A container is a standard unit of software that packages up code and all its dependencies so the application runs quickly and reliably from one computing environment to another. A Docker container image is a lightweight, standalone, executable package of software that includes everything needed to run an application: code, runtime, system tools, system libraries, and settings.
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.
Why do you need Docker?
We were facing compatibility issues in all these different services that were not compatible with the version of OS. We have to search for different versions and then try to run with these versions in different os and test all functionality with code coverage, automation test which is very time consuming & costly.
And sometimes need to upgrade any service which is very difficult to support in different os. Another point is — when new developers join to work on this project, need to set up all service which is also time consuming & costly because generally, they are also using different os.
So I needed something that could help us with the compatibility issue and something that will allow us to modify or change these components without affecting the other components and even modify the underlying operating systems as not required.
And that search landed me on Docker. By using docker we have relieved for the below points:
- Compatibility/Dependency Problem
- Long Set-up time
- Different environment (Dev/Prod/Test) Issue
Comparing Containers and Virtual Machines
Containers and virtual machines have similar resource isolation and allocation benefits but function differently because containers virtualize the operating system instead of hardware. Containers are more portable and efficient.
Uses a client-server architecture. 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 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.
The 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.
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.
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 which 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 own images or you might only use those created by others and published in a registry. To build your own 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 from 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.
Run Docker Containers
We can install docker desktop in windows or mac local pc by seeing the official installation documentation.
We can install docker engine in linux machine by seeing the official documentation.
After installing docker we can check that docker is running successfully by running below command & get the bellow's information if everything is okay:
docker — helpUsage: docker [OPTIONS] COMMANDA self-sufficient runtime for containersOptions:
--config string Location of client config files (default
-c, --context string Name of the context to use to connect to the
daemon (overrides DOCKER_HOST env var and
default context set with "docker context use")
-D, --debug Enable debug mode
-H, --host list Daemon socket(s) to connect to
-l, --log-level string Set the logging level
--tls Use TLS; implied by --tlsverify
--tlscacert string Trust certs signed only by this CA (default
--tlscert string Path to TLS certificate file (default
--tlskey string Path to TLS key file (default
--tlsverify Use TLS and verify the remote
-v, --version Print version information and quitManagement Commands:
builder Manage builds
buildx* Build with BuildKit (Docker Inc., v0.6.1-docker)
compose* Docker Compose (Docker Inc., v2.0.0-rc.3)
config Manage Docker configs
container Manage containers
context Manage contexts