MSC4222: Adding state_after to /sync

proposals/4222-sync-v2-state-after.md

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+# MSC4222: Adding `state_after` to sync v2
+
+The current sync v2 API does not differentiate between state events in the timeline and updates to state, and so can
+cause the client's view of the current state of the room to diverge from the actual state of the room. This is
+particularly problematic for use-cases that rely on state being consistent between different clients.
+
+This behavior stems from the fact that the clients update their view of the current state with state events that appear
+in the timeline. To handle gappy syncs, the `state` section includes state events that are from *before* the start of
+the timeline, and so are replaced by any matching state events in the timeline. This provides little opportunity for the
+server to ensure that the clients come to the correct conclusion about the current state of the room.
+
+In [MSC4186 - Simplified Sliding Sync](https://github.com/matrix-org/matrix-spec-proposals/pull/4186) this problem is
+solved by the equivalent `required_state` section including all state changes between the previous sync and the end of
+the current sync, and clients do not update their view of state based on entries in the timeline.
+
+
+## Proposal
+
+This change is gated behind the client adding a `?use_state_after=true` (the unstable name is
+`org.matrix.msc4222.use_state_after`) query param.
+
+When enabled, the Homeserver will **omit** the `state` section in the room response sections. This is replaced by
+`state_after` (the unstable field name is `org.matrix.msc4222.use_state_after`), which will include all state changes between the
+previous sync and the *end* of the timeline section of the current sync. This is in contrast to the old `state` section
+that only included state changes between the previous sync and the *start* of the timeline section. Note that this does
+mean that a new state event will (likely) appear in both the timeline and state sections of the response.
+
+This is basically the same as how state is returned in [MSC4186 - Simplified Sliding
+Sync](https://github.com/matrix-org/matrix-spec-proposals/pull/4186).
+
+State events that appear in the timeline section **MUST NOT** update the current state. The current state **MUST** only be
+updated with the contents of `state_after`.
+
+Clients can tell if the server supports this change by whether it returns a `state` or `state_after` section in the
+response.
+
+### Examples
+
+#### Example 1 \- Common case
+
+Let’s take a look at the common case of a state event getting sent down an incremental sync, which is non-gappy.
+
+<table>
+<tr><th>Previously</th><th>Proposed</th></tr>
+<tr>
+<td>
+
+```json
+{
+  "timeline": {
+    "events": [ {
+      "type": "org.matrix.example",
+      "state_key": ""
+    } ],
+    "limited": false,
+  },
+  "state": {
+    "events": []
+  }
+}
+```
+
+</td>
+<td>
+
+```json
+{
+  "timeline": {
+    "events": [ {
+      "type": "org.matrix.example",
+      "state_key": ""
+    } ],
+    "limited": false,
+  },
+  "state_after": {
+    "events": [ {
+      "type": "org.matrix.example",
+      "state_key": ""
+    } ]
+  }
+}
+```
+
+</td>
+</tr>
+</table>
+
+Since the current state of the room will include the new state event, it's included in the `state_after` section.
+
+> [!NOTE]
+> In the proposed API the state event comes down both in the timeline section *and* the state section.
+
+
+#### Example 2 - Receiving “outdated” state
+
+Next, let’s look at what would happen if we receive a state event that does not take effect, i.e. that shouldn’t cause the client to update its state.
+
+<table>
+<tr><th>Previously</th><th>Proposed</th></tr>
+<tr>
+<td>
+
+```json
+{
+  "timeline": {
+    "events": [ {
+      "type": "org.matrix.example",
+      "state_key": ""
+    } ],
+    "limited": false,
+  },
+  "state": {
+    "events": []
+  }
+}
+```
+
+</td>
+<td>
+
+```json
+{
+  "timeline": {
+    "events": [ {
+      "type": "org.matrix.example",
+      "state_key": ""
+    } ],
+    "limited": false,
+  },
+  "state_after": {
+    "events": []
+  }
+}
+```
+
+</td>
+</tr>
+</table>
+
+Since the current state of the room does not include the new state event, it's excluded from the `state_after` section.
+
+> [!IMPORTANT]
+> Both responses are the same, but the client **MUST NOT** update its state with the event.
+
+
+## Potential issues
+
+With the proposed API the common case for receiving a state update will cause the event to come down in both the
+`timeline` and `state` sections, potentially increasing bandwidth usage. However, it is common for the HTTP responses to
+be compressed, heavily reducing the impact of having duplicated data.
+
+Clients will not be able to tell when a state change happened within the timeline. This was used by some clients to
+render e.g. display names of users at the time they sent the message (rather than their current display name), though
+e.g. Element clients have moved away from this UX. This behavior can be replicated in the same way that clients dealt
+with messages received via pagination (i.e. calling `/messages`), by walking the timeline backwards and inspecting the
+`unsigned.prev_state` field. While this can lead to incorrect results, this is no worse than the previous situation.
+
+
+## Alternatives
+
+There are a number of options for encoding the same information in different ways, for example the response could
+include both the `state` and a `state_delta` section, where `state_delta` would be any changes that needed to be applied
+to the client calculated state to correct it. However, since
+[MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186) is likely to replace the sync v2 API, we may as
+well use the same mechanism. This also has the benefit of showing that the proposed API shape can be successfully
+implemented by clients, as the MSC is implemented and in use by clients.
+
+Another option would be for server implementations to try and fudge the state and timeline responses to ensure that
+clients came to the correct view of state. For example, if the server detects that a sync response will cause the client
+to come to an incorrect view of state it could either a) "fixup" the state in the `state` section of the *next* sync
+response, or b) remove or add old state events to the timeline section. While both these approaches are viable, they're
+both suboptimal to just telling the client the correct information in the first place. Since clients will need to be
+updated to handle the new behavior for future sync APIs anyway, there is little benefit from not updating clients now.
+
+We could also do nothing, and instead wait for [MSC4186](https://github.com/matrix-org/matrix-spec-proposals/pull/4186)
+(or equivalent) to land and for clients to update to it.
+
+
+## Security considerations
+
+There are no security concerns with this proposal, as it simply encodes the same information sent to clients in a
+different way
+
+## Unstable prefix
+
+| Name | Stable prefix | Unstable prefix |
+| - | - | - |
+| Query param | `use_state_after` | `org.matrix.msc4222.use_state_after` |
+| Room response field | `state_after` | `org.matrix.msc4222.state_after` |
+
+## Dependencies
+
+None