How to do file versioning with CDN and a loadbalancer?
So I'm using a very simple CDN service. You point to your website and if you call it through their HostName they'll cache it for you after the first call.
I use this for all my static content, like JavaScript files and images.
This all works perfect - and I like that it has very little maintenance or setup cost.
Problem starts when rolling out new versions of JavaScript files. New JavaScript files automatically get a new hash if the files changes.
Because roll out over multiple instances is not simultaneously a problem occurs though. I tried to model it in this diagram:
In words:
- Request hits server with new version
- Requests Js file with new version hash
- CDN detects correctly that the file is not cached
- CDN requests the original file with the new hash from the load balancer
- loadbalancer serves request of CDN to a random server - accidently serving from a server with the old version
- CDN caches old version with the new hash
- everyone gets served old versions from the CDN
There are some ways I know how to fix this - i.e. manually uploading files to a seperate storage with the hash baked in, etc. But this needs extra code and has more "moving parts" that makes maintenance more complicated.
I would prefer to have something that works as seamlessly as the normal CDN behavior. I guess this is a common problem for sites that are running on multiple instances, but I can't find a lot of information about this.
What is the common way to solve this?
Edit
I think another solution would be to somehow force the CDN to go to the same instance for the .js file as the original html file - but how?
I fixed this in the end by only referencing to the CDN version after a few minutes of runtime.
So if the runtime is less then 5 minutes it refers to:
/scripts/example.js?v=351
After 5 minutes it refers to the CDN version:
https://cdn.example.com/scripts/example.js?v=351
After 5 minutes we are pretty sure that all instances are running the new version, so that we don't accidently cache an old version with the new hash.
The downside is that on very busy moments you don't have the advantage of the CDN if you would redeploy, but I haven't seen a better alternative yet.
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