What is Docker?

 

Docker:

Docker is an open-source platform for building, deploying, and managing containerized applications. Let's learn about containers, how they compare to VMs, and why Docker is so widely adopted and used.

The main difference lies in their architecture, demonstrated below.

Virtual machines have host OS and the guest OS inside each VM. Guest OS can be any OS, like Linux or Windows, irrespective of host OS. In contrast, Docker containers host on a single physical server with a host OS, which shares among them. Sharing the host OS between containers makes them light and decreases the boot time. Docker containers are considered suitable to run multiple applications over a single OS kernel; whereas, virtual machines are needed if the applications or services required to run on different OS. 

"https://www.docker.com/" is the official website of Docker.

Features of Docker:

> Docker has the ability to reduce the size of development by providing a smaller footprint of the operating system via containers.

> With containers, it becomes easier for teams across different units, such as development, QA and Operations to work seamlessly across applications.

> You can deploy Docker containers anywhere, on any physical and virtual machines and even on the cloud.

> Since Docker containers are pretty lightweight, they are very easily scalable.

Docker tools and terms:

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 commands 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.

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.

Docker Hub: Docker Hub is the public repository of Docker images that calls itself the “world’s largest library and community for container images.” It holds over 100,000 container images sourced from commercial software vendors, open-source projects, and individual developers.

Docker deployment and orchestration:

Docker Engine: If you’re running only a few containers, it’s fairly simple to manage your application within Docker Engine itself.

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.

Docker architecture:

Docker uses a client-server architecture. The Docker client talks to the Docker daemon, which does the heavy lifting of building, running, and distributing your Docker containers. The Docker client and daemon can run on the same system, or you can connect a Docker client to a remote Docker daemon. The Docker client and daemon communicate using a REST API, over UNIX sockets or a network interface. Another Docker client is Docker Compose, that lets you work with applications consisting of a set of containers.

Docker daemon:

The Docker daemon (dockerd) listens for Docker API requests and manages Docker objects such as images, containers, networks, and volumes. A daemon can also communicate with other daemons to manage Docker services.

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. The docker command uses the Docker API. The Docker client can communicate with more than one daemon.

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.

Installing Docker Engine on CentOS:

> Uninstall the old versions of docker if already installed on the machine. Older versions of Docker were called "docker" or "docker-engine".

$ sudo yum remove docker \
                  docker-client \
                  docker-client-latest \
                  docker-common \
                  docker-latest \
                  docker-latest-logrotate \
                  docker-logrotate \
                  docker-engine

The latest version of the Docker Engine package is now called "docker-ce".

> Setup the yum repository for docker.

$ sudo yum install -y yum-utils

$ sudo yum-config-manager \
    --add-repo \
    https://download.docker.com/linux/centos/docker-ce.repo

> Install the latest version of Docker Engine and containerd.

$ sudo yum install docker-ce docker-ce-cli containerd.io

Docker is installed but not started. The "docker" group is created, but no users are added to the group.

> To start Docker run the below command.

$ sudo systemctl start docker

> To verify Docker version installed run the below command.

$ sudo docker version

> To get Docker info run the below command.

$ sudo docker info

> We can verify that Docker Engine is installed correctly by running the hello-world image.

$ sudo docker run hello-world

> To use Docker as a non-root user, you should add your user to the “docker” group using below command.

$ sudo usermod -aG docker <your-user>

Remember to log out and back in for this to take effect!