Supported container types for images

[Ian Booth]

Hi folks,

You will be aware we are implementing containers for Juju. Machines can be asked
to create an LXC, KVM, XEN (et al) container. Not all machine instances will be
able to run all containers. eg kernel patches may be required to support XEN or
whatever and the image may not have been set up with that capability. Or LXC
doesn't make sense on Windows images etc.

Juju needs to know what types of container an instance can run. Ideally, we
would know this before starting an instance from an image, since the add-machine
command would optionally be asked to create a machine with a given container.
We'd love to be able to get this information from the simplestreams image
metadata. Theoretically it should be possible because whoever creates the images
knows their capabilities. And we already have access to this kind of data now,
like the storage type or virtualisation type (for EC2). The alternative is to
wait for a machine instance to boot and query the capabilities then but this
opens up a whole world of hurt we'd rather not deal with.

So, is what we're asking for viable? Can the simplestreams image metadata be
made to include the types of supported containers? Is there a better way to
achieve our goals?