Design
Defining features of the Rhapsody operating system included the Mach microkernel, a BSD operating system layer (based on 4.4BSD), the object-oriented Yellow Box API framework, the Blue Box compatibility environment for running "Classic" Mac OS applications, and a Java Virtual Machine.
The user interface was modeled after Mac OS 8's "Platinum" appearance. The file management functions served by the Finder in previous Mac OS versions were instead handled by a port of OPENSTEP's Workspace Manager. Additional features inherited from OPENSTEP and not found in Classic Mac OS's Finder were included, such as the column view, and would later make their way to Mac OS X's Finder.
Rhapsody's Blue Box environment, available only when running on the PowerPC architecture, was responsible for providing runtime compatibility with existing Mac OS applications. Compared to the more streamlined and integrated Classic compatibility layer that was later featured in Mac OS X, Blue Box's interface presented users with a distinct barrier between emulated legacy software and native Rhapsody applications. All emulated applications and their associated windows were encapsulated within a single Blue Box emulation window instead of being interspersed with the other applications using the native Yellow Box API. This limited cross-environment interoperability and caused various user interface inconsistencies.
To avoid the pitfalls of running within the emulation environment and take full advantage of Rhapsody's features, software needed to be rewritten to use the new Yellow Box API. Inherited from OPENSTEP, Yellow Box used an object-oriented model completely unlike the procedural model used by the Classic APIs. The large difference between the two frameworks meant transition of legacy code required significant changes and effort on the part of the developer. The consequent lack of adoption as well as objections by prominent figures in the Macintosh software market, including Adobe Systems and Microsoft, became major factors in Apple's decision to cancel the Rhapsody project in 1998.
However, most of Yellow Box and other Rhapsody technologies went on to be used in Mac OS X's Cocoa API. Bowing to developers' wishes, Apple also ported existing Classic Mac OS technologies into the new operating system and implemented the Carbon API to provide Classic Mac OS API compatibility. Widely used Mac OS libraries like QuickTime and AppleScript were ported and made available to developers. Carbon allowed developers to maintain full compatibility and native functionality using their current codebases, while enabling them to take advantage of new features at their discretion.
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