We host our static files on S3 and they are served via fastly, because of which they are delivered through another domain. The error tracking snippet could not log the errors because ‘window.onerror’ would not return the necessary stack trace and information.
Many of you may have heard about CORS, it is a mechanism that allows restricted resources (e.g. fonts) on a web page to be requested from another domain outside the domain from which the resource originated. For security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts. The only way to solve this and get errors to be posted to errorception was to allow CORS in the header of the static files.
After some research (errorception also has a good blog on it), we finally managed to have the error logging implemented. Below are the steps we had to go through:
S3 has this unnecessarily complicated “CORS configuration” that you need to create. Here’s the steps to get that right:
<?xml version="1.0" encoding="UTF-8"?> <CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"> <CORSRule> <AllowedOrigin>*</AllowedOrigin> <AllowedMethod>GET</AllowedMethod> <MaxAgeSeconds>3000</MaxAgeSeconds> <AllowedHeader>Authorization</AllowedHeader> </CORSRule> </CORSConfiguration>
You should look at Amazon’s docs to see what this configuration means.
Make sure you add <?xml ?> declaration, if you omit these, Amazon will fail silently, showing you a happy looking green tick!
Once you’ve saved the configuration, give it a couple of minutes.
Test if everything’s looking right. You could use a tool like curl to specify the additional headers needed for a “correct” CORS request:
$ curl -sI -H "Origin: example.com" -H "Access-Control-Request-Method: GET" https://s3.amazonaws.com/bucket/script.js HTTP/1.1 200 OK Date: Wed, 05 Nov 2014 13:37:20 GMT Access-Control-Allow-Origin: * Access-Control-Allow-Methods: GET Access-Control-Max-Age: 3000 Vary: Origin, Access-Control-Request-Headers, Access-Control-Request-Method Cache-Control: max-age=604800, public ...snip...
You should see the “Access-Control-Allow-Origin: *” header, and the “Vary: Origin” header in the output. If you do, you’re golden.
For configuring fastly, we just had to follow a few simple steps and then we were done:
Set up a custom HTTP header via the Content pane for your service.
Then, create a custom header with the following information that adds the required “Access-Control-Allow-Origin” header for all requests.
Name - CORS S3 Allow Type/Action - Cache Set Destination - http.Access-Control-Allow-Origin Source - "*" Ignore if Set - No Priority - 10
Access-Control-Allow-Origin: http://your-hostname.tld Access-Control-Allow-Methods: GET Access-Control-Expose-Headers: Content-Length, Connection, Date...
Once we had the “Access-Control-Allow-Origin” set up for both S3 and Fastly, we had one major thing left to do, to add crossorigin=’anonymous’ to all our script tags. For this we used a simple regex to modify the existing script tags in all the files:
Find: (<script)(.*?)(STATIC_URL)(.*?)(.js)(.*?)("|')(>) Replace: $1$2$3$4$5$6$7 crossorigin=’anonymous’$8
After this we just bumped the version of all js files (to clear cache) and then we were done. We finally had the js logger implemented.
And as always, *Happy Coding!*