PRD: Implement MSC4186 Simplified Sliding Sync

PRD: Implement MSC4186 Simplified Sliding Sync

Problem: Dendrite currently lacks support for the new MSC4186 Simplified Sliding Sync API, which provides significantly better sync performance and scalability compared to the existing /sync endpoint. As user room counts grow, clients using traditional /sync experience slow initial syncs and sluggish incremental syncs.

Solution: Implement the full MSC4186 specification in Dendrite, introducing a new POST /_matrix/client/v4/sync endpoint that supports selective room syncing via lists and subscriptions, sliding windows, required_state filtering, lazy member loading, and multi-worker safety.

Detailed PRD: See prds/1-simplified-sliding-sync.md

Implementation Phases: 11 phases (40-60 days estimated effort)

  • Phases 1-7: Core features + multi-worker safety (essential for production)
  • Phases 8-11: Optimization, extensions, testing

Priority: Medium

Spec Reference: https://github.com/matrix-org/matrix-spec-proposals/blob/erikj/sss/proposals/4186-simplified-sliding-sync.md

Branch: sliding-sync

Edited by Jack Maninov