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authorEric Eastwood <erice@element.io>2022-09-02 14:05:39 -0500
committerGitHub <noreply@github.com>2022-09-02 14:05:39 -0500
commit877bdfa889fa07d09f385df297a9282d51d50dae (patch)
tree2bfe6a364ac9d0b681d7879f08be25b68b00125e /docs/development
parentUpdate docs to make enabling metrics more clear (#13678) (diff)
downloadsynapse-877bdfa889fa07d09f385df297a9282d51d50dae.tar.xz
Clarify `(room_id, event_id)` global uniqueness (#13701)
Summarized from @richvdh's reply at https://github.com/matrix-org/synapse/pull/13589#discussion_r961116999
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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).
+