A-7E Avionics System Case Study

An object-oriented program’s structure often bears little resemblance to its code structure.

The code structure is frozen at compile-time; it consists of classes in fixed inheritance relationships.

A program’s runtime structure consists of rapidly changing networks of communicating objects.

The structures are largely independent and nearly impossible to understand one from the other.

The system is responsible for reading sensors and updating cockpit displays that help the pilot drop weapons on a target.

It does not fly the aircraft.

The architecture is centered around three structures: Decomposition, a structure of modules Uses, a structure of modules Process, a structure of components and

connectors The module decomposition is based on

information hiding

Goal How AchievedEase of change to: weapons, platform, symbology, input

Information hiding

Understand anticipated changes Formal evaluation procedures to take advantage of experience of domain experts

Assign work teams so that their interactions were minimized

Modules structured as a hierarchy; each work team assigned to a second-level module and all of its descedants

Hardware-Hiding Module Extended Computer Module Device Interface Module

Behavior-Hiding Module Function Driving Module Shared Services Module

Software Decision Module Application Data Type Module Data Banker Module Filter Behavior Module Physical Models Module Software Utility Module System Generation Module

Goal How AchievedIncrementally build and test system functions

Create “is-allowed-to-use” structure for programmers that limits procedures each can use

Design for platform change Restrict number of procedures that use platform directly

Produce usage guidance of manageable size

Where appropriate, define uses to be a relationship among modules

Goal How AchievedMap input to output Each process implemented as

cycle that samples inputs, computes, and presents output

Maintain real-time constraints Identify process through process structure and then perform offline scheduling

Provide results of time-consuming calculations immediately

Perform calculations in background and return most recent value when queried