MSC2409: Proposal to send EDUs to appservices

proposals/2409-appservice-edus.md

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+# MSC2409: Proposal to send EDUs to appservices
+
+*Node: This proposal is a continuation of [MSC1888](https://github.com/matrix-org/matrix-doc/pull/1888)
+and deprecates that one.*
+
+The [appservice /transactions API](https://matrix.org/docs/spec/application_service/r0.1.2#put-matrix-app-v1-transactions-txnid)
+currently supports pushing PDU events (regular message and state events)
+however it doesn't provison for EDU events (typing, presence and more). This means that bridges cannot
+react to Matrix users who send any typing or presence information in a room the service is part of.
+
+There is an interest amongst the community to have equal bridging on both sides of a bridge, so that
+read reciepts and typing notifications can be seen on the remote side. To that end, this proposal
+specifies how these can be pushed to an appservice.
+
+## Proposal
+
+### Changes to the registration file
+
+In order that appservices don't get flooded with EDUs, appservices have to opt-in to receive them by
+setting `push_ephemeral` to true. A registration file could look like following:
+
+```yaml
+id: "IRC Bridge"
+url: "http://127.0.0.1:1234"
+as_token: "30c05ae90a248a4188e620216fa72e349803310ec83e2a77b34fe90be6081f46"
+hs_token: "312df522183efd404ec1cd22d2ffa4bbc76a8c1ccf541dd692eef281356bb74e"
+sender_localpart: "_irc_bot"
+# We want to receive EDUs
+push_ephemeral: true
+namespaces:
+  users:
+    - exclusive: true
+      regex: "@_irc_bridge_.*"
+  aliases:
+    - exclusive: false
+      regex: "#_irc_bridge_.*"
+  rooms: []
+```
+
+### Changes to the /transactions/ API
+
+The `PUT /_matrix/app/v1/transactions/{txnId}` API currently supports sending PDUs
+via the `events` array.
+
+```json
+{
+  "events": [
+    {
+      "content": {
+        "membership": "join",
+        "avatar_url": "mxc://domain.com/SEsfnsuifSDFSSEF#auto",
+        "displayname": "Alice Margatroid"
+      },
+      "type": "m.room.member",
+      "event_id": "$143273582443PhrSn:domain.com",
+      "room_id": "!jEsUZKDJdhlrceRyVU:domain.com",
+      "sender": "@example:domain.com",
+      "origin_server_ts": 1432735824653,
+      "unsigned": {
+        "age": 1234
+      },
+      "state_key": "@alice:domain.com"
+    }
+  ]
+}
+```
+
+This proposal would extend the `PUT /_matrix/app/v1/transactions/` endpoint to include a new key
+`ephemeral` to behave similar to the various sections of the CS API `/sync` endpoint.
+
+```json
+{
+  "ephemeral": [
+    {
+      "type": "m.typing",
+      "room_id": "!jEsUZKDJdhlrceRyVU:domain.com",
+      "content": {
+        "user_ids": [
+          "@alice:example.com"
+        ]
+      }
+    },
+    {
+      "type": "m.receipt",
+      "room_id": "!jEsUZKDJdhlrceRyVU:domain.com",
+      "content": {
+        "$1435641916114394fHBLK:matrix.org": {
+          "m.read": {
+            "@rikj:jki.re": {
+              "ts": 1436451550453
+            }
+          }
+        }
+      }
+    }
+  ],
+  "events": [
+    // ...
+  ]
+}
+```
+
+The reason for a new key rather than bundling the events into `events` is that
+existing appservices may mistake them for PDUs and could cause undefined behaviour.
+While `events` may now be a somewhat misleading name, this is an acceptable tradeoff.
+
+Note that the EDU is otherwise formatted as it would for client-server API transport.
+
+To-device messages are a bit special as they are aimed at a particular user/device ID
+combo. These events are annotated by the server with a `to_device_id` and `to_user_id`
+field at the top level of the message for transport to the appservice:
+
+```json5
+{
+  "type": "org.example.to_device_event_type",
+  "sender": "@alice:example.com",
+  "to_user_id": "@_irc_bob:example.org",
+  "to_device_id": "ABCDEF123",
+  "content": {
+    "hello": "world"
+  }
+}
+```
+
+Unlike other ephemeral events, to-device messages are included at a top level `to_device`
+array in the transaction. If there are no messages to be sent, the array can be omitted.
+This is primarily due to how to-device messages work over federation: they get wrapped in
+an EDU (`m.direct_to_device`) but that parent EDU is stripped out before sending the message
+off to clients. This can lead to potential conflict where if down the line we support EDUs
+and to-device messages with the same event type: consumers would be uncertain as to whether
+they are handling an EDU or to-device message.
+
+A complete example of the transaction with all 3 arrays populated would be:
+
+```json5
+{
+  "ephemeral": [
+    {
+      "type": "m.typing",
+      "room_id": "!jEsUZKDJdhlrceRyVU:domain.com",
+      "content": {
+        "user_ids": [
+          "@alice:example.com"
+        ]
+      }
+    }
+  ],
+  "events": [
+    {
+      "content": {
+        "membership": "join",
+        "avatar_url": "mxc://domain.com/SEsfnsuifSDFSSEF#auto",
+        "displayname": "Alice Margatroid"
+      },
+      "type": "m.room.member",
+      "event_id": "$143273582443PhrSn:domain.com",
+      "room_id": "!jEsUZKDJdhlrceRyVU:domain.com",
+      "sender": "@example:domain.com",
+      "origin_server_ts": 1432735824653,
+      "unsigned": {
+        "age": 1234
+      },
+      "state_key": "@alice:domain.com"
+    }
+  ],
+  "to_device": [
+    {
+      "type": "org.example.to_device_event_type",
+      "sender": "@alice:example.com",
+      "to_user_id": "@_irc_bob:example.org",
+      "to_device_id": "ABCDEF123",
+      "content": {
+        "hello": "world"
+      }
+    }
+  ]
+}
+```
+
+### Expectations of when an EDU should be pushed to an appservice
+
+It is not clear at face value what should be pushed to an appservice. Appservices claim
+namespaces of users which registers "interest" in the rooms where those users reside, as
+well as claiming namespaces of rooms for explicit interest. However, not all EDUs are
+associated with a single room (presence, etc).
+
+If the EDU is capable of being associated to a particular room, it should be sent to the
+appservice under the same rules as regular events (interest in the room means sending it).
+For EDUs which are not associated with a particular room, the appservice receives the EDU
+if it contextually *would* apply. For example, a presence update for a user an appservice
+shares a room with (or is under the appservice's namespace) would be sent to the appservice.
+
+To-device messages for devices belonging to the appservice's user namespaces should always
+be sent.
+
+### Implementation detail: when to delete a to-device message
+
+Not defined by this MSC is an explicit algorithm for when to delete a to-device message (mark
+it as sent). This is left as an implementation detail, though a suggested approach is as
+follows:
+
+* If the message is sent to a user under an appservice's *exclusive* namespace, mark it as sent
+  and delete it. Note that retried transactions will still need to include the message.
+* If the message is sent to a user under an appservice's *inclusive* namespace, mark it as sent
+  to the appservice but do not delete it until a `/sync` request is completed which includes the
+  message. Note that retried transactions will still need to include the message.
+
+This approach is largely to align with how namespaces are used by appservices in practice, but
+is not applicable to all scenarios (and thus is an implementation detail). The majority of known
+appservices use exclusive namespaces, which typically also means that those users will not be
+calling `/sync`. Because of this, the server may never get another opportunity to delete the
+messages until it has confirmed that the appservice received the transaction successfully. Inclusive
+namespaces are typically used when the appservice wants to impact a subset of users, but without
+controlling those users explicitly. Typically, inclusive users are also calling `/sync` and so
+the appservice should be CC'd on the to-device messages that would normally go down `/sync`.
+
+## Potential issues
+
+Determining which EDUs to transmit to the appservice could lead to quite some overhead on the
+homeservers side. Additionally, more network traffic is produced, potentially straining the local
+network and the appservice more. As such, appservices have to opt-in to receive EDUs.
+
+## Security considerations
+
+The homeserver needs to accuratley determine which EDUs to send to the appservice, as to not leak
+any metadata about users. Particularly `m.presence` could be tricky, as no `room_id` is present in
+that EDU.
+
+## Unstable prefix
+
+In the transaction body, instead of `ephemeral`, `de.sorunome.msc2409.ephemeral` is used.
+
+In the transaction body, instead of `to_device`, `de.sorunome.msc2409.to_device` is used.
+
+In the registration file, instead of `push_ephemeral`, `de.sorunome.msc2409.push_ephemeral` is used.