Runtime

1. What is Container Runtime?

   • A container runtime is software responsible for running containers, managing their lifecycle (start, stop, execute), and interacting with the operating system to create and manage containers.

2. Kubernetes and Container Runtime:

   • Kubernetes interacts with the container runtime through the Container Runtime Interface (CRI).
   • The container runtime is responsible for pulling container images, creating containers, running them, and managing their lifecycle on a node.

3. Popular Container Runtimes in Kubernetes:

   • Docker (historically the most common, though deprecated in Kubernetes as of v1.20+ in favor of other runtimes).
   • containerd: A high-performance container runtime used by Kubernetes, focused on running containers.
   • CRI-O: A lightweight container runtime specifically built for Kubernetes, adhering strictly to the Kubernetes Container Runtime Interface (CRI).
   • runc: The low-level container runtime that creates and runs containers based on the OCI (Open Container Initiative) standards; often used by containerd and CRI-O.

4. Container Runtime Interface (CRI):

   • A Kubernetes API that allows the kubelet to communicate with various container runtimes.
   • Ensures that Kubernetes can support multiple runtimes (e.g., Docker, containerd, CRI-O) by abstracting runtime-specific details.

5. Runtime Features:

   • Container Image Management: Pulling, caching, and running images.
   • Container Lifecycle Management: Starting, stopping, and cleaning up containers.
   • Namespaces & Cgroups: Providing isolation for containers (ensuring they have their own process space, network, etc.).

6. Runtime in Kubernetes Workflow:

   • Kubelet requests container runtimes to start or stop containers on a node.
   • PodSpec in Kubernetes specifies what containers to run; the runtime manages the execution.

7. Runtime Security Considerations:

   • Using runtime security tools to scan container images and ensure compliance with security standards.
   • Enforcing security policies through runtime, such as restricting container privileges (user IDs, rootless containers).

8. Transition from Docker to containerd/CRI-O:

   • As of Kubernetes 1.20+, Docker is no longer the default container runtime.
   • Docker is replaced with containerd or CRI-O for better integration with Kubernetes.

In the KCNA exam, understanding how Kubernetes interacts with container runtimes and the different runtime options is essential, particularly focusing on how Kubernetes uses the Container Runtime Interface (CRI) to communicate with container runtimes like containerd or CRI-O.