System design

• transformation of an analysis model into a system design model
• developers
  • define the design goals of the project and decompose the system into smaller subsystems that can be realized by individual teams
  • select strategies for building the system, such as the hardware/software strategy, the persistent data management strategy, the global control flow, the access control policy, and the handling of boundary conditions
  • must make trade-offs among many design goals that often conflict with each other
  • cannot anticipate all design issues that they will face because they do not yet have a clear picture of the solution domain
• is not algorithmic
• the whole is divided into simpler components and interfaces, while considering nonfunctional and functional requirements

scope

• bridge the gap between a problem and an existing system in a manageable way

• by using Divide & Conquer:

  1. Identify design goals
  2. Model the new system design as a set of subsystems

  3.8. Address the major design goals

concepts

• subsystem

  • replaceable part of the system with well-defined interfaces that encapsulates the state and behavior of its contained components
  • typically corresponds to the amount of work that a single developer or a single development team can tackle
  • by decomposing the system into relatively independent subsystems, concurrent teams can work on individual subsystems with minimal communication overhead

• service

  • set of related operations that share a common purpose
  • during system design, we define the subsystems in terms of the services they provide
  • during object design, we define the subsystem interface in terms of the operations it provides

• coupling

  measures

  • the dependencies between two subsystems

• cohesion

  measures

  • the dependencies among classes within a subsystem

minimize coupling and maximize cohesion

• layering
  • allows a system to be organized as a hierarchy of subsystems, each providing higher level services to the subsystem above it by using lower-level services from the subsystems below it
• partitioning
  • organizes subsystems as peers that mutually provide different services to each other

From Analysis to Design