In a response to the blog I wrote earlier on the public launch of Onshape, a LinkedIn member asked how could the geometry be useful for anything other than development ideas without having files. He was worried that the lack of files for the models would not enable linking the geometry to physical properties for solid geometry. Files are not what is important for representing physical properties associated with geometry, models are.
CAD users have become accustomed to files being the container for the geometry and even equating part numbers and file names in many instances. The CAD software is working with the files through the computer's operating system. Things get more complicated when a document management system, shared drive, PDM, or PLM system is used to manage the files. There are no laws of physics that require the virtual representation of product (the model) to have a tight relationship to the data storage structure (files on 'disk').
There are many modeling tools that represent what we think of as systems or assemblies that do not capture components at the file-level. Most Finite Element Models (FEA) that represent a structural assembly store the model information as well as the simulation parameters as a single document. Same situation with many Multi-Body Dynamics (MBD) models and the system-level representation model used in Model-Based Systems Engineering (MBSE). There may be interoperability challenges for Onshape for CAE, 3D Printing, and other forms of digital manufacturing, but this is not a new challenge and certainly not one that should be of concern for most people.
Onshape's cloud-enabled data model dispenses with the traditional file-based structure (parts and assemblies being contained in individual files). Models of the solid geometry exist in parts and assemblies within the context of a project-level "Document" that Onshape manages. There are no individual part or assembly files that users see within Onshape. I'm not (yet) aware of how Onshape manages the information on the server side, but I'm not sure that it really matters. I recall that when I was developing very large assemblies in the early 1990s using I-DEAS all the parts, assemblies, Finite Element meshes, simulation information (both FEA and MBD), etc. was all in a single database file.
It is important to keep in mind are that Onshape can import and export geometric models in many of the popular CAD data formats (Parasolid x_t, IGES, STEP, ACIS SAT, SolidWorks, etc). So one way to work will be to use files to input to analysis and other tools. Additionally, Onshape has an API that allows both desktop and cloud applications to access Onshape model data directly. I expect announcements on their work with partners to be made some time in the future.
Personally, I'm very excited about being able to focus on the development of systems with Onshape in this structure; the flexibility and freedom that comes from not having to manage files is a significant opportunity.