Friday, December 26, 2014


Docker is a very hot topic lately and i've gather some info as to what is docker and why using docker.

What is Linux Container

LXC (Linux Containers) is an operating system-level lightweight virtualization method for running multiple isolated Linux systems (containers) on a single control host.
The Linux kernel comprise cgroups for resource isolation (CPU, memory, block I/O, network, …) that does not require starting any virtual machines on an emulation of physical hardware, thus the performance of application in LXC is the same as if it’s on the host. It also provides namespace isolation to completely isolate applications’ view of operating environment.
You can run a single application within a container (an application container) whose namespace is isolated from the other processes on the system in a similar manner to a chroot jail. I.e. starting a container is like starting a normal process on the machine, except they have a stamp/flag to indicate it belongs to a container. It’s very similar to UID checking of a process, thus, it has very little extra code path when compared to normal process.
The main use of Linux Containers is to allow you to run a complete copy of Linux operating system in a container (a system container) without the overhead of running a level-2 hypervisor such as VirtualBox. But one thing hypervisors can do that containers can’t is to use different operating systems or kernels. So, for example, you can use Microsoft Azure to run both instances of Windows Server 2012 and SUSE Linux Enterprise Server, at the same time. All containers must use the same operating system and kernel.
It can bind-mount a directory with zero overhead to host or another container running on the same host. The directory can contain named pipes (FIFOs), UNIX sockets and memory-mapped files. It can also share the host’s network stack (by reusing its network namespace) with native performance.
“Ops” functions (backups, logging…) can be performed in separate containers. Application containers can run unchanged in various environments.

What is Docker

Docker is built on top of LXC to automates the deployment of application inside LXC, and provides the capability to package an application with its runtime dependencies into a container. Docker is not about containers but more about standardizing software unit, empowering infrastructure around such software unit. So we have a standard interface to deploy any “dockerized” application no matter if it’s in Java, C or nodejs.
It provides a Docker CLI command line tool for the lifecycle management of image-based containers. Docker works with the following fundamental components.
  • Container – an application sandbox. Each container is based on an image that holds necessary configuration data. When you launch a container from an image, a writable layer is added on top of this image. Every time you commit a container (using the docker commit command), a new image layer is added to store your changes.
  • Image – a static snapshot of the containers' configuration. Image is a read-only layer that is never modified, all changes are made in top-most writable layer, and can be saved only by creating a new image. Each image depends on one or more parent images.
  • Platform Image – an image that has no parent. Platform images define the runtime environment, packages and utilities necessary for containerized application to run. The platform image is read-only, so any changes are reflected in the copied images stacked on top of it.
  • Registry – a repository of images. Registries are public or private repositories that contain images available for download. Some registries allow users to upload images to make them available to others.
  • Dockerfile – a configuration file with build instructions for Docker images. Dockerfiles provide a way to automate, reuse, and share build procedures.
  • Docker Daemon – Docker running in daemon mode.

Why use Docker

Docker brings in an API for container management, an image format and a possibility to use a remote registry for sharing containers. This scheme benefits both developers and system administrators with advantages such as:
  • Rapid application deployment – containers include the minimal runtime requirements of the application, reducing their size and allowing them to be deployed quickly.
  • Portability across machines – an application and all its dependencies can be bundled into a single container that is independent from the host version of Linux kernel, platform distribution, or deployment model. This container can be transferred to another machine that runs Docker, and executed there without compatibility issues.
  • Version control and component reuse – you can track successive versions of a container, inspect differences, or roll-back to previous versions. Containers reuse components from the preceding layers, which makes them noticeably lightweight.
  • Sharing – you can use a remote repository to share your container with others. Red Hat provides a registry for this purpose, and it is also possible to configure your own private repository.
  • Lightweight footprint and minimal overhead – Docker images are typically very small, which facilitates rapid delivery and reduces the time to deploy new application containers.
  • Simplified maintenance – Docker reduces effort and risk of problems with application dependencies.
Docker also makes it possible to setup local development environments that are exactly like a live server, and developer can pack, ship and run the application as a lightweight, portable, self-sufficient LXC container that can virtually run anywhere.

Especially when deploying microservices, we’ll need an extremely fast and convenient way to deploy a lot of services and many times a day. Using virtual machine and configuration management like puppet or chef will take a very long time to just redeploy the application and maybe restart the VMs. Also, since VM are heavy, we’ll have to put multiple services in the same VM which make scaling less efficient as we’ll have to bring up all those services together, even though only one of them are needed. Putting them in an application container (each service has its own container) have the advantage to scale out a service easily, without the added overhead of starting a virtual machine.   


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