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Basic Concepts

Container​

Containers are lightweight, portable, and self-sufficient units that include everything needed to run an application, such as code, runtime, operating system, libraries, and dependencies.

  • Isolation: Containers run in isolation from each other and from the host, ensuring that changes in one container do not affect other containers or the host operating system.
  • Efficiency: Since containers share the same host operating system, they are more resource-efficient than traditional virtual machines.

Image​

A Docker image is a static file that contains a filesystem and all the resources required to run a container. It serves as a blueprint for a container.

  • Layering: Images are built in layers, allowing for the reuse of layers and reducing the amount of data needed for storage and transmission.
  • Creation: Images are created from Dockerfiles, which contain a set of instructions on how to build the image.

Dockerfile​

A Dockerfile is a text file script that contains a set of instructions to build a Docker image. Each instruction in the Dockerfile adds a new layer to the image.

  • Example:
FROM python:3.8-slim
WORKDIR /app
COPY . /app
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

In the example above:

  • FROM specifies the base image.
  • WORKDIR sets the working directory inside the container.
  • COPY copies files from the host to the container.
  • RUN executes a command during the image build.
  • CMD specifies the default command to run when the container starts.

Docker Engine​

Docker Engine is the core of the Docker platform, consisting of several key components.

  • Components:
    • Docker Daemon: A background process that manages containers and images on the host.
    • REST API: An interface that allows communication with the daemon using HTTP.
    • CLI (Command Line Interface): A command-line tool that enables users to interact with Docker.

Docker Registry​

A Docker registry is a service for storing and distributing Docker images. The most well-known public registry is Docker Hub.

  • Public vs Private: Docker Hub offers both public and private image repositories. Organizations can also create their own private registries for internal use.

Volumes​

Volumes are a mechanism for managing data in containers. They allow persistent storage of data independent of the container's lifecycle.

  • Types: There are three main types of volumes:
    • Named Volumes: Managed by Docker and can be shared between containers.
    • Anonymous Volumes: Created dynamically without a name.
    • Host Volumes: Mapped directly to directories on the host.

Networks​

Docker offers various types of networks to connect containers to each other and to external networks.

  • Types:
    • Bridge Network: The default network that allows communication between containers on the same host.
    • Host Network: Uses the host's network without network isolation between the container and the host.
    • Overlay Network: Enables communication between containers on different hosts, useful in Docker Swarm clusters.