Docker - 1

 

Comprehensive Overview of Docker

Introduction

Docker is a powerful platform designed to make it easier to create, deploy, and run applications by using containers. Containers allow a developer to package up an application with all of the parts it needs, such as libraries and other dependencies, and ship it all out as one package.

Docker Architecture

Docker Engine The Docker Engine is a client-server application with three major components:

• A server which is a type of long-running program called a daemon process (the dockerd command).
• A REST API which specifies interfaces that programs can use to talk to the daemon and instruct it what to do.
• A command-line interface (CLI) client (docker command).

Docker Daemon The daemon (dockerd) listens for Docker API requests and manages Docker objects such as images, containers, networks, and volumes.

Docker Client The Docker client (docker) is the primary way that many Docker users interact with Docker. When you use commands such as docker run, the client sends these commands to dockerd, which carries them out.

Docker Registries A Docker registry stores Docker images. Docker Hub is a public registry that anyone can use, and Docker is configured to look for images on Docker Hub by default. You can even run your own private registry.

Docker Objects

• Images
• Containers
• Networks
• Volumes

Key Docker Concepts

Images An image is a read-only template with instructions for creating a Docker container. Often, an image is based on another image, with some additional customization.

Containers A container is a runnable instance of an image. You can create, start, stop, move, or delete a container using the Docker API or CLI. You can connect a container to one or more networks, attach storage to it, or even create a new image based on its current state.

Services In a distributed application, different pieces of the app are called “services”. For example, a database and a web front end are separate services. In Docker, these services can be defined using a docker-compose.yml file.

Using Docker

Installation Installation of Docker varies based on the platform. Docker provides packages that easily configure the Docker environment on macOS, Windows, and Linux.

Basic Commands

• docker run creates and starts a container in one operation.
• docker stop stops a running container.
• docker build creates an image from a Dockerfile.
• docker pull pulls an image or a repository from a registry.
• docker push pushes an image or a repository to a registry.
• docker export exports a container’s filesystem as a tar archive.
• docker exec runs a command in a run-time container.

Dockerfiles A Dockerfile is a script comprised of various commands and arguments listed successively to automatically perform actions on a base image. These actions create a new image and include it capable of launching new containers that will operate as defined by the Dockerfile.

Example Dockerfile:

# Use an official Python runtime as a parent image
FROM python:3.7-slim
# Set the working directory to /app
WORKDIR /app
# Copy the current directory contents into the container at /app
ADD . /app
# Install any needed packages specified in requirements.txt
RUN pip install --trusted-host pypi.python.org -r requirements.txt
# Make port 80 available to the world outside this container
EXPOSE 80
# Define environment variable
ENV NAME World
# Run app.py when the container launches
CMD ["python", "app.py"]

Networking in Docker Docker’s networking subsystem is pluggable, using drivers. Several drivers exist by default, and provide core networking functionality:

• bridge: The default network driver.
• host: For standalone containers, remove network isolation between the container and the Docker host.
• overlay: Connect multiple Docker daemons together and enable swarm services to communicate with each other.
• macvlan: Assign a MAC address to a container, making it appear as a physical device on your network.

Docker Best Practices

• Use a .dockerignore file to exclude files that are not necessary for building a Docker image, such as logs, cache files, and other artifacts.
• Minimize the number of layers within images. Try to organize commands and their dependencies together.
• Use multi-stage builds to keep images small in size.
• Leverage build cache.
• Use environment variables for passing secrets or other variables that can change between deployments.

Advanced Topics

• Docker Compose: Tool for defining and running multi-container Docker applications.
• Docker Swarm: Native clustering functionality for Docker containers, which turns a group of Docker engines into a single virtual Docker engine.
• Kubernetes Integration: Docker can be integrated with Kubernetes to manage containers across a cluster.