Testing Complex Applications - Software Testing: Testing Across the Entire Software Development Life Cycle - page 241

Alternatively, consider the situation when the tier 2 server is tested fi rst. The
server data management functionality can be tested directly. This usually involves
building simple test harnesses that support business function activity via native Data
Manipulation Language (DML) calls. The most common form of DML for accessing
and manipulating databases is the ANSI Standard Query Language (SLQ), which
is supported by all major database management systems such as Oracle, DB2,
Sybase, Informix, Microsoft SQL Server, Microsoft Access, and others. The extra
effort needed to build and run the test harness is more than offset by identifying
application database design defects long before the client software attempts to use
the databases.
Once the application database design has been validated on the server, the
test team then moves the test harness from the server to a client. This positions
the test team to validate the tier-to-tier connectivity next. The SQL commands
that worked correctly on the server should now produce the same results on a
client if the tier-to-tier connectivity layer is working correctly. If defects arise dur-
ing the tier-to-tier connectivity testing, the diagnosis effort can focus just on the
connectivity components because the server side has been validated and is known
to work correctly.
Once the server side and tier-to-tier connectivity have been validated, then
the test team is ready to test the client side user screens and functions. If de-
fects arise now, the diagnosis effort can focus just on the client side application
software because both the server side and tier-to-tier connectivity have been
validated and known to work correctly. This represents a relatively advanta-
geous situation of introducing only one new variable at a time compared to the
relatively disadvantageous situation where the testing is started on the client
side; a defect is discovered, and none of the tiers can be excluded from the
diagnosis.
The development team receives an additional benefit of this tier 2-to-tier
1 server-to-client testing strategy. Recall that the test team uses a testing
harness and SQL to test the server side. When the development team writes
the client side application code, they will eventually need to use some de-
rivative of the test SQL. Whether the chosen development language supports
SQL calls directly or SQL calls by means of Application Program Interfaces
(APIs), the same business task must be accomplished from the user screen
that was accomplished by the test harness. So the development team is given
a substantial programming starting point with the tested and verified harness
SQL.
14.4 3-TIERAPPLICATIONS
The classic example of a 3-tier application is any Internet business application.
Figure 14.7 shows a reasonably mature Internet business application example.