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+# MSC3664: Notifications for relations
+
+Relations are very powerful and are becoming a platform to build new features
+like replies, edits, reactions, threads, polls and much more on.
+
+On the other hand there is currently no way to control what you are getting
+notified for. Some people want to get notified when someone replies to their
+message. Some want to get notified for reactions to their message. Some people
+explicitly do not want that. You might want to be able to mute a thread and you
+may want to get notified for poll responeses or not. Some people like getting
+notified for edits, others prefer to not get notified, when someone fixes typos
+20 times in a row for a long message they sent a week ago.
+
+We should extend push rules so that a server can provide sane defaults and users
+can adjust them to their own wishes.
+
+## Proposal
+
+### New push rule condition: `related_event_match`
+
+Notifications for relation based features need to distinguish what type of
+relation was used and potentially match on the content of the related-to event.
+
+To do that we introduce a new type of condition: `related_event_match`. This is
+largely similar to the existing `event_match`, but operates on the related-to
+event. Such a condition could look like this:
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.in_reply_to",
+  "include_fallbacks": false,
+  "key": "sender",
+  "pattern": "@me:my.server"
+}
+```
+
+This condition can be used to notify me whenever someone sends a reply to my
+messages.
+
+- `rel_type` is the relation type. For the sake of compatibility, replies
+    should be matched as if they were sent in the relation format from
+    [MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674) with a
+    `rel_type` of `m.in_reply_to`. If the event has any relation of this type,
+    the related event should be matched using `pattern` and `key` as if matching
+    that related event using an event_match rule.
+- `include_fallbacks` decides if the relation should be followed even for
+    fallbacks (i.e. relations with the `is_falling_back` property set to `true`
+    like for [threads](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3440-threading-via-relations.md#backwards-compatibility).
+    Defaults to `false` so only actual relations are counted.
+- `key` (optional): The dot-separated field of the event to match, e.g. `content.body`
+    or `sender`. If it is not present, the condition should match all events,
+    that have a relation of type `rel_type`.
+- `pattern` (optional): The glob-style pattern to match against.
+
+`key` and `pattern` have exactly the same meaning as in 
+[`event_match` conditions](https://spec.matrix.org/unstable/client-server-api/#conditions-1).
+
+`key` and `pattern` are optional to allow you to enable or suppress all
+notifications for a specific relation type. For example one could suppress
+notifications for all events with a relation from
+[threads](https://spec.matrix.org/v1.4/client-server-api/#threading) and all
+[edits](https://spec.matrix.org/v1.4/client-server-api/#event-replacements) with the following
+two conditions:
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.replace"
+}
+```
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.thread"
+}
+```
+
+Without a `key` and `pattern` the push rule can be evaluated without fetching
+the related to event. If one of those two fields is missing, a server should
+prevent those rules from being added with the appropriate error code. (A client
+which sees a `related_event_match` condition with one, but not both, of `key` and `pattern` should
+ignore the `key`/`pattern` property.
+
+A client can check for the `related_event_match` condition being supported by
+testing for an existing `.m.rule.reply` in the default rules.
+
+### A push rule for replies
+
+To enable notifications for replies without relying on the reply fallback, but
+with similar semantics we also define a new default push rule. The proposed
+push rule differs slightly from the old behaviour, because it only notifies you
+for replies to your events, but it does not notify you for replies to events
+containing your display name or matrix ID. The rule should look like this:
+
+```json5
+{
+    "rule_id": ".m.rule.reply",
+    "default": true,
+    "enabled": true,
+    "conditions": [
+        {
+            "kind": "related_event_match",
+            "rel_type": "m.in_reply_to",
+            "key": "sender",
+            "pattern": "[the user's Matrix ID]"
+        }
+    ],
+    "actions": [
+        "notify",
+        {
+            "set_tweak": "sound",
+            "value": "default"
+        },
+        {
+            "set_tweak": "highlight"
+        }
+    ]
+}
+```
+
+This should be an override rule, since it can't be a content rule and should
+not be overridden when setting a room to mentions only. It should be placed just
+before `.m.rule.contains_display_name` in the list. This ensures you get
+notified for replies to all events you sent. The actions are the same as for
+`.m.rule.contains_display_name` and `.m.rule.contains_user_name`.
+
+No other rules are proposed as no other relations are in the specification as of
+writing this MSC to decrease dependencies.
+
+## Potential issues
+
+Most push rules for relations will need a lookup into a second event. This
+causes additional implementation complexity and can potentially be expensive.
+Looking up one event shouldn't be that heavy, but it is overhead that wasn't
+there before and it needs to be evaluated for every event, so it clearly is
+somewhat performance sensitive.
+
+If the related to event is not present on the homeserver, evaluating the push
+rule may be delayed or fail completely. For most rules this should not be an
+issue. You can assume the event was not sent by a user on your server if the
+event is not present on your server.  In general clients and servers should do
+their best to evaluate the condition. If they fail to do so (possibly because
+they can't look up the event asynchronously) in a timely manner, the rule
+may be ignored/the condition evaluated to false. This should affect only a
+subset of events, because in general relations happen to events in close
+proximity. There is a risk of missing notifications for replies to very old
+messages and similar relations.
+
+[Threads](https://github.com/matrix-org/matrix-doc/pull/3440) use replies
+[as a fallback](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3440-threading-via-relations.md#backwards-compatibility).
+This would cause a notification with the new `.m.rule.reply` rule. To prevent
+that this MSC adds the `include_fallbacks` key to the rule, so that reply
+relations only added as a fallback are ignored. (Currently `is_falling_back` key
+is in a bit of a weird location. Maybe this can be amended in the threading MSC
+to be a bit more generic before it is added to the spec.)
+
+Adding a new rule that causes notifications will force users to change their
+notification settings again. In this case, a user who disabled notifications
+for mentions (or set them to silent) may be surprised to suddenly start
+receiving noisy notifications for replies. Worse, in the transition period,
+clients might not have a UI to disable the new notifications.
+This is a risk with all push rule changes and since it allows for a much better
+control over what notifies you, the tradeoff should be acceptable. Many users
+disable mention based pings, because they
+[can be error prone](https://github.com/matrix-org/matrix-doc/pull/3517), but
+they may not actually have intended to also disable notifications for
+replies, which should only trigger for actual replies to your messages. So for a
+significant chunk of people disabling mentions this should be an improvement.
+
+## Alternatives
+
+- One could add an optional `rel_type` key to all existing conditions. This
+    would allow you to also easily match by `contains_display_name`,
+    `sender_notification_permission` and `room_member_count`. Out of those
+    conditions only `contains_display_name` seems to be useful in a related
+    event context. Having a potentially expensive key like `rel_type` available
+    for all conditions would also increase implementation complexity. As such
+    this MSC proposes the minimum amount of conditions to support push rules for
+    most relations, although allowing `rel_type` on `contains_display_name` and
+    `event_match` could be a good alternative.
+- Beeper has a
+    [similar feature in their synapse](https://gitlab.com/beeper/synapse/-/commit/44a1728b6b021f97900c89e0c00f7d1a23ce0d43),
+    but it does not allow you to filter by relation type.
+
+
+
+## Security considerations
+
+- These pushrules could be used to increase load on the homeserver. Apart from
+    that there shouldn't be any potential security issues.
+
+## Unstable prefix
+
+While this proposal is still in progress, implementations should use the
+unstable prefix `im.nheko.msc3664` for the `related_event_match` condition. As
+a result it should be called `im.nheko.msc3664.related_event_match`.
+
+Clients can check the capabilities for `im.nheko.msc3664.related_event_match` to
+see if this MSC is implemented and enabled on the homeserver until this MSC is
+included in a spec release.
+