[CloudFormation] Automating API deployment of API Gateway (Finale)

[CloudFormation] Automating API deployment of API Gateway (Finale)

A Lambda-backed custom resource for AWS API Gateway API Deployment !


7 min read

Want to catch up from the start? Click here to read the first part of the series


In my previous article, I figured out the manual steps of provisioning an AWS API Gateway and API deployments via awscli.

Today's Objective

In this article, I will be creating the Lambda-backed Custom Resource that will be triggered when an update is detected in a CloudFormation changeset.


Learning Points

  • It is important to draw the line between provisioning and subsequent deployment of resources when using Infrastructure as Code (IaC).

  • In our case of using CloudFormation to provision and deploy AWS API Gateway, creating a new API deployment may not be most suitable via native CloudFormation.

  • A Lambda-backed custom resource to perform an API invocation to update the deployment may be a more suitable solution for retaining the deployment history and documentation version.

  • I agree with the response provided by the user Federico in this Stack Overflow post. Do give it a read!

TLDR; Source Code

  1. Please ensure you fulfil the following prerequisites:

    • You have an existing S3 bucket with versioning enabled

    • You have an API JSON file with multiple versions

      • You can use the sample API JSON files here.

      • Remember to rename api-full.json to api.json when uploading to replace the api.json file in the S3 bucket

  2. An update changeset will be detected by the CloudFormation stack, due to the changes for the following parameters

    • RestApiFileVersion: Version of the API JSON file

    • StageNameToUpdate: Target stage to deploy the new version of API to

    • SemanticVersion: Semantic versioning for the APIs

  3. You can download the CloudFormation template at https://github.com/bernicecpz/hashnode_resources/tree/main/aws_series/cf_templates/03_apigw_automation/cf_templates/02_final

    • You can read through the CloudFormation template at your own pace and get your hands dirty!

    • You can always refer back to this article for more explanation

  4. If you find this Lambda-backed custom resource too much to digest, try referring to [this article](bernicecpz.hashnode.dev/cloudformation-crea..) which uses a much simpler use case


In this section, we will focus on creating the required resources for the Lambda-backed custom resource.

  • Lambda execution IAM role

  • Lambda function

  • CloudFormation Custom Resource that will trigger the Lambda function

Lambda execution IAM role

The IAM role created in this snippet will have the following permission: - The IAM role can be assumed by AWS Lambda - Can pipe logs to AWS CloudWatch - Can create and update elements in AWS Gateway except for DELETE operations

    Type: AWS::IAM::Role
        Version: '2012-10-17'
        - Effect: Allow
            - lambda.amazonaws.com
          - sts:AssumeRole
      Path: "/"
      - PolicyName: CustomResourceLambdaExecutionPolicy
          Version: '2012-10-17'
          - Effect: Allow
            - logs:CreateLogGroup
            - logs:CreateLogStream
            - logs:PutLogEvents
            Resource: arn:aws:logs:*:*:*
          - Effect: Allow
            - apigateway:POST
            - apigateway:GET
            - apigateway:PUT
            - apigateway:PATCH
            Resource: "*"

Lambda function

The Lambda function consists of

  • cfn_send function to send the response back to the CloudFormation.

  • lambda_handler function to perform the API invocation for API deployment

import json, urllib3, boto3

http = urllib3.PoolManager()

# cfn_send function has been exclude here for brevity

def lambda_handler(event, context):
response = ""
    apigw = boto3.client('apigateway')

    rest_api_id = event["ResourceProperties"]["RestApiId"]
    deployment_description = event["ResourceProperties"]["DeployDesc"]
    stage_name = event["ResourceProperties"]["StageName"]
    semantic_versioning = event["ResourceProperties"]["SemanticVersion"]

    if stage_name == 'NA':
        # Don't deploy the API that's updated in stage
        cfn_send(event, context, SUCCESS, {})

        # Deploy API by creating deployment
        response = apigw.create_deployment(

        # Update documentation version by creating new documentation version

        cfn_send(event, context, SUCCESS, response)
except Exception as err:
    cfn_send(event, context, FAILED, response)

CloudFormation Custom Resource

The custom resource will invoke the Lambda function for each create and update changeset in the CloudFormation Stack.

    Type: Custom::ApiGWDeploymentFunction
      ServiceToken: !GetAtt ApiGWDeploymentFunction.Arn
      RestApiId: !If [ CreateApiGateway, !Ref MyApiGateway, !Ref MyPrivateRestApiGateway]
      DeployDesc: !Sub "Deploy API file ${RestApiFilename}, version id ${RestApiFileVersion}, semantic versioning ${SemanticVersion}"
      StageName: !Ref StageNameToUpdate
      SemanticVersion: !Ref SemanticVersion

Additional Notes

Testing inputs from CloudFormation to Lambda

To troubleshoot the Lambda function, you can use the cloudFormation-create-request template. You can include the parameter values under the ResourceProperties.

Lambda test event - Using cloudformation-create-request

Further changes

Initially, I included the Documentation Version as part of the AWS resource.

    Type: AWS::ApiGateway::DocumentationVersion
      Description: "Documentation Version for ${RestApiFilename} Version ID: ${RestApiFileVersion}"
      DocumentationVersion: !Ref RestApiFileVersion
      RestApiId: !If [ CreateApiGateway, !Ref MyApiGateway, !Ref MyPrivateRestApiGateway]

However, I realised the Documentation Version was still being overwritten for the subsequent deployment. As such, I also included the creation as part of the API deployment Lambda function.

  • Considering that the documentation version can be any string, I decided to change it to use the semantic versioning rather than the S3 file version ID.

  • It would be easier to identify with and perhaps more meaningful in terms of versioning.

  • For the documentation description, I put it to be the same as the deployment description for illustration purposes.

# Lambda code snippet for creating documentation version
apigw = boto3.client('apigateway')


Execution Outcome


The following parameters will ensure there is an update changeset detected by the CloudFormation stack

  • RestApiFileVersion: Version of the API JSON file

  • StageNameToUpdate: Target stage to deploy the new version of API to

  • SemanticVersion: Semantic versioning for the APIs

Results from the execution

  1. In the Events section,

    • The successful API deployments are illustrated in blue boxes

    • The events annotated by the yellow box are an update to the API in the stages but not yet deployed

      CloudFormation stack, Events section reflecting the changes made for the API deployment

  2. In the API Gateway Dashboard, we can see that the deployment history and documentation history are intact and not overwritten after the execution of update changesets in the CloudFormation Stack.

    • Documentation History

    • Deployment History


The current CloudFormation template's design focuses mainly on the workflow and functionality of automating the API invocation for API deployment on API Gateway. Below are some customization considerations:

1. Scope of execution

  • The current IAM role is lenient and can be applied to any Lambda in your AWS account

  • You may want to reduce the scope of execution by limiting it to specific Lambda functions

2. Stage Name

  • In the current implementation, the Lambda function will create a new stage if the stage name doesn't exist.

  • If you want to limit the stage name, you can consider enforcing the allowed values in the CloudFormation parameters using AllowedValues

    Type: String
    Default: "staging"
    Description: "To indicate recent stage that was deployed to. Indicate NA if you do not want to deploy the API for the changeset"
      - "staging"
      - "NA"

3. S3 File Version ID

  • Agreeably, using the S3 file version ID may not be intuitive as it will be hard to determine the versions of the API.

  • It may be wiser to name the api.json with the semantic version, e.g. api-${SEMANTIC-VERSION}.json.

  • However, some manual effort is still required to retrieve the file's version ID. Since it is required as part of the CloudFormation API Gateway definition when using the BodyS3Location property.

4. End-to-End Process

The current implementation is only partially automated.

  • The API JSON file is manually uploaded

  • The CloudFormation stack still needs to be updated manually

Via S3 Event Notifications

  1. Upon detecting a PUT event in S3 bucket, trigger the Lambda function to do an API deployment

    • In this case, a different implementation will be required to cater to a different event response for the Lambda function

Via CI/CD pipeline

  1. The API JSON file is stored in a repository.

  2. Depending on the pipeline CI/CD flow,

    • The API JSON file is uploaded to a version-enabled S3 bucket

    • Upon uploading, store the file's version ID and filename

    • Using the awscli command, call the CloudFormation update-stack option with the required values to update the CloudFormation stack and invoke the custom resource to trigger the API deployment

Final Thoughts

It's been one long ride to finally implement this Lambda-backed custom resource. Looking back, the problem statement may not be a complex one. Yet, it is surprisingly not easy to craft the solution πŸ™ƒ

Glad to say I have learnt something new and understand how to implement Lambda-backed custom resources πŸ™ŒπŸΌ

I hope this gives you a good peek into using Lambda-backed custom resources for API Gateway API deployment!

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