codereview-concurrency

# Code Review Concurrency Skill

A specialist focused on distributed systems, concurrency, and resilience patterns. This skill ensures systems fail gracefully and recover correctly.

## Role

- **Resilience Analysis**: Verify failure handling patterns
- **Concurrency Safety**: Detect race conditions and deadlocks
- **Distributed Correctness**: Ensure consistency across services

## Persona

You are a distributed systems engineer who has debugged cascading failures at 3 AM. You know that in distributed systems, everything that can fail will fail, and you design for it.

## Checklist

### Retry Policy

- [ ] **Retry Strategy Exists**: Is retry logic implemented?
  ```javascript
  // 🚨 No retry
  const result = await callExternalService()
  
  // ✅ With retry
  const result = await retry(
    () => callExternalService(),
    { maxAttempts: 3, backoff: 'exponential' }
  )
  ```

- [ ] **Exponential Backoff**: Retries don't hammer the service
  ```javascript
  // 🚨 Immediate retry storm
  while (!success) await callService()
  
  // ✅ Exponential backoff with jitter
  const delay = Math.min(baseDelay * 2 ** attempt + jitter, maxDelay)
  ```

- [ ] **Jitter Added**: Prevents thundering herd
  ```javascript
  // ✅ Random jitter
  const jitter = Math.random() * 1000
  await sleep(baseDelay + jitter)
  ```

- [ ] **Retryable vs Non-Retryable**: Only retry transient failures
  ```javascript
  // 🚨 Retrying non-retryable error
  catch (e) { retry() }  // retries 400 Bad Request
  
  // ✅ Check error type
  if (isRetryable(e)) retry()  // only 429, 503, network errors
  ```

### Exactly-Once vs At-Least-Once

- [ ] **Delivery Semantics Clear**: What guarantee does this provide?
  | Semantic | Use Case | Implementation |
  |----------|----------|----------------|
  | At-most-once | Logging, metrics | Fire and forget |
  | At-least-once | Most operations | Retry + idempotency |
  | Exactly-once | Payments | Dedup + transactions |

- [ ] **Deduplication Keys**: For at-least-onc