Design Patterns MCP Server

/** * Object Pool Pattern Implementation for Prepared Statements * Prevents memory leaks by limiting the number of cached prepared statements * Uses LRU (Least Recently Used) eviction strategy */ interface PooledStatement { statement: any; lastUsed: number; useCount: number; } interface StatementPoolConfig { maxSize: number; enableMetrics: boolean; } export interface PoolMetrics { size: number; hits: number; misses: number; evictions: number; hitRate: number; } export class StatementPool { private pool = new Map<string, PooledStatement>(); private config: StatementPoolConfig; private metrics: PoolMetrics = { size: 0, hits: 0, misses: 0, evictions: 0, hitRate: 0, }; constructor(config: Partial<StatementPoolConfig> = {}) { this.config = { maxSize: config.maxSize || 100, enableMetrics: config.enableMetrics ?? true, }; } /** * Get or create a prepared statement (with error recovery) */ getOrCreate(sql: string, factory: () => any): any { const pooled = this.pool.get(sql); if (pooled) { // Statement exists in pool - verify it's still valid try { // Test statement validity if possible if (pooled.statement && typeof pooled.statement.safeIntegers === 'function') { pooled.statement.safeIntegers(); // SQLite test call } pooled.lastUsed = Date.now(); pooled.useCount++; this.updateMetrics(true); return pooled.statement; } catch (error) { // Statement corrupted - remove and recreate this.pool.delete(sql); // Fall through to create new one } } // Statement not in pool, create new one this.updateMetrics(false); // Check if we need to evict if (this.pool.size >= this.config.maxSize) { this.evictLRU(); } // Create and store new statement with error handling try { const statement = factory(); this.pool.set(sql, { statement, lastUsed: Date.now(), useCount: 1, }); this.metrics.size = this.pool.size; return statement; } catch (error) { const errorMessage = error instanceof Error ? error.message : String(error); // For CREATE INDEX/TABLE statements that fail due to "already exists" or schema issues, // return a no-op statement that does nothing when run if ( (errorMessage.includes('already exists') || errorMessage.includes('no such column') || errorMessage.includes('no such table')) && (sql.toUpperCase().includes('CREATE INDEX') || sql.toUpperCase().includes('CREATE TABLE')) ) { // Create a dummy statement that does nothing const dummyStatement = { run: () => ({ changes: 0, lastInsertRowid: undefined }), get: () => undefined, all: () => [], finalize: () => {}, }; this.pool.set(sql, { statement: dummyStatement as any, lastUsed: Date.now(), useCount: 1, }); this.metrics.size = this.pool.size; return dummyStatement as any; } // Factory failed - propagate error but don't corrupt pool throw new Error(`Failed to create prepared statement: ${errorMessage}`); } } /** * Check if statement exists in pool */ has(sql: string): boolean { return this.pool.has(sql); } /** * Remove statement from pool */ remove(sql: string): boolean { const pooled = this.pool.get(sql); if (pooled) { // Free the statement if it has a free method if (pooled.statement && typeof pooled.statement.free === 'function') { pooled.statement.free(); } this.pool.delete(sql); this.metrics.size = this.pool.size; return true; } return false; } /** * Clear all statements from pool */ clear(): void { // Free all statements for (const [_, pooled] of this.pool) { if (pooled.statement && typeof pooled.statement.free === 'function') { pooled.statement.free(); } } this.pool.clear(); this.metrics.size = 0; } /** * Get pool size */ size(): number { return this.pool.size; } /** * Get pool metrics */ getMetrics(): PoolMetrics & { statements: Array<{ sql: string; useCount: number; age: number }>; } { const statements = Array.from(this.pool.entries()).map(([sql, pooled]) => ({ sql: sql.substring(0, 50) + (sql.length > 50 ? '...' : ''), useCount: pooled.useCount, age: Date.now() - pooled.lastUsed, })); return { ...this.metrics, size: this.pool.size, hitRate: this.calculateHitRate(), statements, }; } /** * Evict least recently used statement */ private evictLRU(): void { let lruKey: string | null = null; let lruTime = Date.now(); for (const [sql, pooled] of this.pool.entries()) { if (pooled.lastUsed < lruTime) { lruTime = pooled.lastUsed; lruKey = sql; } } if (lruKey) { this.remove(lruKey); this.metrics.evictions++; } } /** * Update metrics */ private updateMetrics(hit: boolean): void { if (!this.config.enableMetrics) return; if (hit) { this.metrics.hits++; } else { this.metrics.misses++; } } /** * Calculate hit rate */ private calculateHitRate(): number { const total = this.metrics.hits + this.metrics.misses; return total > 0 ? this.metrics.hits / total : 0; } /** * Reset metrics */ resetMetrics(): void { this.metrics = { size: this.pool.size, hits: 0, misses: 0, evictions: 0, hitRate: 0, }; } }