How to enable CORS on Google Cloud Functions?

Simply put, Cross-Origin Resource Sharing (CORS) is a way for the server to say, “I will accept your request, even though you came from a different origin.”

It’s a mechanism that enables web pages to request restricted resources from a domain outside the domain where the resource originated. This allows for sharing resources between different domains and provides greater flexibility for web developers in designing and building web applications.

Let’s find out below how to enable CORS on Google Cloud Functions.

How to Enable CORS on Google Cloud Functions?

Enabling CORS on Google Cloud Functions is a simple process. First, you need to add some code to your function; this is an example of a code for Python:

import functions_framework

@functions_framework.http
def cors_enabled_function(request):
    # For more information about CORS and CORS preflight requests, see:
    # https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request

    # Set CORS headers for the preflight request
    if request.method == 'OPTIONS':
        # Allows GET requests from any origin with the Content-Type
        # header and caches preflight response for an 3600s
        headers = {
            'Access-Control-Allow-Origin': '*',
            'Access-Control-Allow-Methods': 'GET',
            'Access-Control-Allow-Headers': 'Content-Type',
            'Access-Control-Max-Age': '3600'
        }

        return ('', 204, headers)

    # Set CORS headers for the main request
    headers = {
        'Access-Control-Allow-Origin': '*'
    }

    return ('Hello World!', 200, headers)

This code will enable CORS for all requests, allowing requests from any origin. You can also specify an origin if you wish. For example, if you want to allow requests from example.com, you can use the following code:

def cors_enabled(request):
    # Set CORS headers for the preflight request
    if request.method == 'OPTIONS':
        # Allows GET requests from example.com with the Content-Type
        # header and caches preflight response for an 3600s
        headers = {
            'Access-Control-Allow-Origin': 'example.com',
            'Access-Control-Allow-Methods': 'GET',
            'Access-Control-Allow-Headers': 'Content-Type',
            'Access-Control-Max-Age': '3600'
        }

        return ('', 204, headers)

    # Set CORS headers for the main request
    headers = {
        'Access-Control-Allow-Origin': 'example.com'
    }

    return (request.get_json(), 200, headers)

Once you have added the code to your function, you can deploy it to Google Cloud Functions. You can then test your function to ensure that CORS is enabled correctly.

Conclusion

This blog post discussed allowing Cross-Origin Resource Sharing (CORS) on Google Cloud Functions. We also went over adding the required code to your function.

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FAQs

Q1: What is Cross-Origin Resource Sharing (CORS)?

Cross-Origin Resource Sharing (CORS) is a security mechanism that allows a server to accept requests for resources from a domain that is different from its own. It provides web developers with greater flexibility when building applications that need to share resources across different domains.

Q2: How do you enable CORS in a Google Cloud Function written in Python?

To enable CORS, you must add code within your function to handle incoming requests and set the appropriate HTTP response headers. The code needs to check if the request is a preflight request (OPTIONS) and set specific CORS headers, and also set the necessary origin header for the main request.

Q3: What is the purpose of checking if request.method == 'OPTIONS' in the code?

This check is used to identify and handle a "preflight request." Before a web browser sends the actual request (e.g., a GET request), it may send an OPTIONS request to the server to check if the CORS protocol is understood and if the main request will be permitted.

Q4: What specific headers are set for a preflight request in the provided examples?

For a preflight (OPTIONS) request, the example code sets the following headers:

• Access-Control-Allow-Origin: Specifies which origins can make requests (e.g., '*' for any origin or 'example.com' for a specific one).
• Access-Control-Allow-Methods: Defines the allowed HTTP methods, such as 'GET'.
• Access-Control-Allow-Headers: Lists the request headers that are allowed, such as 'Content-Type'.
• Access-Control-Max-Age: Specifies how long the results of the preflight request can be cached by the browser, in this case for '3600' seconds.

Q5: How can I restrict requests to a specific domain instead of allowing any origin?

To allow requests only from a specific domain, you must change the value of the Access-Control-Allow-Origin header from '*' to the domain you want to permit, such as 'example.com'. This must be done for both the preflight and main request headers.

Q6: After adding the CORS code, what is the next step?

Once you have added the CORS handling code to your function, you need to deploy it to Google Cloud Functions. After deployment, you should test the function to ensure that CORS is enabled and configured correctly.