Introduction to Docker

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Docker provides, Package Once Deploy Anywhere, or PODA.

Docker Concepts:

Docker simplifies software delivery of distributed applications in three ways:

Build:

Provides tools to create containerized applications. Developer package the application, its dependencies and infrastructure as read only templates. These are called the Docker image.

Ship:

 Allows to share these applications in a secure and collaborative manner. Docker images are stored, shared and manged in a Docker registry. Docker Hub is a publicly available registry. This is the default registry for all images.

 Run:

The ability to deploy,manage and scale these applications. Docker container is a runtime representation of an image. Container can be run, started, scaled, moved and deleted.

What is  Docker Client ?

The client is a Docker binary that accepts commands from the user and communicates back and forth with the Docker Engine.

What is Docker Host:

A machine, either physical or virtual, is identified to run the Docker Engine.

Configure docker client and host:


Configure Docker Client:

Docker client library is downloaded on a machine and configured to talk to this Docker Engine. The client and Docker Engine are located on the same machine. Docker engine could be on a different host in the network as well.

Client downloads or build an image

Client can pull a prebuilt image from the pre-configured registry using the pull command, create a new image using the build command, or run a container using the run command. 

Docker host downloads the image from the registry

Docker Engine checks to see if the image already exist on the host. If not, then it downloads the image from the registry.

Client runs the container

The new container can be created using the run command, which runs the container using the image definition.

Docker Images and Containers

Docker images are read only templates from which docker containers are launched. Each image consists of a series of  layers.Docker makes uses of union filesystem to combine these layers into a single image. Union filesystems allow files and directories of separate filesystem, known as branches, to be transparently overlaid, forming a single coherent filesystem.

One of the reasons Docker is so lightweight is because of these layers.

Docker images are built on Docker Engine, distributed using the registry and run as containers.



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