matrix-spec-proposals/proposals/4343-modify-msc2244-use-new-event-type.md

MSC4343: Making mass redactions use a new event type

MSC2244 (accepted) introduces a concept of "mass redactions", where a single m.room.redaction event can redact upwards of about 1500 other events. This massively improves scalability of the redactions system in Matrix, though by reusing m.room.redaction (and the redacts) key, the MSC becomes backwards incompatible for clients which are expecting redacts to be a string.

MSC2244 does call out that it should have landed prior to MSC2174 (merged) to minimize this breaking change, but unfortunately that did not happen in practice.

This proposal suggests moving mass redactions to a new m.room.redactions (plural of m.room.redaction) event type to minimize breakage in clients.

Proposal

Instead of modifying the existing m.room.redaction event schema, MSC2244 is updated to say it uses a new event type: m.room.redactions.

This change is primarily to maintain support for outdated clients (and to a degree, servers) which may not support the redacts key being an array instead of string.

Process note

MSC2244 is accepted under an older MSC process and has no known implementations. Typically, changes such as those described above would happen prior to acceptance as they would have been caught during implementation.

To honour the older MSC process as best as possible, MSC2244 is considered to remain accepted, but is modified directly by this MSC as a pre-merge change. This MSC requires an implementation before the modifications are accepted into the spec, and further MSC2244. This is why the PR for this MSC modifies MSC2244 itself.

Potential issues

Using another event type for redactions isn't great. Ideally, there would be a non-conflicting key or fallback we could provide to redacts which has the same meaning, however it's difficult to represent an array as a single string target within the same event.

Alternatives

No significant alternatives. Proposals like MSC4292 may prove valuable in gating MSC2244 behind a (breaking) room version, though such areas of thinking are experimental as of writing.

Security considerations

No significant security concerns apply.

Unstable prefix

While this proposal is not considered stable, implementations should use org.matrix.msc2244.redactions in place of m.room.redactions. Further, MSCs should note the room version requirements of MSC2244.

Dependencies

This proposal has a mandatory dependency on MSC2244.