MSC3051: scalable relation format

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

proposals/3051-scalable-relations.md

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+# MSC3051: Scalable relations
+
+Edits, reactions, replies, threads, message annotations and other MSCs have
+shown, that relations between events are very powerful and useful. Currently the
+format from [MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674) is
+used. That format however limits each event to exactly one relation. As a result
+events rely on other ways to represent secondary relations. For example edits
+keep the relation from the previous event. Their support to change or delete
+that relation is limited. In theory you could pass that in `m.new_content`, but
+clients don't seem to support that and the actual deletion of a relation is
+unexplored as well.
+
+There are many cases where 2 or more relations on an event would be useful. This
+MSC proposes a simple way to do that and replace the currently proposed format.
+
+## Proposal
+
+To support multiple relations per file this MSC proposes the following format:
+
+```json
+{
+  "content": {
+    "m.relations": [
+      {
+        "event_id": "$some-other-event",
+        "rel_type": "m.in_reply_to"
+      },
+      {
+        "event_id": "$some-third-event",
+        "rel_type": "m.replaces"
+      },
+      {
+        "event_id": "$event-four",
+        "rel_type": "org.example.custom_relation",
+        "key": "some_aggregation_key"
+      }
+    ]
+  },
+  "event_id": "$something",
+  "type": "m.room.message"
+}
+```
+
+This has a few benefits:
+
+- You can relate to multiple events at the same time. (I.e. you have a
+    description for multiple files you sent.)
+- You can have multiple different relation types at once. (I.e. an edit, that
+    is also a reply, or a reaction inside a thread.)
+- You don't need to look up reply relations in multiple events for edits. The
+  edited event is canonical and can be used standalone, without having to look
+  up the original event to figure out, what was replied to. You can also remove
+  a relation with an edit now. (Useful if you replied to the wrong message or
+  didn't mean to reply to anyone.)
+- This format is conceptually a lot simpler, if an event has multiple relations.
+    You don't run into issues with packing relations into `m.new_content`,
+    especially for encrypted events, etc. You just have a list of relations.
+
+If clients want to stay backwards compatible (for a while at least), in many
+instances it is possible to generate an `m.relates_to` object from the relations
+list. This can be done by picking a primary relation, i.e. the edit relation,
+and then packaging up the remaining relations in `m.new_content` or simply
+throwing them away. Since this proposal uses `m.relations`, this does not
+conflict with the current relations from the other MSCs. One can also generate
+the relations object from this MSC from the old relations, since the new
+relations are a strict superset, which may be useful to make handling inside of
+a client easier.
+
+## Potential issues
+
+### Ordering
+
+The list of relations is not hierarchical. As such there is no order like where
+you have a top level relation and a lower level relation like an edit having
+priority over a reply.
+
+I don't believe that is an issue in practice. If you edit a message with a
+reply, there is a natural meaning to the combination of both relations. You can
+even apply them in any order, imo. But there may be other relations, where this
+causes more issues. An MSC introducing such a relation should specify how to
+handle conflicts then.
+
+### Conflicting relations
+
+Some relation types should probably not be combined. For example you may
+disallow editing a reaction, because clients probably won't be handling that
+correctly. This MSC however does not disallow that. Relations should specify,
+how clients should handle that and clients sending such combinations should be
+aware, that those probably won't get handled. I don't think just allowing 1
+relation is the solution to handling such conflicts and I don't think they will
+happen much in practice.
+
+## Alternatives
+
+- We could just stick with the existing proposal to only have 1 relation per
+    event. This is obviously limiting, but works well enough for a lot of
+    relation types.
+- There are a few other ways to structure relations like using an object instead
+    of an array, etc. I believe this is the most usable one.
+
+## Security considerations
+
+Multiple releations may increase load on the server and the client and provide
+more opportunities to introduce bad data. Servers and clients should take
+additional care and validate accordingly. It should not be considerably worse
+than single relations though and servers may limit relations to a reasonable
+amount (like they do for devices already).
+
+## Unstable prefix
+
+Clients should use `im.nheko.relations.v1.relations` instead of `m.relations`
+and `im.nheko.relations.v1.in_reply_to` as the relation type for replies in the
+mean time.
+