What is Docker

Docker is an open source platform for building, deploying, and managing containerized applications.

It enables developers to package applications into containers—standardized executable components combining application source code with the operating system (OS) libraries and dependencies required to run that code in any environment.

Developers can create containers without Docker, but the platform makes it easier, simpler, and safer to build, deploy and manage containers.

Docker is essentially a toolkit that enables developers to build, deploy, run, update, and stop containers using simple commands and work-saving automation through a single API.

Containers are made possible by process isolation and virtualization capabilities built into the Linux kernel.

Docker tools

DockerFile

Every Docker container starts with a simple text file containing instructions for how to build the Docker container image. DockerFile automates the process of Docker image creation. It’s essentially a list of command-line interface (CLI) instructions that Docker Engine will run in order to assemble the image.

Docker images

Docker images contain executable application source code as well as all the tools, libraries, and dependencies that the application code needs to run as a container.

When you run the Docker image, it becomes one instance (or multiple instances) of the container.

Multiple Docker images can be created from a single base image, and they’ll share the commonalities of their stack.

Docker images are made up of layers, and each layer corresponds to a version of the image. Whenever a developer makes changes to the image, a new top layer is created, and this top layer replaces the previous top layer as the current version of the image. Previous layers are saved for rollbacks or to be re-used in other projects.

Each time a container is created from a Docker image, yet another new layer called the container layer is created. Changes made to the container—such as the addition or deletion of files—are saved to the container layer only and exist only while the container is running.

This iterative image-creation process enables increased overall efficiency since multiple live container instances can run from just a single base image, and when they do so, they leverage a common stack.

Docker containers

Docker containers are the live, running instances of Docker images. While Docker images are read-only files, containers are live, ephemeral, executable content. Users can interact with them, and administrators can adjust their settings and conditions using docker commands.

Docker Hub

Docker Hub is the public repository of Docker images It includes images that have been produced by Docker, Inc., certified images belonging to the Docker Trusted Registry, and many thousands of other images.

All Docker Hub users can share their images at will. They can also download predefined base images from the Docker filesystem to use as a starting point for any containerization project.

Docker daemon

Docker daemon is a service running on your operating system This service creates and manages your Docker images for you using the commands from the client, acting as the control center of your Docker implementation.

Docker registry

A Docker registry is a scalable open-source storage and distribution system for docker images. The registry enables you to track image versions in repositories, using tagging for identification. This is accomplished using git, a version control tool.

Docker Compose

If you’re building an application out of processes in multiple containers that all reside on the same host, you can use Docker Compose to manage the application’s architecture. Docker Compose creates a YAML file that specifies which services are included in the application and can deploy and run containers with a single command. Using Docker Compose, you can also define persistent volumes for storage, specify base nodes, and document and configure service dependencies.

Kubernetes

To monitor and manage container lifecycles in more complex environments, you’ll need to turn to a container orchestration tool. While Docker includes its own orchestration tool (called Docker Swarm), most developers choose Kubernetes instead.

Kubernetes is an open-source container orchestration platform descended from a project developed for internal use at Google. Kubernetes schedules and automates tasks integral to the management of container-based architectures, including container deployment, updates, service discovery, storage provisioning, load balancing, health monitoring, and more. In addition, the open source ecosystem of tools for Kubernetes—including Istio and Knative—enables organizations to deploy a high-productivity Platform-as-a-Service (PaaS) for containerized applications and a faster on-ramp to serverless computing.