MSC2409: Proposal to send typing, presence and receipts 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 `receive_edus` 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
+receive_edus: 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 match the CS APIs `/sync`.
+
+```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.
+
+### 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. An appservice
+registers interests in rooms and (usually) it's own users, however EDU events are not
+tied to a single room in all situations and as such there needs to be a specified way of
+forwarding these events.
+
+An EDU should be sent to an appservice if the `room_id` is shared by any of the registered appservices
+users, if possible. For EDUs where that isn't the case, that is `m.presence`, the EDU should be sent
+if the sender is present in a room that is shared by any of the registered appservices users.
+
+## 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.