How Docker Works
Docker overview
Docker is an open platform for developing, shipping, and running applications. Docker enables you to separate your applications from your infrastructure so you can deliver software quickly. With Docker, you can manage your infrastructure in the same ways you manage your applications. By taking advantage of Docker’s methodologies for shipping, testing, and deploying code, you can significantly reduce the delay between writing code and running it in production.
Key Components
1. Developer: The person who writes the code and prepares it for deployment.
2. Client: The Docker client is a command-line tool that allows developers to interact with the Docker daemon. Common commands include:
– `docker build`: Builds an image from a Dockerfile.
– `docker pull`: Downloads an image from a Docker registry.
– `docker run`: Runs a container from an image.
– `docker push`: Uploads an image to a Docker registry.
3. Dockerfile: A script containing a series of instructions on how to build a Docker image. For example:
– `FROM postgres:latest`: Specifies the base image.
– `ENV POSTGRES_USER myuser`: Sets environment variables.
– `EXPOSE 5432`: Exposes a port for network connections.
4. Registry: A storage and distribution system for Docker images. Public registries like Docker Hub and private registries can be used to store and manage images.
– Images: These are templates used to create Docker containers. They are immutable and can include applications and their dependencies.
– Extensions & Plugins: Additional components that can be added to Docker images to extend their functionality.
5. Docker Host: The machine (physical or virtual) where the Docker daemon runs. This daemon is responsible for managing Docker containers and images.
6. Docker Daemon: A background service running on the Docker host that manages Docker images, containers, networks, and storage volumes.
7. Images: Once a Dockerfile is processed with the `docker build` command, it creates an image that can be stored in a registry and used to create containers.
8. Containers: Runtime instances of Docker images. Containers are isolated environments where applications run. They include everything needed to run an application: code, runtime, libraries, and configuration files.
Workflow
1. Building an Image:
– The developer writes a Dockerfile and uses the `docker build` command to create an image.
– This image contains the application and all its dependencies.
2. Pushing an Image:
– The built image can be pushed to a Docker registry using the `docker push` command.
– This makes the image available for others to pull and use.
3. Pulling an Image:
– Other developers or production systems can pull the image from the registry using the `docker pull` command.
– This fetches the image from the registry to the local Docker host.
4. Running a Container:
– The `docker run` command is used to create and start a container from an image.
– Containers are lightweight and start quickly, as they share the host system’s kernel.
About Author
