Docker is a set of products that includes platform as a service (PaaS) options delivers software in packages called containers using OS-level virtualization. Containers are independent and isolated from each other and bundle their software, libraries, and configuration files but can communicate through well-defined channels. Since all the containers share the service provided by the single operating system (OS) kernel, they can only use fewer resources than virtual machines. The services that are provided have both free and premium tiers. Docker engine is the container that hosts the software. 

Docker packages an application with dependencies in a virtual container that can execute on Linux, Windows, or macOS computers. This helps the container run on different locations, like on-premise, in a public cloud, or private cloud. Docker uses resource isolation features when working on Linux kernel and a union-capable file system to enable containers to run in a single instance on Linux. This avoids the overhead of starting and maintaining virtual machines. On macOS, Docker uses a Linux virtual machine to run containers. 

Docker containers are light-weighted. So, a single server or a virtual machine can run several containers in parallel. A typical Docker use case can run eight containers per host, and many organizations run 18 or more per host. 

The Linux kernel primarily isolates an application's view of the environment while supporting namespaces. This includes process trees, networks, user IDs, and mounted file systems. Kernel's cgroups give resource limiting for memory and CPU. Docker 0.9 version and above includes its components called libcontainer to use virtualization capabilities supported by Linux kernel and abstracted visualization interfaces through libvirt, LXC, and systemd-nspawn.

Docker is capable of implementing high-level API to support lightweight containers that run processes in isolation. The containers are:

• Standard processes.

• Hence making it possible to use kernel features to monitor executions, for example.

• The use of tools such as strace to observe and intercede with the system calls. 

The Docker software as a service has three components. They are:

• Software: The docked is the Docker daemon is a continuous process managing Docker containers and handling container objects. It listens to requests sent through the Docker Engine API. Docker is a client program that provides a common-line interface (CLI) to allow users to communicate with Docker daemons. 

• Objects: The objects in Docker are various entities used to assemble a software application. The main Docker object classes are images, containers, and services.

• Registries: A registry is a repository of Docker images. The clients connect to registries to pull images to upload and use them that are built. They can be public or private; Docker Hub and Docker Cloud are the two central public registries. Docker Hub is the default registry where images are looked for. These registries allow the creation of notifications based on events.  

The free Docker course offered by Great Learning will help you understand Docker, its services offered, features, registries, containers, and their working in a very easy and better way. You will be able to work with the application proficiently at the end of this session, y. You can also learn WordPress with Docker, Docker Swarm, Docker Orchestration, and many other courses for free. Enroll in Great Learning today to avail the course for free. You will also earn a certificate after completing the course successfully. Happy Learning!