What is Docker?

An introduction to Docker, a new paradigm in developing and shipping applications.

By Karl Matthias and Sean Kane

May 10, 2016

Card catalog (source: Stuart Caie via Wikimedia Commons)

Docker was first introduced to the world—with no pre-announcement and little fanfare—by Solomon Hykes, founder and CEO of dotCloud, in a five-minute lightning talk at the Python Developers Conference in Santa Clara, California, on March 15, 2013. At the time of this announcement, only about 40 people outside dotCloud been given the opportunity to play with Docker.

Within a few weeks of this announcement, there was a surprising amount of press. The project was quickly open-sourced and made publicly available on GitHub, where anyone could download and contribute to the project. Over the next few months, more and more people in the industry started hearing about Docker and how it was going to revolutionize the way software was built, delivered, and run. And within a year, almost no one in the industry was unaware of Docker, but many were still unsure what it was exactly, and why people were so excited about.

Learn faster. Dig deeper. See farther.

Join the O'Reilly online learning platform. Get a free trial today and find answers on the fly, or master something new and useful.

Learn more

Docker is a tool that promises to easily encapsulate the process of creating a distributable artifact for any application, deploying it at scale into any environment, and streamlining the workflow and responsiveness of agile software organizations.

The Promise of Docker

While ostensibly viewed as a virtualization platform, Docker is far more than that. Docker’s domain spans a few crowded segments of the industry that include technologies like KVM, Xen, OpenStack, Mesos, Capistrano, Fabric, Ansible, Chef, Puppet, SaltStack, and so on. There is something very telling about the list of products that Docker competes with, and maybe you’ve spotted it already. For example, most engineers would not say that virtualization products compete with configuration management tools, yet both technologies are being disrupted by Docker. The technologies in that list are also generally acclaimed for their ability to improve productivity and that’s what is causing a great deal of the buzz. Docker sits right in the middle of some of the most enabling technologies of the last decade.

If you were to do a feature-by-feature comparison of Docker and the reigning champion in any of these areas, Docker would very likely look like a middling competitor. It’s stronger in some areas than others, but what Docker brings to the table is a feature set that crosses a broad range of workflow challenges. By combining the ease of application deployment tools like Capistrano and Fabric, with the ease of administrating virtualization systems, and then providing hooks that make workflow automation and orchestration easy to implement, Docker provides a very enabling feature set.

Lots of new technologies come and go, and a dose of skepticism about the newest rage is always healthy. Without digging deeper, it would be easy to dismiss Docker as just another technology that solves a few very specific problems for developers or operations teams. If you look at Docker as a virtualization or deployment technology alone, it might not seem very compelling. But Docker is much more than it seems on the surface.

It is hard and often expensive to get communication and processes right between teams of people, even in smaller organizations. Yet we live in a world where the communication of detailed information between teams is increasingly required to be successful. A tool that reduces the complexity of that communication while aiding in the production of more robust software would be a big win. And that’s exactly why Docker merits a deeper look. It’s no panacea, and implementing Docker well requires some thought, but Docker is a good approach to solving some real-world organizational problems and helping enable companies to ship better software faster. Delivering a well-designed Docker workflow can lead to happier technical teams and real money for the organization’s bottom line.

So where are companies feeling the most pain? Shipping software at the speed expected in today’s world is hard to do well, and as companies grow from one or two developers to many teams of developers, the burden of communication around shipping new releases becomes much heavier and harder to manage. Developers have to understand a lot of complexity about the environment they will be shipping software into, and production operations teams need to increasingly understand the internals of the software they ship. These are all generally good skills to work on because they lead to a better understanding of the environment as a whole and therefore encourage the designing of robust software, but these same skills are very difficult to scale effectively as an organization’s growth accelerates.

The details of each company’s environment often require a lot of communication that doesn’t directly build value in the teams involved. For example, requiring developers to ask an operations team for release 1.2.1 of a particular library slows them down and provides no direct business value to the company. If developers could simply upgrade the version of the library they use, write their code, test with the new version, and ship it, the delivery time would be measurably shortened. If operations people could upgrade software on the host system without having to coordinate with multiple teams of application developers, they could move faster. Docker helps to build a layer of isolation in software that reduces the burden of communication in the world of humans.

Beyond helping with communication issues, Docker is opinionated about software architecture in a way that encourages more robustly crafted applications. Its architectural philosophy centers around atomic or throwaway containers. During deployment, the whole running environment of the old application is thrown away with it. Nothing in the environment of the application will live longer than the application itself and that’s a simple idea with big repercussions. It means that applications are not likely to accidentally rely on artifacts left by a previous release. It means that ephemeral debugging changes are less likely to live on in future releases that picked them up from the local filesystem. And it means that applications are highly portable between servers because all state has to be included directly into the deployment artifact and be immutable, or sent to an external dependency like a database, cache, or file server.

This leads to applications that are not only more scalable, but more reliable. Instances of the application container can come and go with little repercussion on the uptime of the frontend site. These are proven architectural choices that have been successful for non-Docker applications, but the design choices included in Docker’s own design mean that Dockerized applications will follow these best practices by requirement and that’s a good thing.

Benefits of the Docker Workflow

It’s hard to cohesively group into categories all of the things Docker brings to the table. When implemented well, it benefits organizations, teams, developers, and operations engineers in a multitude of ways. It makes architectural decisions simpler because all applications essentially look the same on the outside from the hosting system’s perspective. It makes tooling easier to write and share between applications. Nothing in this world comes with benefits and no challenges, but Docker is surprisingly skewed toward the benefits. Here are some more of the things you get with Docker:

Packaging software in a way that leverages the skills developers already have.: Many companies have had to create positions for release and build engineers in order to manage all the knowledge and tooling required to create software packages for their supported platforms. Tools like rpm, mock, dpkg, and pbuilder can be complicated to use, and each one must be learned independently. Docker wraps up all your requirements together into one package that is defined in a single file.
Bundling application software and required OS filesystems together in a single standardized image format.: In the past, you typically needed to package not only your application, but many of the dependencies that it relied on, including libraries and daemons. However, you couldn’t ever ensure that 100 percent of the execution environment was identical. All of this made packaging difficult to master, and hard for many companies to accomplish reliably. Often someone running Scientific Linux would resort to trying to deploy a community package tested on Red Hat Linux, hoping that the package was close enough to what they needed. With Docker you deploy your application along with every single file required to run it. Docker’s layered images make this an efficient process that ensures that your application is running in the expected environment.
Using packaged artifacts to test and deliver the exact same artifact to all systems in all environments.: When developers commit changes to a version control system, a new Docker image can be built, which can go through the whole testing process and be deployed to production without any need to recompile or repackage at any step in the process.
Abstracting software applications from the hardware without sacrificing resources.: Traditional enterprise virtualization solutions like VMware are typically used when people need to create an abstraction layer between the physical hardware and the software applications that run on it, at the cost of resources. The hypervisors that manage the VMs and each VM’s running kernel use a percentage of the hardware system’s resources, which are then no longer available to the hosted applications. A container, on the other hand, is just another process that talks directly to the Linux kernel and therefore can utilize more resources, up until the system or quota-based limits are reached.

When Docker was first released, Linux containers had been around for quite a few years, and many of the other technologies that it is built on are not entirely new. However, Docker’s unique mix of strong architectural and workflow choices combine together into a whole that is much more powerful than the sum of its parts. Docker finally makes Linux containers, which have been around for more than a decade, approachable to the average technologist. It fits containers relatively easily into the existing workflow and processes of real companies. And the problems discussed above have been felt by so many people that interest in the Docker project has been accelerating faster than anyone could have reasonably expected.

In the first year, newcomers to the project were surprised to find out that Docker wasn’t already production-ready, but a steady stream of commits from the open source Docker community has moved the project forward at a very brisk pace. That pace seems to only pick up steam as time goes on. As Docker has now moved well into the 1.x release cycle, stability is good, production adoption is here, and many companies are looking to Docker as a solution to some of the serious complexity issues that they face in their application delivery processes .

What Docker Isn’t

Docker can be used to solve a wide breadth of challenges that other categories of tools have traditionally been enlisted to fix; however, Docker’s breadth of features often means that it lacks depth in specific functionality. For example, some organizations will find that they can completely remove their configuration management tool when they migrate to Docker, but the real power of Docker is that although it can replace some aspects of more traditional tools, it is usually compatible with them or even augmented by combining with them, as well. In the following list, we explore some of the tool categories that Docker doesn’t directly replace but that can often be used in conjunction to achieve great results:

Enterprise Virtualization Platform (VMware, KVM, etc.): A container is not a virtual machine in the traditional sense. Virtual machines contain a complete operating system, running on top of the host operating system. The biggest advantage is that it is easy to run many virtual machines with radically different operating systems on a single host. With containers, both the host and the containers share the same kernel. This means that containers utilize fewer system resources, but must be based on the same underlying operating system (i.e., Linux).
Cloud Platform (Openstack, CloudStack, etc.): Like Enterprise virtualization, the container workflow shares a lot of similarities on the surface with cloud platforms. Both are traditionally leveraged to allow applications to be horizontally scaled in response to changing demand. Docker, however, is not a cloud platform. It only handles deploying, running, and managing containers on pre-existing Docker hosts. It doesn’t allow you to create new host systems (instances), object stores, block storage, and the many other resources that are typically associated with a cloud platform.
Configuration Management (Puppet, Chef, etc.): Although Docker can significantly improve an organization’s ability to manage applications and their dependencies, it does not directly replace more traditional configuration management. Dockerfiles are used to define how a container should look at build time, but they do not manage the container’s ongoing state, and cannot be used to manage the Docker host system.
Deployment Framework (Capistrano, Fabric, etc.): Docker eases many aspects of deployment by creating self-contained container images that encapsulate all the dependencies of an application and can be deployed, in all environments, without changes. However, Docker can’t be used to automate a complex deployment process by itself. Other tools are usually still needed to stitch together the larger workflow automation.
Workload Management Tool (Mesos, Fleet, etc.): The Docker server does not have any internal concept of a cluster. Additional orchestration tools (including Docker’s own Swarm tool) must be used to coordinate work intelligently across a pool of Docker hosts, and track the current state of all the hosts and their resources, and keep an inventory of running containers.
Development Environment (Vagrant, etc.): Vagrant is a virtual machine management tool for developers that is often used to simulate server stacks that closely resemble the production environment in which an application is destined to be deployed. Among other things, Vagrant makes it easy to run Linux software on Mac OS X and Windows-based workstations. Since the Docker server only runs on Linux, Docker originally provided a tool called Boot2Docker to allow developers to quickly launch Linux-based Docker machines on various platforms. Boot2Docker is sufficient for many standard Docker workflows, but it doesn’t provide the breadth of features found in Docker Machine and Vagrant .

Wrapping your head around Docker can be challenging when you are coming at it without a strong frame of reference. In the next chapter we will lay down a broad overview of Docker, what it is, how it is intended to be used, and what advantages it brings to the table when implemented with all of this in mind.

Post topics: Infrastructure