MSC2674: Event Relationships

proposals/2674-event-relationships.md

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+# MSC2674: Event relationships
+
+It's common to want to send events in Matrix which relate to existing events -
+for instance, reactions, edits and even replies/threads.
+
+This proposal is one in a series of proposals that defines a mechanism for
+events to relate to each other.  Together, these proposals replace
+[MSC1849](https://github.com/matrix-org/matrix-doc/pull/1849).
+
+* This proposal defines a standard shape for indicating events which relate to
+  other events.
+* [MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675) defines APIs to
+  let the server calculate the aggregations on behalf of the client, and so
+  bundle the related events with the original event where appropriate.
+* [MSC2676](https://github.com/matrix-org/matrix-doc/pull/2676) defines how
+  users can edit messages using this mechanism.
+* [MSC2677](https://github.com/matrix-org/matrix-doc/pull/2677) defines how
+  users can annotate events, such as reacting to events with emoji, using this
+  mechanism.
+* [MSC3267](https://github.com/matrix-org/matrix-doc/pull/3267) defines how events
+  can make a reference to other events.
+* [MSC3389](https://github.com/matrix-org/matrix-doc/pull/3389) defines changes to
+  the redaction algorithm, to preserve the type and target id of a relation.
+
+
+## Proposal
+
+This proposal introduces the concept of relations, which can be used to
+associate new information with an existing event.
+
+A relationship is an object with a field `rel_type`, which is a string describing the type of relation,
+and a field `event_id`, which is a string that represents the event_id of the target event of this relation.
+The target event must exist in the same room as the relating event is sent.
+Both of those fields are required. An event is said to contain a relationship if its `content` contains
+a relationship with all the required fields under the `m.relates_to` key. If any of these conditions is not met,
+clients and servers should treat the event as if it does not contain a relationship.
+Servers should reject events not meeting these conditions with an HTTP 400 error when
+they are received via the client-server API.
+
+Here's a (partial) example of an event relating to another event:
+
+```json
+{
+  "content": {
+      "m.relates_to": {
+          "rel_type": "m.replace",
+          "event_id": "$abc:server.tld"
+      }
+  }
+}
+```
+
+All the information about the relationship lives under the `m.relates_to` key.
+
+If it helps, you can think of relations as a "subject verb object" triple,
+where the subject is the relation event itself; the verb is the `rel_type`
+field of the `m.relates_to` and the object is the `event_id` field.
+
+We consciously do not support multiple different relations within a single event,
+in order to keep the API simple.  This means that if event A relates to event B
+in two different ways you would send two events to describe the two relations,
+rather than bundling them into a single event.  Another MSC,
+like [MSC 3051](https://github.com/matrix-org/matrix-doc/pull/3051),
+can propose a change to add support for multiple relations if it turns out that
+this would facilitate certain use cases.
+
+Relations do not yet replace the 
+[reply mechanism currently defined in the spec](https://matrix.org/docs/spec/client_server/r0.6.1#rich-replies).
+
+### Relation types
+
+Any values for `rel_type` should abide the
+[general guidelines for identifiers](https://github.com/matrix-org/matrix-doc/pull/3171).
+
+The `rel_type` property determines how an event relates to another and can be used
+by clients to determine how and in what context a relation should be displayed.
+
+[MSC 2675](https://github.com/matrix-org/matrix-doc/pull/2675) proposes to also interpret the `rel_type` server-side.
+
+It is left up to the discretion of other MSCs building on this one whether they introduce
+`rel_type`s that are specific to their use case or that can serve a broad range
+of use cases. MSCs may define additional properties on the relation object for a given `rel_type`.
+
+Currently, a few `rel_type`s are already proposed. Here's a non-exhaustive list:
+
+ - `m.replace` in [MSC 2676](https://github.com/matrix-org/matrix-doc/pull/2676).
+ - `m.annotation` in [MSC 2677](https://github.com/matrix-org/matrix-doc/pull/2677).
+ - `m.reference` in [MSC 3267](https://github.com/matrix-org/matrix-doc/pull/3267).
+ - `m.thread` in [MSC 3440](https://github.com/matrix-org/matrix-doc/pull/3440).
+
+
+### Sending relations
+
+Related events are normal Matrix events, and can be sent by the normal `/send`
+API.
+
+The server should postprocess relations if needed before sending them into a
+room, as defined by the relationship type. For example, a relationship type
+might only allow a user to send one related message to a given event.
+
+### Receiving relations
+
+Relations are received like other non-state events, with `/sync`,
+`/messages` and `/context`, as normal discrete Matrix events. As explained
+in the limitations, clients may be unaware of some relations using just these endpoints.
+
+[MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675) defines ways in
+which the server may aid clients in processing relations by aggregating the
+events.
+
+### Redactions
+
+Events with a relation may be redacted like any other event.
+
+[MSC3389](https://github.com/matrix-org/matrix-doc/pull/3389) proposes that
+the redaction algorithm should preserve the type and target id of a relation.
+
+However, event relationships can still be used in existing room versions, but
+the user experience may be worse if redactions are performed.
+
+## Potential issues
+
+### Federation considerations
+
+We have a problem with resynchronising relations after a gap in federation:
+We have no way of knowing that an edit happened in the gap to one of the events
+we already have.  So, we'll show inconsistent data until we backfill the gap.
+ * We could write this off as a limitation.
+ * Or we could make *ALL* relations a DAG, so we can spot holes at the next
+   relation, and go walk the DAG to pull in the missing relations?  Then, the
+   next relation for an event could pull in any of the missing relations.
+   Socially this probably doesn't work as reactions will likely drop off over
+   time, so by the time your server comes back there won't be any more reactions
+   pulling the missing ones in.
+ * Could we also ask the server, after a gap, to provide all the relations which
+   happened during the gap to events whose root preceeded the gap.
+   * "Give me all relations which happened between this set of
+     forward-extremities when I lost sync, and the point i've rejoined the DAG,
+     for events which preceeded the gap"?
+   * Would be hard to auth all the relations which this api coughed up.
+     * We could auth them based only the auth events of the relation, except we
+       lose the context of the nearby DAG which we'd have if it was a normal
+       backfilled event.
+     * As a result it would be easier for a server to retrospectively lie about
+       events on behalf of its users.
+     * This probably isn't the end of the world, plus it's more likely to be
+       consistent than if we leave a gap.
+       * i.e. it's better to consistent with a small chance of being maliciously
+         wrong, than inconsistent with a guaranteed chance of being innocently
+         wrong.
+   * We'd need to worry about pagination.
+   * This is probably the best solution, but can also be added as a v2.
+ * In practice this seems to not be an issue, which is worth complicating
+   the s-s API over. Clients very rarely jump over the federation gap to an edit.
+   In most cases they scroll up, which backfills the server and we have all the
+   edits, when we reach the event before the gap.
+
+## Limitations
+
+Based solely on this MSC, relations are only received as discrete events in
+the timeline, so clients may only have an incomplete image of all the relations
+with an event if they do not fill gaps (syncs with a since token that have 
+`limited: true` set in the sync response for a room) in the timeline.
+
+In practice, this has proven not to be too big of a problem, as reactions
+(as proposed in [MSC 2677](https://github.com/matrix-org/matrix-doc/pull/2677))
+tend to be posted close after the target event in the timeline.
+
+A more complete solution to this has been deferred to
+[MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675). 
+
+## Tradeoffs
+
+### Event shape
+
+Shape of
+
+```json
+"content": {
+    "m.relates_to": {
+        "m.reference": {
+            "event_id": "$another:event.com"
+        }
+    }
+}
+```
+versus
+
+```json
+"content": {
+    "m.relates_to": {
+        "rel_type": "m.reference",
+        "event_id": "$another:event.com"
+    }
+}
+```
+
+The reasons to go with `rel_type` is:
+ * This format is now in use in the wider matrix ecosystem without a prefix,
+   in spite of the original MSC 1849 not being merged. This situation is not ideal
+   but we still don't want to break compatibility with several clients.
+ * We don't need the extra indirection to let multiple relations apply to a given pair of
+   events, as that should be expressed as separate relation events.
+ * If we want 'adverbs' to apply to 'verbs' in the subject-verb-object triples which
+   relations form, then we apply it as mixins to the relation data itself rather than trying
+   to construct subject-verb-verb-object sentences.
+ * We decided to not adopt the format used by `m.in_reply_to` as it allows for multiple relations
+   and is hence overly flexible. Also, the relation type of `m.in_reply_to` is also overly specific
+   judged by the guidelines for `rel_type`s laid out in this MSC. Having replies use the same
+   format as relations is postponed to a later MSC, but it would likely involve replies
+   adopting the relation format with a more broadly useful `rel_type` (possibly the `m.reference`
+   type proposed in [MSC3267](https://github.com/matrix-org/matrix-doc/pull/3267)),
+   rather than relations adopting the replies format.
+
+## Historical context
+
+pik's MSC441 has:
+
+Define the JSON schema for the aggregation event, so the server can work out
+which fields should be aggregated.
+
+```json
+"type": "m.room._aggregation.emoticon",
+"content": {
+    "emoticon": "::smile::",
+    "msgtype": "?",
+    "target_id": "$another:event.com"
+}
+```
+
+These would then be aggregated, based on target_id, and returned as annotations on
+the source event in an `aggregation_data` field:
+
+```json
+"content": {
+    ...
+    "aggregation_data": {
+        "m.room._aggregation.emoticon": {
+            "aggregation_data": [
+                {
+                    "emoticon": "::smile::",
+                    "event_id": "$14796538949JTYis:pik-test",
+                    "sender": "@pik:pik-test"
+                }
+            ],
+            "latest_event_id": "$14796538949JTYis:pik-test"
+        }
+    }
+}
+```