Facade Design Pattern in Ruby

1. Definition

The Facade Design Pattern provides a unified interface to a set of interfaces in a subsystem. The facade defines higher-level operations that make the subsystem easier to use.

2. Problem Statement

When working with complex systems or libraries, there can be a vast number of intricate, interconnected interfaces. This can be overwhelming to work with, especially for common tasks that require interaction with multiple components of the system.

3. Solution

By introducing a Facade, you can encapsulate the complexity of the subsystem behind a simpler, higher-level interface. Clients then interact with the Facade instead of the subsystem directly, leading to easier integration and reduced complexity.

4. Real-World Use Cases

1. Simplifying interactions with a complex video transcoding library.

2. A power switch acting as a facade for all the intricate processes inside a computer.

3. An API gateway in microservice architectures that provides a single entry point for external consumers.

5. Implementation Steps

1. Identify a simpler, unified interface for a subsystem.

2. Implement this interface by delegating calls to methods from the subsystem's classes.

3. Direct the client code to use the facade instead of subsystem classes directly.

6. Implementation in Ruby

# Subsystem classes
class CPU
  def boot_up
    "CPU booting up..."
  end
end
class Memory
  def load
    "Memory loading..."
  end
end
class HardDrive
  def read
    "HardDrive reading..."
  end
end
# Facade
class ComputerFacade
  def initialize
    @cpu = CPU.new
    @memory = Memory.new
    @hard_drive = HardDrive.new
  end
  def start
    @cpu.boot_up + " " + @memory.load + " " + @hard_drive.read
  end
end
# Client code
computer = ComputerFacade.new
puts computer.start

Output:

CPU booting up... Memory loading... HardDrive reading...

Explanation:

1. CPU, Memory, and HardDrive are classes of the subsystem with their individual methods.

2. ComputerFacade is the facade class that wraps around the subsystem classes and provides a simplified interface via the start method.

3. The client code interacts only with the ComputerFacade, abstracting away the complexity of booting up individual components.

7. When to use?

Use the Facade Design Pattern when:

1. You want to provide a simple interface to a complex subsystem.

2. The system has many dependencies or components, and a simplified API is needed.

3. You want to decouple the client code from the subsystem's components.