containerd

containerd is an industry-standard container runtime that manages the complete container lifecycle on a host system. It is designed to provide core functionalities for running and managing containers, such as image management, container execution, and storage and network interfaces, without including higher-level orchestration features. It is widely used as the runtime behind platforms like Docker and Kubernetes.

Short History

• 2015: Docker spun out containerd as a separate project to isolate the core container runtime functionality from Docker’s higher-level tooling.
• 2017: containerd became a CNCF (Cloud Native Computing Foundation) project, gaining community governance and standardization.
• 2018–Present: It has matured as a stable runtime, integrated deeply with Kubernetes (as a CRI – Container Runtime Interface implementation) and other container platforms.

Essentially, containerd evolved from being a component inside Docker to becoming a standalone, robust runtime that other systems can rely on.

Usages

1. Container Execution: Runs containers on Linux and Windows hosts, handling namespaces, cgroups, and container processes.
2. Image Management: Pulls, pushes, and stores container images from registries.
3. Storage and Networking: Manages container filesystem layers and networking plugins.
4. Kubernetes Integration: Acts as a CRI-compatible runtime, often used with CRI-O or Docker shim.
5. Cloud-Native Applications: Provides a lightweight runtime for cloud platforms requiring fast container startup and lifecycle management.

Core Concepts

1. Containers: Isolated execution environments managed by containerd.
2. Images: Immutable filesystem snapshots used to create containers.
3. Namespaces: Logical partitions within containerd for separating containers, images, and tasks.
4. Tasks: Represents a running container instance; containerd distinguishes between the container definition and the running task.
5. Snapshots: Filesystem layers used by containers, often implemented with overlayfs or other storage drivers.
6. Content Store: Manages storage of images and their layers in a standardized format.
7. gRPC API: containerd exposes a gRPC API for programmatic control of containers and images.
8. CRI Integration: Supports Kubernetes’ Container Runtime Interface, allowing Kubernetes to manage containers via containerd.

Usage

containerd is used with Kubernetes directly, or via Docker.