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With AWS services we have the Web application running from the S3 bucket and accessing the data through the REST API from Load Balancer (which is set of Node.js applications running on EC2 instance).

Currently we have specified URL's as following:

  • API Load Balancer:       api.somedomain.com
  • Static Web App on S3:        somedomain.com

But having this setup brought us a set of problems since requests are CORS with this setup. We could workaround CORS with special headers, but that doesn't work with all browsers.

What we want to achieve is running API on the same domain but with different path:

  • API Load Balancer:            somedomain.com/api
  • Static Web App on S3:       somedomain.com

One of the ideas was to attach the API Load Balancer to the CDN and forward all request to Load Balancer if query is coming on the "/api/*" path. But that doesn't work since our API is using not only HEAD and GET requests, but also POST, PUT, DELETE.

Another idea is using second EC2 instance instead of S3 bucket to host website (using some web server like nginx or apache). But that gives too much overhead when everything is in place already (S3 static content hosting). Also if using this scenario we wouldn't get all the benefits of Amazon CloudFront performance.

So, could your recommend how to combine Load Balancer and S3, so they would run on same domain, but with different paths? (API on somedomain.com/api and Web App on somedomain.com)

Thank you!

user1671010
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2 Answers2

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You can't have an EC2 instance and an S3 bucket with the same host name. Consider what happens when a web browser makes a request to that host name. DNS resolves it to an IP address (or addresses) and the packets of the request are delivered to that address. The address either terminates at the EC2 instance or the S3 bucket, not both.

As I understand your situation, you have static web pages hosted on S3 that include JavaScript code that makes various HTTP requests to the EC2 instance. If the S3 web pages are on a different host than the EC2 instance then the same origin policy will prevent the browser from even attempting some of the requests.

The only solutions I can see are:

  • Make all requests to the EC2 instance, with it fetching the S3 contents and delivering it to the browser whenever a web page is asked for.
  • Have your JavaScript use iframes and change the document.domain in the the web pages to a common parent origin. For example, if your web pages are at www.example.com and your EC2 instance is at api.example.com, the JavaScript would change document.domain to just example.com and the browser would permit iframes from from www.example.com to communicate with api.example.com.
  • Bite the bullet and use CORS. It's really not hard, and it's supported in all remotely recent browsers (IE 8 and 9 do it, but not in a standard way).

The first method is no good, because you almost might as well not use S3 at all in that case.

The second case should be okay for you. It should work in any browser, because it's not really CORS. So no CORS headers are needed. But it's tricky.

The third, CORS, approach should be just fine. Your EC2 instance just has to return the proper headers telling web pages from the S3 bucket that it's safe for them to talk to the EC2 instance.

Charles Engelke
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Just wanted to add an additional bit to the answer that, if we go with CORS approach and preflight requests adds an overhead to the server and network bandwidth, we may even consider adding header "Access-Control-Max-Age" to the CORS response

Access-Control-Max-Age

Jayavardhan Gange
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