When using AWS SAM (Serverless Application Model) to run a local API with sam local start-api, you might encounter CORS (Cross-Origin Resource Sharing) issues. Here are a few steps you can take to address CORS problems in SAM local:

  1. Enable CORS in your Lambda function: Ensure that your Lambda function is configured to handle CORS requests. This typically involves adding appropriate CORS headers to the HTTP response. In your Lambda function code, you can include the necessary headers in the response object. For example:

    def lambda_handler(event, context): # Your code logic here # Set CORS headers response_headers = { 'Access-Control-Allow-Origin': '*', # or specific domain 'Access-Control-Allow-Headers': 'Content-Type', 'Access-Control-Allow-Methods': 'OPTIONS,POST,GET' } # Construct response response = { 'statusCode': 200, 'headers': response_headers, 'body': json.dumps(response_body) } return response

    Adjust the headers based on your specific requirements.

  2. Configure CORS in your SAM template: Add CORS-related configuration to your SAM template (template.yaml) to instruct the API Gateway to handle CORS properly. Below is an example of how to define CORS configuration for a SAM API:

    Resources: MyApi: Type: AWS::Serverless::Api Properties: # other properties Cors: AllowMethods: "'OPTIONS,POST,GET'" AllowHeaders: "'Content-Type'" AllowOrigin: "'*'" # or specific domain

    Update the MyApi resource with the appropriate CORS configuration. This informs the API Gateway to include the necessary CORS headers in the responses.

  3. Preflight OPTIONS requests: For CORS requests, the browser often sends a preflight OPTIONS request to determine if the subsequent request is allowed. Ensure that your Lambda function responds properly to OPTIONS requests by handling them separately and returning the appropriate CORS headers.

    def lambda_handler(event, context): # Handle OPTIONS requests for CORS if event['httpMethod'] == 'OPTIONS': response_headers = { 'Access-Control-Allow-Origin': '*', # or specific domain 'Access-Control-Allow-Headers': 'Content-Type', 'Access-Control-Allow-Methods': 'OPTIONS,POST,GET', 'Access-Control-Max-Age': '86400' # Optional: specify the max age of preflight requests } response = { 'statusCode': 200, 'headers': response_headers, 'body': '' } return response # Handle other requests # Your code logic here

    By explicitly handling OPTIONS requests and returning the proper CORS headers, you allow the browser to proceed with subsequent requests.

  4. Clear browser cache: If you've made previous requests without proper CORS headers, your browser might have cached those responses. Clear your browser cache or try accessing the API from a different browser or incognito mode to ensure you're testing with fresh CORS handling.

By implementing these steps, you can address CORS issues when running a local API with AWS SAM using sam local start-api. Remember to customize the CORS headers and configuration based on your specific needs.

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