Docker Architecture

Last Updated: 11th January, 2024

This tutorial delves into the architecture of Docker which contains client-server model of Docker, its key components like Docker Client, Docker Daemon, Docker Registries, and Docker Objects, and how they interact to enable efficient creation, deployment, and management of applications in a containerized environment. It also covers Docker Swarm, a built-in orchestration tool in Docker Engine for managing a cluster of Docker engines. This tutorial provides a firm foundation for understanding Docker's architecture, making it easier to harness Docker's full potential for building, shipping, and running applications consistently and efficiently.

What is Docker Architecture?

The Docker architecture is based on a client-server model, where the Docker client communicates with the Docker daemon to handle tasks such as constructing, executing, and distributing Docker containers. The Docker client and daemon can exist on the same machine or the client can interact with a remote Docker daemon. They communicate via a REST API, either through UNIX sockets or a network interface. Docker Compose is another Docker client that allows you to manage applications composed of multiple containers.

Docker Architecture Diagram

Docker Architecture Components

Docker's architecture consists of several key components that enable the creation, deployment, and management of applications in a containerized environment as depicted in above Docker Architecture Diagram.

Docker Client

The Docker client is the primary interface that users interact with when using Docker. It's a command-line tool that communicates with the Docker daemon to execute Docker commands. The Docker client can run on the same system as the Docker daemon, or it can connect to a remote Docker daemon.

When you issue commands such as docker run, the Docker client sends these commands to the Docker daemon (dockerd), which executes them. The Docker client uses the Docker API to send these commands. Importantly, the Docker client can communicate with more than one Docker daemon, which allows it to manage Docker services across different machines or environments.

Docker Daemon

The Docker daemon, also known as dockerd, is a persistent background process that manages Docker containers and handles Docker objects such as images, containers, networks, and volumes. It listens for Docker API requests and performs actions based on these requests. The Docker daemon can also communicate with other daemons to manage Docker services.

Customizing the Docker daemon is achieved through a JSON configuration file. To configure the Docker daemon via this method, you create a file at /etc/docker/daemon.json on Linux or C:\\\\ProgramData\\\\docker\\\\config\\\\daemon.json on Windows. This file empowers you to set options such as running the Docker daemon in debug mode, implementing TLS, and specifying an IP address and port for traffic routing.

All Docker-related data, encompassing containers, images, volumes, service definitions, and secrets, is stored in a singular directory by the Docker daemon. If you wish to designate an alternative directory, you can achieve this by configuring the Docker daemon through the data-root configuration option.

The Docker daemon can be run on any host operating system that supports Docker, although it currently only runs on Linux due to its dependency on certain Linux kernel features. However, there are ways to run Docker on MacOS and Windows.

Docker Registries

Docker registries are essentially libraries of images and act as repositories where Docker images are stored. They allow users to distribute and share Docker images across different environments. Docker maintains a public registry known as Docker Hub, but there are also several other registries provided by various vendors such as Azure Container Registry, Amazon Elastic Container Registry, and Google Container Registry.

Docker Hub is the world's largest container registry and provides both public and private repositories for Docker images. When you push an image to Docker Hub, it gets stored in a repository associated with your Docker Hub account. Other users can then pull these images from Docker Hub to their local machines or any other environment.

Here's an example of how to push an image to Docker Hub:

# Tag the image with your Docker Hub username and the name of the repository
docker tag my-image:latest myusername/myrepository:latest

# Push the image to Docker Hub
docker push myusername/myrepository:latest

And here's how to pull an image from Docker Hub:

docker pull myusername/myrepository:latest

Private Docker registries can be hosted on-premises or in the cloud. These are recommended when your images must not be shared publicly due to confidentiality, or when you want to minimize network latency between your images and your chosen deployment environment.

It's also possible to set up your own private Docker registry. This could be useful if you want to keep your Docker images completely private, or if you need to store large amounts of images that exceed the limits of Docker Hub.

Docker Objects

Docker operates on several key objects that are essential to its functioning. These include Images, Containers, Networks, Volumes, and Plugins.

Docker Images

Docker Image is a lightweight, stand-alone, and executable software package that contains all the essentials to run software like code, runtime, libraries, environment variables, and system tools. Images are created using a Dockerfile, which defines the configuration and setup of the containerized application. Docker images consist of multiple layers. When you download a Docker image, it is kept on the server until you manually remove it.

Docker Containers

Docker Containers are instances of Docker images. They create isolated and independent environments for applications and their dependencies. Containers ensure consistent and reproducible execution, regardless of the underlying infrastructure or operating system.

Docker Volumes

Docker Volumes are a method of storing data generated or used by a Docker container. Docker manages volumes, which are separate from the container’s filesystem, making them less prone to data loss and allowing for more efficient data sharing between containers. Volumes can be used to store database files, application configurations, and user-uploaded content, among other things.

Docker Networks

Docker Networks provide networking stack to containers for communication. Each Docker network corresponds to a network namespace. Multiple containers can share the same network, enabling them to communicate with each other.

Docker Plugins

Docker Plugins are out-of-process extensions which add capabilities to the Docker Engine. Plugins are available for Networking, Storage and Authorization purposes.

Docker Swarm Architecture

Docker Swarm is a container orchestration tool built into Docker Engine. It allows you to manage a cluster of Docker engines, also known as a swarm. Docker Swarm uses the Docker CLI for creating a swarm, deploying application services, and managing swarm behavior.

Docker Swarm Architecture

In Docker Swarm, there are two types of nodes: Manager nodes and Worker nodes:

• Manager Nodes: These nodes act as the 'brain' of the cluster. They're responsible for orchestrating and managing cluster operations. They utilize the Raft Consensus Algorithm to decide task assignments and maintain the swarm's state. Manager nodes can also run services as worker nodes by default but can be configured to carry out manager tasks exclusively.
• Worker Nodes: These nodes function as the 'muscles' of the cluster, receiving and executing tasks as assigned by the Manager nodes.

The architecture and working of Docker Swarm are simple: you declare the applications as stacks of services and let Docker handle the rest. The key components of a Docker Swarm are Docker Nodes, Docker Services, and Docker Tasks. Applications are deployed by submitting a service definition to a manager node. These service definitions detail the ideal state of the service, including factors such as the number of replicas, network and storage resource allocation, external-facing ports, and container images.

Once the manager node receives this information, it distributes the running containers (tasks) to worker nodes. After that, Docker does all the work to maintain the desired state. For example, if a worker node becomes inaccessible, Docker will allocate that node’s tasks to other healthy nodes.

When a service needs to be accessible from requests outside the network, the manager node intervenes. It assigns a specific port (the publishedPort, within the range of 30000 to 32767) and, if an external host makes a connection request to this port on any swarm node, the swarm's routing mesh redirects it to the appropriate task. The manager node employs an ingress load balancer to distribute these incoming requests among the service’s replicas.

Docker Swarm has several features that make it a powerful tool for managing and scaling your applications:

• Decentralized design: Docker Engine handles differentiation between node roles at runtime, meaning you can build an entire swarm from a single disk image.
• Declarative service model: You can declare your desired state instead of having to manage everything manually. All your resources, services, labeling or constraints can be declared in a YAML file.
• Scaling: Docker Swarm can handle provisioning and manages where containers get deployed. If you have a cluster of several nodes, it will balance out your workloads across multiple nodes within your cluster. This is great for high availability and scaling.
• Load Balancing: Docker Swarm also load balances traffic across multiple containers.
• Security: The communication between the manager and worker nodes is highly secure within the swarm.
• Rollback: Docker Swarm allows you to roll back environments to previous safe environments.

Conclusion

In conclusion, Docker's architecture is a key component in facilitating the creation, deployment, and management of applications in a containerized environment. It consists of several key components including the Docker client, Docker daemon, Docker registries, and Docker objects, each of which play a significant role in the overall functionality of Docker.

The Docker client is the primary user interface for Docker, allowing users to execute Docker commands and interact with the Docker daemon. The Docker daemon is a persistent background process that handles the management of Docker containers and Docker objects. Docker registries act as repositories for Docker images, facilitating the distribution and sharing of these images across different environments. Docker objects include Docker images, containers, networks, volumes, and plugins, which collectively enable the operation of Docker.

In addition to these components, Docker provides Docker Swarm, a container orchestration tool that allows management of a cluster of Docker engines. Docker Swarm provides features such as decentralized design, declarative service model, scaling, load balancing, security, and rollback, making it a powerful tool for managing and scaling applications.

Understanding Docker's architecture allows developers and system administrators to effectively utilize Docker's containerization technology, thereby streamlining application deployment, improving resource utilization, and enhancing software development and operations workflows. Docker's architecture is not only pivotal in the world of containerization but also in shaping the future of software development and deployment.

Key Takeaways

1. Docker's architecture is based on a client-server model where the Docker client interacts with the Docker daemon to manage tasks like building, running, and distributing Docker containers.

2. Docker Client is the primary user interface that sends commands to the Docker daemon (dockerd) using Docker API.

3. Docker Daemon (dockerd) is a persistent background process that manages Docker containers and handles Docker objects like images, containers, networks, and volumes.

4. Docker Registries are repositories where Docker images are stored, allowing users to distribute and share Docker images across different environments. Docker Hub is the public registry maintained by Docker, but users can also set up their own private registries.

5. Docker Objects include Docker images, containers, networks, volumes, and plugins. Docker Images are lightweight, standalone, executable software packages with all essentials to run software. Docker Containers are instances of Docker images that create isolated and independent environments for applications and their dependencies.

6. Docker Swarm is a container orchestration tool built into Docker Engine. It allows managing a cluster of Docker engines, known as a swarm. Docker Swarm uses the Docker CLI for creating a swarm, deploying application services, and managing swarm behavior.

7. Docker Swarm architecture consists of Manager nodes that orchestrate and manage cluster operations, and Worker nodes that execute tasks assigned by the Manager nodes.

8. Docker's architecture and components enable the efficient creation, deployment, and management of applications in a containerized environment, making it a crucial tool in the world of containerization and DevOps.