ADR-0009 — Patches, not snapshots

Status: accepted

Context

Whole-file saves are slow for large files, lossy for concurrent edits, and useless for undo replay or future collaboration.

Decision

All text mutations travel as patches inside core::Transaction ops with expected versions. Sources apply patches (folder: read-modify-write atomically; databases: as updates).

Consequences

• Undo/redo = replay.
• A CRDT (e.g. loro/yrs) can be introduced later behind the same op stream; not adopted now.
• Sources that cannot patch (a read-only DB) return Unsupported per op.