Lambda

AWS Lambda serverless compute for running code without provisioning or managing servers.

Protocol: RestJson1Signing name: lambdaPersistent: No (function code and configuration are lost on restart)

Quick Start

Package a function, create it, and invoke it:

# 1. Create a minimal Node.js handler file
cat > index.js << 'EOF'
exports.handler = async (event) => {
  console.log('Event:', JSON.stringify(event));
  return { statusCode: 200, body: JSON.stringify({ message: 'Hello!', input: event }) };
};
EOF

# 2. Package it
zip function.zip index.js

# 3. Create the function
aws --endpoint-url http://localhost:4566 lambda create-function \
  --function-name hello-world \
  --runtime nodejs20.x \
  --handler index.handler \
  --role arn:aws:iam::000000000000:role/any-role \
  --zip-file fileb://function.zip

# 4. Invoke it
aws --endpoint-url http://localhost:4566 lambda invoke \
  --function-name hello-world \
  --payload '{"name":"Alice"}' \
  response.json && cat response.json

Operations

Functions

Operation	Description
CreateFunction	Create a function from a ZIP file or S3 object. Input: FunctionName, Runtime, Handler (e.g., index.handler), Role (IAM ARN), Code ({ZipFile} or {S3Bucket, S3Key}), Environment ({Variables: {KEY: VALUE}}), Timeout (seconds, default 3), MemorySize (MB)
GetFunction	Get function metadata and code location. Returns Configuration and Code.Location
GetFunctionConfiguration	Get just the configuration (runtime, handler, env vars, etc.)
UpdateFunctionCode	Update the deployment package. Input: FunctionName, ZipFile or S3Bucket/S3Key
UpdateFunctionConfiguration	Update runtime, handler, env vars, timeout, memory. Input: FunctionName plus fields to update
DeleteFunction	Delete a function. Input: FunctionName, optional Qualifier (version or alias)
ListFunctions	List all functions. Returns paginated Functions list

Invocation

Operation	Description
Invoke	Invoke a function synchronously. Input: FunctionName, Payload (JSON body), InvocationType (RequestResponse, Event, DryRun). Returns: StatusCode, Payload (function response), FunctionError (if function threw), LogResult (base64 tail of logs if LogType=Tail)

Versions and Aliases

Operation	Description
PublishVersion	Publish the current $LATEST code as a numbered version. Returns: Version (number string)
ListVersionsByFunction	List all published versions. Returns: Versions list
CreateAlias	Create an alias (e.g., prod) pointing to a version. Input: FunctionName, Name, FunctionVersion
GetAlias	Get alias configuration
DeleteAlias	Delete an alias
ListAliases	List all aliases for a function

Event Source Mappings

Operation	Description
CreateEventSourceMapping	Map SQS, Kinesis, or DynamoDB Streams as a trigger. Input: FunctionName, EventSourceArn, BatchSize, StartingPosition (TRIM_HORIZON or LATEST for Kinesis)
GetEventSourceMapping	Get mapping configuration
DeleteEventSourceMapping	Remove a trigger
ListEventSourceMappings	List all mappings
UpdateEventSourceMapping	Update batch size or enable/disable the mapping

Layers

Operation	Description
PublishLayerVersion	Publish a layer version. Input: LayerName, Content ({ZipFile}), CompatibleRuntimes
GetLayerVersion	Get layer version metadata
ListLayers	List layers
ListLayerVersions	List versions of a layer

Function URL Configs

Operation	Description
CreateFunctionUrlConfig	Create a function URL for a function. Input: FunctionName, AuthType (NONE or AWS_IAM), optional Cors. Returns FunctionUrl, FunctionArn, AuthType
GetFunctionUrlConfig	Get the URL configuration for a function. Returns FunctionUrl, FunctionArn, AuthType, Cors
DeleteFunctionUrlConfig	Remove the URL config from a function
ListFunctionUrlConfigs	List URL configs for a function (zero or one entry)

Tags

Operation	Description
TagResource	Add or update tags on a function. Input: Resource (function ARN or name), Tags (object)
UntagResource	Remove tags from a function. Input: Resource, TagKeys (list)
ListTags	List tags on a function. Input: Resource. Returns Tags object

Resource Policy / Permissions

Operation	Description
GetPolicy	Return the function's resource-based policy as a JSON string. Returns 404 if no permissions have been added
AddPermission	Grant a principal (e.g., S3, SNS, EventBridge) permission to invoke the function. Input: FunctionName, StatementId, Action, Principal, optional SourceArn, SourceAccount
RemovePermission	Remove a policy statement from the function. Input: FunctionName, StatementId

Account Settings

Operation	Description
GetAccountSettings	Return account-level Lambda limits (total code size, concurrency limits, function count). Useful for SDK health checks

Curl Examples

# 1. Create a function using base64-encoded ZIP
ZIP_B64=$(base64 -w 0 function.zip)
curl -s -X POST http://localhost:4566/2015-03-31/functions \
  -H "Content-Type: application/json" \
  -H "Authorization: AWS4-HMAC-SHA256 Credential=test/20260421/us-east-1/lambda/aws4_request, SignedHeaders=host, Signature=fake" \
  -d "{\"FunctionName\":\"my-fn\",\"Runtime\":\"nodejs20.x\",\"Handler\":\"index.handler\",\"Role\":\"arn:aws:iam::000000000000:role/any\",\"Code\":{\"ZipFile\":\"$ZIP_B64\"}}"

# 2. Invoke a function
curl -s -X POST http://localhost:4566/2015-03-31/functions/my-fn/invocations \
  -H "Content-Type: application/json" \
  -H "Authorization: AWS4-HMAC-SHA256 Credential=test/20260421/us-east-1/lambda/aws4_request, SignedHeaders=host, Signature=fake" \
  -d '{"key":"value","nested":{"items":[1,2,3]}}'

# 3. List all functions
curl -s http://localhost:4566/2015-03-31/functions \
  -H "Authorization: AWS4-HMAC-SHA256 Credential=test/20260421/us-east-1/lambda/aws4_request, SignedHeaders=host, Signature=fake"

SDK Example

import { LambdaClient, CreateFunctionCommand, InvokeCommand, UpdateFunctionCodeCommand } from '@aws-sdk/client-lambda';
import { readFileSync } from 'fs';

const lambda = new LambdaClient({
  region: 'us-east-1',
  endpoint: 'http://localhost:4566',
  credentials: { accessKeyId: 'test', secretAccessKey: 'test' },
});

// Create function from ZIP
const zipBytes = readFileSync('function.zip');
await lambda.send(new CreateFunctionCommand({
  FunctionName: 'my-function',
  Runtime: 'nodejs20.x',
  Handler: 'index.handler',
  Role: 'arn:aws:iam::000000000000:role/lambda-role',
  Code: { ZipFile: zipBytes },
  Environment: {
    Variables: {
      TABLE_NAME: 'my-table',
      STAGE: 'staging',
    },
  },
  Timeout: 30,
  MemorySize: 256,
}));

// Invoke
const { Payload, StatusCode, FunctionError } = await lambda.send(new InvokeCommand({
  FunctionName: 'my-function',
  Payload: JSON.stringify({ userId: '123', action: 'getProfile' }),
  LogType: 'Tail',
}));

if (FunctionError) {
  console.error('Function error:', FunctionError);
}

const result = JSON.parse(Buffer.from(Payload!).toString());
console.log('Status:', StatusCode); // 200
console.log('Result:', result);

// Update code after changes
const newZip = readFileSync('function-v2.zip');
await lambda.send(new UpdateFunctionCodeCommand({
  FunctionName: 'my-function',
  ZipFile: newZip,
}));

Supported Runtimes

• nodejs* — any string starting with nodejs, e.g. nodejs18.x, nodejs20.x, nodejs22.x
• python* — any string starting with python, e.g. python3.11, python3.12, python3.13

See Lambda Execution for full runtime details and limitations.

Behavior Notes

• Lambda actually executes Node.js and Python code as local processes on the AWSim host machine.
• Functions share the host filesystem and network — no container/sandbox isolation. A function can read /etc/passwd or make outbound network calls.
• Invoke always behaves synchronously even if InvocationType: "Event" is specified.
• Function output (stdout/stderr) is captured and written to CloudWatch Logs at /aws/lambda/{function-name}.
• Layers are stored as metadata but their contents are not merged into the function's execution environment.
• Function code and configuration are not persisted — you must re-create functions after an AWSim restart.
• Environment variables are injected as real OS environment variables during execution.
• SQS event source mappings are actively polled every 2 seconds — messages received on the queue trigger the Lambda function automatically.