MSC4140: Delayed events

proposals/4140-delayed-events-futures.md

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+# MSC4140: Delayed events (Futures)
+
+In the context of matrixRTC where we want
+to ignore expired state events of users who left the call without sending a new
+state empty `m.call.member` event.
+
+We would like the homeserver to mark this event as expired/send an expired version
+in a reasonable time window after a user disconnected.
+
+Currently there is no mechanism for a client to provide a reliable way of
+communicating that an event is still valid.
+The only way to update an event is to post a new one.
+In some situations the client just looses connection or fails to sent the expired
+version of the event.
+
+A generic way in which one can automate expirations is desired.
+
+The described usecase is solved if we allow to send an event in advance
+to the homeserver but let the homeserver compute when its actually added to the
+DAG.
+The condition for actually sending the delayed event would could be a timeout.
+
+## Proposal
+
+To make this as generic as possible, the proposed solution is to allow sending
+multiple presigned events and delegate the control of when to actually send these
+events to an external services. This allows to exactly define what expiration means,
+since any event that will be sent once expired can be defined.
+
+We call those events `Futures`.
+
+A new endpoint is introduced:
+`PUT /_matrix/client/v3/rooms/{roomId}/send/future/{txnId}`
+It behaves exactly like the normal send endpoint except that that it allows
+to send a list of event contents. The body looks as following:
+
+```json
+{
+  "m.timeout": 10,
+  "m.send_on_timeout": {
+    "content": sendEventBody0,
+    "type": "m.room.message",
+  },
+
+  "m.send_on_action:${actionName}": {
+    "content": sendEventBody1,
+    "type": "m.room.message"
+  },
+
+  // optional
+  "m.send_now": {
+    "content": sendEventBody2,
+    "type": "m.room.message"
+    },
+}
+```
+
+Each of the `sendEventBody` objects are exactly the same as sending a normal
+event.
+
+There can be an arbitrary amount of `actionName`s.
+
+All of the fields are optional except the `timeout` and the `send_on_timeout`.
+This guarantees that all tokens will expire eventually.
+
+The homeserver can set a limit to the timeout and return an error if the limit
+is exceeded.
+
+### Response
+
+The response will mimic the request:
+
+```json
+{
+  "m.send_on_timeout": {
+    "eventId": "id_hash"
+  },
+  "m.send_on_action:${actionName}": {
+    "eventId": "id_hash"
+  },
+
+  "future_token": "token",
+
+  // optional
+  "m.send_now": { "eventId": "id_hash" }
+}
+```
+
+### Delegating futures
+
+The `token` can be used to call another future related endpoint:
+`PUT /_matrix/client/v3/futures/refresh` and `PUT /_matrix/client/v3/futures/action/${actionName}`.
+where the body is:
+
+```json
+{
+  "future_token": "token"
+}
+```
+
+The information required to call this endpoint is very limited so that almost
+no metadata is leaked. This allows to share a refresh link to a different
+service. This allows to delegate the send time. An SFU for instance, that tracks the current client connection state,
+and pings the HS to refresh and call a dedicated action to communicate
+that the user has intentionally left the conference.
+
+The homeserver does the following when receiving a Future.
+
+- It **sends** the optional `m.send_now` event.
+- It **generates** a `future_token` and stores it alongside with the time
+  of retrieval, the event list and the timeout duration.
+- **Starts a timer** for the stored `future_token`.
+
+  - If a `PUT /_matrix/client/v3/futures/refresh` is received, it
+    **restarts the timer** with the stored timeout duration.
+  - If a `PUT /_matrix/client/v3/futures/action/${actionName}` is received, it **sends the associated action event**
+    `m.action:${actionName}`.
+  - If the timer times out, **it sends the timeout event** `m.send_timeout`.
+  - If the future is a state event and includes a `m.send_now` event
+    the future is only valid while the `m.send_now`
+    is still the current state:
+
+    - This means, if the homeserver receives
+      a new state event for the same state key, the **`future_token`**
+      **gets invalidated and the associated timer is stopped**.
+
+    - There is no race condition here since a possible race between timeout and
+      new event will always converge to the new event:
+      - Timeout -> new event: the room state will be updated twice. once by
+        the content of the `m.send_on_timeout` event but later with the new event.
+      - new event -> timeout: the new event will invalidate the future. No
+
+- After the homeservers sends a timeout or action future event, the associated
+  timer and `future_token` is canceled/invalidated.
+
+So for each Future the client sends, the homeserver will send one event
+conditionally at an unknown time that can trigger logic on the client.
+This allows for any generic timeout logic.
+
+Timed messages/reminders or ephemeral events could be implemented using this where
+clients send a redact as a future or a room event with intentional mentions.
+
+In some scenarios it is important to allow to send an event with an associated
+future at the same time.
+
+- One example would be redacting an event. It only makes sense to redact the event
+  if it exists.
+  It might be important to have the guarantee, that the redact is received
+  by the server at the time where the original message is sent.
+- In the case of a state event we might want to set the state to `A` and after a
+  timeout reset it to `{}`. If we have two separate request sending `A` could work
+  but the event with content `{}` could fail. The state would not automatically
+  reset to `{}`.
+
+For this usecase an optional `m.send_now` field can be added to the body.
+
+## Usecase specific considerations
+
+### MatrixRTC
+
+We want can use the actions and the timeout for matrix rtc for the following situations
+
+- If the client takes care of its membership, we use a short timeout value (around 5-20 seconds)
+  The client will have to ping the refresh endpoint approx every 2-19 seconds.
+- When the SFU is capable of taking care of managing our connection state and we trust the SFU to
+  not disconnect a really long value can be chosen (approx. 2-10hours). The SFU will then only send
+  an action once the user disconnects or looses connection (it could even be a different action for both cases
+  handling them differently on the client)
+  This significantly reduces the amount of calls for the `/future` endpoint since the sfu only needs to ping
+  once per session (per user) and every 2-5hours (instead of every `X` seconds.)
+
+### Self destructing messages
+
+This MSC also allows to implement self destructing messages:
+
+`PUT /_matrix/client/v3/rooms/{roomId}/send/{eventType}/{txnId}`
+
+```json
+{
+  "m.text": "my msg"
+}
+```
+
+`PUT /_matrix/client/v3/rooms/{roomId}/send/future/{txnId}`
+
+```json
+{
+  "m.timeout": 10*60,
+  "m.send_on_timeout": {
+    "type":"m.room.readact",
+    "content":{
+      "redacts": "EvId"
+    }
+  }
+}
+```
+
+## EventId template variable
+
+It would be useful to be able to send redactions and edits as one http request.
+This would make sure that the client cannot loose connection after sending the first event.
+For instance sending a self destructing message without the redaction.
+
+The optional proposal is to introduce template variables that are only valid in `Future` events.
+`$m.send_now.event_id` in the content of one of the `m.send_on_action:${actionName}` and
+`m.send_on_timeout` contents this template variable can be used.
+The **Self destructing messages** example would simplify to:
+
+`PUT /_matrix/client/v3/rooms/{roomId}/send/future/{txnId}`
+
+```json
+{
+  "m.send_now":{
+    "type":"m.room.message",
+    "content":{
+      "m.text": "my msg"
+    }
+  },
+  "m.timeout": 10*60,
+  "m.send_on_timeout": {
+    "type":"m.room.readact",
+    "content":{
+      "redacts": "$m.send_now.event_id"
+    }
+  }
+}
+```
+
+## Potential issues
+
+## Alternatives
+
+[MSC4018](https://github.com/matrix-org/matrix-spec-proposals/pull/4018) also
+proposes a way to make call memberships reliable. It uses the client sync loop as
+an indicator to determine if the event is expired. Instead of letting the SFU
+inform about the call termination or using the call app ping loop like we propose
+here.
+
+---
+
+The following names for the endpoint are considered
+
+- Future
+- DelayedEvents
+- RetardedEvents
+
+## Security considerations
+
+We are using an unauthenticated endpoint to refresh the expirations. Since we use
+the token it is hard to guess a correct request and force one of the actions
+events of the Future.
+
+It is an intentional decision to not provide an endpoint like
+`PUT /_matrix/client/v3/futures/room/{roomId}/event/{eventId}`
+where any client with access to the room could also `end` or `refresh`
+the expiration. With the token the client sending the event has ownership
+over the expiration and only intentional delegation of that ownership
+(sharing the token) is possible.
+
+On the other hand the token makes sure that the instance gets as little
+information about the matrix metadata of the associated `future` event. It cannot
+even tell with which room or user it is interacting.
+
+## Unstable prefix
+
+## Dependencies