MSC2314: A Method to Backfill Room State

proposals/2314-backfill-current-state.md

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+# MSC-XXXX: Backfilling Current State
+
+If a server experiences data loss, it is difficult for it to recover membership in federation without having a user be reinvited, as backfill alone can not easily retrieve the data that is required to operate in a room (namely the current state and auth chain).
+
+This MSC introduces S2S APIs to provide a given room's auth chain and current state events, provided the requesting server is in the room specified.
+
+## Existing APIs
+
+If one knows the room ID and an event ID, `/_matrix/federation/v1/state/{roomId}?event_id={eventId}` can be used to retrieve the auth chain and current state events for that given event. However, this requires knowing an event ID (which cannot be assumed), as well as the version of the room (which can be assumed to be contained within the current state delivered). If an event arrives for a room that a Matrix server does not know about, the event ID could be used to backfill the state, but this is impractical for rooms which may be low-traffic yet valuable to the end-user.
+
+## Proposal
+
+Add a new v2 state API that returns the server's present auth chain and state PDUs if an event ID is not provided, as well as specifying the room version to ease the parsing of the given events.
+
+```
+GET /_matrix/federation/v2/state/{roomId}
+
+{
+  "room_version": "3",
+  "auth_chain": [
+    {
+      "type": "m.room.minimal_pdu",
+      "room_id": "!somewhere:example.org",
+      "content": {
+        "see_room_version_spec": "The event format changes depending on the room version."
+      }
+    }
+  ],
+  "pdus": [
+    {
+      "type": "m.room.minimal_pdu",
+      "room_id": "!somewhere:example.org",
+      "content": {
+        "see_room_version_spec": "The event format changes depending on the room version."
+      }
+    }
+  ]
+}
+```
+
+This requires the following changes to the existing API:
+
+- The "event_id" query parameter's definition changes to "Optional. An event ID in the room to retrieve the state at. If this is not provided, the results are the receiving server's latest current state."
+- "room_version" is added to the response, defined as "Type: string. Required. The version of the room that the state was queried of."
+
+## Potential issues
+
+Although the creating server is part of the room ID, a server using this API as a client may find that the target server does not presently know about the room (for example, it has been shut down or deleted). Finding servers that will successfully return results from this API is outside of the scope of this MSC.
+
+Users may not know the room ID for a given room, only a room alias. Translating this alias into a room ID is outside of the scope of this MSC. Users of this API may want to use it in conjunction with `/_matrix/federation/v1/query/directory` to resolve aliases to room IDs as part of an end-user focused API.
+
+Excessively large rooms may cause performance problems for servers implementing this API (including in its v1 incarnation). Discretionary rate limiting of this API may be required.
+
+## Security considerations
+
+In the case of a domain-name hijack, this may make recovering rooms that the domain name was in easier. However, since a domain name hijack will lead to other servers potentially sending PDUs with the required event IDs to allow backfill and state querying, this does not constitute a meaningful increase in attack surface. Proposals such as MSC-1228 are expected to mitigate.