Update: See this Docker image.

People sometimes ask about making GemStone available in a Docker container. The demand seems to be driven by a desire for a simplified install/deployment process. In preparation for my ESUG talk on a cloud-hosted GemStone IDE I decided to do some investigation.

Docker is an alternative to a virtual machine (which I’ve written about extensively on this blog) where the guest environment (“container”) shares not just the hardware with the host and other guests, but also the OS kernel. What is isolated is the application and supporting libraries. It does allow simplified deployment and ensures that each installation has exactly the same libraries and other components. The container can also be isolated so that (by default) it does not write outside its boundaries. These are attractive features.

Docker

A typical description of Docker suggests that each container uses the host OS, but it really uses the host OS kernel, and on macOS it runs an embedded Linux instance. Thus, a single Docker container can run on various OS hosts. Contrary to my initial misconception, you don’t need a separate Docker container for every OS and version.

A more challenging issue is what should go in the container. Obviously, it should include GemStone (and you would need a different container for each GemStone version), but what else? Should it include a web server? If so, which one? Perhaps not, but it does increase the complexity if you need to install, configure, and coordinate multiple components.

One hurdle is that once built, the software in a container is essentially fixed. You do not upgrade the contained software, you replace the container. Furthermore, I hope your container does not have any persistent data – it’s not supposed to, meaning thou shalt not run a database inside a container. Or, at least, not a production database; running a development database inside a container might be a good idea.

More specifically, you need to make sure that any persistent data is held outside the container, meaning that your container is not so isolated. Volumes are the preferred mechanism for persisting data generated by and used by Docker containers. And you would still need to manage backups and related system administration tasks.

For things like Topaz (a command-line interface to GemStone), you could run a command in the container. For RPC Gems (used by most client applications and IDE tools such as Jade), you should need to have only one port open into the container since the server-side Gem would be inside the container as well.

Overall, I don’t find it very difficult to install GemStone, but if someone were more likely to investigate GemStone if a Docker container were available, then it shouldn’t be too difficult to provide it.

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