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Diffstat (limited to 'develop/development')
| -rw-r--r-- | develop/development/database_schema.html | 16 |
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diff --git a/develop/development/database_schema.html b/develop/development/database_schema.html index 8bbd1c46da..901808363e 100644 --- a/develop/development/database_schema.html +++ b/develop/development/database_schema.html @@ -308,6 +308,22 @@ default value is the <strong>string</strong> <code>"FALSE"</code> - wh in Python, evaluates to <code>True</code>.</p> </li> </ul> +<h2 id="event_id-global-uniqueness"><a class="header" href="#event_id-global-uniqueness"><code>event_id</code> global uniqueness</a></h2> +<p>In room versions <code>1</code> and <code>2</code> it's possible to end up with two events with the +same <code>event_id</code> (in the same or different rooms). After room version <code>3</code>, that +can only happen with a hash collision, which we basically hope will never +happen.</p> +<p>There are several places in Synapse and even Matrix APIs like <a href="https://spec.matrix.org/v1.1/server-server-api/#get_matrixfederationv1eventeventid"><code>GET /_matrix/federation/v1/event/{eventId}</code></a> +where we assume that event IDs are globally unique.</p> +<p>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 +<code>event_id</code> in the database schema, it should be also accompanied by <code>room_id</code> +(<code>PRIMARY KEY (room_id, event_id)</code>) and lookups should be done through the pair +<code>(room_id, event_id)</code>.</p> +<p>There has been a lot of debate on this in places like +https://github.com/matrix-org/matrix-spec-proposals/issues/2779 and +<a href="https://github.com/matrix-org/matrix-spec-proposals/pull/2848">MSC2848</a> which +has no resolution yet (as of 2022-09-01).</p> </main> |