From 87fb070eefff4270805da2ec6d389ff6c704e353 Mon Sep 17 00:00:00 2001 From: Erik Johnston Date: Tue, 29 Oct 2024 14:55:51 +0000 Subject: [PATCH] First draft of MSC4222 --- proposals/4222-sync-v2-state-after.md | 188 ++++++++++++++++++++++++++ 1 file changed, 188 insertions(+) create mode 100644 proposals/4222-sync-v2-state-after.md diff --git a/proposals/4222-sync-v2-state-after.md b/proposals/4222-sync-v2-state-after.md new file mode 100644 index 0000000000..6b767ab178 --- /dev/null +++ b/proposals/4222-sync-v2-state-after.md @@ -0,0 +1,188 @@ +# 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.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.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. + + + + + + + +
PreviouslyProposed
+ +```json +{ + "timeline": { + "events": [ { + "type": "org.matrix.example", + "state_key": "" + } ], + "limited": false, + }, + "state": { + "events": [] + } +} +``` + + + +```json +{ + "timeline": { + "events": [ { + "type": "org.matrix.example", + "state_key": "" + } ], + "limited": false, + }, + "state_after": { + "events": [ { + "type": "org.matrix.example", + "state_key": "" + } ] + } +``` + +
+ +> [!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. + + + + + + + +
PreviouslyProposed
+ +```json +{ + "timeline": { + "events": [ { + "type": "org.matrix.example", + "state_key": "" + } ], + "limited": false, + }, + "state": { + "events": [] + } +} +``` + + + +```json +{ + "timeline": { + "events": [ { + "type": "org.matrix.example", + "state_key": "" + } ], + "limited": false, + }, + "state_after": { + "events": [] + } +} +``` + +
+ +> [!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 ways of 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. + +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 do clients in a +different way + +## Unstable prefix + +| Name | Stable prefix | Unstable prefix | +| - | - | - | +| Query param | `use_state_after` | `org.matrix.use_state_after` | +| Room response field | `state_after` | `org.matrix.state_after` | + +## Dependencies + +None