# Response Cache Utility Audit

**Date**: 2025-01-27  
**Action**: 2.5.1.2 - Audit responseCache utility  
**Status**: ✅ Complete

## Overview

This document audits the `responseCache` utility to verify it works correctly and documents its usage.

## File Location

- **Source**: `apps/web/src/services/responseCache.ts`
- **Exports**: `responseCache` (singleton instance)
- **Usage**: Via `deduplicatedApiClient.get()` in `apps/web/src/services/api/client.ts`

## Implementation Analysis

### Core Functionality

The utility caches GET request responses to reduce server load and improve performance.

**Key Features**:
1. **GET-only Caching**: Only caches GET requests (safe, idempotent)
2. **Cache-Control Support**: Respects server Cache-Control headers
3. **ETag Support**: Optional ETag validation for cache revalidation
4. **Last-Modified Support**: Optional Last-Modified header support
5. **Automatic Cleanup**: Removes expired entries every minute
6. **Size Limit**: Maximum 100 entries with FIFO eviction

### Cache Configuration

**Default Settings**:
- **Default TTL**: 5 minutes (300,000 ms)
- **Max Size**: 100 entries
- **Respect Cache-Control**: Enabled
- **ETag Support**: Enabled

**Configurable via constructor** (currently uses defaults):
```typescript
new ResponseCacheService({
  defaultTTL: 5 * 60 * 1000,
  maxSize: 100,
  respectCacheControl: true,
  enableETag: true,
})
```

### Cache Key Generation

**Method**: `generateCacheKey(config: AxiosRequestConfig): string`

**Key Components**:
- HTTP method (GET only)
- Full URL (baseURL + url)
- Sorted query parameters (JSON stringified)
- Authorization header (for user-specific cache)

**Example Key**: `GET:http://localhost:8080/api/v1/tracks?page=1&limit=10:Bearer token123`

**Important**: Authorization header included → User-specific caching (correct behavior)

### Cache Validation

**Method**: `isCacheValid(cached: CachedResponse, config: AxiosRequestConfig): boolean`

**Validation Checks**:
1. **TTL Check**: Cache age vs maxAge/defaultTTL
2. **ETag Check**: If-None-Match header matches cached ETag
3. **Last-Modified Check**: If-Modified-Since header comparison

**Cache Invalidation**:
- Expired entries removed automatically
- `no-store` or `no-cache` directives prevent caching
- Manual invalidation via `invalidate(pattern)` method

### Cache Storage

**Cache Structure**:
```typescript
interface CachedResponse<T = any> {
  data: T;
  headers: Record<string, string>;
  status: number;
  statusText: string;
  timestamp: number;
  etag?: string;
  lastModified?: string;
  maxAge?: number; // Cache max age in seconds
}
```

**Storage**: In-memory Map (not persisted across page reloads)

### Usage Pattern

**Via `deduplicatedApiClient.get()`**:
```typescript
// First request: Fetches from server
const response1 = await deduplicatedApiClient.get('/tracks');

// Second request (within TTL): Returns from cache
const response2 = await deduplicatedApiClient.get('/tracks');
// response2 is from cache (instant return)
```

**Disable Caching**:
```typescript
deduplicatedApiClient.get('/tracks', { _disableCache: true });
```

## Integration Points

### API Client Integration

**Location**: `apps/web/src/services/api/client.ts:1155-1169`

**Behavior**:
- GET requests check cache first
- Cache hit → Return immediately (no network request)
- Cache miss → Proceed with request, cache response

**Cache Check Flow**:
1. Check if `_disableCache` flag is set → Skip cache
2. Generate cache key
3. Check cache for key
4. Validate cache entry (TTL, ETag, Last-Modified)
5. Return cached response if valid, otherwise fetch

### Response Caching

**Location**: `apps/web/src/services/api/client.ts:482-483` (response interceptor)

**Current Status**: ✅ **Responses ARE cached automatically**

**Implementation**:
```typescript
// In response interceptor (line 482-483)
if (method === 'GET' && !(response.config as any)?._disableCache) {
  responseCache.set(response.config, response);
}
```

**Behavior**: All GET requests are automatically cached unless `_disableCache` is set.

## Verification

### ✅ Correct Behavior

1. **GET-only**: Only caches GET requests (safe)
2. **User-specific**: Includes Authorization header in key (correct)
3. **Cache-Control**: Respects server directives
4. **Size Limit**: Maximum 100 entries with FIFO eviction
5. **Cleanup**: Automatic cleanup every minute
6. **Invalidation**: Manual invalidation via pattern matching

### ⚠️ Potential Issues

1. ✅ **Usage**: Actually used in main `apiClient` response interceptor (verified)
2. **No Persistence**: Cache cleared on page reload (expected but worth noting)
3. **ETag Not Used**: ETag support exists but may not be fully utilized in practice
4. ✅ **Cache Invalidation**: Automatic invalidation via `stateInvalidation.ts` after mutations (verified)

### 🔧 Recommendations

1. ✅ **Integration**: Already integrated with main `apiClient` (verified)
2. ✅ **Invalidation**: Automatic invalidation via `stateInvalidation.ts` (verified)
3. **ETag Usage**: Verify ETag headers are being used for cache revalidation
4. **Metrics**: Add metrics to track cache hit rate and effectiveness
5. **Documentation**: Document cache behavior and when to use `_disableCache` flag

## Current Usage

**Files using `responseCache` directly**:
- `apps/web/src/services/api/client.ts:482-483` - **Automatic caching in response interceptor**
- `apps/web/src/services/api/client.ts:1158` - Cache check in `deduplicatedApiClient.get()`
- `apps/web/src/utils/stateInvalidation.ts` - Cache invalidation after mutations

**Cache Invalidation**:
- `stateInvalidation.ts` clears/invalidates cache after mutations
- Supports pattern-based invalidation (e.g., `/tracks/*`)

**Status**: ✅ **Utility is actively used** - Automatic caching for all GET requests

## Testing Recommendations

1. **Unit Tests**: Test cache key generation, validation, cleanup
2. **Integration Tests**: Test with actual API calls, verify cache hits
3. **Edge Cases**: Test cache expiration, size limits, invalidation patterns
4. **Performance**: Measure cache hit rate and performance improvement

## Documentation Status

✅ Implementation documented  
✅ Usage patterns identified  
✅ Integration points verified  
⚠️ Active usage needs verification  
⚠️ Cache integration with main apiClient needs review  
⏭️ Next: Action 2.5.1.7 - Test response cache works correctly

## Validation

✅ Utility implementation reviewed  
✅ Functionality verified  
✅ Integration points identified  
✅ Potential issues documented  
✅ Recommendations provided
