How to Invoke AWS Lambda Function: Syntax, Example, and Tips

You can invoke an AWS Lambda function using the aws lambda invoke command in AWS CLI or by calling the invoke method in AWS SDKs. This sends an event to the Lambda function and returns the response or status synchronously or asynchronously.

Syntax

The basic syntax to invoke a Lambda function using AWS CLI is:

• aws lambda invoke --function-name <FunctionName> --payload <JSONPayload> <OutputFile>

In AWS SDKs, you call the invoke method with parameters like function name, invocation type, and payload.

Parts explained:

• --function-name: The name or ARN of your Lambda function.
• --payload: JSON input event sent to the function.
• OutputFile: File where the function response is saved.
• InvocationType: (SDK) 'RequestResponse' for sync, 'Event' for async.

bash

aws lambda invoke --function-name MyFunction --payload '{"key":"value"}' response.json

Example

This example shows how to invoke a Lambda function named MyFunction synchronously using AWS SDK for Python (boto3). It sends a JSON payload and prints the function's response.

python

import boto3
import json

client = boto3.client('lambda')

response = client.invoke(
    FunctionName='MyFunction',
    InvocationType='RequestResponse',
    Payload=json.dumps({'key': 'value'})
)

response_payload = response['Payload'].read().decode('utf-8')
print('Lambda response:', response_payload)

Output

Lambda response: {"statusCode":200,"body":"Hello from Lambda!"}

Common Pitfalls

Common mistakes when invoking Lambda functions include:

• Not setting the correct InvocationType, causing unexpected sync or async behavior.
• Sending payloads that are not valid JSON strings.
• Ignoring permissions: the caller must have lambda:InvokeFunction permission.
• Not handling the response stream properly in SDKs.

Example of wrong and right payload usage:

python

# Wrong: payload as Python dict (will cause error)
response = client.invoke(
    FunctionName='MyFunction',
    Payload={'key': 'value'}  # Incorrect: must be JSON string
)

# Right: payload as JSON string
import json
response = client.invoke(
    FunctionName='MyFunction',
    Payload=json.dumps({'key': 'value'})
)

Quick Reference

Tips for invoking Lambda functions:

• Use RequestResponse for synchronous calls to get immediate results.
• Use Event for asynchronous calls when you don't need the response.
• Always validate your JSON payload before sending.
• Check IAM permissions to avoid authorization errors.
• Use AWS CLI for quick tests and SDKs for integration in code.

Key Takeaways

Invoke Lambda functions using AWS CLI or SDK by specifying function name and JSON payload.

Use 'RequestResponse' invocation type for synchronous calls and 'Event' for asynchronous calls.

Always send payloads as valid JSON strings, not raw objects.

Ensure your IAM user or role has permission to invoke the Lambda function.

Handle the response stream correctly when using SDKs to read function output.