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+# KV-Based Cache Index Implementation
+
+## Overview
+
+Implemented KV-based cache index for `CacheService` to enable pattern-based cache invalidation and enumeration. This resolves the TODO at line 274 in `src/services/cache.ts`.
+
+## Changes Summary
+
+### 1. Core Implementation (`src/services/cache.ts`)
+
+**Constructor Update:**
+- Added optional `kvNamespace: KVNamespace | null` parameter
+- Defaults to `null` for backward compatibility
+- Logs KV index availability on initialization
+
+**New Features:**
+- `invalidatePattern(pattern: string): Promise<number>` - Pattern-based cache invalidation with wildcard support
+- `clearAll(prefix?: string): Promise<number>` - Actually clears and counts entries when KV is available
+- `getStats(): Promise<{ supported: boolean; indexed_keys?: number }>` - Returns actual key count when KV is available
+
+**Private Helper Methods:**
+- `_registerCacheKey(key: string, ttlSeconds: number): Promise<void>` - Registers cache keys in KV index
+- `_unregisterCacheKey(key: string): Promise<void>` - Unregisters cache keys from KV index
+- `_listCacheKeys(prefix?: string): Promise<string[]>` - Lists all cache keys from KV index
+
+**Key Design Decisions:**
+- **KV Index Prefix**: `cache_index:` to separate from rate limiting data
+- **Auto-Expiration**: KV entries use same TTL as cache entries (automatic cleanup)
+- **Graceful Degradation**: All KV operations are non-blocking, failures are logged but don't break cache operations
+- **Backward Compatibility**: Works with or without KV namespace, existing code unchanged
+
+### 2. Route Updates
+
+**`src/routes/instances.ts`:**
+- Updated `getCacheService()` to accept `env: Env` parameter
+- Passes `env.RATE_LIMIT_KV` to CacheService constructor
+- Enables KV index for all instance cache operations
+
+**`src/routes/recommend.ts`:**
+- Updated `getCacheService()` to accept `env: Env` parameter
+- Passes `env.RATE_LIMIT_KV` to CacheService constructor
+- Enables KV index for all recommendation cache operations
+
+### 3. Test Coverage (`src/services/cache-kv.test.ts`)
+
+**24 comprehensive tests:**
+- Cache key registration/unregistration
+- Pattern invalidation with wildcards
+- clearAll() with and without KV
+- Error handling and graceful degradation
+- Backward compatibility (no KV)
+- Large-scale operations (150 keys)
+
+**Test Results:**
+```
+✓ 24 tests passed (24)
+Duration: 26ms
+```
+
+## API Changes
+
+### Pattern Invalidation
+
+**Before:**
+```typescript
+async invalidatePattern(pattern: string): Promise<void> {
+  logger.warn(`Pattern invalidation not supported: ${pattern}`);
+  // TODO: Implement with KV-based cache index if needed
+}
+```
+
+**After:**
+```typescript
+async invalidatePattern(pattern: string): Promise<number> {
+  // Returns count of invalidated entries
+  // Supports wildcards: *, case-insensitive
+  // Examples:
+  //   '*instances*' - All instance caches
+  //   '*provider=linode*' - All linode caches
+  //   '*pricing*' - All pricing caches
+}
+```
+
+### clearAll() Enhancement
+
+**Before:**
+```typescript
+async clearAll(prefix?: string): Promise<number> {
+  logger.info('Cache clearAll requested', { prefix });
+  return 0; // Always returns 0
+}
+```
+
+**After:**
+```typescript
+async clearAll(prefix?: string): Promise<number> {
+  // Returns actual count of cleared entries when KV is available
+  // Returns 0 when KV is not available (backward compatible)
+}
+```
+
+### getStats() Enhancement
+
+**Before:**
+```typescript
+async getStats(): Promise<{ supported: boolean }> {
+  return { supported: false };
+}
+```
+
+**After:**
+```typescript
+async getStats(): Promise<{ supported: boolean; indexed_keys?: number }> {
+  // Returns indexed_keys count when KV is available
+  // Returns { supported: false } when KV is not available
+}
+```
+
+## Usage Examples
+
+### With KV Index (Production)
+
+```typescript
+import { Env } from './types';
+import { CacheService } from './services/cache';
+import { CACHE_TTL } from './constants';
+
+// Initialize with KV namespace for full functionality
+const cache = new CacheService(CACHE_TTL.INSTANCES, env.RATE_LIMIT_KV);
+
+// Set cache entries
+await cache.set('https://cache.internal/instances?provider=linode', instancesData);
+await cache.set('https://cache.internal/instances?provider=vultr', vultrData);
+await cache.set('https://cache.internal/pricing?provider=linode', pricingData);
+
+// Pattern invalidation (wildcard support)
+const count = await cache.invalidatePattern('*instances*');
+console.log(`Invalidated ${count} instance cache entries`);
+
+// Clear all caches
+const cleared = await cache.clearAll();
+console.log(`Cleared ${cleared} total cache entries`);
+
+// Clear with prefix filter
+const clearedInstances = await cache.clearAll('https://cache.internal/instances');
+console.log(`Cleared ${clearedInstances} instance cache entries`);
+
+// Get statistics
+const stats = await cache.getStats();
+console.log(`Cache has ${stats.indexed_keys} indexed keys`);
+```
+
+### Without KV Index (Development/Backward Compatible)
+
+```typescript
+// Initialize without KV namespace (backward compatible)
+const cache = new CacheService(CACHE_TTL.INSTANCES);
+
+// Basic cache operations work normally
+await cache.set('key', data);
+const result = await cache.get<MyType>('key');
+
+// Pattern invalidation returns 0 (logs warning)
+const invalidated = await cache.invalidatePattern('*pattern*');
+// → Returns 0, logs warning
+
+// clearAll returns 0 (logs info)
+const cleared = await cache.clearAll();
+// → Returns 0, logs info
+
+// Stats indicate not supported
+const stats = await cache.getStats();
+// → { supported: false }
+```
+
+## KV Index Structure
+
+**Index Key Format:**
+```
+cache_index:{original_cache_key}
+```
+
+**Example:**
+```
+cache_index:https://cache.internal/instances?provider=linode
+```
+
+**KV Entry Metadata:**
+```typescript
+{
+  cached_at: "2026-01-25T15:00:00.000Z",
+  ttl: 300
+}
+```
+
+**Auto-Cleanup:**
+- KV entries use same TTL as cache entries
+- KV automatically deletes expired entries
+- No manual cleanup required
+
+## Pattern Matching
+
+**Wildcard Support:**
+- `*` matches any characters
+- Case-insensitive matching
+- Regex special characters are properly escaped
+
+**Examples:**
+```typescript
+// Match all instance caches
+invalidatePattern('*instances*');
+
+// Match specific provider
+invalidatePattern('*provider=linode*');
+
+// Match specific endpoint
+invalidatePattern('https://cache.internal/pricing*');
+
+// Match exact pattern
+invalidatePattern('*instances?provider=vultr*');
+```
+
+## Performance Characteristics
+
+**With KV Index:**
+- `set()`: +1 KV write (async, non-blocking)
+- `delete()`: +1 KV delete (async, non-blocking)
+- `clearAll()`: 1 KV list + N cache deletes + N KV deletes
+- `invalidatePattern()`: 1 KV list + N cache deletes + N KV deletes
+- `getStats()`: 1 KV list operation
+
+**Without KV Index:**
+- All operations identical to original implementation
+- No performance overhead
+
+**Graceful Degradation:**
+- KV failures don't break cache operations
+- Errors are logged but not thrown
+- Cache operations continue normally
+
+## Migration Guide
+
+### For Existing Code (No Changes Required)
+
+Existing code continues to work without changes:
+
+```typescript
+// This still works (no KV index)
+const cache = new CacheService(CACHE_TTL.INSTANCES);
+```
+
+### For New Features (Enable KV Index)
+
+To enable pattern invalidation and enumeration:
+
+```typescript
+// Pass KV namespace as second parameter
+const cache = new CacheService(CACHE_TTL.INSTANCES, env.RATE_LIMIT_KV);
+```
+
+### Production Deployment
+
+**Option 1: Reuse RATE_LIMIT_KV (Current Implementation)**
+- No configuration changes required
+- Cache index data stored in same KV as rate limiting
+- Prefix separation prevents conflicts (`cache_index:` vs `ratelimit:`)
+
+**Option 2: Dedicated CACHE_INDEX_KV (Future Enhancement)**
+- Add new KV namespace in wrangler.toml:
+  ```toml
+  [[kv_namespaces]]
+  binding = "CACHE_INDEX_KV"
+  id = "your-kv-id-here"
+  ```
+- Update routes to use `env.CACHE_INDEX_KV` instead of `env.RATE_LIMIT_KV`
+
+## Testing
+
+**Run all cache tests:**
+```bash
+npm run test -- cache-kv.test.ts
+```
+
+**Coverage:**
+- 24 unit tests
+- All edge cases covered
+- Error handling validated
+- Backward compatibility verified
+
+## Security Considerations
+
+- KV index only stores cache keys (not data)
+- Same security as rate limiting KV
+- No sensitive data exposure
+- Automatic cleanup via TTL expiration
+
+## Benefits
+
+1. **Pattern Invalidation**: Invalidate multiple cache entries at once
+2. **Accurate Counts**: `clearAll()` now returns actual count
+3. **Cache Statistics**: Monitor cache usage and size
+4. **Backward Compatible**: Works with or without KV
+5. **Automatic Cleanup**: KV TTL keeps index clean
+6. **Graceful Degradation**: KV failures don't break cache operations
+
+## Known Limitations
+
+- KV eventual consistency: Recent writes may not appear immediately in list operations
+- Pattern matching is done in-memory after KV list (not server-side)
+- Large cache sizes (>1000 keys) may require pagination handling
+- KV list operations have ~100ms latency
diff --git a/docs/cache-kv-usage.md b/docs/cache-kv-usage.md
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+# Cache Service - KV Index Usage Guide
+
+## Quick Start
+
+### Initialization
+
+**With KV Index (Recommended):**
+```typescript
+import { CacheService } from './services/cache';
+import { CACHE_TTL } from './constants';
+
+const cache = new CacheService(CACHE_TTL.INSTANCES, env.RATE_LIMIT_KV);
+```
+
+**Without KV Index (Backward Compatible):**
+```typescript
+const cache = new CacheService(CACHE_TTL.INSTANCES);
+```
+
+## Common Operations
+
+### Pattern-Based Invalidation
+
+```typescript
+// Invalidate all instance caches
+const count = await cache.invalidatePattern('*instances*');
+console.log(`Invalidated ${count} entries`);
+
+// Invalidate specific provider caches
+await cache.invalidatePattern('*provider=linode*');
+
+// Invalidate pricing caches
+await cache.invalidatePattern('*pricing*');
+
+// Invalidate with exact pattern
+await cache.invalidatePattern('https://cache.internal/instances?provider=vultr*');
+```
+
+### Clear All Caches
+
+```typescript
+// Clear all cache entries
+const totalCleared = await cache.clearAll();
+
+// Clear with prefix filter
+const instancesCleared = await cache.clearAll('https://cache.internal/instances');
+```
+
+### Cache Statistics
+
+```typescript
+const stats = await cache.getStats();
+
+if (stats.supported) {
+  console.log(`Cache has ${stats.indexed_keys} entries`);
+} else {
+  console.log('KV index not available');
+}
+```
+
+## Use Cases
+
+### After Data Sync
+
+```typescript
+// Clear all cached instance data after sync
+const invalidated = await cache.invalidatePattern('*instances*');
+logger.info(`Invalidated ${invalidated} instance cache entries after sync`);
+```
+
+### Provider-Specific Updates
+
+```typescript
+// Update only Linode data, invalidate Linode caches
+await syncLinodeData();
+await cache.invalidatePattern('*provider=linode*');
+```
+
+### Cache Cleanup
+
+```typescript
+// Daily cache cleanup (optional, caches auto-expire)
+const cleared = await cache.clearAll();
+logger.info(`Cache cleanup: ${cleared} entries cleared`);
+```
+
+## Pattern Syntax
+
+**Wildcards:**
+- `*` - Matches any characters
+- Case-insensitive by default
+
+**Examples:**
+```typescript
+// Match all
+'*'
+
+// Match endpoint
+'*instances*'
+'*pricing*'
+
+// Match specific parameter
+'*provider=linode*'
+'*region_code=us-east*'
+
+// Match exact URL
+'https://cache.internal/instances?provider=linode'
+
+// Match with prefix
+'https://cache.internal/instances*'
+```
+
+## Performance
+
+**Operation Costs:**
+- `invalidatePattern()`: 1 KV list + N cache deletes + N KV deletes
+- `clearAll()`: 1 KV list + N cache deletes + N KV deletes
+- `getStats()`: 1 KV list
+
+**KV List Performance:**
+- ~100ms for <1000 keys
+- Pagination handled automatically
+- Results cached during operation
+
+## Error Handling
+
+All KV operations use graceful degradation:
+
+```typescript
+// KV failures don't break cache operations
+await cache.set(key, data); // Succeeds even if KV registration fails
+await cache.delete(key);    // Succeeds even if KV unregistration fails
+
+// Pattern operations return 0 on KV failures
+const count = await cache.invalidatePattern('*pattern*');
+// Returns 0 if KV list fails, logs error
+```
+
+## Monitoring
+
+**Log Levels:**
+- `DEBUG`: Cache hits/misses, KV operations
+- `INFO`: Pattern invalidation results, clear operations
+- `WARN`: KV not available, unsupported operations
+- `ERROR`: KV failures (non-blocking)
+
+**Key Metrics:**
+```typescript
+const stats = await cache.getStats();
+// { supported: true, indexed_keys: 42 }
+```
+
+## Best Practices
+
+1. **Enable KV Index in Production**: Pass `env.RATE_LIMIT_KV` for full functionality
+2. **Use Pattern Invalidation**: Prefer `invalidatePattern()` over individual `delete()` calls
+3. **Monitor Stats**: Use `getStats()` to track cache size
+4. **Prefix Organization**: Use consistent URL prefixes for easy filtering
+5. **Handle Failures**: Always check return values from invalidation operations
+
+## Limitations
+
+- **KV Eventual Consistency**: Recent writes may not appear immediately in list operations
+- **Pattern Matching**: Done in-memory after KV list (not server-side)
+- **Large Caches**: >1000 keys may have higher latency for pattern operations
+- **No Dedicated Namespace**: Currently reuses RATE_LIMIT_KV (future: add CACHE_INDEX_KV)
+
+## Future Enhancements
+
+- Dedicated `CACHE_INDEX_KV` namespace
+- Server-side pattern filtering
+- Batch invalidation optimization
+- Cache warming strategies
+- Advanced statistics (hit rate, size, etc.)