Multiple Cloudfront Origins with Behavior Path Redirection

Question

I have two S3 buckets that are serving as my Cloudfront origin servers:

example-bucket-1
example-bucket-2

The contents of both buckets live in the root of those buckets. I am trying to configure my Cloudfront distribution to route or rewrite based on a URL pattern. For example, with these files

example-bucket-1/something.jpg
example-bucket-2/something-else.jpg

I would like to make these URLs point to the respective files

http://example.cloudfront.net/path1/something.jpg
http://example.cloudfront.net/path2/something-else.jpg

I tried setting up cache behaviors that match the path1 and path2 patterns, but it doesn't work. Do the patterns have to actually exist in the S3 bucket?

Answer 1

Update: the original answer, shown below, is was accurate when written in 2015, and is correct based on the built-in behavior of CloudFront itself. Originally, the entire request path needed to exist at the origin.

If the URI is /download/images/cat.png but the origin expects only /images/cat.png then the CloudFront Cache Behavior /download/* will not do what you might assume -- the cache behavior's path pattern is only for matching -- the matched prefix isn't removed.

By itself, CloudFront doesn't provide a way to remove elements from the path requested by the browser when sending the request to the origin. The request is always forwarded as it was received, or with extra characters at the beginning, if the origin path is specified.

However, the introduction of Lambda@Edge in 2017 changes the dynamic.

Lambda@Edge allows you to declare trigger hooks in the CloudFront flow and write small Javascript functions that inspect and can modify the incoming request, either before the CloudFront cache is checked (viewer request), or after the cache is checked (origin request). This allows you to rewrite the path in the request URI. You could, for example, transform a request path from the browser of /download/images/cat.png to remove /download, resulting in a request being sent to S3 (or a custom orgin) for /images/cat.png.

This option does not modify which Cache Behavior will actually service the request, because this is always based on the path as requested by the browser -- but you can then modify the path in-flight so that the actual requested object is at a path other than the one requested by the browser. When used in an Origin Request trigger, the response is cached under the path requested by the browser, so subsequent responses don't need to be rewritten -- they can be served from the cache -- and the trigger won't need to fire for every request.

Lambda@Edge functions can be quite simple to implement. Here's an example function that would remove the first path element, whatever it may be.

'use strict';

// lambda@edge Origin Request trigger to remove the first path element
// compatible with either Node.js 6.10 or 8.10 Lambda runtime environment

exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request;           // extract the request object
    request.uri = request.uri.replace(/^\/[^\/]+\//,'/');  // modify the URI
    return callback(null, request);                        // return control to CloudFront
};

That's it. In .replace(/^\/[^\/]+\//,'/'), we're matching the URI against a regular expression that matches the leading / followed by 1 or more characters that must not be /, and then one more /, and replacing the entire match with a single / -- so the path is rewritten from /abc/def/ghi/... to /def/ghi/... regardless of the exact value of abc. This could be made more complex to suit specific requirements without any notable increase in execution time... but remember that a Lambda@Edge function is tied to one or more Cache Behaviors, so you don't need a single function to handle all requests going through the distribution -- just the request matched by the associated cache behavior's path pattern.

To simply prepend a prefix onto the request from the browser, the Origin Path setting can still be used, as noted below, but to remove or modify path components requires Lambda@Edge, as above.

---

Original answer.

Yes, the patterns have to exist at the origin.

CloudFront, natively, can prepend to the path for a given origin, but it does not currently have the capability of removing elements of the path (without Lambda@Edge, as noted above).

If your files were in /secret/files/ at the origin, you could have the path pattern /files/* transformed before sending the request to the origin by setting the "origin path."

The opposite isn't true. If the files were in /files at the origin, there is not a built-in way to serve those files from path pattern /download/files/*.

You can add (prefix) but not take away.

A relatively simple workaround would be a reverse proxy server on an EC2 instance in the same region as the S3 bucket, pointing CloudFront to the proxy and the proxy to S3. The proxy would rewrite the HTTP request on its way to S3 and stream the resulting response back to CloudFront. I use a setup like this and it has never disappointed me with its performance. (The reverse proxy software I developed can actually check multiple buckets in parallel or series and return the first non-error response it receives, to CloudFront and the requester).

Or, if using the S3 Website Endpoints as the custom origins, you could use S3 redirect routing rules to return a redirect to CloudFront, sending the browser back with the unhandled prefix removed. This would mean an extra request for each object, increasing latency and cost somewhat, but S3 redirect rules can be set to fire only when the request doesn't actually match a file in the bucket. This is useful for transitioning from one hierarchical structure to another.

http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/distribution-web-values-specify.html

http://docs.aws.amazon.com/AmazonS3/latest/dev/HowDoIWebsiteConfiguration.html