Background

Related: #173 (Replace StorageBackend in V2 engine)

The Problem: Replacing StorageBackend in entire V2 engine is too complex.

Better Approach: Instead of refactoring V2 engine globally, decouple V3 (mutation/query) from V2, then deprecate V2 entirely.

Why? Once V3 mutation and query no longer directly call V2 components, we can discard V2 engine wholesale without maintaining backward compatibility.

Path: /graph/v3/databases/{database}/tables/{table}/edges

• EdgeMutationController → V3MutationService → V3CompatibleTableBinding

Currently makes direct v2 calls:

• graph.wal.write(), graph.cdc.write() (V3MutationService)
• label.findHashEdge(), label.acquireLock/releaseLock, label.handleDeferredRequests() (V3CompatibleTableBinding)

Goal: Decouple via MutationEngine abstraction → all v2 calls become indirect → V2 engine becomes replaceable/discardable.

Task

• Define MutationEngine interface (abstracts v2 operations)
• Implement V2BackedMutationEngine (v2 backend wrapper)
• Refactor V3MutationService (remove direct Graph/label calls)
• Spring configuration (register MutationEngine bean)
• Testing (verify API behavior unchanged)

Done When

• V3MutationService has no direct Graph reference
• All v2 calls go through MutationEngine
• /graph/v3/... path fully decoupled from V2 engine internals

Notes

• V3 implementation separate (future)
• Same pattern applies to V3Query path
• After both V3Mutation and V3Query decouple, V2 engine can be discarded