Using Azure Blob Storage as a cache backend for Go acme/autocert

December 21, 2018 • 3 minutes read

If you’re not familiar with acme/autocert package on Go, I’d recommend you to start with Free and Automated SSL Certificates with Go post. It’ll show you how you can use acme/autocert to provision a Let’s Encrypt Certificate for free in a fully automated manner.

By default, acme/autocert stores provisioned certificates on local disk for long-term caching. What happens is that the next time an user visits the same page, this package will fetch the certificate from the local disk instead of provisioning a new one (except if it needs to be renewed, but that’s not relevant here).

The problem here is that if you have a cluster and your service is deployed across multiple machines, having a local disk cache is not very helpful as each machine would have its own cache. Sure you could use NFS to solve this, but there are more cloud-native ways of doing so.

Thankfully this package allow us to switch the cache strategy with a custom implementation where you can choose where cache the certificates.

Azure Blob Storage

github.com/goenning/azcertcache is a Go package that implements a custom Cache strategy that allows you so store certificates in a Azure Blob Storage container.

To use this package you’ll need the Account Name and Account Key that can be found here:

And this is all the setup you need to do:

containerName := "autocertcache"
cache, err := azcertcache.New("<account name>", "<account key>", containerName)
if err != nil {
  // Handle error
}

m := autocert.Manager{
  Prompt:     autocert.AcceptTOS,
  Cache:      cache, // <-- this used to be autocert.DirCache("<folder name>"),
}

s := &http.Server{
  Addr:      ":https",
  TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},
}

s.ListenAndServeTLS("", "")

Ta-da! 🎉

The internal workflow is:

  1. A requests is initialized for mysuperdomain.com
  2. autocert checks if mysuperdomain.com certificate is in the in-memory cache and return it to client
  3. if it’s not found, autocert checks if mysuperdomain.com certificate is in the long-term cache, which in this case is Azure Blob Storage and return it to client
  4. if it’s not found, autocert fetches a new cerificates from let’s encrypt and store in both in-memory and long-term cache.

NOTE: the in-memory cache is lost when the process restarts, hence why it’s so important to keep these certificates on a long-term cache.

BONUS!

What if you’re not using Azure Blob Storage? Well then you still have at least three other options:

  1. https://github.com/goenning/sqlcertcache to store certificates on a SQL Database
  2. https://github.com/danilobuerger/autocert-s3-cache to store certificates on a S3 compatible backend (like AWS S3, DigitalOcean Spaces, Self Hosted Minio…)
  3. NFS and continue to use autocert.DirCache.

Thanks!