Last updated 10/06/2021
Nowadays each organization is turning into a software company, and there is a lot of stuff going around causing software development to happen at record speeds.
In the present cloud market, there are numerous DevOps instruments and approaches that are arising each day. Individuals have such countless alternatives to look over that competition has arrived at its pinnacle, which in turn has squeezed these software firms to constantly deliver products and services even better than their competitors.
As the cloud approach is strongly acquiring prominence, numerous organizations are beginning to accept cloud practices and ideas like containerization, which means DevOps tools like Docker are sought after. In this article, we will understand what exactly is Docker and see a few concepts related to Docker that are helpful for developers and architects.
Docker is an open platform for developing, shipping, and running applications. Docker enables you to separate your applications from your infrastructure so you can deliver software quickly. With Docker, you can manage your infrastructure in the same ways you manage your applications. By taking advantage of Docker’s methodologies for shipping, testing, and deploying code quickly, you can significantly reduce the delay between writing code and running it in production.
Docker provides the ability to package and run an application in a loosely isolated environment called a container. The isolation and security allow you to run many containers simultaneously on a given host. Containers are lightweight and contain everything needed to run the application, so you do not need to rely on what is currently installed on the host. You can easily share containers while you work, and be sure that everyone you share with gets the same container that works in the same way.
Docker provides tooling and a platform to manage the lifecycle of your containers:
Docker streamlines the development lifecycle by allowing developers to work in standardized environments using local containers which provide your applications and services. Containers are great for continuous integration and continuous delivery (CI/CD) workflows.
Consider the following example scenario:
Docker’s container-based platform allows for highly portable workloads. Docker containers can run on a developer’s local laptop, on physical or virtual machines in a data center, on cloud providers, or in a mixture of environments.
Docker’s portability and lightweight nature also make it easy to dynamically manage workloads, scaling up or tearing down applications and services as business needs dictate, in near real-time.
Docker is lightweight and fast. It provides a viable, cost-effective alternative to hypervisor-based virtual machines, so you can use more of your compute capacity to achieve your business goals. Docker is perfect for high-density environments and for small and medium deployments where you need to do more with fewer resources.
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, which lets you work with applications consisting of a set of containers.
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.
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.
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 private registry.
When you use the docker pull or docker run commands, the required images are pulled from your configured registry. When you use the docker push command, your image is pushed to your configured registry.
When you use Docker, you are creating and using images, containers, networks, volumes, plugins, and other objects. This section is a brief overview of some of those objects.
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. For example, you may build an image that is based on the ubuntu image but installs the Apache web server and your application, as well as the configuration details needed to make your application run.
You might create your images or you might only use those created by others and published in a registry. To build your image, you create a Dockerfile with a simple syntax for defining the steps needed to create the image and run it. Each instruction in a Dockerfile creates a layer in the image. When you change the Dockerfile and rebuild the image, only those layers which have changed are rebuilt. This is part of what makes images so lightweight, small, and fast when compared to other virtualization technologies.
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.
By default, a container is relatively well isolated from other containers and its host machine. You can control how isolated a container’s network, storage, or other underlying subsystems are from other containers or the host machine.
A container is defined by its image as well as any configuration options you provide to it when you create or start it. When a container is removed, any changes to its state that are not stored in persistent storage disappear.
Example docker run command
The following command runs an Ubuntu container, attaches interactively to your local command-line session, and runs /bin/bash.
$ docker run -i -t ubuntu /bin/bash
When you run this command, the following happens (assuming you are using the default registry configuration):
Obviously, Docker is important – and its prevalence in the job market is incredible. A recent search on LinkedIn revealed 28,941 jobs across the country are available. With cloud and Docker becoming more linked every day, that demand will only grow. Thus, you should know Docker to have a wonderful future in DevOps.
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