AWS Lambda Exec Module

This module will execute a Lambda function and return its result(s). It allows Lambdas to be used as a form of Data Source (via a resource) but it also allows them to perform other functions upon e.g. when destroy is called.

There are some very specific constraints on how the Lambda function should work: under the hood it is using a CloudFormation "CustomResource" to execute the Lambda.

Possible uses include,

• Advanced AMI Queries: look for an AMI but return a default if not found;
• Get an SSM Parameter but return a default if it is not found (see this example)
• Get the latest RDS Snapshot Identifier, but return an empty string if it does not exist

Usage

If your Lambda function has inputs, pass them using the lambda_inputs map. The actual inputs you pass will depend on the lambda function being run.

If your Lambda function has outputs, specify them with the lambda_outputs array. IMPORTANT: the lambda MUST return these outputs or else Cloudformation will rollback. So even if you want to return an error you must return (an empty) value for all inputs.

Outputs must be alphanumeric (no underscores or hyphens).

data "aws_lambda_function" "my_ami_query" {
  name="my_ami_query"
}

module "execute" {
  source="connect-group/lambda-exec/aws"
  name                = "my_lambda_execution"
  lambda_function_arn = "${data.aws_lambda_function.lambda.arn}"
  custom_name         = "CreateUUID"

  lambda_inputs = {
    ami_tag = "tag"
  }

  lambda_outputs = [
    "ami_id",
    "Error",
  ]
}

Run the Lambda on every Apply

If you wish to run the Lambda on every Apply, then you need to insert an input that changes on every apply: typically using the timestamp() interpolation function.

module "execute_lambda" {
  ...

  lambda_inputs = {
    run_on_every_apply = "${timestamp()}"
  }

  ...
}

Writing a Compatible Lambda

• Inputs to the Lambda function are supplied in the Lambda event['ResourceProperties'] object
• You should return the same set of outputs on success and on error, as Cloudformation will generate an unrecoverable error if a result is missing from the outputs.
• You must send the result of the lambda to a signed URL - you can't just return a result. Take a look at the examples!
• Outputs can only be strings, not maps or lists.
• You can detect if the CloudFormation stack is being updated or destroyed and respond accordingly. The event['RequestType'] indicates the status,
  • Create
  • Update
  • Delete

Examples

• Node.js Reverse input strings
• Python Get SSM Parameter or return default
• For a Java example, see cloudformation-custom-resources on GitHub

Authors

Currently maintained by these awesome contributors. Module managed by Adam Perry and Connect Group

License

Apache 2 Licensed. See LICENSE for full details.