The main alternative runtime that I know of (at the level of containerd) is CRI-O. These runtimes are at the level of fetching images, preparing manifests etc.

The main alternative runtime that I know of (at the level of containerd) is CRI-O. These runtimes are at the level of fetching images, preparing manifests etc. I'm not really sure what benefits they provide. CRI-O is intended to be kubernetes specific, and thus lacks any features that containerd would have that k8s does not need. This in theory ought to mean smaller, lighter, more easily auditable code. There is another lower level of runtime, the OCI runtime, of which the main implementation is runc. Alternatives have interesting attributes, like `runv` running containers in VMs with their own kernel to get even grater isolation, `runhcs` which is the OCI runtime for running windows containers, etc. Most if not all of the higher level runtimes allow switching out the OCI runtime, but in general sticking with the default of `runc` is fine.