MSC4262: Sliding Sync Extension: Profile Updates

proposals/4262-sliding-sync-profiles.md

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+# MSC4262: Sliding Sync Extension: Profile Updates
+
+This MSC is an extension to [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575)
+(and its proposed successor [MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186))
+which adds support for receiving profile updates via Sliding Sync. It complements
+[MSC4259](https://github.com/matrix-org/matrix-spec-proposals/pull/4259) which handles
+federation-level profile updates.
+
+## Proposal
+
+MSC3575 currently does not include support for receiving global profile field updates through the
+`/sync` endpoint. This extension adds support for receiving profile updates for users who are
+members of rooms the client is subscribed to.
+
+The proposal introduces a new extension called `profiles`. It processes the core extension
+arguments `enabled`, `rooms`, and `lists`, and adds the following optional arguments:
+
+```json5
+{
+    "enabled": true,           // sticky
+    "lists": ["rooms", "dms"], // sticky
+    "rooms": ["!abcd:example.com"], // sticky
+    "fields": ["displayname", "avatar_url"], // optional filter for specific profile fields
+    "include_history": false   // optional, defaults to false
+}
+```
+
+If `enabled` is `true`, then the sliding sync response MAY include profile updates in the following format:
+
+```json5
+{
+    "users": {
+        "@alice:example.com": {
+            "displayname": "Alice",
+            "avatar_url": "mxc://example.com/abc123",
+            "org.example.language": "en-GB"
+        },
+        "@bob:example.com": {
+            "displayname": null  // Field removal
+        }
+    }
+}
+```
+
+### Behaviour
+
+1. The extension only returns profile updates for users who are members of rooms that the client is
+   subscribed to via either:
+   - Room IDs explicitly listed in the `rooms` argument
+   - Rooms that fall within the sliding windows specified in `lists`
+
+2. The optional `fields` argument allows clients to filter which profile fields they want to receive
+   updates for. If omitted, all profile field updates are included.
+
+3. The optional `include_history` argument controls whether the initial sync includes recent
+   historical profile changes:
+   - If false (default), only current profile states are sent on initial sync
+   - If true, the server MAY include recent profile changes that occurred before the sync
+
+4. On an initial sync:
+   - Profile data MUST only be sent for rooms returned in the sliding sync response
+   - If `include_history` is false, only current profile states are sent
+   - If `include_history` is true, recent profile changes MAY be included
+
+5. When live streaming:
+   - Profile updates MUST be sent as the server receives them
+   - For rooms which initially appear (`initial: true`) due to direct subscriptions or rooms moving
+     into the sliding window, current profile states MUST be included
+   - A null value for a field indicates the field has been removed
+   - Omitted fields should be considered unchanged
+
+### Implementation Notes
+
+- Servers SHOULD implement appropriate batching and rate limiting of profile updates to prevent
+  overwhelming clients
+
+- While this extension provides real-time profile updates, implementations should note:
+  - Network issues could cause missed updates
+  - Clients MAY implement periodic full profile refreshes if they require stronger consistency guarantees
+  - The frequency of such refreshes should be balanced against resources and desired freshness
+
+- Profile updates are typically infrequent compared to other real-time events like typing
+  notifications, so including them in sliding sync is considered efficient
+
+## Potential Issues
+
+1. Large Initial Sync Payload
+   - With `include_history` enabled, the initial sync could be large for rooms with many members
+   - Servers should consider implementing reasonable limits on historical profile data
+
+2. Update Frequency
+   - Some users might update profiles frequently
+   - Implementations should consider rate limiting and batching updates
+
+## Alternatives
+
+1. Separate Profile Sync API
+   - Could provide more granular control over profile syncing
+   - Would increase complexity by requiring another connection
+   - Would duplicate much of sliding sync's room subscription logic
+
+2. Push-based Profile Updates
+   - Could use a separate WebSocket connection for profile updates
+   - Would increase complexity and connection overhead
+   - Would duplicate room subscription logic
+
+## Security Considerations
+
+1. Profile information is considered public data in Matrix
+
+2. The extension respects existing privacy boundaries:
+   - Only returns updates for users in rooms the client can access
+   - Follows the same authentication and authorization as the main sliding sync endpoint
+
+3. Rate limiting helps prevent abuse
+
+## Unstable Prefix
+
+No unstable prefix as Sliding Sync is still in review. To enable this extension, add this to your
+request JSON:
+
+```json
+{
+    "extensions": {
+        "profiles": {
+            "enabled": true
+        }
+    }
+}
+```
+
+## Dependencies
+
+This MSC builds on:
+
+- [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) (Sliding Sync) or its
+  proposed successor [MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186)
+  (Simplified Sliding Sync), neither of which are yet accepted into the spec