Sommerville-Software-Engineering-10ed

21.2 ■ Architectural patterns for real-time software 625
Figure 21.12
The Process
Pipeline pattern
Name
Description
Stimuli
Responses
Processes
Used in
Process Pipeline
A pipeline of processes is set up with data moving in sequence
from one end of the pipeline to another. The processes are often
linked by synchronized buffers to allow the producer and
consumer processes to run at different speeds. The culmination
of a pipeline may be display or data storage, or the pipeline may
terminate in an actuator.
Input values from the environment or some other process
Output values to the environment or a shared buffer
Producer, Buffer, Consumer
Data acquisition systems, multi-media systems
Figure 21.13 Process
Pipeline process
structure
Producer
process
Produced
data
Buffer
process
Consumed
data
Consumer
process
...
wheels. This monitors the state of each wheel to check if the wheel is turning or locked.
A separate process monitors the pressure on the brake pedal exerted by the car driver.
The system includes an anti-skid feature, which is triggered if the sensors indicate
that a wheel is locked when the brake has been applied. This means that there is
insufficient friction between the road and the tire; in other words, the car is skidding.
If the wheel is locked, the driver cannot steer that wheel. To counteract this effect,
the system sends a rapid sequence of on/off signals to the brake on that wheel, which
allows the wheel to turn and control to be regained.
The Wheel monitor process monitors whether or not each wheel is turning. If a
wheel is skidding (not turning), it informs the Analysis process. This then signals the
processes associated with the wheels that are skidding to initiate anti-skid braking.
21.2.3 Process pipeline
Many real-time systems are concerned with collecting analog data from the system’s
environment. They then digitize that data for analysis and processing by the system.
The system may also convert digital data to analog data, which it then sends to its
environment. For example, a software radio accepts incoming packets of digital data
representing the radio transmission and transforms the data into a sound signal that
people can listen to.
The data processing involved in many of these systems has to be carried out very
quickly. Otherwise, incoming data may be lost and outgoing signals may be broken
up because essential information is missing. The Process Pipeline pattern makes
this rapid processing possible by breaking down the required data processing into a
sequence of separate transformations. Each of these transformations is implemented