However, they also increase the level of abstraction from the bottom
up, on the implementation side, whereas in aDSMlanguage, the level of abstraction is
also raised on the problem domain side.
At one extreme, the target environment can be plain hardware, but more typical are
those with a component framework, a library, or even off-the-shelf engines or
standalone programs, like messaging servers or databases. Which of these are used
depends on the case.
The support provided by programming languages, virtual machines, or operating
systems is usually leveraged in a similar way to libraries. They are used by the created
application. Libraries provide prede?¬?ned building blocks but do not enforce a
particular design for an application. A framework can be considered to be more than a
library. It is an abstract speci?¬?cation of a kind of application. It expects a certain
programming model that the developer must follow. Frameworks are generally
classi?¬?ed as white box or black box. In a white-box framework, the user adds
FIGURE 4.7 Layered architecture of a target environment
86 ARCHITECTURE OF DSM
functionality to existing code. This necessitates that the implementation details must
be understood at least to some extent. In a black-box framework, existing building
blocks are applied as such and their internal implementation is not visible.
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