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| -rw-r--r-- | docs/development/database_schema.md | 24 |
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diff --git a/docs/development/database_schema.md b/docs/development/database_schema.md index d996a7caa2..e9b925ddd8 100644 --- a/docs/development/database_schema.md +++ b/docs/development/database_schema.md @@ -191,3 +191,27 @@ There are three separate aspects to this: flavour will be accepted by SQLite 3.22, but will give a column whose default value is the **string** `"FALSE"` - which, when cast back to a boolean in Python, evaluates to `True`. + + +## `event_id` global uniqueness + +In room versions `1` and `2` it's possible to end up with two events with the +same `event_id` (in the same or different rooms). After room version `3`, that +can only happen with a hash collision, which we basically hope will never +happen. + +There are several places in Synapse and even Matrix APIs like [`GET +/_matrix/federation/v1/event/{eventId}`](https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid) +where we assume that event IDs are globally unique. + +But hash collisions are still possible, and by treating event IDs as room +scoped, we can reduce the possibility of a hash collision. When scoping +`event_id` in the database schema, it should be also accompanied by `room_id` +(`PRIMARY KEY (room_id, event_id)`) and lookups should be done through the pair +`(room_id, event_id)`. + +There has been a lot of debate on this in places like +https://github.com/matrix-org/matrix-spec-proposals/issues/2779 and +[MSC2848](https://github.com/matrix-org/matrix-spec-proposals/pull/2848) which +has no resolution yet (as of 2022-09-01). + |