One Design Pattern a Week: Week 4

Try to solve this real problem: Request Handling

• Complex Decision Logic: Without proper structure, the code becomes filled with nested if-else statements
• Tight Coupling: Processing steps are tightly coupled, making it difficult to add or remove steps
• Code Maintainability: Changes to the processing logic require modifying existing code

An example of bad design

// language: ruby
class RequestProcessor
  def process_request(request)
    # Check authentication
    if !authenticate(request)
      return "Authentication failed"
    end

    # Validate request
    if !validate(request)
      return "Validation failed"
    end

    # Check authorization
    if !authorize(request)
      return "Authorization failed"
    end

    # Process the request
    "Request processed successfully"
  end

  private

  def authenticate(request)
    # Authentication logic
    true
  end

  def validate(request)
    # Validation logic
    true
  end

  def authorize(request)
    # Authorization logic
    true
  end
end

# Usage
processor = RequestProcessor.new
puts processor.process_request("some request")

1. Rigid Structure: The processing steps are hardcoded and cannot be easily modified
2. Single Responsibility Principle Violation: The RequestProcessor class handles multiple responsibilities
3. Poor Extensibility: Adding new processing steps requires modifying existing code

Chain of Responsibility Pattern

• Decoupling: Separates senders of requests from receivers
• Dynamic Chain: Handlers can be added or removed dynamically
• Single Responsibility: Each handler focuses on one specific task

// language: ruby
# Abstract Handler
class Handler
  attr_writer :next_handler

  def handle(request)
    return @next_handler.handle(request) if @next_handler
    nil
  end
end

# Concrete Handlers
class AuthenticationHandler < Handler
  def handle(request)
    if authenticate(request)
      puts "Authentication successful"
      super(request)
    else
      "Authentication failed"
    end
  end

  private

  def authenticate(request)
    # Authentication logic
    true
  end
end

class ValidationHandler < Handler
  def handle(request)
    if validate(request)
      puts "Validation successful"
      super(request)
    else
      "Validation failed"
    end
  end

  private

  def validate(request)
    # Validation logic
    true
  end
end

class AuthorizationHandler < Handler
  def handle(request)
    if authorize(request)
      puts "Authorization successful"
      super(request)
    else
      "Authorization failed"
    end
  end

  private

  def authorize(request)
    # Authorization logic
    true
  end
end

# Client code
authentication = AuthenticationHandler.new
validation = ValidationHandler.new
authorization = AuthorizationHandler.new

# Set up the chain
authentication.next_handler = validation
validation.next_handler = authorization

# Process request
result = authentication.handle("some request")
puts result

• Flexible Chain Construction: Handlers can be arranged in any order
• Easy to Add/Remove Steps: New handlers can be added without modifying existing code
• Single Responsibility: Each handler focuses on one specific task
• Loose Coupling: Handlers are independent of each other

More Practical Examples

// language: ruby
# Base Middleware Handler
class Middleware
  attr_writer :next_handler

  def handle(request)
    return @next_handler.handle(request) if @next_handler
    request
  end
end

# CORS Middleware
class CORSMiddleware < Middleware
  def handle(request)
    # Add CORS headers
    request.headers['Access-Control-Allow-Origin'] = '*'
    request.headers['Access-Control-Allow-Methods'] = 'GET, POST, PUT, DELETE'
    
    puts "CORS headers added to request"
    super(request)
  end
end

# Authentication Middleware
class JWTAuthMiddleware < Middleware
  def handle(request)
    token = request.headers['Authorization']
    
    if token.nil?
      return { status: 401, message: 'No token provided' }
    end

    begin
      decoded_token = JWT.decode(token, ENV['JWT_SECRET'], true, algorithm: 'HS256')
      request.user = decoded_token['user']
      puts "User authenticated successfully"
      super(request)
    rescue JWT::DecodeError
      { status: 401, message: 'Invalid token' }
    end
  end
end

# Rate Limiting Middleware
class RateLimitMiddleware < Middleware
  def initialize
    @redis = Redis.new
    @max_requests = 100  # 100 requests
    @time_window = 3600  # per hour
  end

  def handle(request)
    client_ip = request.ip
    current_count = @redis.get(client_ip).to_i

    if current_count >= @max_requests
      return { status: 429, message: 'Rate limit exceeded' }
    end

    @redis.multi do
      @redis.incr(client_ip)
      @redis.expire(client_ip, @time_window)
    end

    super(request)
  end
end

# Usage
class Request
  attr_accessor :headers, :ip, :user
  
  def initialize
    @headers = {}
    @ip = '127.0.0.1'
  end
end

# Set up middleware chain
rate_limit = RateLimitMiddleware.new
cors = CORSMiddleware.new
auth = JWTAuthMiddleware.new

rate_limit.next_handler = cors
cors.next_handler = auth

# Process request
request = Request.new
request.headers['Authorization'] = 'valid.jwt.token'
result = rate_limit.handle(request)

Conclusion