resources We provide abstract interfaces, amenable to both users and applications, through (virtual) file trees. Unlike other systems that use files for interfaces (Plan 9, for example), we don't provide pixmaps or other low-level artifacts to support GUIs. We support user interfaces by providing a file tree in which a different file represents each UI element (that is, widget). The same applies to other resources.These files aren't on disk but are provided by resource servers, such as in Plan 9 or Unix's /proc. 3 In addition, we don't propose to intercept file system calls. Rather, we take the Plan 9 approach to the limit and provide all services by implement-
Plan B maps abstract interfaces to files and adapts to file tree availability. It is easy to program, offers a general-purpose computing environment, and supports smart spaces without using middleware.The system ensures that the namespace reflects which resources are available and which ones aren't, cleanly removing unavailable resources from the file trees. JULY-SEPTEMBER 2007 PERVASIVE computing 63 Figure 2. A typical Plan B screen, serviced by the Omero UI volume.
Nowadays computing environments are made of heterogeneous networked resources, but unlike environments used a decade ago, the current e n vironments are highly dynamic. During a computing session, new resources are likely to appear and some are likely to go o i n e o r t o m o ve to some other place. The operating system is supposed to hide most of the complexity o f such e n vironments and make it easy to write applications using them. However, that is not the case with our current operating systems. Plan B is a new operating system that attempts to allow the applications and their programmers select and use whatever resources are available without forcing them to deal with the problems created by their dynamic distributed and heterogeneous environments. It does so by using constraints along with a new abstraction used to replace the traditional le abstraction.
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