclusive namespace prevents humans and other application 
  services from creating/deleting entities in that namespace. Typically,
  exclusive namespaces are used when the rooms represent real rooms on
  another service (e.g. IRC). Non-exclusive namespaces are used when the
  application service is merely augmenting the room itself (e.g. providing
  logging or searching facilities).
- Namespaces are represented by POSIX extended regular expressions, 
  e.g:

.. code-block:: yaml

   users:
     - exclusive: true
       regex: @irc.freenode.net_.*


Home Server -> Application Service API
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This contains application service APIs which are used by the home server. All
application services MUST implement these APIs.

User Query
++++++++++

This API is called by the HS to query the existence of a user on the Application
Service's namespace.

Inputs:
 - User ID
 - HS Credentials
Output:
 - Whether the user exists.
Side effects:
 - User is created on the HS by the AS via CS APIs during the processing of this request.
API called when:
 - HS receives an event for an unknown user ID in the AS's namespace, e.g. an
   invite event to a room.
Notes:
 - When the AS receives this request, if the user exists, it must create the user via
   the CS API.
 - It can also set arbitrary information about the user (e.g. display name, join rooms, etc)
   using the CS API.
 - When this setup is complete, the AS should respond to the HS request. This means the AS 
   blocks the HS until the user is created.
 - This is deemed more flexible than alternative methods (e.g. returning a JSON blob with the
   user's display name and get the HS to provision the user).
Retry notes:
 - The home server cannot respond to the client's request until the response to
   this API is obtained from the AS.
 - Recommended that home servers try a few times then time out, returning a
   408 Request Timeout to the client.
   
::

 GET /users/$user_id?access_token=$hs_token
 
 Returns:
   200 : User is recognised.
   404 : User not found.
   401 : Credentials need to be supplied.
   403 : HS credentials rejected.
 
 
   200 OK response format
 
   {}
   
Room Alias Query
++++++++++++++++
This API is called by the HS to query the existence of a room alias on the 
Application Service's namespace.

Inputs:
 - Room alias
 - HS Credentials
Output:
 - Whether the room exists.
Side effects:
 - Room is created on the HS by the AS via CS APIs during the processing of 
   this request.
API called when:
 - HS receives an event to join a room alias in the AS's namespace.
Notes:
 - When the AS receives this request, if the room exists, it must create the room via
   the CS API.
 - It can also set arbitrary information about the room (e.g. name, topic, etc)
   using the CS API.
 - It can send messages as other users in order to populate scrollback.
 - When this setup is complete, the AS should respond to the HS request. This means the AS 
   blocks the HS until the room is created and configured.
 - This is deemed more flexible than alternative methods (e.g. returning an initial sync
   style JSON blob and get the HS to provision the room). It also means that the AS knows
   the room ID -> alias mapping.
Retry notes:
 - The home server cannot respond to the client's request until the response to
   this API is obtained from the AS.
 - Recommended that home servers try a few times then time out, returning a
   408 Request Timeout to the client.
 
::

 GET /rooms/$room_alias?access_token=$hs_token
 
 Returns:
   200 : Room is recognised.
   404 : Room not found.
   401 : Credentials need to be supplied.
   403 : HS credentials rejected.
 
 
   200 OK response format
 
   {}

Pushing
+++++++
This API is called by the HS when the HS wants to push an event (or batch of 
events) to the AS.

Inputs:
 - HS Credentials
 - Event(s) to give to the AS
 - HS-generated transaction ID
Output:
 - None. 

Data flows:

::

 Typical
 HS ---> AS : Home server sends events with transaction ID T.
    <---    : AS sends back 200 OK.
    
 AS ACK Lost
 HS ---> AS : Home server sends events with transaction ID T.
    <-/-    : AS 200 OK is lost.
 HS ---> AS : Home server retries with the same transaction ID of T.
    <---    : AS sends back 200 OK. If the AS had processed these events 
              already, it can NO-OP this request (and it knows if it is the same
              events based on the transacton ID).
            

Retry notes:
 - If the HS fails to pass on the events to the AS, it must retry the request.
 - Since ASes by definition cannot alter the traffic being passed to it (unlike
   say, a Policy Server), these requests can be done in parallel to general HS
   processing; the HS doesn't need to block whilst doing this.
 - Home servers should use exponential backoff as their retry algorithm.
 - Home servers MUST NOT alter (e.g. add more) events they were going to 
   send within that transaction ID on retries, as the AS may have already 
   processed the events.
    
Ordering notes:
 - The events sent to the AS should be linearised, as they are from the event
   stream.
 - The home server will need to maintain a queue of transactions to send to 
   the AS.

::

  PUT /transactions/$transaction_id?access_token=$hs_token
 
  Request format
  {
    events: [
      ...
    ]
  }

Client-Server v2 API Extensions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Passive application services can utilise a more powerful version of the 
client-server API by identifying itself as an application service to the
home server.

Identity assertion
++++++++++++++++++
The client-server API infers the user ID from the ``access_token`` provided in 
every request. It would be an annoying amount of book-keeping to maintain tokens
for every virtual user. It would be preferable if the application service could
use the CS API with its own ``as_token`` instead, and specify the virtual user
they wish to be acting on behalf of. For real users, this would require 
additional permissions granting the AS permission to masquerade as a matrix user.

Inputs:
 - Application service token (``access_token``)

 Either:
   - User ID in the AS namespace to act as.
 Or:
   - OAuth2 token of real user (which may end up being an access token) 
Notes:
 - This will apply on all aspects of the CS API, except for Account Management.
 - The ``as_token`` is inserted into ``access_token`` which is usually where the
   client token is. This is done on purpose to allow application services to 
   reuse client SDKs.

::

 /path?access_token=$token&user_id=$userid

 Query Parameters:
   access_token: The application service token
   user_id: The desired user ID to act as.
   
 /path?access_token=$token&user_token=$token

 Query Parameters:
   access_token: The application service token
   user_token: The token granted to the AS by the real user

Timestamp massaging
+++++++++++++++++++
The application service may want to inject events at a certain time (reflecting
the time on the network they are tracking e.g. irc, xmpp). Application services
need to be able to adjust the ``origin_server_ts`` value to do this.

Inputs:
 - Application service token (``as_token``)
 - Desired timestamp
Notes:
 - This will only apply when sending events.
 
::

 /path?access_token=$token&ts=$timestamp

 Query Parameters added to the send event APIs only:
   access_token: The application service token
   ts: The desired timestamp

Server admin style permissions
++++++++++++++++++++++++++++++
The home server needs to give the application service *full control* over its
namespace, both for users and for room aliases. This means that the AS should
be able to create/edit/delete any room alias in its namespace, as well as
create/delete any user in its namespace. No additional API changes need to be
made in order for control of room aliases to be granted to the AS. Creation of
users needs API changes in order to:

- Work around captchas.
- Have a 'passwordless' user.

This involves bypassing the registration flows entirely. This is achieved by
including the AS token on a ``/register`` request, along with a login type of
``m.login.application_service`` to set the desired user ID without a password.

::

  /register?access_token=$as_token
  
  Content:
  {
    type: "m.login.application_service",
    user: "<desired user localpart in AS namespace>"
  }

Application services which attempt to create users or aliases *outside* of
their defined namespaces will receive an error code ``M_EXCLUSIVE``. Similarly,
normal users who attempt to create users or aliases *inside* an application
service-defined namespace will receive the same ``M_EXCLUSIVE`` error code,
but only if the application service has defined the namespace as ``exclusive``.

ID conventions
~~~~~~~~~~~~~~
.. TODO-spec
  - Giving HSes the freedom to namespace still feels like the Right Thing here.
  - Exposing a public API provides the consistency which was the main complaint
    against namespacing.
  - This may have knock-on effects for the AS registration API. E.g. why don't
    we let ASes specify the *URI* regex they want?

This concerns the well-defined conventions for mapping 3P network IDs to matrix
IDs, which we expect clients to be able to do by themselves.

User IDs
++++++++
Matrix users may wish to directly contact a virtual user, e.g. to send an email.
The URI format is a well-structured way to represent a number of different ID
types, including:

- MSISDNs (``tel``)
- Email addresses (``mailto``)
- IRC nicks (``irc`` - https://tools.ietf.org/html/draft-butcher-irc-url-04)
- XMPP (XEP-0032)
- SIP URIs (RFC 3261)

As a result, virtual user IDs SHOULD relate to their URI counterpart. This
mapping from URI to user ID can be expressed in a number of ways:

- Expose a C-S API on the HS which takes URIs and responds with user IDs.
- Munge the URI with the user ID.

Exposing an API would allow HSes to internally map user IDs however they like,
at the cost of an extra round trip (of which the response can be cached).
Munging the URI would allow clients to apply the mapping locally, but would force
user X on service Y to always map to the same munged user ID. Considering the
exposed API could just be applying this munging, there is more flexibility if
an API is exposed. 

::

  GET /_matrix/app/v1/user?uri=$url_encoded_uri
  
  Returns 200 OK:
  {
    user_id: <complete user ID on local HS>
  }

Room Aliases
++++++++++++
We may want to expose some 3P network rooms so Matrix users can join them directly,
e.g. IRC rooms. We don't want to expose every 3P network room though, e.g.
``mailto``, ``tel``. Rooms which are publicly accessible (e.g. IRC rooms) can be
exposed as an alias by the application service. Private rooms
(e.g. sending an email to someone) should not
be exposed in this way, but should instead operate using normal invite/join semantics.
Therefore, the ID conventions discussed below are only valid for public rooms which 
expose room aliases.

Matrix users may wish to join XMPP rooms (e.g. using XEP-0045) or IRC rooms. In both
cases, these rooms can be expressed as URIs. For consistency, these "room" URIs 
SHOULD be mapped in the same way as "user" URIs.

::

  GET /_matrix/app/v1/alias?uri=$url_encoded_uri
  
  Returns 200 OK:
  {
    alias: <complete room alias on local HS>
  }
  
Event fields
++++++++++++
We recommend that any events that originated from a remote network should
include an ``external_url`` field in their content to provide a way for Matrix
clients to link into the 'native' client from which the event originated.
For instance, this could contain the message-ID for emails/nntp posts, or a link
to a blog comment when bridging blog comment traffic in & out of Matrix.