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proposals/3395-synthetic-appservice-events.md

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+# MSC3395: Synthetic Appservice Events
+
+Most services today have the concept of a webhook, which is used to inform an external service about
+the state of another service. For instance when a new user is registered on a platform, it is useful
+to alert a bot to send a welcome message.
+
+In Matrix we already have the concept of services which complement the homeserver, known as Application
+Services. However, they can only be informed of user actions that have taken place in rooms (with
+the notable exception of presence / device updates via EDUs). This proposal aims to extend the existing
+AS spec to include "synthetic" events where the homeserver can inform the appservice when a variety of
+things have happened on the homeserver.
+
+For clarity, this MSC introduces a modest set events that could be extended by further MSCs:
+
+- User registration
+- User login
+- User logout
+- User deactivation
+
+## Proposal
+
+An appservice must subscribe to the changes that it wishes to listen for. This can be done by setting a new
+key in the appservice registration file: `m.synthetic_events`, under the `namespaces` key:
+
+```yaml
+namespaces:
+  users:
+    - exclusive: false # This needs to be false to allow for registrations
+      regex: "@.*:mydomain"
+      m.events: false # Allow or disallow normal events to be sent from this namespace.
+      m.synthetic_events:
+          events:
+              - "m.user.registration"
+              - "m.user.login"
+              - "m.user.logout"
+              - "m.user.deactivated"
+```
+
+Currently only the `namespaces.users` field can contain this key, though the door is left open for
+future MSCs to expand upon this feature and allow synthetic events for different contexts.
+
+Then, when the homeserver wishes to inform a appservice of an event it would send the event over the
+appservice `/transaction` API. If the application service is down, these events SHOULD be retried when
+the appservice comes back up.
+
+As the synthetic events are namespaced, the AS should only be sent events for users in that namespace.
+
+Because a namespace is listed for these users, the AS will also recieve room events for these users by default. To opt-out
+of sending room events to the AS (and allow only synthetic events to be sent), the key `m.events` can be set to `false`.
+
+```
+PUT /_matrix/app/v1/transactions/{txnId}
+```
+
+```json5
+{
+    "m.synthetic_events": [{
+        "type": "m.user.deactivated",
+        "content": {
+            "user_id": "@alice:example.com"
+        },
+        "ts": 1432735824653,
+    }],
+    "events": [/* ... */]
+}
+```
+
+For each of the event types given above, there is an expected schema. As with all Matrix
+event contents, this can be extended to include implementation specific metadata.
+
+### `m.user.registration`
+
+This should be sent when a new user registers on the homeserver.
+
+```json5
+{
+        "type": "m.user.registration",
+        "content": {
+            "user_id": "@alice:example.com"
+        }
+}
+```
+
+### `m.user.login`
+
+This should be sent when an existing user logs in on the homeserver.
+
+```json5
+{
+        "type": "m.user.login",
+        "content": {
+            "user_id": "@alice:example.com",
+            "device_id": "ABCDEF"
+        }
+}
+```
+
+### `m.user.logout`
+
+This should be sent when an existing user logs out of their session.
+
+```json5
+{
+        "type": "m.user.logout",
+        "content": {
+            "user_id": "@alice:example.com",
+            "soft_logout": true,
+            "device_id": "ABCDEF"
+        }
+}
+```
+
+### `m.user.deactivated`
+
+This should be sent when an existing user deactivates their account.
+
+```json5
+{
+        "type": "m.user.logout",
+        "content": {
+            "user_id": "@alice:example.com"
+        }
+}
+```
+
+## Potential issues
+
+Appservices can now request permissions to reveal quite intimate details about each user, which means that homeserver
+administrators will need to be more careful when adding new appservice registrations. However, it has always been the
+case that appservices should be considered powerful tools that need review before being connected. The ability to 
+namespace the events sent allows for fine-grained control.
+
+This proposal would not work over a federated context, as federated homeservers are not aware of foregin appservices.
+That being said the events suggested in this proposal are sensitive and are expected to only be shared with immediate
+appservices connected to the homeserver. It would be possible for an appservice to "proxy" these events to a seperate room
+if federation of the information is desired, though.
+
+## Alternatives
+
+One alternative would be to have homeserver implementations make an "events" room, which sends these events
+as normal room events rather than synthetic events. Appservices would be joined and left to this room and would 
+recieve events organically. However there are some problems with this approach:
+
+- It increases the burden on the homeserver implementor to manage these rooms, including membership of 
+  each appservice.
+- It would require each of these events to be stored in the homeserver database, which would increase storage 
+  costs over time and would require homeservers to store lots of activity in the database.
+- While varying by implementation, it would be slower to store an event in a room and send it to an AS, than
+  just to send it.
+- Each event type would need it's own room to allow fine grained control over which events the appservice would
+  recieve, at least until a form of per-message ACLs lands in the spec. 
+
+For these reasons, the suggested proposal was chosen.
+
+## Security considerations
+
+These events will be sent down the same channel as other AS events, and so the security footprint
+is not impacted.
+
+## Unstable prefix
+
+All keys mentioned in this document beginning with `m.` will use `uk.half-shot.msc3395.` as the prefix.