VITAL provides a way of building information systems
constructed from generalized modules that can be reused and
shared.It specifies how to construct and provide interfaces
for these modules. A cornerstone of Apple's client/server
architecture is Data Access Language (DAL). DAL
consists of software components for the client and the
server (Macintosh System 7 includes the client component,
DOS and Windows clients are sold separately). DAL sits
between the client and server components.
This approach allows varied clients to communicate with
many different servers. With DAL, a change in the DBMS
does not affect the front-end;
it simply requires the appropriate DAL link.