Docker is a platform for developing, shipping and running applications inside containers. Containers are lightweight, portable, and self-sufficient units that encapsulate an application and its dependencies. Docker uses containerization technology to make it easier to build, deploy, and manage applications.
There are 3 components in Docker architecture:
1. Docker client
The docker client talks to the Docker daemon.
2. Docker host
The Docker daemon listens for Docker API requests and manages Docker objects such as images, containers, networks, and volumes.
3. Docker registry
A Docker registry stores Docker images. Docker Hub is a public registry that anyone can use.
Let’s take the “docker run” command as an example.
- Docker pulls the image from the registry.
- Docker creates a new container.
- Docker allocates a read-write filesystem to the container.
- Docker creates a network interface to connect the container to the default network.
- Docker starts the container.
Here's an overview of how Docker works:
1. Containerization Technology:
Docker leverages containerization technology, which is based on OS-level virtualization. Containers are isolated environments that share the host OS kernel but have their own file systems and resources.
2. Docker Engine:
Docker is powered by the Docker Engine, which includes the Docker daemon (server) and the Docker client. The Docker daemon manages containers and images, while the Docker client communicates with the daemon, allowing users to interact with containers.
3. Images:
Docker images are the blueprints for containers. An image is a read-only template that includes an application, its dependencies, and runtime configurations. Images are stored in a repository and can be versioned.
4. Dockerfile:
To create an image, developers use a text file called a Dockerfile. This file specifies the base image, application code, dependencies, and configuration settings. Docker then builds an image based on the Dockerfile's instructions.
5. Containers:
Containers are instances of Docker images. They run in isolated environments, separate from the host system. Each container has its own file system, processes, and network, but shares the host OS kernel.
6. Isolation and Resource Control:
Docker provides process and resource isolation, ensuring that containers do not interfere with each other. Resource control features allow you to allocate CPU and memory limits to containers.
7. Docker Registry:
Docker images can be stored in a Docker Registry, such as Docker Hub, which is a public registry, or a private registry within an organization. This makes it easy to share and distribute images.
8. Docker Compose:
Docker Compose is a tool that allows you to define and manage multi-container applications. It uses a YAML file to specify the services, networks, and volumes required for the application.
9. Docker Networking:
Docker provides networking features to connect containers to each other and to external networks. Containers can be connected through bridges, overlay networks, or other networking modes.
10. Docker Orchestration:
For managing containerized applications at scale, Docker offers orchestration tools like Docker Swarm and Kubernetes. These tools automate the deployment, scaling, and management of containers across multiple hosts or nodes.
11. Portability and Consistency:
Docker containers are highly portable and consistent. An application and its environment are bundled together in a container image, ensuring that the application runs consistently across different environments, from a developer's laptop to a production server.
12. Security:
Docker implements security features like namespaces and control groups to isolate containers and restrict their access to the host system. Images can be scanned for vulnerabilities, and access control policies can be applied.
Docker simplifies application development and deployment by packaging applications and their dependencies in containers, allowing for easy testing, scaling, and distribution. It has become a popular tool for DevOps and application deployment, as it streamlines the process and provides a consistent environment for applications to run.
