ContainerRuntimes

ContainerRuntimes are software components that execute and manage containerized processes on a host. They provide process isolation, resource isolation, and lifecycle management by leveraging kernel features such as namespaces and control groups. Runtimes are typically invoked by container orchestration systems or by higher-level container engines to start, pause, resume, and terminate containers, while handling interactions with storage, networking, and images.

Architecturally, a runtime may be a low-level OCI-compliant component (for example, the OCI runtime-spec) that actually launches containers, or it may sit behind a daemon and shim that orchestrates multiple containers, images, and volumes. The most common combination in modern systems is a daemon such as containerd or CRI-O that delegates to an OCI runtime like runc or crun.

Container runtimes are critical to Kubernetes and other orchestrators through the Container Runtime Interface (CRI), which abstracts the details of the runtime. Popular runtimes include containerd, CRI-O, as well as other options using runc-based or VM-based isolation such as gVisor's runsc and Kata Containers. These choices affect startup latency, isolation level, and maintenance responsibilities.

Standards from the Open Container Initiative (OCI) define the runtime-spec used to create containers and the image-spec used to package container images, enabling portability across runtimes. Security features include namespace isolation, cgroups, seccomp, AppArmor, and, for stronger isolation, sandboxing runtimes that run containers in lightweight virtual machines.

History and ecosystem: container runtimes emerged to separate image management from execution, enabling flexible deployment models and better security boundaries. They remain central to modern cloud-native deployments, where multi-tenant clusters rely on diverse runtimes and orchestration platforms.