Docker is a software platform for building applications based on containers—small and lightweight execution environments that make shared use of the operating system kernel but otherwise run in isolation from one another. While containers have been used in Linux and Unix systems for some time, Docker, an open source project launched in 2013, helped popularize the technology by making it easier than ever for developers to package their software to “build once and run anywhere.”

A brief history of Docker

Founded as DotCloud in 2008 by Solomon Hykes in Paris, what we now know as Docker started out as a platform as a service (PaaS) before pivoting in 2013 to focus on democratizing the underlying software containers its platform was running on.

Hykes first demoed Docker at PyCon in March 2013, explaining that Docker was created because developers kept asking for the underlying technology powering the DotCloud platform. “We did always think it would be cool to be able to say, ‘Yes, here is our low-level piece. Now you can do Linux containers with us and go do whatever you want, go build your platform.’ So that’s what we are doing.”

And so, Docker was born, with the open source project quickly picking up traction with developers and attracting the attention of high-profile technology providers like Microsoft, IBM, and Red Hat, as well as venture capitalists willing to pump millions of dollars into the innovative startup. The container revolution had begun.

What are containers?

As Hykes described it in his PyCon talk, containers are “self-contained units of software you can deliver from a server over there to a server over there, from your laptop to EC2 to a bare-metal giant server, and it will run in the same way because it is isolated at the process level and has its own file system.”

The components for doing this have long existed in operating systems like Linux. By simplifying their use and giving these bits a common interface, Docker quickly became close to a de facto industry standard for containers. Docker let developers deploy, replicate, move, and back up a workload in a single, streamlined way, using a set of reusable images to make workloads more portable and flexible than previously possible.

Also see: Why you should use Docker and OCI containers.

In the virtual machine (VM) world, something similar could be achieved by keeping applications separate while running on the same hardware. But each VM requires its own operating system, meaning VMs are typically large, slow to start up, difficult to move around, and cumbersome to maintain and upgrade.

Containers represent a defined shift from the VM era, in that they isolate execution environments while sharing the underlying OS kernel. As a result, they are speedier and far more lightweight than VMs.

Docker: The component parts

Docker took off with software developers as a novel way to package the tools required to build and launch a container. It was more streamlined and simplified than anything previously possible. Broken down into its component parts, Docker consists of the following:

• Dockerfile: Each Docker container starts with a Dockerfile. This text file provides a set of instructions to build a Docker image, including the operating system, languages, environmental variables, file locations, network ports, and any other components it needs to run. Provide someone with a Dockerfile and they can recreate the Docker image wherever they please, although the build process takes time and system resources.
• Docker image: Like a snapshot in the VM world, a Docker image is a portable, read-only executable file. It contains the instructions for creating a container and the specifications for which software components to run and how the container will run them. Docker images are far larger than Dockerfiles but require no build step: They can boot and run as-is.
• Docker run utility: Docker’s run utility is the command that launches a container. Each container is an instance of an image, and multiple instances of the same image can be run simultaneously.
• Docker Hub: Docker Hub is a repository where container images can be stored, shared, and managed. Think of it as Docker’s own version of GitHub, but specifically for containers.
• Docker Engine: Docker Engine is the core of Docker. It is the underlying client-server technology that creates and runs the containers. The Docker Engine includes a long-running daemon process called dockerd for managing containers, APIs that allow programs to communicate with the Docker daemon, and a command-line interface.
• Docker Compose: Docker Compose is a command-line tool that uses YAML files to define and run multicontainer Docker applications. It allows you to create, start, stop, and rebuild all the services from your configuration and view the status and log output of all running services.
• Docker Desktop: All of these component parts are wrapped in Docker’s Desktop application, providing a user-friendly way to build and share containerized applications and microservices.

Advantages of Docker

Docker containers provide a way to build applications that are easier to assemble, maintain, and move around than previous methods allowed. That provides several advantages to software developers:

• Docker containers are minimalistic and enable portability: Docker helps to keep applications and their environments clean and minimal by isolating them, which allows for more granular control and greater portability.
• Docker containers enable composability: Containers make it easier for developers to compose the building blocks of an application into a modular unit with easily interchangeable parts, which can speed up development cycles, feature releases, and bug fixes.
• Docker containers make orchestration and scaling easier: Because containers are lightweight, developers can launch many of them for better scaling of services, and each container instance launches many times faster than a VM. These clusters of containers do then need to be orchestrated, which is where a platform like Kubernetes typically comes in.

Also see: How to succeed with Kubernetes.

Drawbacks of Docker

Containers solve a great many problems, but they don’t solve them all. Common complaints about Docker include the following:

• Docker containers are not virtual machines: Unlike virtual machines, containers use controlled portions of the host operating system’s resources, which means elements aren’t as strictly isolated as they would be on a VM.
• Docker containers don’t provide bare-metal speed: Containers are significantly more lightweight and closer to the metal than virtual machines, but they do incur some performance overhead. If your workload requires bare-metal speed, a container will get you close but not all the way there.
• Docker containers are stateless and immutable: Containers boot and run from an image that describes their contents. That image is immutable by default—once created, it doesn’t change. But a container instance is transient. Once removed from system memory, it’s gone forever. If you want your containers to persist state across sessions, like a virtual machine, you need to design for that persistence.

Docker today

Container usage has continued to grow in tandem with cloud-native development, now the dominant model for building and running software. But these days, Docker is only a part of that puzzle.

Docker grew popular because it made it easy to move the code for an application and its dependencies from the developer’s laptop to a server. But the rise of containers led to a shift in the way applications were built—from monolithic stacks to networks of microservices. Soon, many users needed a way to orchestrate and manage groups of containers at scale.

Launched at Google, the Kubernetes open source project quickly emerged as the best way to orchestrate containers, superseding Docker’s own attempts to solve this problem with Docker Swarm (RIP). Amidst increasing funding trouble, Docker eventually sold its enterprise business to Mirantis in 2019, which has since absorbed Docker Enterprise into the Mirantis Kubernetes Engine.

The remains of Docker—which includes the original open source Docker Engine container runtime, Docker Hub image repository, and Docker Desktop application—live on under the leadership of company veteran Scott Johnston, who is looking to reorient the business around its core customer base of software developers.

The Docker Business subscription service, and the revised Docker Desktop product, both reflect those new goals: Docker Business offers tools for managing and rapidly deploying secure Docker instances, and Docker Desktop requires paid usage for organizations with more than $10 million in annual revenue and 250 or more employees. But there’s also the Docker Personal subscription tier, for individuals and companies that fall below those thresholds, so end users still have access to many of Docker’s offerings.

Docker has other offerings suited to the changing times. Docker Hardened Images, available in both free and enterprise tiers, provide application images with smaller attack surfaces and checked software components for better security. And, in step with the AI revolution, the Docker MCP Catalog and Toolkit provide Dockerized versions of tools that give AI applications broader functionality (such as by allowing access to the file system), making it easier to deploy AI apps with less risk to the surrounding environment.