Alien is basically a huge state machine where every API call that mutates the environment is a discrete step, and the full state is durably persisted after each one.

If something fails mid-update, it resumes from exactly where it stopped. You can also point a deployment to a previous release and it walks back. This catches and recovers from issues that something like Terraform would just leave in a broken state.

For on-prem: we're working on Kubernetes as a deployment target (e.g. bare metal OpenShift)