CRI-O is an implementation of the Kubernetes CRI (Container Runtime Interface) to enable using OCI (Open Container Initiative) compatible runtimes. It is a lightweight alternative to using Docker as the runtime for Kubernetes. It allows Kubernetes to use any OCI-compliant runtime as the container runtime for running pods. Today it supports runc and Kata Containers as the container runtimes but any OCI-conformant runtime can be plugged in principle.
CRI-O supports OCI container images and can pull from any container registry. It is a lightweight alternative to using Docker, Moby or rkt as the runtime for Kubernetes.
Table of Contents
Contributors to CRI-O:
CRI-O is developed by maintainers and contributors from these companies and others. It is a community-driven, open-source project. Feedback, users, and of course, contributors, are always welcome via the cri-o/cri-o GitHub project.
The architectural components are as follows:
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- Kubernetes contacts the kubelet to launch a pod.
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- Pods are a Kubernetes concept consisting of one or more containers sharing the same IPC, NET and PID namespaces and living in the same cgroup.
- The kubelet forwards the request to the CRI-O daemon VIA Kubernetes CRI (Container runtime interface) to launch the new POD.
- CRI-O uses the containers/image library to pull the image from a container registry.
- The downloaded image is unpacked into the container’s root filesystems, stored in COW file systems, using containers/storage library.
- After the rootfs has been created for the container, CRI-O generates an OCI runtime specification json file describing how to run the container using the OCI Generate tools.
- CRI-O then launches an OCI Compatible Runtime using the specification to run the container proceses. The default OCI Runtime is runc.
- Each container is monitored by a separate conmon process. The conmon process holds the pty of the PID1 of the container process. It handles logging for the container and records the exit code for the container process.
- Networking for the pod is setup through use of CNI, so any CNI plugin can be used with CRI-O.
Components
CRI-O is made up of several components that are found in different GitHub repositories.
- OCI compatible runtime
- containers/storage
- containers/image
- networking (CNI)
- container monitoring (conmon)
- security is provided by several core Linux capabilities
Alternative Solutions to CRI-O
While CRI-O is a robust and Kubernetes-focused container runtime, several alternatives are available, each with unique advantages:
- Containerd:
- Originally developed as part of Docker, containerd is now an independent, CNCF-hosted project.
- It provides a simple and reliable container runtime with native support for Kubernetes.
- Offers a similar level of performance and resource efficiency as CRI-O, while also being widely adopted.
- Docker:
- Although Kubernetes has deprecated support for the Dockershim adapter (as of Kubernetes v1.24), Docker can still be used indirectly by leveraging containerd (which Docker uses internally).
- Docker is ideal for developers who need comprehensive tools for container building and management.
- Kata Containers:
- Kata Containers provide an added layer of isolation by running containers inside lightweight virtual machines.
- This is a great choice for workloads requiring enhanced security and isolation, albeit at the cost of additional resource overhead.
- Podman:
- A daemonless, Linux-native container runtime that emphasizes security and simplicity.
- While Podman isn’t a direct CRI implementation, it can be integrated into Kubernetes workflows using tools like CRI-O or containerd.
- gVisor:
- Developed by Google, gVisor is a container runtime that focuses on isolating workloads for enhanced security.
- It is particularly suitable for multi-tenant environments where security is a top priority.
How CRI-O Fits into Kubernetes
Architecture
CRI-O acts as a bridge between Kubernetes and the lower-level container runtime. It interacts with the kubelet via the CRI to:
Pull container images from registries.
Start, stop, and manage containers and pods.
Handle logs, stats, and other operational details.
Under the hood, CRI-O relies on an OCI-compliant runtime, such as runc, to create and manage containers.
Benefits for Kubernetes Deployments
- Simplicity: CRI-O’s minimalistic approach reduces complexity, which translates to easier maintenance and troubleshooting.
- Performance: By eliminating unnecessary overhead, CRI-O ensures efficient resource utilization, making it a great choice for resource-constrained environments.
- Security: Its built-in support for modern security features aligns with the security-first approach of containerized environments.
- Community and Support: As a CNCF project, CRI-O benefits from a vibrant community and regular updates.
Comparison with Docker and Containerd
While Docker and containerd are popular options, CRI-O offers distinct advantages:
- Purpose-built for Kubernetes: Unlike Docker, CRI-O doesn’t include extra features unrelated to Kubernetes.
- Direct CRI Implementation: While Docker requires the Dockershim adapter (deprecated as of Kubernetes v1.24), CRI-O directly implements the CRI.
- Lightweight Alternative to Containerd: Compared to containerd, CRI-O is often seen as simpler and more Kubernetes-focused.
NOTE:
CRI-O is not supported as a stand-alone container engine. You must use CRI-O as a container engine for a Kubernetes installation, such as OpenShift Container Platform. To run containers without Kubernetes or OpenShift Container Platform, use podman.
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Conclusion
CRI-O is a powerful, lightweight, and Kubernetes-focused container runtime that simplifies the management of containers in Kubernetes environments. Its alignment with Kubernetes’ needs, adherence to open standards, and emphasis on performance and security make it an excellent choice for modern cloud-native deployments.
Whether you’re looking to optimize resource usage, enhance security, or embrace Kubernetes-native tools, CRI-O deserves a spot on your radar. Give it a try and experience the simplicity and efficiency it brings to container orchestration.
Whether you’re looking to optimize resource usage, enhance security, or embrace Kubernetes-native tools, CRI-O deserves a spot on your radar. Give it a try and experience the simplicity and efficiency it brings to container orchestration.
