I’ve been playing with Docker a bit recently, and decided to take a look at the nova-docker driver for OpenStack.

The nova-docker driver lets Nova, the OpenStack Compute service, spawn Docker containers instead of hypervisor-based servers. For certain workloads, this leads to better resource utilization than you would get with a hypervisor-based solution, while at the same time givin you better support for multi-tenancy and flexible networking than you get with Docker by itself.

The Docker driver wiki was mostly sufficient for getting the nova-docker driver installed in my existing OpenStack deployment, although I did make a few small changes to the wiki to reflect some missing steps. Other than that, the installation was relatively simple and I was soon able to spin up Docker containers using nova boot ...

The one problem I encountered is that it is not possible to pass environment variable to Docker containers via the nova-docker driver. Many existing images (such as the official MySQL image) expect configuration information to be passed in using environment variables; for example, the mysql image expects to be started like this:

docker run --name some-mysql \
  -e MYSQL_ROOT_PASSWORD=mysecretpassword -d mysql

I have proposed a patch to the nova-docker driver that permits one to provide environment variables via the Nova metadata service. With this patch in place, I would start the mysql container like this:

nova boot --image mysql --flavor m1.small \
  --meta ENV_MYSQL_ROOT_PASSWORD=mysecretpassword \
  some-mysql

That is, the driver looks for metadata items that begin with ENV_ and transforms these into Docker environment variables after stripping ENV_ from the name.

While this patch works great in my testing environment, it’s unlikely to get accepted. Generally, the metadata provided by Nova belongs to the tenant and is not meant to be operationally significant to the compute driver itself.

It sounds as if there is a lot of work going on right now regarding container support in OpenStack, so it is very likely that a better solution will show up in the near future.

In the absence of that support, I hope others find this patch helpful.