First impressions of Docker on Windows

Not to be confused with Docker for Windows (Docker inside a Linux Hyper-V VM on Windows 10), these are my initial opinions on Microsoft’s implementation of Docker, running on Windows 2016 Server, which was released a few weeks ago. You can also enable it, minus any settings, via the beta version of Docker for Windows on Windows 10.

Like the current release of .net core, Docker on Windows feels like a beta product not quite ready “for production”. There’s unlikely to be any support for it on AWS ECS, Kubernetes, Rancher and Mesos for a while which means you’ll end up deploying and managing containers yourself, probably through RDP on the server (or WinRM if you’re feeling wild), and no doubt the magic of an Azure pipeline will come soon.

My biggest complaints

A few of us spent around half a day making an Exceptionless Windows Docker image. Exceptionless is a great web-based UI for aggregating and grouping .NET exceptions on a per-project level (not an alternative to ELK, but something that lives with it). As I support the Exceptionless Chocolatey package, whose upgrade experience is currently non-existence, the Docker format makes perfect sense for Exceptionless. We built the Docker image successfully in the end, but with a fair number of desk thumping moments:

You can’t use the Docker Linux eco system

I wanted to use the Elasticsearch image and couldn’t as it’s a Linux image. There are a lot of mature images on the official registry right now that you’d have to use on a separate Docker host.

No Docker logs

Forwarding of logs from the system/application eventlog isn’t automatic - you have to docker exec -it powershell and use Get-Logs application, unless I am missing something.

The original Docker images are (weirdly) not open source

The problem with not being able to see the base Dockerfile for all the microsoft/xyz Docker registry images is you don’t know what’s going on under the hood. This is useful if you don’t know what’s happening in the base - for example what the ENTRYPOINT does.

Maybe there’s some tooling to come that will allow this - something similar to Packer that generates the Docker image based off an iso and an autounattend.xml file.

They all use cmd.exe as their default entrypoint

The only image that seems to have Powershell as the default shell is the microsoft/iis one. This makes Dockerfiles platform dependent (file paths are Windowerised and commands like mkdir don’t exist). It’s also a weird choice for a new technology, when Microsoft are pushing Powershell as the default shell, picking cmd.exe and batch feels like you’re moving backwards.

Docker build uses cmd.exe

When you enter a RUN command in your Dockerfile, it’s using cmd.exe not Powershell. I ended up with lines of RUN powershell.exe <my command>.

Minor moans

• All but the Nano image are 10gb in size. It’s not an issue until you create your own ones based of microsoft/xyz and watch your registry use 10gb at a time. But thanks for the Docker layering system these are probably less than 10gb
• IIS is slow to launch.
• A bug in the NAT implementation of the Docker for Windows beta means you can’t use localhost, you have to use the multi-cast address (172…) of the container, using its internal port.

Ending on a positive

It’s easy to be negative - Docker for Windows appears to have been rushed out as Microsoft seem very keen catch up with the Linux world as part of their Azure vision. Give Docker on Windows a year and it’s going to make Windows applications (primarily web ones I imagine) far easier to deploy, scale up and upgrade. I’m currently working on getting Roadkill into Docker (albeit cross-platform with .net core) for this reason.

I’m hoping Microsoft find a way of combining all Docker images in Windows, maybe through some Hyper-V or Linux Subsystem magic. The tooling is also likely to rapidly improve as it’s Microsoft we’re talking about, the kings of developer tooling.

I’m hoping to get the Exceptionless image out shortly, which will let you run

docker run -p 50000:50000 -e ELASTICSEARCH_URL=http://elasticsearch -e SMTP_HOST=mysmtp yetanotherchris/exceptionless

I'm Chris Small, a software engineer working in London. This is my tech blog. Find out more about me via Github, Stackoverflow, Resume