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// config/env.ts
import dotenv from 'dotenv';
import path from 'path';

// Load environment-specific .env file
const envFile = `.env.${process.env.NODE_ENV || 'development'}`;
dotenv.config({ path: path.resolve(process.cwd(), envFile) });

// Validate required variables
const required = ['DATABASE_URL', 'PORT', 'API_KEY'];
const missing = required.filter(key => !process.env[key]);

if (missing.length > 0) {
  throw new Error(`Missing required environment variables: ${missing.join(', ')}`);
}

// Export typed configuration
export const config = {
  env: process.env.NODE_ENV || 'development',
  port: parseInt(process.env.PORT || '3000', 10),
  database: {
    url: process.env.DATABASE_URL!,
    poolSize: parseInt(process.env.DB_POOL_SIZE || '10', 10)
  },
  redis: {
    url: process.env.REDIS_URL || 'redis://localhost:6379'
  },
  logging: {
    level: process.env.LOG_LEVEL || 'info'
  },
  api: {
    key: process.env.API_KEY!,
    timeout: parseInt(process.env.API_TIMEOUT || '5000', 10)
  }
} as const;

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# Database
DATABASE_URL = os.getenv('DATABASE_URL')
DB_POOL_SIZE = int(os.getenv('DB_POOL_SIZE', 10))

# Redis
REDIS_URL = os.getenv('REDIS_URL', 'redis://localhost:6379')

# API
API_KEY = os.getenv('API_KEY')
API_TIMEOUT = int(os.getenv('API_TIMEOUT', 5000))

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// config/config.ts
import deepmerge from 'deepmerge';

// Base configuration (shared across all environments)
const baseConfig = {
  app: {
    name: 'MyApp',
    version: '1.0.0'
  },
  server: {
    timeout: 30000,
    bodyLimit: '100kb'
  },
  database: {
    poolSize: 10,
    idleTimeout: 30000
  },
  logging: {
    format: 'json',
    destination: 'stdout'
  }
};

// Environment-specific overrides
const developmentConfig = {
  server: {
    port: 3000
  },
  database: {
    url: 'postgresql://localhost:5432/myapp_dev',
    logging: true
  },
  logging: {
    level: 'debug',
    prettyPrint: true
  }
};

const productionConfig = {
  server: {
    port: 8080,
    trustProxy: true
  },
  database: {
    url: process.env.DATABASE_URL,
    ssl: true,
    logging: false
  },
  logging: {
    level: 'info',
    prettyPrint: false
  }
};

// Merge configurations
const configs = {
  development: deepmerge(baseConfig, developmentConfig),
  production: deepmerge(baseConfig, productionConfig),
  test: deepmerge(baseConfig, {
    database: { url: 'postgresql://localhost:5432/myapp_test' }
  })
};

export const config = configs[process.env.NODE_ENV || 'development'];

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```typescript
// Load YAML config
import yaml from 'js-yaml';
import fs from 'fs';
import path from 'path';

function loadYamlConfig(env: string) {
  const defaultConfig = yaml.load(
    fs.readFileSync(path.join(__dirname, 'config/default.yml'), 'utf8')
  );

  const envConfig = yaml.load(
    fs.readFileSync(path.join(__dirname, `config/${env}.yml`), 'utf8')
  );

  return deepmerge(defaultConfig, envConfig);
}

export const config = loadYamlConfig(process.env.NODE_ENV || 'development');

// secrets/aws-secrets-manager.ts
import { SecretsManagerClient, GetSecretValueCommand } from '@aws-sdk/client-secrets-manager';

export class SecretManager {
  private client: SecretsManagerClient;
  private cache = new Map<string, { value: any; expiry: number }>();
  private cacheTtl = 300000; // 5 minutes

  constructor() {
    this.client = new SecretsManagerClient({ region: process.env.AWS_REGION });
  }

  async getSecret(secretName: string): Promise<any> {
    // Check cache
    const cached = this.cache.get(secretName);
    if (cached && cached.expiry > Date.now()) {
      return cached.value;
    }

    try {
      const command = new GetSecretValueCommand({ SecretId: secretName });
      const response = await this.client.send(command);

      const secret = JSON.parse(response.SecretString || '{}');

      // Cache the secret
      this.cache.set(secretName, {
        value: secret,
        expiry: Date.now() + this.cacheTtl
      });

      return secret;
    } catch (error) {
      throw new Error(`Failed to retrieve secret ${secretName}: ${error.message}`);
    }
  }

  async getDatabaseCredentials(): Promise<DatabaseCredentials> {
    return this.getSecret('prod/database/credentials');
  }

  async getApiKey(service: string): Promise<string> {
    const secrets = await this.getSecret('prod/api-keys');
    return secrets[service];
  }
}

// Usage
const secretManager = new SecretManager();

async function connectDatabase() {
  const credentials = await secretManager.getDatabaseCredentials();

  return createConnection({
    host: credentials.host,
    port: credentials.port,
    username: credentials.username,
    password: credentials.password,
    database: credentials.database
  });
}

// secrets/vault.ts
import vault from 'node-vault';

export class VaultClient {
  private client: any;

  constructor() {
    this.client = vault({
      apiVersion: 'v1',
      endpoint: process.env.VAULT_ADDR || 'http://localhost:8200',
      token: process.env.VAULT_TOKEN
    });
  }

  async getSecret(path: string): Promise<any> {
    try {
      const result = await this.client.read(path);
      return result.data.data;
    } catch (error) {
      throw new Error(`Failed to read secret from ${path}: ${error.message}`);
    }
  }

  async getDatabaseConfig(): Promise<DatabaseConfig> {
    return this.getSecret('secret/data/database');
  }

  async getApiKeys(): Promise<Record<string, string>> {
    return this.getSecret('secret/data/api-keys');
  }

  // Dynamic database credentials (rotated automatically)
  async getDynamicDBCredentials(): Promise<Credentials> {
    const result = await this.client.read('database/creds/readonly');
    return {
      username: result.data.username,
      password: result.data.password,
      leaseId: result.lease_id,
      leaseDuration: result.lease_duration
    };
  }
}

// secrets/secret-provider.ts
export interface SecretProvider {
  getSecret(key: string): Promise<string>;
}

// Development: Use .env file
export class EnvFileSecretProvider implements SecretProvider {
  async getSecret(key: string): Promise<string> {
    const value = process.env[key];
    if (!value) {
      throw new Error(`Secret ${key} not found in environment`);
    }
    return value;
  }
}

// Production: Use AWS Secrets Manager
export class AWSSecretProvider implements SecretProvider {
  private secretManager: SecretManager;

  constructor() {
    this.secretManager = new SecretManager();
  }

  async getSecret(key: string): Promise<string> {
    const secrets = await this.secretManager.getSecret('prod/secrets');
    return secrets[key];
  }
}

// Factory
export function createSecretProvider(): SecretProvider {
  if (process.env.NODE_ENV === 'production') {
    return new AWSSecretProvider();
  }
  return new EnvFileSecretProvider();
}

// feature-flags/feature-flag.ts
export interface FeatureFlag {
  enabled: boolean;
  rolloutPercentage?: number;
  allowedUsers?: string[];
  allowedEnvironments?: string[];
}

export class FeatureFlagManager {
  private flags: Map<string, FeatureFlag>;

  constructor(flags: Record<string, FeatureFlag>) {
    this.flags = new Map(Object.entries(flags));
  }

  isEnabled(
    flagName: string,
    context?: { userId?: string; environment?: string }
  ): boolean {
    const flag = this.flags.get(flagName);
    if (!flag) return false;

    // Check if disabled globally
    if (!flag.enabled) return false;

    // Check environment restriction
    if (flag.allowedEnvironments && context?.environment) {
      if (!flag.allowedEnvironments.includes(context.environment)) {
        return false;
      }
    }

    // Check user whitelist
    if (flag.allowedUsers && context?.userId) {
      if (flag.allowedUsers.includes(context.userId)) {
        return true;
      }
    }

    // Check rollout percentage
    if (flag.rolloutPercentage !== undefined && context?.userId) {
      const hash = this.hashUserId(context.userId);
      return (hash % 100) < flag.rolloutPercentage;
    }

    return true;
  }

  private hashUserId(userId: string): number {
    let hash = 0;
    for (let i = 0; i < userId.length; i++) {
      hash = ((hash << 5) - hash) + userId.charCodeAt(i);
      hash |= 0;
    }
    return Math.abs(hash);
  }
}

// Configuration
const featureFlags = {
  'new-dashboard': {
    enabled: true,
    rolloutPercentage: 50 // 50% of users
  },
  'experimental-feature': {
    enabled: true,
    allowedUsers: ['user-123', 'user-456'],
    allowedEnvironments: ['development', 'staging']
  },
  'beta-api': {
    enabled: true,
    rolloutPercentage: 10
  }
};

const flagManager = new FeatureFlagManager(featureFlags);

// Usage
app.get('/api/dashboard', (req, res) => {
  if (flagManager.isEnabled('new-dashboard', {
    userId: req.user.id,
    environment: process.env.NODE_ENV
  })) {
    return res.json(getNewDashboard());
  }

  return res.json(getOldDashboard());
});

// feature-flags/launchdarkly.ts
import LaunchDarkly from 'launchdarkly-node-server-sdk';

export class LaunchDarklyClient {
  private client: LaunchDarkly.LDClient;

  async initialize() {
    this.client = LaunchDarkly.init(process.env.LAUNCHDARKLY_SDK_KEY!);
    await this.client.waitForInitialization();
  }

  async isEnabled(flagKey: string, user: LaunchDarkly.LDUser): Promise<boolean> {
    return this.client.variation(flagKey, user, false);
  }

  async getVariation<T>(
    flagKey: string,
    user: LaunchDarkly.LDUser,
    defaultValue: T
  ): Promise<T> {
    return this.client.variation(flagKey, user, defaultValue);
  }

  close() {
    this.client.close();
  }
}

// Usage
const ldClient = new LaunchDarklyClient();
await ldClient.initialize();

app.get('/api/dashboard', async (req, res) => {
  const user = {
    key: req.user.id,
    email: req.user.email,
    custom: {
      groups: req.user.groups
    }
  };

  const showNewDashboard = await ldClient.isEnabled('new-dashboard', user);

  if (showNewDashboard) {
    return res.json(getNewDashboard());
  }

  return res.json(getOldDashboard());
});

// config/twelve-factor.ts

/**
 * 12-Factor App Configuration Principles
 *
 * III. Config - Store config in the environment
 * - Strict separation of config from code
 * - Config varies between deploys, code does not
 * - Store in environment variables
 */

// ✅ Good: Configuration from environment
export const config = {
  database: {
    url: process.env.DATABASE_URL!,
    poolMin: parseInt(process.env.DB_POOL_MIN || '2', 10),
    poolMax: parseInt(process.env.DB_POOL_MAX || '10', 10)
  },
  redis: {
    url: process.env.REDIS_URL!
  },
  s3: {
    bucket: process.env.S3_BUCKET!,
    region: process.env.AWS_REGION!
  },
  sendgrid: {
    apiKey: process.env.SENDGRID_API_KEY!
  }
};

// ❌ Bad: Hardcoded configuration
const badConfig = {
  database: {
    host: 'prod-db.example.com',  // Hardcoded!
    password: 'secretpassword'     // Secret in code!
  }
};

/**
 * Backing Services - Treat backing services as attached resources
 * - Database, cache, message queue, etc. are accessed via URLs
 * - Should be swappable without code changes
 */

// ✅ Good: Backing service as URL
const db = createConnection(process.env.DATABASE_URL);
const cache = createClient(process.env.REDIS_URL);

// Can swap services by changing environment variable
// DATABASE_URL=postgresql://localhost/dev  (local dev)
// DATABASE_URL=postgresql://prod-db/app     (production)

/**
 * Disposability - Fast startup and graceful shutdown
 */
function startServer() {
  const server = app.listen(config.port, () => {
    console.log(`Server started on port ${config.port}`);
  });

  // Graceful shutdown
  process.on('SIGTERM', async () => {
    console.log('SIGTERM received, shutting down gracefully');

    server.close(() => {
      console.log('HTTP server closed');
    });

    await db.close();
    await cache.quit();

    process.exit(0);
  });
}

// config/validation.ts
import Joi from 'joi';

const configSchema = Joi.object({
  NODE_ENV: Joi.string()
    .valid('development', 'production', 'test')
    .default('development'),

  PORT: Joi.number()
    .port()
    .default(3000),

  DATABASE_URL: Joi.string()
    .uri()
    .required(),

  REDIS_URL: Joi.string()
    .uri()
    .default('redis://localhost:6379'),

  LOG_LEVEL: Joi.string()
    .valid('debug', 'info', 'warn', 'error')
    .default('info'),

  API_KEY: Joi.string()
    .min(32)
    .required(),

  API_TIMEOUT: Joi.number()
    .min(1000)
    .max(30000)
    .default(5000),

  ENABLE_METRICS: Joi.boolean()
    .default(false)
});

export function validateConfig() {
  const { error, value } = configSchema.validate(process.env, {
    allowUnknown: true,  // Allow other env vars
    stripUnknown: true   // Remove unknown vars
  });

  if (error) {
    throw new Error(`Configuration validation error: ${error.message}`);
  }

  return value;
}

// Usage
const validatedConfig = validateConfig();