Lambda

Learning objectives#

• Write a Lambda handler
• Connect event sources
• Grant permissions with an execution role
• Configure memory, timeout and environment variables

Explanation#

A handler is a function that receives an event (an HTTP request, S3 upload, SQS message, schedule…) and a context object.

Event sources include API Gateway, function URLs, S3, SQS, SNS, DynamoDB Streams and EventBridge schedules.

The execution role defines what the function may access. Memory (128 MB–10 GB) also scales CPU; the maximum timeout is 15 minutes.

Pricing is per request plus GB-seconds of compute. Cold starts happen when a new execution environment is created.

Code example: An HTTP API handler#

1export const handler = async (event) => {
2  const name = event.queryStringParameters?.name ?? 'world';
3  console.log(JSON.stringify({ msg: 'greeting', name }));   // goes to CloudWatch Logs
4

5  return {
6    statusCode: 200,
7    headers: { 'content-type': 'application/json' },
8    body: JSON.stringify({ message: `Hello, ${name}!` }),
9  };
10};
11

12// zip function.zip index.mjs
13// aws lambda create-function --function-name hello --runtime nodejs22.x \
14//   --handler index.handler --zip-file fileb://function.zip \
15//   --role arn:aws:iam::123456789012:role/hello-lambda-role
16// aws lambda create-function-url-config --function-name hello --auth-type NONE

GET https://<url-id>.lambda-url.ap-south-1.on.aws/?name=Asha
{"message":"Hello, Asha!"}

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. The handler reads a query parameter from the event.
2. console.log output is captured in CloudWatch Logs automatically.
3. The return value is turned into an HTTP response.
4. A function URL gives the function its own HTTPS endpoint.

Real-world use case#

Common mistakes#

Best practices#

• One execution role per function, least privilege
• Initialise clients outside the handler
• Log structured JSON
• Use dead-letter queues or on-failure destinations for async events

Remember this#

Practice exercise#

1aws lambda update-function-configuration --function-name hello --memory-size 512 --timeout 10

Mini quiz#

Want more? Practise all AWS MCQs · AWS interview questions

Related topics#

My notes#

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