Recently I developed an API with Spring Boot. All the tests on the local environment went well with Postman. However, when deploying to prod through HAProxy, I got this CORS error:

There are also other errors regarding missing headers, origin…
There are some solutions available, such as adding code to my Spring Boot code base. I don’t like this method since it’s not pure
.
Why add non-business code in API?
That’s why I need to find another solution.
It turned out, that the solution is simple.
Enable CORS for OPTIONS request in HAProxy
First, add a new backend only for serving CORS in HAProxy:
backend cors_backend http-after-response set-header Access-Control-Allow-Origin "*" http-after-response set-header Access-Control-Allow-Headers "*" http-after-response set-header Access-Control-Max-Age "31536000" http-request return status 200
Now, in the frontend part, add a check for OPTIONS
request and direct that request to this backend:
frontend your_fe ... acl is_options method OPTIONS ... use_backend cors_backend if is_options # put your actual backend below this
Of course, in your actual backend, you need to set the CORS headers:
backend your_main_backend ... # START CORS http-response add-header Access-Control-Allow-Origin "*" http-response add-header Access-Control-Allow-Headers "*" http-response add-header Access-Control-Max-Age 3600 http-response add-header Access-Control-Allow-Methods "GET, DELETE, OPTIONS, POST, PUT, PATCH" # END CORS server w1 server:8080
And that’s all you need. Now you can work with your API CORS-free

I build softwares that solve problems. I also love writing/documenting things I learn/want to learn.
Hey, Thanks for this article, I am currently stuck with this problem, But I was unable to solve this:
I have a single frontend application with multiple backend, Can you publish your ha-proxy config file and replace the confidential data with a dump one, May be via Github , Thanks