I heard we can now run Docker containers on Windows but

  • Are Windows containers and Linux containers the same?
  • Can I run Linux containers on Windows?
  • Can I run Windows containers on Linux?
  • Can I only run .NET Core apps on Windows containers?
  • Can I run .NET (full framework like v4.6.2) apps on Windows containers?
  • Can I run containerized Windows apps on Linux?
  • Can I have a docker cluster of Linux and Windows together?
  • What is Docker for Windows?
  • What was Docker Toolbox for Windows?
  • Windows Nano or Windows Server Core?

If you are bothered by any of these questions read ahead…

Docker started off on Linux. It used the features which the Linux kernel provided (namespaces, cgroups etc.) to create an ‘application isolation’ system. Until then if you wanted to isolate your applications - Virtual Machines were the primary way. Docker caught on and we soon had supporting software like Kubernetes, Docker Swarm, Docker Compose etc.

One such supporting software was Docker Tookbox for Windows. People having Windows computers could now work on Linux containers. The intent was to give developers, having Windows machines a way to work with Docker and Linux containers. It achieved this by installing Oracle VM VirtualBox and creating a Linux VM on it. The docker cli was installed on the Windows machine but the commands were forwarded to the Linux VM where the docker daemon was running. So, you were essentially working with Linux containers in a Linux VM on Oracle VM VirtualBox. In fact you could just install Oracle VM VirtualBox or Hyper-V on a Windows machine, create a Linux VM, ssh to it and install docker on it.

Then Microsoft jumped on the containerization bandwagon. First it made changes to its Windows kernel to support application isolation (Windows Server Containers) and second along with Docker Inc. it released Docker for Windows

Similar to Linux containers, today you can have Windows containers. But that does not mean these are interoperable. Linux containers have their base as Linux and will run on Linux container hosts. Windows containers have their base as Windows and will run on Windows container hosts.

Many developers and administrators for some reason think Microsoft has added support to run Linux containers on Windows. And the confusion is usually based on a loose reading of the blog post Preview: Linux containers on Windows and a totally unrelated event Run Bash on Ubuntu on Windows

Docker for Windows allows developers to work on both Windows and Linux containers on a Windows machine. When in Windows container mode it uses the inbuilt container support from the Windows kernel. When in Linux container mode it uses a Linux VM on Hyper-V (similar to what Docker Toolbox for Windows did) to host the containers.

The next question then is what can you run on Windows containers?

A Docker container image is composed of a series of layers. On Linux, the base os layer is usually a very minimalistic linux distro (alpine linux is just 5 MB). So for the Windows containers, Microsoft launched the Windows Server Nano Server which can act as the base os image (currently 1.1 GB). For .NET developers, the big catch is that it only supports .NET Core and does not support the full .NET Framework. Native developers can use the NanoServerApiScan tool to check if their applications can run on Windows Nano Server. Windows containers can also run Windows Server Core as their base os image (currently 10.4 GB). If you use microsoft/windowsservercore as your base image you can then run the full .NET framework along with all Win32 applications that Windows Server supports. Your Windows Containers can have either microsoft/nanoserver or microsoft/windowsservercore as the base os layer.

  • Are Windows containers and Linux containers the same?

    No. Linux containers use the features provided by the Linux kernel to achieve application isolation and need Linux host machines to run them. Windows containers use the features provided by the Windows kernel to achieve application isolation and need Windows host machines to run them.

  • Can I run Linux containers on Windows?

    As a developer yes. In production no. Docker for Windows allows developers with Windows machines to run Linux containers on a Windows host machine (it uses a Linux VM on Hyper-V to host the actual Linux container). But orchestrators like Kubernetes or Docker Swarm are not aware of this feature.

  • Can I run Windows containers on Linux?

    No.

  • Can I only run .NET Core apps on Windows containers?

    If your container base image is microsoft/nanoserver, it only supports .NET Core. But you can also use microsoft/windowsservercore as your base os image.

  • Can I run .NET (full framework like v4.6.2) apps on Windows containers?

    Yes you can with microsoft/windowsservercore as your base os image.

  • Can I run containerized Windows apps on Linux?

    .NET Core applications can run natively on Linux. So a Linux container built with .NET Core apps can run on a Linux host machine. But you cannot run Win32 applications or .NET framework applications on Linux containers. Also Windows containers cannot be run on Linux host machines.

  • Can I have a docker cluster of Linux and Windows together?

    Yes. Docker Swarm has built in support for running Linux+Windows mixed OS clusters. Similarly Kubernetes is building support for Windows Server Containers

  • What is Docker for Windows?

    Docker for Windows is a development environment where, on a Windows developer host machine you can run Windows or Linux containers. For Linux containers a Linux VM running on Hyper-V is used as the host.

  • What was Docker Toolbox for Windows?

    Before Docker for Windows, Docker Inc had released Docker Toolbox for Windows which allowed developers to work with Linux containers on a Windows host machine. It used a Linux VM on Oracle VM VirtualBox to host the actual Linux container.

  • Windows Nano or Windows core?

    If you are starting a new project today, you should target microsoft/nanoserver. For legacy applications use microsoft/windowsservercore. Certain SDKs / Frameworks might force you to use microsoft/windowsservercore due to compatibility reasons.

Apart from the base os images, Microsoft also has a list of official images like microsoft/dotnet-framework, microsoft/wcf etc, that can be used to containerize specific applications. The full list is at https://hub.docker.com/u/microsoft.

What about hosting a production-ready docker cluster ?

If you are using Microsoft Azure as your cloud provider you have 3 ways to run a Docker cluster.

  • Manually create the cluster using a bunch of networked VMs in Azure
    • Manual administration effort.
    • You can use either Docker Swarm or Kubernetes as the orchestrator and create a cluster
    • You get support for hybrid os clusters as provided by the orchestrator
  • Azure Container Service (ACS)
    • Cluster creation is managed.
    • You get Docker Swarm, Kubernetes and a few other options as the orchestrator
    • You get support for hybrid os clusters only using acs-engine
    • Is GA
  • Azure Containers Service Managed (AKS)
    • Cluster creation/administration is managed.
    • You get Kubernetes as the orchestrator
    • Is in BETA, and hybrid os clusters support is still missing.

Why AKS when you had ACS ?

Well AKS is a truely managed kubernetes offering from Azure (K8S as a Service). In ACS your typical minimal cluster would have 3 master nodes and 3 worker agents. In AKS that cluster would only have 3 worker agents (the stuff the master nodes used to do, is now managed by Azure and for free).