# Using AWS Lambda with CloudFormation
<a name="services-cloudformation"></a>

In an AWS CloudFormation template, you can specify a Lambda function as the target of a custom resource. Use custom resources to process parameters, retrieve configuration values, or call other AWS services during stack lifecycle events.

The following example invokes a function that's defined elsewhere in the template.

**Example – Custom resource definition**  

```
Resources:
  primerinvoke:
    Type: [AWS::CloudFormation::CustomResource](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-cfn-customresource.html)
    Version: "1.0"
    Properties:
      ServiceToken: !GetAtt primer.Arn
      FunctionName: !Ref randomerror
```

The service token is the Amazon Resource Name (ARN) of the function that CloudFormation invokes when you create, update, or delete the stack. You can also include additional properties like `FunctionName`, which CloudFormation passes to your function as is.

CloudFormation invokes your Lambda function [asynchronously](invocation-async.md) with an event that includes a callback URL.

**Example – CloudFormation message event**  

```
{
    "RequestType": "Create",
    "ServiceToken": "arn:aws:lambda:us-east-1:123456789012:function:lambda-error-processor-primer-14ROR2T3JKU66",
    "ResponseURL": "https://cloudformation-custom-resource-response-useast1.s3-us-east-1.amazonaws.com/arn%3Aaws%3Acloudformation%3Aus-east-1%3A123456789012%3Astack/lambda-error-processor/1134083a-2608-1e91-9897-022501a2c456%7Cprimerinvoke%7C5d478078-13e9-baf0-464a-7ef285ecc786?AWSAccessKeyId=AKIAIOSFODNN7EXAMPLE&Expires=1555451971&Signature=28UijZePE5I4dvukKQqM%2F9Rf1o4%3D",
    "StackId": "arn:aws:cloudformation:us-east-1:123456789012:stack/lambda-error-processor/1134083a-2608-1e91-9897-022501a2c456",
    "RequestId": "5d478078-13e9-baf0-464a-7ef285ecc786",
    "LogicalResourceId": "primerinvoke",
    "ResourceType": "AWS::CloudFormation::CustomResource",
    "ResourceProperties": {
        "ServiceToken": "arn:aws:lambda:us-east-1:123456789012:function:lambda-error-processor-primer-14ROR2T3JKU66",
        "FunctionName": "lambda-error-processor-randomerror-ZWUC391MQAJK"
    }
}
```

The function is responsible for returning a response to the callback URL that indicates success or failure. For the full response syntax, see [Custom resource response objects](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html).

**Example – CloudFormation custom resource response**  

```
{
    "Status": "SUCCESS",
    "PhysicalResourceId": "2019/04/18/[$LATEST]b3d1bfc65f19ec610654e4d9b9de47a0",
    "StackId": "arn:aws:cloudformation:us-east-1:123456789012:stack/lambda-error-processor/1134083a-2608-1e91-9897-022501a2c456",
    "RequestId": "5d478078-13e9-baf0-464a-7ef285ecc786",
    "LogicalResourceId": "primerinvoke"
}
```

CloudFormation provides a library called `cfn-response` that handles sending the response. If you define your function within a template, you can require the library by name. CloudFormation then adds the library to the deployment package that it creates for the function.

If your function that a Custom Resource uses has an [Elastic Network Interface](configuration-vpc.md#configuration-vpc-enis) attached to it, add the following resources to the VPC policy where **region** is the Region the function is in without the dashes. For example, `us-east-1` is `useast1`. This will allow the Custom Resource to respond to the callback URL that sends a signal back to the CloudFormation stack.

```
arn:aws:s3:::cloudformation-custom-resource-response-region",
"arn:aws:s3:::cloudformation-custom-resource-response-region/*",
```

The following example function invokes a second function. If the call succeeds, the function sends a success response to CloudFormation, and the stack update continues. The template uses the [AWS::Serverless::Function](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-resource-function.html) resource type provided by AWS Serverless Application Model.

**Example – Custom resource function**  

```
Transform: 'AWS::Serverless-2016-10-31'
Resources:
  primer:
    Type: [AWS::Serverless::Function](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-resource-function.html)
    Properties:
      Handler: index.handler
      Runtime: nodejs16.x
      InlineCode: |
        var aws = require('aws-sdk');
        var response = require('cfn-response');
        exports.handler = function(event, context) {
            // For Delete requests, immediately send a SUCCESS response.
            if (event.RequestType == "Delete") {
                response.send(event, context, "SUCCESS");
                return;
            }
            var responseStatus = "FAILED";
            var responseData = {};
            var functionName = event.ResourceProperties.FunctionName
            var lambda = new aws.Lambda();
            lambda.invoke({ FunctionName: functionName }, function(err, invokeResult) {
                if (err) {
                    responseData = {Error: "Invoke call failed"};
                    console.log(responseData.Error + ":\n", err);
                }
                else responseStatus = "SUCCESS";
                response.send(event, context, responseStatus, responseData);
            });
        };
      Description: Invoke a function to create a log stream.
      MemorySize: 128
      Timeout: 8
      Role: !GetAtt role.Arn
      Tracing: Active
```

If the function that the custom resource invokes isn't defined in a template, you can get the source code for `cfn-response` from [cfn-response module](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/cfn-lambda-function-code-cfnresponsemodule.html) in the AWS CloudFormation User Guide.

For more information about custom resources, see [Custom resources](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/template-custom-resources.html) in the *AWS CloudFormation User Guide*.