# 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.)