Introduction to AWS Lambda

AWS Lambda is Amazon's serverless compute service. You upload your code, configure a trigger, and Lambda runs your function in response to events — scaling automatically from zero to thousands of concurrent executions. There are no servers to provision, no operating systems to patch, and no capacity to plan.

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How Lambda Works — The Execution Model

When an event triggers a Lambda function, the following sequence occurs:

graph LR
  subgraph Event Source
    ES["API GW<br/>S3 Event<br/>SQS Msg<br/>Schedule"]
  end
  subgraph Lambda Service
    LS["Invoke API<br/>Route event<br/>Scale if needed"]
  end
  subgraph Execution Environment
    EE["Init runtime<br/>Load function code<br/>Execute handler()<br/>Return response"]
  end
  ES --> LS --> EE

1. Event arrives — an HTTP request, S3 upload, SQS message, or scheduled timer
2. Lambda service receives the invocation request
3. Execution environment is created (cold start) or reused (warm start)
4. Handler function runs with the event payload
5. Response is returned to the caller (synchronous) or acknowledged (asynchronous)

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Supported Runtimes

Lambda supports a wide range of programming languages through managed runtimes:

Runtime	Language	Typical Cold Start
nodejs20.x	JavaScript / TypeScript	~100–300 ms
python3.12	Python	~100–300 ms
java21	Java	~500–3000 ms
dotnet8	C# / F#	~400–1500 ms
provided.al2023	Any (custom runtime)	Varies
ruby3.3	Ruby	~200–500 ms

Custom runtimes let you bring any language — Go, Rust, C++, PHP — by implementing the Lambda Runtime API. Compiled languages like Go and Rust achieve very fast cold starts (~10–50 ms).

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Anatomy of a Lambda Function

Every Lambda function consists of a handler — the entry point that the runtime calls when an event arrives.

Node.js Example

// index.mjs
export const handler = async (event, context) => {
  console.log('Event:', JSON.stringify(event));

  const name = event.queryStringParameters?.name || 'World';

  return {
    statusCode: 200,
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ message: `Hello, \${name}!` }),
  };
};

Python Example

# lambda_function.py
import json

def handler(event, context):
    print(f"Event: {json.dumps(event)}")

    name = event.get("queryStringParameters", {}).get("name", "World")

    return {
        "statusCode": 200,
        "headers": {"Content-Type": "application/json"},
        "body": json.dumps({"message": f"Hello, {name}!"}),
    }

The event Object

The event parameter contains the data from the trigger source. Its structure varies by source:

Trigger Source	Event Contents
API Gateway	HTTP method, path, headers, query params, body
S3	Bucket name, object key, event type
SQS	Message body, message attributes, receipt handle
DynamoDB Streams	New/old item images, event type (INSERT, MODIFY, REMOVE)
EventBridge	Source, detail-type, detail payload
Scheduled (cron)	Scheduled event metadata

The context Object

The context parameter provides runtime information:

{
  functionName: 'my-function',
  functionVersion: '$LATEST',
  memoryLimitInMB: '256',
  logGroupName: '/aws/lambda/my-function',
  logStreamName: '2024/01/15/[$LATEST]abc123',
  awsRequestId: 'a1b2c3d4-5678-90ab-cdef-example',
  getRemainingTimeInMillis: [Function]  // Time left before timeout
}

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Invocation Types

Lambda supports two invocation modes:

Synchronous Invocation

The caller waits for the function to complete and receives the response directly.

sequenceDiagram
  participant Caller
  participant Lambda
  Caller->>Lambda: request
  Note over Caller,Lambda: blocks until complete
  Lambda-->>Caller: response

Used by: API Gateway, Application Load Balancer, SDK Invoke calls

Asynchronous Invocation

The caller sends the event and receives an immediate acknowledgement. Lambda manages retries.