• Patterns
  • Are observed through experience
  • Prevent reinventing the wheel
  • Exist at different levels of abstraction – 
    • design, architectural, analysis, creational, structural, behavioral
  • Are reusable artifacts
  • Undergo continuous improvement
  • Communicate designs and best practices
  • Can be used together to solve a larger problem.
• During pattern identification, we may observe differing implementation strategies
  • where the solutions are similar a pattern may lurk.
• Patterns vs. Strategies
  • Patterns exist at a higher level of abstraction than strategies.
  • Patterns include recommended implementations as strategies.
  • Strategies provide an extensible point for patterns 
    • new implementation strategies emerge over time.

B. Pattern Template

• A standardized way to document multiple patterns.

Context – Server application conducting integral units of work that must not be interrupted (akin to the database concept of Atomic operation).

Problem – How to shutdown the server app without interrupting app during a critical phase (within some unit of work that would be hard to recover from if disrupted).

Forces – Unix offers “signals,” Windows-NT offers “Services” – each a platform-dependent device to control server startup and shutdown.

Java “… run anywhere” begs for a platform-independent approach.

Java language has necessary features and constructs to support a solution.

Solution – Use Java networking and threading language features to implement a portable mechanism as a reusable design pattern.

-- Strategies – Different strategies relate to the level of encapsulation of functional pieces, and the amount of multithreading in the solution. (For example, while there must be a thread blocking on a SeverSocket.accept() call, there may or may not be additional worker threads that must be notified that it’s time to quit – SeverSocket.setSoTimeout() allows other strategies.)

-- Structure - Sequence diagrams

                

Consequences – More threading increases complexity, albeit along fairly standardized lines. Worker threads allow encapsulation and scalability of function without additional design burden or code complexity.

Sample Code – See running examples portmanager-basic and portmanager-full.