AWS Client Singleton Pattern

AWS SDK clients initialized lazily as singletons. Shared across Lambda invocations. Conditional credentials for frontend vs backend.

Lazy Singleton Pattern

Module-level null variable, getter function creates on first call:

// From frontend/lib/aws/clients.ts
let dynamoClient: DynamoDBClient | null = null;

export function getDynamoDBClient(): DynamoDBClient {
  if (!dynamoClient) {
    dynamoClient = new DynamoDBClient({
      region: process.env.AWS_REGION || "us-east-1",
      ...(process.env.AWS_ACCESS_KEY_ID && process.env.AWS_SECRET_ACCESS_KEY
        ? {
            credentials: {
              accessKeyId: process.env.AWS_ACCESS_KEY_ID,
              secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY,
            },
          }
        : {}),
    });
  }
  return dynamoClient;
}

Why: Lambda reuses containers. Singleton persists across invocations. Saves ~50ms per call.

Conditional Credentials

Frontend needs explicit credentials. Backend uses IAM role:

// Spread pattern - adds credentials only if env vars exist
...(process.env.AWS_ACCESS_KEY_ID && process.env.AWS_SECRET_ACCESS_KEY
  ? {
      credentials: {
        accessKeyId: process.env.AWS_ACCESS_KEY_ID,
        secretAccessKey: process.env.AWS_SECRET_ACCESS_KEY,
      },
    }
  : {})

Frontend: Set AWSACCESSKEYID + AWSSECRETACCESSKEY Backend: Omit env vars, Lambda role auto-used

DynamoDBDocumentClient Wrapper

Use DocumentClient for auto JSON marshalling:

// From functions/lib/pipeline/StateManager.ts
constructor() {
  const client = new DynamoDBClient({
    region: process.env.AWS_REGION || "us-east-1",
  });

  this.docClient = DynamoDBDocumentClient.from(client, {
    marshallOptions: {
      convertEmptyValues: false,
      removeUndefinedValues: true,
    },
  });
}

Options used:

• convertEmptyValues: false - Don't convert empty strings to NULL
• removeUndefinedValues: true - Strip undefined properties

Inline Client Initialization

When singleton not needed (class-scoped):

// From functions/lib/agents/EmailFetcherAgent.ts
constructor(private costTracker: CostTracker) {
  // Direct initialization - scoped to class instance
  const dynamoClient = new DynamoDBClient({
    region: process.env.AWS_REGION || "us-east-1",
  });
  this.dynamodb = DynamoDBDocumentClient.from(dynamoClient);
}

When to use:

• Class manages own client lifecycle
• Need instance-specific configuration
• Not shared across requests

Token Storage Pattern

Singleton with fallback for testing:

// From functions/lib/gmail/token-storage.ts
let client: DynamoDBClient | null = null;

function getClient(): DynamoDBClient {
  if (!client) {
    client = new DynamoDBClient({
      region: process.env.AWS_REGION || "us-east-1",
    });
  }
  return client;
}

// Use in functions
export async function getStoredToken(): Promise<TokenData | null> {
  const response = await getClient().send(
    new GetItemCommand({
      TableName: TABLE_NAME,
      Key: marshall({ userId }),
    })
  );
  // ...
}

Pattern: Private getter, public functions use it. Easy to mock for tests.

All Clients Used

// frontend/lib/aws/clients.ts exports these
getSFNClient()      // Step Functions
getLambdaClient()   // Lambda invocation
getDynamoDBClient() // DynamoDB operations

All follow same pattern:

1. Module-level null variable
2. Lazy initialization
3. Conditional credentials
4. Region from env with us-east-1 fallback

Region Configuration

Always use environment variable with fallback:

region: process.env.AWS_REGION || "us-east-1"

Terraform sets AWS_REGION in Lambda env vars. Local dev defaults to us-east-1.

Client Reuse

Lambda containers persist between invocations. Module-level variables survive:

Invocation 1: dynamoClient = null -> create new client
Invocation 2: dynamoClient exists -> reuse (faster)
Invocation 3: dynamoClient exists -> reuse (faster)
[Container recycled]
Invocation 4: dynamoClient = null -> create new client

Don't recreate clients in handler functions. Use singletons.

Anti-Patterns

Don't: Create client in every handler call

// BAD - new client per invocation
export async function handler() {
  const client = new DynamoDBClient({ region: "us-east-1" });
  // ...
}

Don't: Hardcode credentials

// BAD - credentials in code
new DynamoDBClient({
  credentials: {
    accessKeyId: "AKIAIOSFODNN7EXAMPLE",
    secretAccessKey: "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY",
  },
});

Don't: Hardcode region

// BAD - hardcoded region
new DynamoDBClient({ region: "us-east-1" });

// GOOD - from env with fallback
new DynamoDBClient({ region: process.env.AWS_REGION || "us-east-1" });

Don't: Mix raw client and DocumentClient

// BAD - inconsistent marshalling
const client = new DynamoDBClient({});
await client.send(new PutItemCommand({
  Item: { id: { S: "123" } } // Manual marshalling
}));

const docClient = DynamoDBDocumentClient.from(client);
await docClient.send(new PutCommand({
  Item: { id: "123" } // Auto marshalling
}));

// GOOD - pick one and stick with it
// Use DocumentClient unless you need low-level control

Key Files

• frontend/lib/aws/clients.ts - Frontend singleton exports
• functions/lib/gmail/token-storage.ts - Private getter pattern
• functions/lib/pipeline/StateManager.ts - DocumentClient wrapper with options
• functions/lib/agents/EmailFetcherAgent.ts - Class instance pattern

When Each Pattern

Module singleton (frontend/lib/aws/clients.ts):

• Shared across entire application
• Frontend API routes
• Multiple handlers use same client

Private getter (functions/lib/gmail/token-storage.ts):

• Module-private singleton
• Only internal functions use it
• Easy test mocking

Class instance (functions/lib/agents/EmailFetcherAgent.ts):

• Agent lifecycle management
• Instance-specific config
• Not shared globally

Inline DynamoDBDocumentClient:

• Most backend code uses this
• Automatic JSON marshalling
• Simpler than raw DynamoDBClient