# Concurrency

> Use when implementing optimistic locking (@Version), pessimistic locking (SELECT FOR UPDATE), isolation levels, and state transition concurrency protections.

- Skill: `sahilkhan30/concurrency` (Agent Skill)
- Install (CLI): `npx skillmds@latest add sahilkhan30/concurrency`
- Raw SKILL.md: https://api.skillmd.com/api/skills/sahilkhan30/concurrency/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: SahilKhan30 (https://skillmd.com/u/sahilkhan30)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/sahilkhan30/concurrency

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# Concurrency & Locking Controls Skill

## Purpose
Protect financial domain state transitions against concurrent race conditions, lost updates, and double-spends.

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## 1. Rules of Engagement

### CONDITIONAL
- **Optimistic Locking (`@Version`)**: Apply JPA `@Version` optimistic locking fields on write-mutable domain entities subject to concurrent state updates (`Transaction`, `AccountBalance`, `DisputeRecord`). Non-mutated or append-only entities do not require `@Version`.
- **Pessimistic Locking (`SELECT FOR UPDATE`)**: Use `PESSIMISTIC_WRITE` for high-contention financial balance updates where optimistic lock retries cause high abort rates.

### DOMAIN SEMANTIC LOCKING RULE
- When `OptimisticLockException` or state conflict occurs:
  - Do **NOT** blindly retry mutation in a loop.
  - If the entity state was already transitioned to the desired final state by a concurrent worker, return the current state or 409 Conflict based on business rules.