What is Docker?
In simple terms, the Docker platform is all about making it easier to create, deploy and run applications by using containers. Containers let developers package up an application with all of the necessary parts, such as libraries and other elements it is dependent upon, and then ship it all out as one package. By keeping an app and associated elements within the container, developers can be sure that the apps will run on any Linux machine no matter what kind of customized settings that machine might have, or how it might differ from the machine that was used for writing and testing the code. This is helpful for developers because it makes it easier to work on the app throughout its life cycle.
Docker is kind of like a virtual machine, but instead of creating a whole virtual operating system (OS), it lets applications take advantage of the same Linux kernel as the system they’re running on. That way, the app only has to be shipped with things that aren’t already on the host computer instead of a whole new OS. This means that apps are much smaller and perform significantly better than apps that are system dependent. It has a number of additional benefits.
Docker is an open platform for distributed applications for developers and system admins. It provides an integrated suite of capabilities for an infrastructure agnostic CaaS model. With Docker, IT operations teams are able to secure, provision and manage both infrastructure resources and base application content while developers are able to build and deploy their applications in a self-service manner.
- Open Source: Another key aspect of Docker is that it is completely open source. This means anyone can contribute to the platform and adapt and extend it to meet their own needs if they require extra features that don’t come with Docker right out of the box. All of this makes it an extremely convenient option for developers and system administrators.
- Low-Overhead: Because developers don’t have to provide a truly virtualized environment all the way down to the hardware level, they can keep overhead costs down by creating only the necessary libraries and OS components that make it run.
- Agile: Docker was built with speed and simplicity in mind and that’s part of the reason it has become so popular. Developers can now very simply package up any software and its dependencies into a container. They can use any language, version and tooling because they are packaged together into a container that, in effect, standardizes all elements without having to sacrifice anything.
- Portable: Docker also makes application containers completely portable in a totally new way. Developers can now ship apps from development to testing and production without breaking the code. Differences in the environment won’t have any effect on what is packaged inside the container. There’s also no need to change the app for it to work in production, which is great for IT operations teams because now they can avoid vendor lock in by moving apps across data centers.
- Control: Docker provides ultimate control over the apps as they move along the life cycle because the environment is standardized. This makes it a lot easier to answer questions about security, manageability and scale during this process. IT teams can customize the level of control and flexibility needed to keep service levels, performance and regulatory compliance in line for particular projects.
How Was It Created and How Did It Come About?
Apps used to be developed in a very different fashion. There were tons of private data centers where off-the-shelf software was being run and controlled by gigantic code bases that had to be updated once a year. With the development of the cloud, all of that changed. Also, now that companies worldwide are so dependent on software to connect with their customers, the software options are getting more and more customized.
As software continued to get more complex, with an expanding matrix of services, dependencies and infrastructure, it posed many challenges in reaching the end state of the app. That’s where Docker comes in.
In 2013, Docker was developed as a way to build, ship and run applications anywhere using containers. Software containers are a standard unit of software that isn’t affected by what code and dependencies are included within it. This helped developers and system administrators deal with the need to transport software across infrastructures and various environments without any modifications.
Docker was launched at PyCon Lightning Talk – The future of Linux Containers on March 13, 2013. The Docker mascot, Moby Dock, was created a few months later. In September, Docker and Red Hat announced a major alliance, introducing Fedora/RHEL compatibility. The company raised $15 million in Series B funding in January of 2014. In July 2014 Docker acquired Orchard (Fig) and in August 2014 the Docker Engine 1.2 was launched. In September 2014 they closed a $40 million Series C funding and by December 31, 2014, Docker had reached 100 million container downloads. In April 2015, they secured another $95 million in Series D funding and reached 300 million container downloads.
How Does It Work?
Docker is a Container as a Service (CaaS). To understand how it works, it’s important to first look at what a Linux container is.
In a normal virtualized environment, virtual machines run on top of a physical machine with the aid of a hypervisor (e.g. Xen, Hyper-V). Containers run on user space on top of an operating system’s kernel. Each container has its own isolated user space, and it’s possible to run many different containers on one host. Containers are isolated in a host using two Linux kernel features: Namespaces and Control Groups.
There are six namespaces in Linux and they allow a container to have its own network interfaces, IP address, etc. The resources that a container uses are managed by control groups, which allow you to limit the amount of CPU and memory resources a container should use.
Docker is a container engine that uses the Linux Kernel features to make containers on top of an OS and automates app deployment on the container. It provides a lightweight environment to run app code in order to create a more efficient workflow for moving your app through the life cycle. It runs on a client-server architecture. The Docker Daemon is responsible for all the actions related to the containers, and this daemon gets the commands from the Docker client through cli or REST APIs.
The containers are built from images, and these images can be configured with apps and used as a template for creating containers. They are organized in a layer, and every change in an image is added as a layer on top of it. The Docker registry is where Docker images are stored, and developers use a public or private registry to build and share images with their teams. The Docker-hosted registry service is called Docker Hub, and it allows you to upload and download images from a central location.
Once you have your images, you can create a container, which is a writable layer of the image. The image tells Docker what the container holds, what process to run when the container is launched and other configuration data. Once the container is running, you can manage it, interact with the app and then stop and remove the container when you’re done. It makes it simple to work with the app without having to alter the code.
Why Should a Developer Care?
Docker is perfect for helping developers with the development cycle. It lets you develop on local containers that have your apps and services, and can then integrate into a continuous integration and deployment workflow. Basically, it can make a developer’s life much easier. It’s especially helpful for the following reasons:
Docker makes it easy to keep workloads highly portable. The containers can run on a developer’s local host, as well as on physical or virtual machines or in the cloud. It makes managing workloads much simpler, as you can use it to scale up or tear down apps and services easily and nearly in real time.
Higher Density and More Workloads
Docker is a lightweight and cost-effective alternative to hypervisor-based virtual machines, which is great for high density environments. It’s also useful for small and medium deployments, where you want to get more out of the resources you already have.
Key Vendors and Supporters Behind Docker
The Docker project relies on community support channels like forums, IRC and StackOverflow. Docker has received contributions from many big organizations, including:
- Project Atomic
Docker is supported by many cloud vendors, including:
- Red Hat
Stay tuned for our next installment, where we will dig even deeper into Docker and its capabilities. In the meanwhile, read this blog post to learn how AppDynamics provides complete visibility into Docker Containers.
This blog was originally posted at AppDynamics Blog.