‘Docker‘ is a Platform set as a Service (PaaS) product intended to deliver software in the form of packages, which are termed as containers. It uses OS-level virtualization standards, wherein the kernel allows multiple instances of isolated user-space such as containers, partitions, zones, virtual kernels, etc.
These behave as real computers simulating the way programs are run in them. On a regular operating system, we see the resources the computer program is running. In containers, we can only see the contents and the devices allocated to the container when the programs are run in it.
For several developers in the industry today, Docker is the accepted standard for developing and sharing containerized apps, across the desktop and the cloud. Containers are a standardized unit of software. Developers use it to isolate an app from its environment. Due to their lightweight characteristics, several docker containers (typically above eight containers per host) can be run on a single server or VM, simultaneously.
Docker is intended for developers to build lightweight and portable software containers. The container packages facilitate simplified application development, deployment, and testing. They initially made Docker for Linux OS. However, it now runs on a range of OSs: Linux, Windows, Datacenter, Cloud, Serverless, etc.
Docker, an open-source project, was launched in 2013. Docker Inc. developed it further to adopt cloud-native, which resulted in a trend towards containerization and microservices in the software domain. Docker released its ‘enterprise edition’ in 2017.
Modern software development faces the challenge of managing the applications on a common host or cluster. There is a need to separate the applications from one another to avoid interference and interoperability with regard to operation or maintenance. The association of the packages, libraries, binaries, and other software components required for an application to run is considered crucial for managing application development.
The conventional approach to address this problem has been the use of virtual machines (VMs). Virtual machines used to emulate a computer system.
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Those VMs retain applications on the same hardware, however separating them virtually. They aim to control conflicts arising between software components and minimize hardware resources. However, over a period of time, VMs have turned bulky, in terms of memory size as they require an indigenous OS.
As for the ever-increasing memory requirements, it has become challenging to maintain and upgrade the same as implementations may involve specialized hardware, software, or a combination of the two.
The following are some of the benefits of Docker Containers:
- Environment standardization – production environment can be shared collaboratively to develop, test, or maintain.
- Faster and consistent configuration – The image configuration eases unprivileged users to run quickly.
- Faster adoption of DevOps – Supports in the key automation phases: Deploy, Operate and Optimize.
- Safe disaster recovery – The reduced drag in the DR with minimal recovery time.
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Every container is run by a single operating system kernel, and therefore it uses fewer resources than virtual machines. Containers, densely packed on the same hardware, share the operating system’s underlying kernel with several applications, and yet isolate the execution environments from one another. Containers use far fewer resources than VMs and are fast.
Now, let’s see the operation in the context of Linux. A Docker packages an application and its dependencies in a virtual container and enables it to run on any Linux server in various configurations such as local premises, in a public or private cloud. Docker uses the shared resource of the kernel and saves on the VM overheads.
Containers are isolated from each other. They also bundle specific sets of software, libraries, and configuration files. They can communicate with one another using channels that are well-defined. Therefore, a Docker container is viewed as an open-source software development platform for creating containers and container-based applications.
It’s a category of cloud computing services that provides a platform for developers to create, run, and manipulate applications without bothering about the complex infrastructure requirements for developing and launching an app.
The Docker ‘run’ command is used to create and start a container on the local docker host. On the other hand, the Docker ‘service’ refers to one or more containers with the same configuration running under the Docker’s cloud mode. It is similar to a Docker run wherein a user spins up a container, forming a transposition.
As containers decouple applications from the OS, users get a clean and minimal OS to help run everything else in more than one isolated container. With the operating system abstracted from containers, it becomes possible to move a container across any server that supports the container runtime environment.
- Docker engine: It is a software that hosts the containers. It is the core of Docker and the underlying client-server engine responsible for creating and running the containers.
- Dockerfile: A Docker container starts with a Dockerfile. It is a text file written in a simplified syntax of the instructions used to build a Docker image (instance). Every docker container starts with a designated Dockerfile.
- Docker image: After the Dockerfile is written, Docker build is used to create a static image as specified by Dockerfile instructions. A Docker image is an example of a portable file, essentially a snapshot of a container. It contains a set of specifications that is run by the container for software components. The container images become containers at runtime in Docker containers, and images become containers when they run on Docker Engine.
- Docker run: The ‘run’ command is used to launch a container. Every container is a specific case of an image. Containers are instantaneous by nature, as they run on the fly and are capable of being stopped and restarted. You can run more than one container instance of an image simultaneously.
- Docker registry: It is a repository for Docker images. This is a place where registered clients can share images. You can download (called ‘pull’) images for use in the development or upload (‘push’) existing images. It also allows the creation of notifications based on the given events. A registry can be public or private in type. Docker Hub and Docker Cloud are examples of main public registries. Docker Hub is the default registry in which Docker looks up images.
- Docker hub: It is a SaaS repository used to share and manage containers. It shares official Docker images that have their source in open-source projects, software vendors, and unofficial images posted by users in the public domain.
Containers share operating systems, whereas VMs are designed to emulate virtual hardware. The Docker containers are apt for situations in which multiple applications need to be run over a single operating-system kernel.
You need VMs if you have applications or servers that have to be run on various operating system flavors. During the fast technological advancements of today’s scenarios, Docker, a lightweight resource, is a preferred alternative to virtual machines.
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