2 Why We Need Model-Based Testing

28 Why We Need Model-Based Testing
A realistic test of a client/server system tests one end at a time, the client or the
server. From the point of view of each end, the other end is nondeterministic: its
behavior cannot be predicted. This poses practical problems for coding tests. Here
we avoid these problems by creating a sandbox, a testing environment where we
remove the nondeterminism by controlling both client and server. Both can run on
the same computer, and use the localhost IP address. In fact, we can call both client
and server methods from the same test method. We call the methods in a single
thread, taking care to interleave the method calls in an order that conforms to the
protocol (Figure 2.2). This test exercises most of the methods in the Client and
Server classes. However, it omits the Temperature class and assigns a temperature
value in the test code instead. The assertions check that the command sent by the
client is received by the server, and the temperature sent by the server is received by
the client. The test also writes a progress message to the console, which is displayed
by the test runner.
Here is a test run.
nunit-console /include=ClientServer Tests.dll
...
Included categories: ClientServer
.Server sent 212.0, client received 212.0
Tests run: 1, Failures: 0, Not run: 0, Time: 0.230 seconds
The test succeeds and the expected progress message appears. Can we conclude
from this that the applications will work?
2.9 Failures in the field
One day we connect up our measurement system (as in Figure 2.1), start the server
and the client, and leave them running. All goes well until the temperature reaches
100 ◦ . At the client, we see:
2006-11-07 16:07:01Z Temperature server at port 8023 reports 99.8 F
2006-11-07 16:07:11Z Temperature server at port 8023 reports 99.9 F
2006-11-07 16:07:21Z Temperature server at port 8023 reports 100.0 F
2006-11-07 16:07:31Z Temperature server at port 8023 reports 0.0 F
2006-11-07 16:07:41Z Temperature server at port 8023 reports 99.9 F
2006-11-07 16:07:51Z Temperature server at port 8023 reports 99.8 F
more free ebooks download links at:
http://www.ebook-x.com