What are Container Runtimes and How do They Work?

Containerization has emerged as a game-changer in the dynamic environment of modern software development. It allows programmers to package applications and their dependencies into compact, transportable units called containers.

But to work with containers, you need container runtimes - containers' execution engines.

Each runtime has advantages and applications, from the tried-and-true containerd to the cutting-edge Firecracker and the well-known Docker.

Come along as we explore these important technologies, their characteristics, and how they foster innovation in the software deployment sector. But first, what are container runtimes?

What are Container Runtimes?

Container runtimes are tools that create, manage, and execute containers. It makes building, running, and orchestrating these independent parts (containers) easier, ensuring that applications function reliably in various settings.

The infrastructure and resources required to support the separation and execution of containerized applications are essentially provided by a container runtime.

Also Read: Which one to Choose - Docker or Podman?

How do Container Runtimes Work?

When you start a container, the container runtime gets going by requesting the specified container image from a repository, which acts as a blueprint for the application and its dependencies.

The container runtime gives the container a secure and segregated environment using Linux namespaces, providing it a unique understanding of the system's resources, including processes, network interfaces, and file systems.

Control groups (cgroups) in the Linux kernel ensures equitable resource distribution, preventing any container from hogging resources and degrading system performance.

When a container is isolated, the runtime executes the program within that environment, allowing easy communication with the host system.

Container runtimes put in a lot of effort to enable effective and dependable application deployment, addressing the constantly changing requirements of contemporary software development, from creation to execution and resource management.

Types of Container Runtimes

Now that we understand what are container runtimes and how they work, let's look at different types of container runtimes.

Low-Level Container Runtimes

Low-level container runtimes, prioritizing essential container functionality, offer the fundamental building blocks for container operation.

These runtimes are often compact and performance-oriented. They use Linux kernel features like namespaces and cgroups to divide containers and manage resource allocation.

A few examples of low-level container runtimes are Containerd, rkt, and LXC.

A high-performance container runtime, containerd was first included in Docker. It provides the essential features for container execution and acts as the backbone of many container systems and orchestrators.

rkt (rocket)

Developed by CoreOS, which is now part of Red Hat, rkt complies with the App Container (appc) specification and highly emphasizes security and usability.

It is designed for environments where security is of the utmost significance and uses a lightweight technique of container execution.

This project has been ended/achieved.

LXC (Linux Containers) and LXD

While LXD gives LXC a more simplified and feature-rich administrative interface, the low-level LXC runtime provides the fundamental container functionality.

System containers are supported by LXD, which combines resource management analogous to virtual machine isolation with containerization.

Also Read: How to Cleanup Docker Resources?

High-Level Container Runtimes

High-level container runtimes add additional capabilities and abstractions to simplify the container administration process and are built on top of low-level runtimes.

Because they usually offer user-friendly tools and interfaces, they are well-liked by developers and operators. In terms of high-level container runtimes, Docker is the most popular.

Containerization became well-known because of Docker, a comprehensive high-level container runtime.

It is accessible to various users and provides graphical tools and a user-friendly command-line interface.

Docker offers tools like Docker Compose for the orchestration of multi-container applications.

Also Read: Docker Commands Cheat Sheet

Sandboxed and Virtualized Container Runtimes

The approaches to containerization used by runtimes for virtualized and sandboxed containers differ, placing a stronger emphasis on security and isolation.

These virtualization-based runtimes, called microVMs, offer compact, secure containers.

AWS developed the microVM-based Firecracker container runtime expressly for serverless and microservices use cases. KVM, which has low overhead and provides rapid and resource-efficient containerization, is used to run MicroVMs.

Container Runtime Interfaces (CRIs)

Container Runtime Interfaces (CRIs) are critical in seamlessly integrating container runtimes and container orchestrators, such as Kubernetes, in the dynamic containerization environment.

CRIs operate as a connection that abstracts away the underlying workings of container runtimes so that container management tools can easily communicate with different runtimes.

Developers and operators can choose the appropriate container runtime for their particular requirements by taking advantage of the openness and flexibility that this abstraction encourages without worrying about incompatibilities.

Container orchestrators don't need to significantly alter their core code to adapt to new runtimes because CRIs provide a uniform language for communication.

CRIs enhance the flexibility and agility of the container ecosystem as a result, empowering companies to handle and develop containerized applications efficiently.

Native Runtimes

Native runtimes, also known as system container runtimes, are a specific kind of container runtime that offers a more straightforward and lightweight approach to containerization.

Using the host operating system's package management and system libraries, native runtimes—as opposed to traditional container runtimes—allow containers to share the same system libraries as the host.

Native runtimes are the greatest option when performance and efficiency are essential due to this unique characteristic, which reduces overhead and optimizes resource utilization.

Native runtimes work well for system-level workloads and programs that require direct access to the host's resources.

Because they don't duplicate libraries inside containers, native runtimes have smaller memory footprints, faster startup times, and superior overall performance.

When aiming to optimize resource usage and boost performance in containerized environments, enterprises typically turn to native runtimes.

Also Read: Docker Swarm vs Kubernetes

OCI Runtimes, the Open Container Initiative

The Open Container Initiative (OCI) is a reputable organization aiming to develop open standards for runtimes and container formats.

The provision of a standard specification for container execution necessitates using OCI runtimes.

The portability and interoperability of container runtimes across different containerization systems are guaranteed by their adherence to the OCI runtime requirements.

The OCI runtime definition, which outlines the interfaces and APIs that container runtimes must implement, makes it simple for container orchestrators like Kubernetes to communicate with various runtimes.

Runtimes like Containerd, runc, and Kata Containers are a few well-liked OCI-compliant ones.

OCI runtimes provide developers and operators with the assurance that their container workloads are built on an open and standardized foundation.

This encouragement leads to a more active, diverse, and dynamic container ecosystem. That further offers numerous opportunities for innovation and growth.

OCI runtimes have subsequently developed into a crucial component of contemporary containerization, encouraging industry-wide cooperation and standardization in container execution.

Also Read: Consul vs Istio vs Linkerd

Common Container Runtimes for Kubernetes

Kubernetes, one of the most popular container orchestration solutions, supports a variety of container runtimes.

Here are a few well-liked Kubernetes container runtimes.

Docker

Docker, once the default runtime for Kubernetes, is undoubtedly the most widely used and well-known container runtime. Because of its user-friendly UI and rich feature set, developers can easily create, distribute, and execute containers.

Also Read: Top Docker Alternatives to Choose from

Containerd

Containerd has supplanted Docker as the dominant container runtime as Kubernetes has grown. A high-performance container runtime focusing on container execution and supervision is containerd. It is a lighter and more efficient alternative.

Cri-o

The alternative container runtime designed specifically for Kubernetes, cri-o, and the Kubernetes Container Runtime Interface (CRI) are compatible.

It was designed to be small, secure, and optimized for using Kubernetes pods and containers.

Containerd-shim

Containerd Shim allows Kubernetes to directly communicate with containerd.

This direct communication eliminates the need for a separate runtime process for each container and helps to optimize resource utilization and reduce startup times.

Firecracker

Firecracker deserves attention due to its unique methodology even though it is not a native Kubernetes runtime.

FAQs

Which container runtimes are best for Kubernetes?

Kubernetes' best container runtimes are containerd, cri-o, and Docker.

What is the difference between container runtime and Docker?

A container runtime executes containers. Whereas Docker is a comprehensive container platform that includes a container runtime and additional tools for container management.

What is the difference between runC and containerd?

runC is a basic container runtime. And containerd is a higher-level runtime with additional features and functionalities built on top of runC.

Further Reading