aws-serverless

AWS Serverless

Patterns

Lambda Handler Pattern

Proper Lambda function structure with error handling

When to use: ['Any Lambda function implementation', 'API handlers, event processors, scheduled tasks']

```javascript
// Node.js Lambda Handler
// handler.js

// Initialize outside handler (reused across invocations)
const { DynamoDBClient } = require('@aws-sdk/client-dynamodb');
const { DynamoDBDocumentClient, GetCommand } = require('@aws-sdk/lib-dynamodb');

const client = new DynamoDBClient({});
const docClient = DynamoDBDocumentClient.from(client);

// Handler function
exports.handler = async (event, context) => {
  // Optional: Don't wait for event loop to clear (Node.js)
  context.callbackWaitsForEmptyEventLoop = false;

  try {
    // Parse input based on event source
    const body = typeof event.body === 'string'
      ? JSON.parse(event.body)
      : event.body;

    // Business logic
    const result = await processRequest(body);

    // Return API Gateway compatible response
    return {
      statusCode: 200,
      headers: {
        'Content-Type': 'application/json',
        'Access-Control-Allow-Origin': '*'
      },
      body: JSON.stringify(result)
    };
  } catch (error) {
    console.error('Error:', JSON.stringify({
      error: error.message,
      stack: error.stack,
      requestId: context.awsRequestId
    }));

    return {
      statusCode: error.statusCode || 500,
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({
        error: error.message || 'Internal server error'
      })
    };
  }
};

async function processRequest(data) {
  // Your business logic here
  const result = await docClient.send(new GetCommand({
    TableName: process.env.TABLE_NAME,
    Key: { id: data.id }
  }));
  return result.Item;
}

# Python Lambda Handler
# handler.py

import json
import os
import logging
import boto3
from botocore.exceptions import ClientError

# Initialize outside handler (reused across invocations)
logger = logging.getLogger()
logger.setLevel(logging.INFO)