How to Debug and Resolve a 412 Error (PreconditionFailedException) in AWS Lambda for SES Email Notifications

Answer: C.

Option A is incorrect as large size code will not trigger a precondition error.

It will trigger a code storage exception error.

Option B is incorrect as the GetPolicy API will return the revissionId of the policy instead of the function needed to correct.

Option C is CORRECT as per the API documentation.

A GetFuncton or GetAlias call will return the right revisionId, and then with that, the new version can be published.

Lambda will trigger an error when the revisionId in the request doesn't align with the current version revisionId.

This is often used to protect against a racing condition when multiple people are working on the same function.

Option D is incorrect.

The update API will not actually work as it will throw an exception for the same reasons.

Reference:

https://docs.aws.amazon.com/lambda/latest/dg/API_PublishVersion.html

The PreconditionFailedException error with a 412 HTTP status code indicates that one or more of the specified precondition headers in the request failed to match the corresponding values on the server. When publishing a version of an AWS Lambda function, AWS verifies that the specified preconditions, such as function version or alias, match the current state of the function before proceeding.

To debug and resolve the error, we need to determine which precondition failed and update it accordingly.

Option A, reducing the size of the code, is unlikely to solve the issue as it does not address the underlying cause of the error.

Option B, calling the GetPolicy API to get the revisionId and publishing a version again, is not relevant to the current error. GetPolicy is used to retrieve the resource-based policy associated with a Lambda function version or alias.

Option C, calling the GetAlias API to get the revisionId and publishing a version again, is also not relevant to the error. GetAlias is used to retrieve the specified alias resource policy and configuration information.

Option D, using the UpdateAlias API to publish a version by updating the Lambda function Alias, is a possible solution. UpdateAlias can be used to update an alias to point to a different version of the Lambda function, which may resolve the precondition failure.

Another possible solution is to check if any of the developers made changes to the function's permissions, roles, or policies, which may have caused the error. In this case, updating the function's IAM policies or roles could resolve the issue.

In summary, the best solution for debugging and resolving the PreconditionFailedException error while publishing a version of an AWS Lambda function would be to investigate the cause of the error, possibly using the AWS CloudWatch logs, and update the relevant preconditions or IAM policies as needed. Option D may be a viable solution in some cases.