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Amazon Cognito InvalidLambdaResponseException

The response from a triggered Lambda function is invalid.

Understanding Amazon Cognito

Amazon Cognito is a robust authentication service provided by AWS that enables developers to add user sign-up, sign-in, and access control to their web and mobile applications. It supports various authentication methods, including social identity providers like Facebook, Google, and enterprise identity providers via SAML 2.0. Cognito also allows for the customization of user authentication flows through AWS Lambda triggers.

Identifying the Symptom

When using Amazon Cognito, you might encounter the InvalidLambdaResponseException. This error typically manifests when a Lambda function, triggered by Cognito, returns a response that does not conform to the expected format. The error can disrupt the authentication flow, leading to failed sign-in or sign-up attempts.

Exploring the Issue

What is InvalidLambdaResponseException?

The InvalidLambdaResponseException is an error code that indicates a problem with the response returned by a Lambda function triggered by Amazon Cognito. This issue arises when the response structure does not match the expected JSON format required by Cognito.

Common Causes

The most common cause of this exception is a misconfigured Lambda function that returns an improperly formatted response. This could be due to missing fields, incorrect data types, or syntax errors in the JSON response.

Steps to Fix the Issue

1. Review the Lambda Function

Start by reviewing the Lambda function associated with the Cognito trigger. Ensure that the function's logic correctly processes the input and generates the expected output. You can use AWS CloudWatch logs to debug and identify any anomalies in the function's execution.

2. Validate the Response Format

Ensure that the Lambda function's response adheres to the expected JSON structure. For example, if the function is triggered during the authentication flow, it should return a response similar to:

{
"statusCode": 200,
"body": {
"message": "Success",
"data": { ... }
}
}

Refer to the AWS Cognito Lambda Trigger Documentation for detailed response format requirements.

3. Test the Lambda Function Independently

Use the AWS Lambda console to test the function independently of Cognito. This can help isolate the issue and ensure that the function behaves as expected when provided with sample input.

4. Update and Deploy the Lambda Function

After making necessary corrections, update the Lambda function code and deploy it. Ensure that the function has the appropriate permissions and is correctly linked to the Cognito trigger.

Additional Resources

For more information on troubleshooting Lambda functions and Cognito triggers, consider visiting the following resources:

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