By using CRI, Kubernetes abstracts the details of container runtimes, making it possible to integrate different runtimes seamlessly, such as containerd, CRI-O, or others, while maintaining the same Kubernetes functionality.

Key Components

  1. Kubelet:
    The kubelet uses CRI to manage the lifecycle of containers on each worker node, including starting, stopping, and retrieving their status.
  2. Container Runtime:
    The runtime is the software that runs containers. Common examples are:
    • containerd: A lightweight runtime that is CNCF-certified and widely used.
    • CRI-O: A runtime optimized for Kubernetes, designed to work with the Open Container Initiative (OCI) standards.
  3. gRPC Protocol:
    Communication between the kubelet and container runtimes is implemented using the gRPC protocol for performance and scalability.

Key Functions

  1. Container Management:
    • Starting and stopping containers.
    • Monitoring container health and resource usage.
  2. Image Management:
    • Pulling container images from registries.
    • Managing image caches and layers.
  3. Pod-Level Networking:
    Works in conjunction with CNI (Container Network Interface) plugins to provide networking for pods.

What is the importance of CRI

  1. Modularity:
    Decouples Kubernetes from specific container runtimes, allowing developers to choose the best runtime for their use case.
  2. Flexibility:
    Supports a variety of container runtimes, enabling Kubernetes to adapt to evolving container technologies.
  3. Standardization:
    Provides a consistent interface for runtime interaction, ensuring stability and interoperability within Kubernetes.
  4. Performance Optimization:
    CRI-optimized runtimes like CRI-O and containerd are designed for Kubernetes, offering better performance and streamlined workflows.

Does Docker Support CRI?

Docker does not natively support the Container Runtime Interface. Instead, Kubernetes initially used an adapter called dockershim to make Docker work with the CRI standard. The dockershim acted as a bridge between Docker and Kubernetes’ kubelet, enabling Kubernetes to interact with Docker as a runtime.

However, starting with Kubernetes v1.20, dockershim was deprecated, and by v1.24, it was fully removed. This means Kubernetes no longer supports Docker as a runtime directly. Users are encouraged to migrate to CRI-compliant container runtimes like containerd or CRI-O, which are optimized for Kubernetes and directly integrate with kubelet via the CRI. Docker itself relies on containerd as its runtime under the hood, so transitioning to containerd is often straightforward for most Kubernetes users.

Use Cases

  1. Kubernetes Deployments:
    Is essential for running containerized workloads in Kubernetes clusters.
  2. Custom Runtime Integration:
    Organizations can develop their own CRI-compliant runtimes tailored to specific needs (e.g., security-focused or lightweight runtimes).
  3. Optimized Performance:
    Switch to CRI-optimized runtimes for faster container management and lower resource overhead.

Further Reading and Resources

  1. Kubernetes Documentation:
  2. CRI-O Project:
  3. containerd Project:
  4. Open Container Initiative (OCI):