LINUX CONTAINERS

LINUX CONTAINERS

What is Linux Container: 

Linux containers have different approach than the Virtualization technology. Simply we can say this is OS level Virtualization, which means all containers run on top of one linux operating system. We can start containers on a hardware running machine or inside of running virtual machine. Each container run's as a fully isolated operating sysem.

In container virtualization rather than having an entire Operating System guest OS, containers isolate the guest but do not virtualize the hardware. For running containers one needs a patched kernel and user tools, the kernel provides process isolation and performs resource management. Thus all containers are running under the same kernel but they still have their own file system, processes, memory etc.

Linux based containers mainly involved with two concepts:

1. Namespaces

2. Cgroups ( Controll Groups)

There are total 6 types of Namespaces:

1. PID Name space.

2. Net Name space.

3. Ipc Name space

4. Mnt Name space

5. Uts Name space

6. User Name space.

Cgroups are already well known technology. We can find more detail about Cgroups here

There is an another important aspect of containers “Copy on write file system” This is also a well known technology in market:

Docker Architecture:

1. Docker Daemon

docker daemon is responsible for all the conainer operations. It runs on the docker host machine. We can't interact with this directly, all the instructions have to be sent through the docker.

2. Docker Client: It is the main interface to connect wit docker daemon. It can be install on the same machine or on the different machine.

3. Docker image: A Docker image is like a golden template. An image consists of OS (Ubuntu, centos etc.,) and applications installed on it.

4. Docker registry: That is a repository for docker images. It can be public or private. Public repository are maintained by docker is called docker hub. Users can upload and download images from there.

5. Docker Container: It is created on top of a docker image and it is completely isolated. Each container has it's own user space, Networking, and security settings associated with it.

INSTALLATION OF DOCKER

Docker can be installed on CentOS 7 without activating extra repository. Only we need to update the system:

# yum update -y

# yum install docker docker-registry

# systemctl start docker

# systemctl enable docker

# systemctl status docker

Now we can launch the container using following command:

# docker run ubuntu:14.04 /bin/echo ‘Welcome to the Container world’

1. docker run ==> is the command to run a container

2. ubuntu:14.04 ==> This is the image name. First it will find the image locally, if this is not there then pull it from docker hub account.

3. /bin/echo 'Welcome to the Container world' ==> This is the command, which will be executed inside if container.

Creating an interactive container

Previous command will create the container, but it was not interactive. To create the interactive container we can use following comamnd.

# docker run -it ubuntu:14.04 /bin/bash

1. -i ==> Make it interactive

2. -t ==> Open the terminal.

3. /bin/bash ==> Command which we want to execute inside the container.

Done...