Kubernetes Runtime Evolution: From Docker to CRI-O

In the early days of Kubernetes, Docker was the default container runtime for managing and orchestrating containers. However, as Kubernetes has evolved, so has its approach to container runtimes. This evolution led to the introduction of the Container Runtime Interface (CRI) and eventually the migration from Docker to CRI-O and containerd. In this article, we will explore the journey from Docker to CRI-O in Kubernetes, explaining why these changes were necessary, what they mean for container orchestration, and how the new runtime landscape affects users.

Why Kubernetes Moved Beyond Docker

Docker was instrumental in popularizing containers, providing a straightforward way to bundle applications and their dependencies. When Kubernetes first emerged as the leading container orchestration tool, Docker was the natural choice for managing these containers. However, as Kubernetes scaled and evolved, certain limitations of Docker became apparent:

Docker’s Monolithic Design: Docker is a monolithic system, which includes not only the container runtime but also other components like the Docker CLI, Docker Engine, and Docker Hub integration. This bundling introduced complexity when only the runtime was needed.
Need for Standardization: Kubernetes introduced the Container Runtime Interface (CRI) to standardize how Kubernetes interacts with various container runtimes. CRI allowed Kubernetes to support multiple runtimes beyond Docker, opening the door to alternatives like containerd and CRI-O.
Dockershim Removal: In Kubernetes v1.24, the dockershim—the intermediary that allowed Kubernetes to communicate with Docker—was officially deprecated and removed. This move was a clear signal that Kubernetes was moving towards leaner, more specialized runtimes.

The Rise of CRI-O and Containerd

With the deprecation of Docker, Kubernetes has embraced two primary container runtimes that comply with the Open Container Initiative (OCI) standards:

CRI-O: A lightweight container runtime designed specifically for Kubernetes. CRI-O implements the CRI standard directly and efficiently, making it the natural choice for Kubernetes environments focused on performance and simplicity.
Containerd: Initially part of Docker, containerd has evolved into a stand-alone runtime. It’s a powerful and extensible container runtime that also fully supports CRI, making it ideal for managing containers within Kubernetes.

Both CRI-O and containerd provide optimized ways to manage containers in Kubernetes, delivering better performance, reduced complexity, and improved resource utilization compared to Docker.

Understanding the Kubernetes Runtime CLI Tools

As Kubernetes transitioned to new runtimes, it introduced specific command-line interfaces (CLIs) to manage and debug containers running on CRI-O and containerd. Here’s an overview of the most important CLI tools that have replaced Docker’s CLI in Kubernetes environments:

Ctr: The Containerd CLI

Ctr is the low-level command-line tool for containerd, allowing administrators to manage containerd’s resources. While not user-friendly, it provides comprehensive control for advanced debugging.

Pulling an image:

ctr image pull docker.io/library/nginx:latest

Listing running containers:
```
ctr container list
```

Running a container:

ctr run --rm docker.io/library/nginx:latest mynginx

Nerdctl: A Docker-Like CLI for Containerd

Nerdctl is a more user-friendly CLI for containerd, offering Docker-like commands and simplifying container management for users familiar with Docker.

Pulling an image:
```
nerdctl pull nginx:latest
```

Running a container:

nerdctl run -d --name mynginx -p 8080:80 nginx:latest

Listing containers:
```
nerdctl ps
```

Crictl: The CRI Debugging Tool

Crictl is the CLI for interacting with CRI-compliant runtimes, including CRI-O and containerd. It is primarily used for debugging Kubernetes clusters and managing pods, containers, and images.

Listing all containers:
```
crictl ps -a
```
Viewing logs from a container:
```
crictl logs <container-id>
```

Executing a command in a container:

crictl exec -i -t <container-id> /bin/sh

Why CRI-O Is the Future of Kubernetes Runtime

CRI-O was designed specifically to integrate seamlessly with Kubernetes. Unlike Docker, CRI-O adheres strictly to the OCI and CRI standards, which makes it more efficient and leaner for Kubernetes use. Here’s why CRI-O is being widely adopted:

Minimal Overhead: CRI-O is lightweight and optimized for Kubernetes workloads, meaning fewer resources are used compared to Docker.
Direct CRI Integration: CRI-O was built to implement the CRI standard, reducing the complexity introduced by using an intermediary like dockershim.
Enhanced Security: CRI-O’s smaller footprint means fewer components, which translates to fewer potential security vulnerabilities.
Better Support for Kubernetes Features: CRI-O is aligned with Kubernetes' release cycle, ensuring better integration and support for new Kubernetes features.

Conclusion: Embracing the Evolution of Kubernetes Runtimes

The shift from Docker to CRI-O and containerd in Kubernetes reflects a broader move towards more specialized, efficient, and secure container runtimes. As Kubernetes has grown, the need for a flexible, standards-based runtime has become clear, and CRI-O is the natural choice for those looking to maximize performance and security.

For Kubernetes administrators and developers, adapting to these new runtimes involves learning new CLI tools like Ctr, Nerdctl, and Crictl, which offer powerful ways to manage and troubleshoot containers in this evolving landscape.

As Kubernetes continues to evolve, CRI-O and containerd will remain at the forefront, offering the flexibility and efficiency needed to power modern, scalable infrastructure. Embracing these runtimes is key to ensuring your Kubernetes clusters are future-proof and ready to handle the next generation of workloads.

Kubernetes Runtime Evolution: From Docker to CRI-O