Lambda Execution

AWSim actually executes Lambda function code. It extracts the deployment package, writes a bootstrap wrapper, and runs it as a local process using the system's Node.js or Python interpreter.

Supported Runtimes

Runtime	Prefix
Node.js	nodejs* (e.g., nodejs18.x, nodejs20.x)
Python	python* (e.g., python3.11, python3.12)

Any runtime starting with nodejs invokes node. Any runtime starting with python invokes python3. Other runtimes return an UnsupportedRuntime error.

How It Works

1. CreateFunction — AWSim stores the base64-encoded ZIP from ZipFile (or fetches from S3 if S3Bucket/S3Key are provided).
2. Invoke — AWSim extracts the ZIP to a temp directory, generates a thin bootstrap script, and runs the handler as a subprocess.
3. Bootstrap — the wrapper script loads the handler module, passes the event JSON and a context object, and captures stdout as the return value.
4. Result — stdout is parsed as the function response. Non-zero exit codes or stderr output is returned as a function error.

Deployment Package Constraints

A deployment package is caller-supplied and is extracted to the local filesystem, so extraction is bounded and contained:

Constraint	Behaviour
Path containment	Every archive member must resolve inside the extraction directory. A member naming ../ or an absolute path is rejected and the whole extraction fails.
Symlinks	Symlink members are refused. A symlink pointing outside the extraction directory would let a later member write through it.
Unpacked size	Capped at 250 MB, matching the real Lambda unzipped limit.
Entry count	Capped at 100,000 members.
Function name	Must match [a-zA-Z0-9-_]{1,64}, since the name forms part of the on-disk cache path.

When extraction is rejected, the error names the offending archive member so the cause is obvious.

Handler Format

The handler is specified as module.function:

index.handler     → loads ./index.js, calls exports.handler
src/app.process   → loads ./src/app.js, calls exports.process

For Python: app.handler → loads app.py, calls handler(event, context).

Context Object

Your function receives a context object with:

{
  functionName: "my-function",
  functionVersion: "$LATEST",
  invokedFunctionArn: "arn:aws:lambda:...",
  memoryLimitInMB: "128",
  awsRequestId: "local",
  logGroupName: "/aws/lambda/my-function",
  logStreamName: "local",
  getRemainingTimeInMillis: () => <timeout_ms>,
}

Environment Variables

Your function receives the environment variables you configured on the function, plus:

• AWS_LAMBDA_FUNCTION_NAME
• AWS_LAMBDA_FUNCTION_MEMORY_SIZE
• AWS_REGION
• AWS_DEFAULT_REGION

Deploying a Function

# Package your code
zip function.zip index.js

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

# Invoke it
aws --endpoint-url http://localhost:4566 lambda invoke \
  --function-name my-function \
  --payload '{"key":"value"}' \
  output.json

cat output.json

Example: Node.js Handler

// index.js
exports.handler = async (event, context) => {
  console.log("Event:", JSON.stringify(event));
  return {
    statusCode: 200,
    body: JSON.stringify({ message: "hello from AWSim Lambda" }),
  };
};

Example: Python Handler

# handler.py
import json

def handler(event, context):
    print("Event:", json.dumps(event))
    return {
        "statusCode": 200,
        "body": json.dumps({"message": "hello from AWSim Lambda"}),
    }

Known Limitations

• No container/Docker runtime — functions run directly as child processes. There is no isolation between functions or from the host filesystem.
• No Lambda layers execution — layers can be created and listed, but their content is not merged into the function runtime environment.
• No VPC networking — VPC configuration is accepted but ignored.
• Timeout enforcement — the timeout is passed to the context object but is not strictly enforced for long-running functions.
• Node.js require only — the bootstrap uses CommonJS require(). ESM (import) works only if your runtime supports it natively.