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

In an Amazon 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 Amazon Web Services 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.amazonaws.cn/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 Amazon CloudFormation invokes when you create, update, or delete the stack. You can also include additional properties like `FunctionName`, which Amazon CloudFormation passes to your function as is.

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

**Example – Amazon CloudFormation message event**  

```
{
    "RequestType": "Create",
    "ServiceToken": "arn:aws-cn:lambda:cn-north-1:123456789012:function:lambda-error-processor-primer-14ROR2T3JKU66",
    "ResponseURL": "https://cloudformation-custom-resource-response-cnnorth1.s3.cn-north-1.amazonaws.com.cn/arn%3Aaws%3Acloudformation%3Acn-north-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-cn:cloudformation:cn-north-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-cn:lambda:cn-north-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.amazonaws.cn/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html).

**Example – Amazon CloudFormation custom resource response**  

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

Amazon 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. Amazon 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 Amazon 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 Amazon CloudFormation, and the stack update continues. The template uses the [AWS::Serverless::Function](https://docs.amazonaws.cn/serverless-application-model/latest/developerguide/sam-resource-function.html) resource type provided by Amazon Serverless Application Model.

**Example – Custom resource function**  

```
Transform: 'AWS::Serverless-2016-10-31'
Resources:
  primer:
    Type: [AWS::Serverless::Function](https://docs.amazonaws.cn/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.amazonaws.cn/AWSCloudFormation/latest/UserGuide/cfn-lambda-function-code-cfnresponsemodule.html) in the Amazon CloudFormation User Guide.

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