Container Runtimes

Introduction

• Containers have revolutionized the way software is developed, deployed, and managed.
• In this blog post, we will explore how containers use the container runtimes for their lifecycle management.

What are Container Engines?

• A container engine is a higher-level software that manages containers.
• It provides the user interface, orchestration, and management capabilities for interacting with containers.
• It often includes features like building, managing, and running containers, and sometimes even networking and storage management.
• Example: Docker Engine

What are Container Runtimes?

• A container runtime is the underlying component responsible for actually running and managing containers on the host system.
• It is responsible for the low-level tasks such as container lifecycle management (starting, stopping, and running containers), resource isolation (using namespaces and cgroups), and managing execution environments.
• Container runtime acts as a middle layer between the host OS and container instances.
• Example: containerd, CRI-O, runc

Responsibilities of Container Engine v/s Runtimes

Aspect	Container Engine	Container Runtime
Primary Function	Manages containers, including orchestration and interfaces.	Manages the lifecycle of containers (starting, stopping, running).
Interaction Level	High-level abstraction; provides APIs and user interfaces.	Low-level abstraction; provides the environment for container execution.
Container Creation	Builds container images and manages container orchestration.	Executes the container from an image provided by the engine.
Container Lifecycle	Manages container deployment, updates, and scaling.	Handles starting, stopping, and maintaining container processes.
Resource Management	Manages containerized application environments. Can include networking and storage management.	Manages resource isolation for containers using namespaces and cgroups.
Isolation	Not typically responsible for isolation; it relies on runtimes.	Provides isolation between containers using Linux namespaces and cgroups.
Networking	Manages networking between containers and outside environments (often integrated with orchestration).	Not typically responsible for networking; relies on container engine.
Storage	Manages container storage and volume management.	Not responsible for storage, uses external systems integrated by the engine.
Orchestration	Orchestrates container deployment, scaling, and management (e.g., in Kubernetes or Docker Swarm).	Does not handle orchestration, relies on container engines or orchestration systems like Kubernetes.
Example	Docker Engine, Podman, Docker Compose	containerd, runc, CRI-O, Kata Containers

Container Runtimes Categories

High-Level Container Runtimes

• High-level container runtimes are responsible for managing the overall lifecycle of containers.
• They typically interact with container orchestration systems (like Kubernetes) and provide abstractions for users to deploy and manage containers.
• These runtimes focus on higher-level management tasks, including pulling container images, managing container processes, and networking.

1. Docker Engine

• Docker Engine is a high-level runtime that enables the creation and management of lightweight, isolated containers for applications and services.
• It provides a simplified and consistent environment for running software, abstracting away the complexities of the underlying infrastructure.
• Docker allows developers to package their applications with all their dependencies into a portable container, ensuring consistent behavior across different environments.
• These containers are isolated from one another and can be easily deployed, scaled, and managed, making Docker a popular choice for deploying and running applications in various settings, from development to production environments.

2. Podman

• It allows users to create, run, and manage containers for applications and services.
• Podman is compatible with the Open Container Initiative (OCI) standards and can run Docker-formatted container images.
• It provides a command-line interface and API for container management, including functions like creating, starting, stopping, and removing containers.
• One key distinction of Podman is that running containers does not require a separate daemon process.
• It operates as a daemonless container engine, meaning it runs containers as individual processes without the need for a centralized daemon.
• Podman interacts with the Linux kernel directly through runc.

3. CRI-O

• CRI-O is another high-level runtime, specifically designed to run containers that adhere to the Kubernetes Container Runtime Interface (CRI) specification.
• It focuses on providing a lightweight, stable, and secure runtime environment for running containers within Kubernetes clusters.
• CRI-O enables the execution of containers by interacting with the Kubernetes control plane.
• It adheres to the CRI standard, which defines the interface between Kubernetes and the container runtime.
• This ensures compatibility and interoperability with Kubernetes, allowing users to seamlessly run and manage containers within their Kubernetes environments.
• Similar to Docker and Podman, CRI-O offers features such as container isolation, image management, and container lifecycle management.
• However, CRI-O is specifically tailored for Kubernetes deployments and is optimized for use within the Kubernetes ecosystem.
• It provides a minimal footprint, improved security, and compatibility with container images that adhere to the Open Container Initiative (OCI) standards.

Low-Level Container Runtimes

• Low-level container runtimes are responsible for directly interacting with the operating system to create and manage containers.
• These runtimes handle the core functionality of containers, such as creating container images, setting up namespaces, cgroups, and providing the execution environment for containers.

1. runc

• runc is a low-level runtime that implements the OCI runtime specification.
• It provides the foundation for running containers by orchestrating container execution and lifecycle management.
• runc is focused on simplicity, portability, and standardization, allowing different containerization platforms and tools to utilize a common runtime interface.

2. crun

• crun is another low-level runtime in C that implements the OCI runtime specification.
• It is designed for lightweight and fast container execution, optimized for performance and resource efficiency.
• crun aims to be a simple and minimalistic alternative to runc, focusing on speed and minimal overhead.
• It provides a compatible runtime environment for running containers while prioritizing simplicity and performance.

3. runhcs

• runhcs (runC with Hypervisor-based Containers) is an experimental project that extends the functionality of runC to support hypervisor-based containers, such as those utilizing the Kernel-based Virtual Machine (KVM).
• It leverages the same OCI runtime specification, allowing users to run containers with the added isolation and security benefits of a hypervisor.
• runhC aims to provide a consistent and familiar interface for managing containers while leveraging the capabilities of hardware virtualization.
• Please note that runhcs is an experimental project, and its adoption and usage may vary. Simply, you can understand as a Windows containers counterpart of runc.

4. containerd

• Containerd is on the line between low-level and high-level.
• It actually leans more toward a high-level container runtime because it provides an API layer on top of an OCI-compatible runtime, but we will include it here because you don’t really interact with it directly.
• There’s always an external layer that you use to interact with it, such as a CLI (like nerdctl or ctr) or CRI on Kubernetes.
• It is one of the most popular option after Docker engine and it is a CNCF graduated project since 2019.

Process-based vs VM-bases Runtimes

Process-based Runtimes	Virtual Machine (VM)-based Runtimes
Examples: Docker, Containerd, CRI-O	Examples: Kata, Firecracker
Rely on operating system features such as namespaces and cgroups for container isolation and resource management.	Run containers within lightweight virtual machines using hypervisor technologies to create and manage VMs.
Lightweight	Enhanced isolation and security
Provide fast startup times	Suitable for scenarios where strict isolation is a priority
Suitable for most container workloads	Typically used in scenarios requiring strong isolation and security.

Uses Cases for Container Runtimes

• Here are some of the widely used container runtimes and their associated use cases:

Docker	Containerd	CRI-O	rocket(rkt)
Development and testing environments	Designed to be lightweight and focuses on executing containers reliably and efficiently	It is specifically designed to work with Kubernetes	Developed by CoreOS (now part of Red Hat)
Enables the deployment of individual microservices	It is the default container runtime in Kubernetes	Provides a runtime environment optimized for Kubernetes-specific use cases	rkt emphasizes strong security features, such as container signature verification and isolation mechanisms
Often used in CI/CD workflows	Can be used in container orchestration		Security-focused deployments
Deploying applications across different cloud providers or hybrid environments	It aims to provide a minimal runtime

List of Tools and Container Runtimes

Method	Tool/Option	Description	Example Command	Container Runtime
1. Docker	docker	A platform for developing, shipping, and running containers. Uses docker CLI to build images and run containers.	Build image: docker build -t mywebsite . Run container: docker run -d -p 8080:80 mywebsite	Docker Engine
2. nerdctl	nerdctl	A tool for managing containers with containerd, mimics Docker’s functionality but uses containerd as the runtime.	Build image: nerdctl build -t mywebsite . Run container: nerdctl run -d -p 8080:80 mywebsite	containerd
3. Podman	podman	A container engine compatible with Docker, but rootless and daemonless, often used as an alternative to Docker.	Build image: podman build -t mywebsite . Run container: podman run -d -p 8080:80 mywebsite	Podman
4. Buildah	buildah	A tool for building OCI and Docker container images, often used with Podman but works independently of Docker.	Build image: buildah bud -t mywebsite . Run container: podman run -d -p 8080:80 mywebsite	Podman, Containerd
5. Kubernetes	kubectl	Orchestrates containerized applications, can manage container images and run them as Pods in clusters.	Deploy to Kubernetes: kubectl run mywebsite --image=mywebsite:latest Expose service: kubectl expose pod mywebsite --type=NodePort	containerd / Docker (Kubernetes runtime)
6. Docker Compose	docker-compose	A tool for defining and running multi-container Docker applications. Often used for complex setups.	Build and run with Compose: docker-compose up --build	Docker Engine