The reason that the session manager web service need to call `get_os_path` is that it starts kernels in the same directory as the notebooks. For now I do think this should be in the base class, but I am not sure how to make that decision for not file system kernels. Maybe we should separate two questions:

* For file system based stores, `get_os_path` which is used internally.
* For all stores, `get_kernel_path` that the session can use to figure out where to start the kernels. The file system backend can simple return `get_os_path` from this.

How does that sound?