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320 PART THREE CONVENTIONAL METHODS FOR SOFTWARE ENGINEERING

FIGURE 12.18
Establishing Homeowner
connections

Control panel Sensor

Security Monitoring
system service

for SafeHome (Section 11.3.3), the following (partial) list of “things” are relevant to
the problem:
• homeowner
• control panel
• sensors
• security system
• monitoring service

Taking these “things” one at a time, connections are explored. To accomplish this,
each object is drawn and lines connecting the objects are noted. For example, refer-
ring to Figure 12.18, a direct connection exists between homeowner and control
panel, security system, and monitoring service. A single connection exists
between sensor and security system, and so forth.
Once all connections have been defined, one or more object/relationship pairs are
identified for each connection. For example, the connection between sensor and
security system is determined to have the following object/relationship pairs:
security system monitors sensor
security system enables/disables sensor
security system tests sensor
security system programs sensor
Each of these object/relationship pairs is analyzed to determine cardinality and modal-
ity. For example, considering the object/relationship pair security system monitors
sensor, the cardinality between security system and sensor is one to many. The
modality is one occurrence of security system (mandatory) and at least one occur-
rence of sensor (mandatory). Using the ERD notation introduced in Section 12.3, the