Containerd and Kubernetes: How are they related?

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Containers are a technology that allows developers to package applications and their dependencies into isolated, lightweight, and portable units. Containers decouple applications from the underlying host infrastructure, which means that they can run on any platform that supports the container runtime. This enables faster development, deployment, and scaling of applications across different environments, such as local machines, cloud servers, or edge devices.

Kubernetes automates deploying, managing, and scaling containerized apps in the cloud. It organizes containers into groups called pods and offers services like load balancing, service discovery, networking, storage, security, and monitoring. Kubernetes works on various cloud providers like Google Cloud Platform, Amazon Web Services, Microsoft Azure, or IBM Cloud.

Containerd is a container runtime designed for Kubernetes and other orchestration systems. It’s a project of the Cloud Native Computing Foundation (CNCF), hosted alongside Kubernetes. Containerd follows the Open Container Initiative (OCI) standards, defining container formats and runtimes. It’s compatible with Docker images, the widely used container format. Containerd simplifies container lifecycle management, from image pulling and unpacking to creation, execution, resource allocation, monitoring, and graceful shutdown.

The Beginnings

Docker, founded in 2013, revolutionized container usage. It started as a spin-off from dotCloud, introducing a tool called docker simplifying Linux container creation and management. Docker popularized Docker images, portable snapshots of containerized apps, stored and shared via Docker Hub. This enabled faster and more reliable application building, shipping, and running.

Kubernetes, created in 2014, emerged from Google’s Borg system. Three Google engineers developed it to share their container orchestration experience with the open-source community. Kubernetes automates deploying, scaling, and managing containerized applications across node clusters, providing features like service discovery, load balancing, networking, storage, security, and monitoring. It quickly became the standard for container orchestration, offering a powerful platform for cloud-native applications.

The challenges of using Docker directly with Kubernetes and the role of Dockershim

Using Docker directly with Kubernetes posed challenges because Docker had some features and components that were not needed or compatible with Kubernetes, such as its own networking and storage drivers clashing with Kubernetes’ solutions. Moreover, Docker’s container format and runtime didn’t align with OCI standards, hindering interoperability.

To address these issues, Kubernetes introduced Dockershim, a thin layer translating Kubernetes’ CRI calls to Docker API calls. While Dockershim facilitated Docker use in Kubernetes, it added performance and security overhead. Maintaining Dockershim became burdensome, keeping up with Docker’s changes.

In 2020, Kubernetes announced the deprecation and removal of Dockershim in future releases, suggesting users switch to more compatible container runtimes like containerd. Containerd aligns with OCI standards, supports Docker images, and simplifies container lifecycle management, offering efficiency from image pulling to deletion.

The Rise of Containerd

Containerd is a container runtime that is designed to work with Kubernetes and other orchestration systems. Containerd is based on the Open Container Initiative (OCI) standards, which define the specifications for container formats and runtimes. Containerd is also compatible with Docker images, which are the most widely used container format in the industry. Containerd provides a simple and efficient way to manage the lifecycle of containers, from image pulling and unpacking, to container creation and execution, to resource allocation and monitoring, to graceful shutdown and deletion.

The Emergence of OCI and Container Standardization

The Open Container Initiative (OCI), launched in 2015 by the Linux Foundation, aims to establish open and interoperable standards for container technologies. It was formed by industry leaders like Docker, Google, Microsoft, IBM, Red Hat, and others to ensure containers work across diverse platforms and systems, avoiding vendor lock-in or fragmentation. OCI produced two key specifications: the OCI Image Specification, governing container image creation, storage, and distribution; and the OCI Runtime Specification, dictating container execution and management. These specifications prioritize simplicity, portability, security, and modularity, and they are compatible with existing container technologies such as Docker and runc.

Docker’s Donation: Extracting Container Runtime to CNCF

Docker, a founding member of OCI, contributed its container format and runtime to OCI specifications. However, Docker’s runtime was part of its broader platform with features like networking and storage. This setup, coupled with a single-point-of-failure daemon, posed challenges for Kubernetes integration. In 2016, Docker introduced containerd, a streamlined version of its runtime, focusing on core container management functions. Containerd, with a gRPC API, aimed for simplicity and stability without Docker’s platform complexity.

In 2017, Docker donated containerd to CNCF, the organization hosting Kubernetes. CNCF supports cloud-native tech development, ensuring vendor-neutral, community-driven, and transparent governance. This move ensured containerd’s maintenance and development by an independent, diverse contributor group aligned with the cloud-native ecosystem’s values.

How are they related?

Now that we know a little about their history and differences, let’s see how they might be related. Kubernetes and containerd communicate through the Container Runtime Interface (CRI), defining interactions between Kubernetes components like kubelet and kube-proxy, and the container runtime. CRI has two services: RuntimeService for container lifecycle operations and ImageService for image operations. It specifies messages and types exchanged between Kubernetes and the container runtime.

Kubernetes sends commands to containerd via its API and components. For instance, to create a pod, Kubernetes sends a CreateContainerRequest to containerd’s RuntimeService, including pod and container configuration. Containerd uses its components to handle tasks like pulling images and creating containers. After completion, Containerd sends a CreateContainerResponse to Kubernetes with pod and container status.

Kubernetes supports various CRI-compliant runtimes, allowing users to choose based on their needs. Containerd, one such runtime, offers simplicity, efficiency, compatibility with OCI standards, interoperability with Docker images, modularity, extensibility, stability, and security.

However, it’s important to note that containerd is not embedded in Kubernetes; they are separate projects communicating through CRI. Containerd is installed and configured alongside Kubernetes on nodes, allowing independent updates, upgrades, and replacements with other runtimes or orchestration systems, provided they support OCI standards and CRI.

Benefits of their synergy

Kubernetes and containerd are key technologies in the cloud-native ecosystem. Kubernetes automates deploying, managing, and scaling containerized apps, while containerd is a standard container runtime for running containers on Linux and Windows. Together, they offer benefits like:

Flexible Runtimes: Kubernetes supports multiple runtimes, including Containerd, giving users choices based on their needs. Containerd, compliant with CRI standards, provides advantages in simplicity, efficiency, compatibility, interoperability, modularity, extensibility, stability, and security. Users can switch runtimes as long as they support OCI standards and CRI.
Scalability and Resilience: Kubernetes offers a distributed orchestration platform, handling cloud complexities. It deploys and manages containers across node clusters, scaling containers based on resource availability. Kubernetes ensures container high availability by monitoring their health and performing self-healing actions in case of failures.
Automated Container Management: Kubernetes and containerd streamline the container lifecycle with an automated workflow. They communicate through CRI, defining interactions between Kubernetes components and the container runtime. Kubernetes issues commands to containerd through its API, simplifying container management with features like services, events, metrics, leases, namespaces, pod sandbox, network plugin, volume plugin, logging, and security.

Frequently Asked Questions (FAQ):

1. Isn’t containerd just part of Kubernetes?

No, containerd is a separate, open-source project that acts as a container runtime engine. Kubernetes utilizes containerd as one of its available CRI-compliant runtimes, but they are distinct entities with different roles.

2. What are the benefits of using containerd outside of Kubernetes?

containerd is lightweight and modular, making it suitable for standalone container execution scenarios. You can use it for building custom container platforms, developing CI/CD pipelines, or even running containers on your laptop.

3. Which container runtime should I use with Kubernetes – containerd, CRI-O, or something else?

The choice depends on your specific needs and priorities. Containerd is a good general-purpose option, while CRI-O offers security-focused features. Research each option and consult documentation to make an informed decision.

4. What’s the future of containerd and Kubernetes?

Both projects are actively developed and have promising futures. Expect deeper integration with emerging technologies like serverless functions and edge computing, increasing security advancements, and the rise of even more sophisticated orchestration tools.

Comparison Table: containerd vs. Kubernetes

Feature Comparison

Feature	containerd	Kubernetes
Type	Container runtime engine	Container orchestration platform
Focus	Low-level container management (pull, run, etc.)	Deploying and managing containerized applications across clusters
Open-source?	Yes	Yes
Standalone use?	Yes	No
CRI compatibility?	Yes	Requires a CRI-compliant runtime like containerd
Scalability	Limited (standalone)	Highly scalable across clusters
Focus on security?	No (general-purpose)	Yes, includes security features

Conclusion

This article introduced containerd and Kubernetes, essential technologies in the cloud-native ecosystem. Containerd serves as a standard container runtime for Linux and Windows, supporting Docker images and offering a modular design. Kubernetes, on the other hand, is a tool that automates deploying, managing, and scaling containerized applications in the cloud, ensuring high availability and reliability.

Together, containerd and Kubernetes enable flexible runtime choices, enhanced scalability, and streamlined container management. They are compatible with various container technologies, supporting OCI standards and the Container Runtime Interface (CRI).

Future improvements may focus on performance, security, expanding functionality, and fostering a diverse and collaborative community.

We hope this overview helped you to see the relationship between Containerd and Kubernetes. Thank you for reading, and happy containerizing!