# YSD-0135 — Collaboration conflict model

- **Status:** approved (2026-07-18)
- **Priority:** P0 EXT — blocking (production rollout gated via YSD-0139)
- **Decision owner:** @GreyChimp
- **Drafted:** 2026-07-18 by Claude Code (claude-fable-5)
- **Approval:** approved as recommended by @GreyChimp on 2026-07-18; outcome
  recorded in decision-log.json; review by 2027-07-18

## Question

CRDT, lock/lease, or bounded hybrid for co-annotation, offline queues, and live
sessions?

## Recommendation

Explicitly bounded hybrid: CRDT-mergeable draft layers for annotations, notes,
and offline queues (drafts are commutative and low-conflict); lease-based
single-writer editing for accepted-record supersession, rights decisions, and
comparison-set freezes (append-only history must not merge); live co-study
sessions use presence + shared playback clock with per-object leases. The
boundary is contractual: anything with review state acceptance semantics is
lease-gated; anything draft is CRDT.

## Options considered

- Pure CRDT — rejected: accepted/immutable records (YSD-0013) cannot be
  last-writer-merged without corrupting review history.
- Pure locking — rejected: kills offline note capture and fluid co-annotation.

## Consequences

- Section 8 collaboration items implement per this boundary; persistence
  (YSD-3005) implements optimistic concurrency for drafts and append-only
  acceptance.

## Machine-enforced outcome (once approved)

Contract-level marker (draft vs accepted) selects the mechanism; tests prove
accepted records never CRDT-merge.