Setting Up CORS and Integration on AWS API Gateway Using CloudFormation

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Cross-Origin Resource Sharing (CORS) is a critical feature when developing APIs that need to be accessed by web applications hosted on different domains. AWS API Gateway, a powerful service for creating, publishing, maintaining, monitoring, and securing RESTful APIs at any scale, supports the setup of CORS directly. However, setting up CORS through CloudFormation can be a bit tricky due to the various configurations required. This article will walk you through the process of setting up CORS and API integration on AWS API Gateway using CloudFormation.

Understanding CORS and Its Importance

CORS is a security feature implemented by web browsers to prevent malicious websites from accessing resources on another domain. For instance, if you have an API hosted on api.mydomain.com and your frontend application is hosted on app.mydomain.com, CORS ensures that only specified domains can access the API.

Without CORS, browsers would block these requests, protecting the user’s data from potential security risks. Configuring CORS properly is essential to ensure that your API can be safely consumed by the intended clients.

Prerequisites for Setting Up CORS with AWS API Gateway

Before diving into the CloudFormation setup, ensure that you have the following prerequisites:

  1. AWS Account: You must have an active AWS account.
  2. Basic Knowledge of AWS CloudFormation: Understanding how CloudFormation works will help you grasp the configuration templates.
  3. Familiarity with API Gateway: Knowledge of API Gateway, its resources, and methods is crucial for setting up CORS correctly.

Define the API Gateway Resources in CloudFormation

The first step in setting up CORS with API Gateway using CloudFormation is defining the necessary resources in your CloudFormation template. This includes the API itself, the resources, methods, and any integrations with backend services like AWS Lambda or DynamoDB.

yaml

Resources:
MyApi:
Type: "AWS::ApiGateway::RestApi"
Properties:
Name: "MyApi"
Description: "API for handling CORS setup using CloudFormation"MyResource:
Type: “AWS::ApiGateway::Resource”
Properties:
ParentId:
Fn::GetAtt:
– MyApi
– RootResourceId
PathPart: “myresource”
RestApiId:
Ref: MyApi

Explanation

  • AWS::ApiGateway::RestApi: This resource defines the API itself.
  • AWS::ApiGateway::Resource: This resource creates a new path (/myresource) under the root API.

Define the API Methods

Next, you need to define the HTTP methods (GET, POST, etc.) that your API will support. Each method requires an integration with a backend service, like AWS Lambda, and appropriate permissions.

yaml

MyMethod:
Type: "AWS::ApiGateway::Method"
Properties:
AuthorizationType: "NONE"
HttpMethod: "GET"
ResourceId:
Ref: MyResource
RestApiId:
Ref: MyApi
Integration:
IntegrationHttpMethod: "POST"
Type: "MOCK"
RequestTemplates:
application/json: '{"statusCode": 200}'
MethodResponses:
- StatusCode: "200"

Explanation

  • AuthorizationType: Set to NONE for simplicity, though you can integrate IAM, Cognito, etc.
  • HttpMethod: The HTTP method type (e.g., GET).
  • Integration: Defines how the method integrates with a backend service. In this example, a MOCK integration is used for simplicity.

Configure CORS Headers

CORS is configured by adding the appropriate headers to the HTTP response. These headers inform the browser which domains are allowed to access the API, which HTTP methods are permitted, and which headers can be sent with the request.

yaml

MyCorsOptions:
Type: "AWS::ApiGateway::Method"
Properties:
AuthorizationType: "NONE"
HttpMethod: "OPTIONS"
ResourceId:
Ref: MyResource
RestApiId:
Ref: MyApi
Integration:
Type: "MOCK"
IntegrationResponses:
- StatusCode: "200"
ResponseParameters:
method.response.header.Access-Control-Allow-Headers: "'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token'"
method.response.header.Access-Control-Allow-Origin: "'*'"
method.response.header.Access-Control-Allow-Methods: "'GET,OPTIONS,POST,PUT'"
RequestTemplates:
application/json: '{"statusCode": 200}'
MethodResponses:
- StatusCode: "200"
ResponseParameters:
method.response.header.Access-Control-Allow-Headers: true
method.response.header.Access-Control-Allow-Origin: true
method.response.header.Access-Control-Allow-Methods: true

Explanation

  • HttpMethod: Set to OPTIONS to handle preflight requests.
  • Integration: Uses a MOCK integration to return a 200 status code.
  • ResponseParameters: Specifies the headers required for CORS, including Access-Control-Allow-Headers, Access-Control-Allow-Origin, and Access-Control-Allow-Methods.

Deploy the API

With the API defined and CORS configured, the next step is to deploy the API. AWS API Gateway deployments are created using the AWS::ApiGateway::Deployment resource in CloudFormation.

yaml

MyApiDeployment:
Type: "AWS::ApiGateway::Deployment"
Properties:
RestApiId:
Ref: MyApi
StageName: "dev"

Explanation

  • StageName: The stage name, such as dev, staging, or prod, where the API will be deployed.

Add Permissions for CORS Headers

To ensure that the CORS headers are included in the API response, you need to add permissions for the API Gateway to include these headers.

yaml

ApiGatewayMethodResponse:
Type: "AWS::ApiGateway::MethodResponse"
Properties:
HttpMethod: "OPTIONS"
ResourceId:
Ref: MyResource
RestApiId:
Ref: MyApi
StatusCode: "200"
ResponseParameters:
method.response.header.Access-Control-Allow-Headers: true
method.response.header.Access-Control-Allow-Methods: true
method.response.header.Access-Control-Allow-Origin: true

Explanation

  • MethodResponse: Configures the API Gateway to return specific headers in the response, ensuring that the CORS configuration is respected.

Testing Your Setup

Once the CloudFormation stack is deployed, you can test the CORS setup by making requests from a web application hosted on a different domain. Use tools like Postman or a simple web page with JavaScript fetch calls to validate the configuration.

Example JavaScript Fetch Call

javascript

fetch('https://your-api-id.execute-api.region.amazonaws.com/dev/myresource', {
method: 'GET',
headers: {
'Content-Type': 'application/json'
}
})
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error('Error:', error));

This JavaScript code snippet sends a GET request to the API Gateway endpoint and logs the response or any errors that occur.

Common Troubleshooting Tips

  1. Missing Headers: Ensure that the CORS headers are correctly configured in the CloudFormation template.
  2. Preflight Failures: If the OPTIONS method is not properly configured, preflight requests may fail.
  3. Invalid Method: Make sure the HTTP methods in your frontend requests match those allowed by the CORS configuration.

Conclusion

Setting up CORS on AWS API Gateway using CloudFormation involves a series of detailed configurations, including defining API resources, methods, and explicitly configuring the CORS headers. While CloudFormation provides a way to automate this setup, careful attention to the configuration details is crucial to ensure your API is accessible from different domains without running into CORS issues.

By following the steps outlined in this article, you can successfully configure CORS for your API, allowing it to be securely accessed by web applications across different domains. Whether you’re developing a single-page application or a complex microservices architecture, understanding and correctly implementing CORS is essential for building secure and functional APIs.

With this comprehensive guide, you should now be able to set up CORS on AWS API Gateway using CloudFormation confidently, ensuring a smooth integration between your frontend applications and backend services.