Introduction to Docker
Docker is an open-source DevOps tool designed to manage the containers. Docker allows us to build our application in a container with all the pre-requisites like libraries, binaries, and dependencies to run the application and run it anywhere. Moreover, containerization technology offers functionalities and benefits like VM, including isolation of applications and cost-effective solutions.
DevOps has brought in a fundamental change in how the Development, Operations, and Testing teams interact to ensure faster, reliable, and secure software delivery. The build release process for application development is automated through DevOps, resulting in operational efficiency, improved code quality, and faster delivery time with the CI/CD pipelines. DevOps as a single team is based on the rule that developers and operations work together in a cloud infrastructure communicating in the same language.
To learn more about DevOps Lifecycle Processes, read this blog.
Why do we need Docker?
- Light Weight – Docker containers are lightweight as they don’t carry the load of an Operating system and hypervisors.
- Portability – Applications can be bundled and deployed into multiple developments, testing, or production environments.
- Resource Optimization – Docker helps you save resources on your hardware.
- Delivery and Scalability – Since Docker uses fewer resources and is a lightweight container, it can be deployed very fast and easily scalable.
- With Docker, you can run multiple operating systems on a single host.
Containers are commonly known as self-contained units of software that can be transferred from server to server, from your laptop to VM in the cloud because Docker is isolated on the process-level and has its file system.”
Docker helps developers deploy, move, replicate, and take backup of their workload very quickly since we can reuse Docker images, which helps us make our workloads easily movable and more flexible than previous methods.
When using Virtual Machines (VM), this could be done by putting the applications separately while running on the same physical hardware. But VMs require their OS, which means they are large, take more time to startup, and are difficult to move, maintain, and upgrade. Instead, we can isolate the execution environments by sharing the same underlying OS kernel when it comes to containers.
Docker comprises different components:
- Docker Client manages different docker components like containers, images, storage volumes, and networks by instructing them to perform various operations like build, pull, create, run, stop, and restart. Docker client communicates with docker host where Docker daemon(dockerd) is present, i.e., Docker Host. It is not required that the docker client be current at the Docker host. Communication between the Docker client and the docker daemon uses REST API over UNIX sockets and Network Interfaces.
- Docker Engine – It is called the core of Docker. It is the underlying server where the docker daemon is running and helps create and manage containers and other resources. It runs a daemon process called dockerd, which manages the containers and API used to interact with the Docker daemon and a CLI.
- Docker Registry – It is known as Docker Hub, which is used to manage the Docker images. We can push and pull the docker images using the docker hub, which means downloading and uploading the docker images.
There are two types of Docker Registry:
a. Docker Hub is publicly accessible over the public internet and maintained by Docker company in the cloud. We can pull the images which are publicly accessible or push the images to the Docker hub to be available online. We can make our images accessible to everyone by making them public, and if we want them to be accessible to us, we can make them private.
b. Local Docker Hub – It is accessible only within the Organization. If we pull the images, it will first check for that image in the local docker hub. If it is not present, it will download that image from the Online Docker Hub.
- Docker Images – It is very similar to snapshot of VMs. Docker images are portable. These images are read-only and executable files that consist of instructions to create a Docker container and its software specifications. You can create a docker image using an existing container in which changes are done based on our requirements. It can also be made from Dockerfile, with instructions in the file.
- Dockerfile – Every container has a Dockerfile with it, using which a Docker container gets created. This text file provides instructions to build a Docker image, which includes OS, libraries, environment variables, network ports, and other components required to run the image.
Using the Docker network, we can make communication between the docker containers. There is the following Docker network that can be used in our Docker environment:
- Bridge – The default network driver gets automatically attached to the container while creating it when no network is specified.
- Host – It can attach the container to the Docker host network directly. We can have a standalone container using the host network.
- None – This network is used for disabling the networking for the containers. Docker containers will not get an IP address when this network is being used.
- Overlay – It is an ingress network in Docker. It is mainly used to connect docker containers hosted on the different docker hosts.
- Macvlan – This network driver is used for assigning a MAC address to the containers. So, whenever a container gets attached to a Macvlan network driver, it looks like a physical network is attached to it. Macvlan can be used for the application which requires a physical network and not a virtual network.
There are three types of storage in Docker:
- Volumes – Volumes can be stored as a part of the host filesystem, which Docker will manage. The location of volume on host filesystem is /var/lib/docker/volumes/ on Linux. (Can be different on different OS) This filesystem cannot be modified by the processes which do not belong to Docker. Data can easily persist in Docker by using Volumes.
- tmpfs mounts – These are stored in the host machine’s memory and cannot be written on the host filesystem.
- Bind mounts – These can be stored anywhere on the host machine. These might be some critical system-related files or directories. It can be modified by any Docker or Non-Docker related processes.
In today’s blog, we have seen components of the docker container and its advantages over a VM. Also, Docker, an essential part of DevOps, is a tool built and designed for developers and system engineers. The developer can focus more on the application code and development rather than the underlying environment where it will run. Every cloud provider like AWS, Azure, and GCP has their respective managed solutions for containers to help set up managed container services.
By leveraging Docker containers, you can solve the build/test/deploy problem in DevOps.
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- What is Docker Image?
A Docker image is a non-editable file containing libraries, source code, tools, and other items required to run applications. Docker images are sometimes known as snapshots because of their read-only nature. Snapshots are representations of an application and its VE (virtual environment) at a specific moment in time. Docker is one of the most well-known virtualization programs ever because of its consistency. Developers may explore and test programs in various scenarios thanks to the ability to stamp time.
- How can we connect to the docker container?
The best and recommended way to connect to the docker container is by using the Docker network. You can view in detail to use the docker network from here.
- Can we lose data when we exit from a docker container?
No. Whatever data your application writes on the disk will be preserved inside the docker container until it is deleted explicitly from the docker container. If the docker container exits, then also file system is conserved.