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CHAPTER 4 Preparing for and Testing Docker Containers 31 These materials are © 2017 John Wiley & Sons, Inc. Any dissemination, distribution, or unauthorized use is strictly prohibited. Union file systems A union file system allows the piecing together of files and direc- tories from separate file systems to be overlaid on one another to create a single, coherent file system. The union file system plays a huge role in the lightweight nature of Docker containers. Let's take the case for updating an applica- tion. When you modify an image, you simply swap out a changed layer for a new one, leaving all the other layers in place. When an image is started up, the union file system attaches a read/write layer to the container. Docker on Windows If you prefer or require Windows Server in the data center, you're in luck! Docker support has come to the Windows-based data center. With the release of Windows Server 2016, Microsoft and Docker have provided an additional platform on which to run container-based workloads. There are, however, some key considerations to keep in mind as you go down the Windows-based container journey: » OS version: Whereas Docker can run on almost all modern Linux variants, only certain versions of Windows 10 and Windows Server 2016 can run Docker. For example, since Hyper-V is required, Windows 10 Home Edition is not an eligible platform, even for testing. Check Docker's compatibility list to make sure your operating system version is supported. » Orchestration: Container orchestration systems that are common to Linux-based Docker implementations aren't always available on Windows. Docker's own Swarm mode, however, is available. Over time, as Docker on Windows becomes more popular, expect to see more orchestration systems become available. » Command line: PowerShell is one of the unsung heroes of the Windows world. In addition to the same CLI used with Linux-based Docker containers, Docker on Windows can also be managed using the PowerShell CLI, easing the transition for Windows users. Of course, if you're a Linux Docker admin making the jump to Docker for Windows, you'll be able to bring over all of your command line knowledge and it will work for you.