How does Docker work internally?
Now that we know what Docker is and what benefits it provides, let's go through the important details one by one.
What is a container? How do they work?
Before diving into the internals of Docker, we first need to understand the concept of containers. Simply put, a container is an isolated and lightweight runtime environment that encapsulates an application and its dependencies.
Unlike traditional virtualization, in which a complete operating system is simulated, containers share the kernel of the host system for more efficient resource utilization.
The following diagram shows the clear difference between containers, virtual machines, and containers:
2. Docker architecture
The core of Docker's architecture is a client-server model consisting of three key components: Docker client, Docker daemon, and Docker registry.
The Docker client serves as the main interface for users to interact with Docker, while the Docker daemon is responsible for building, running, and managing containers.
The Docker registry serves as a centralized warehouse for storing Docker images , which are the building blocks of containers. It's similar to NPM, which hosts Node.js packages, or the Maven repository, which stores many Java libraries.
The image below, from Whizlabs, shows how Docker works and how images are pulled from a registry during a Docker build:
3. Mirrors and layers
To really understand the inner workings of Docker, we need to explore the concept of Docker images.
An image is a read-only template that contains everything needed to run an application, including code, runtime, libraries, and dependencies.
Docker images are built using a layered file system, with each layer representing a change or modification made to the image. This layering mechanism allows efficient storage and sharing of common components across multiple images, reducing redundancy and improving performance.
The following figure is another diagram illustrating the relationship between Dockerfile, Docker image and Docker container:
4. Dockerfile
A Dockerfile is a blueprint for building a Docker image. It is a text file that specifies the instructions needed to create an image. These instructions include defining the base image, adding dependencies, copying files, exposing ports, and commands executed during image building.
Docker intelligently caches the middle layer according to the instructions of the Dockerfile, speeds up the subsequent construction process, and reduces redundancy.
Here's an example Dockerfile so you can see what's in there:
5. Container runtime
When a Docker image is run, it is instantiated as a container using the container runtime. Docker supports several container runtimes, of which Docker Engine (using a default runtime called runc) is the most commonly used.
The container runtime creates an isolated environment, sets up a namespace, allocates resources, manages the network, and controls access to system resources to ensure isolation between containers and the host system.
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6. Container Orchestration and Networking
Docker's flexibility is not limited to running a single container. It provides powerful orchestration tools such as Docker Swarm and Kubernetes to manage containerized applications at scale.
These tools can deploy, scale, and load balance containers across a cluster, ensuring high availability and fault tolerance.
Docker also provides networking capabilities, allowing containers to communicate with each other and the outside world through virtual networks, ports, and routes.
The following diagram shows how containers are used at scale:
Summarize
In this article, we took a deep dive into the inner workings of Docker. We learned about the concept of containers, as well as Docker's architecture and key components. We also explore the concepts of Docker images, Dockerfiles, and container runtimes, and briefly introduce container orchestration and networking.
By gaining a deep understanding of Docker's internals, you can better understand containerization and use and manage Docker more effectively. This is vital knowledge for developers, DevOps engineers, and system administrators.