Collaborations and Mechanisms

Modeling design patterns as parameterized collaborations to realize use cases and operations

In architecture-centric development, collaborations and mechanisms provide a powerful way to bridge high-level requirements (use cases) and low-level implementation details. They let us name and reuse meaningful “chunks” of behavior—societies of classes, interfaces, and interactions that work together to deliver value—while keeping the model clean, understandable, and resilient to change.

Key Concepts

• Collaboration A named collection of structural elements (classes, interfaces, roles) and behavioral specifications (interactions, sequences, state machines) that cooperate to achieve a specific purpose. Unlike packages, a collaboration does not own its elements—it references them from elsewhere in the model. It is a view or slice across the system showing how parts work together for a particular responsibility.
• Mechanism A special kind of collaboration that reifies a design pattern or architecturally significant decision. Mechanisms are reusable, often parameterized, and represent common solutions to recurring problems (Observer, Strategy, Command, Transaction, Pub/Sub, Circuit Breaker, etc.).
• Parameterized Collaboration To maximize reuse, a collaboration or mechanism is defined with template parameters (formal roles like Subject, Observer, Context, Strategy). When applied in a concrete context, these parameters are bound to actual classes, interfaces, or components—creating an isomorphic (structure-preserving) application of the pattern.
• Realization Relationship A collaboration realizes (implements) a use case, an operation, or another higher-level requirement. The dashed arrow with hollow triangle points from the collaboration to the realized element (use case or operation).

This approach allows teams to:

• Capture architecturally significant patterns explicitly
• Reuse proven solutions across use cases
• Keep models and code aligned through clear mappings
• Make complex behavior understandable at different levels of abstraction

Practical Examples of Collaborations and Mechanisms

Here are many concrete, real-world examples showing how parameterized collaborations and mechanisms realize use cases and operations:

• Observer Mechanism (Publish-Subscribe Pattern)
  • Parameterized collaboration: «mechanism» Observer
  • Formal roles: Subject, Observer, update() operation
  • Structural: Subject ↔ Observer (association), notify() method
  • Behavioral: Sequence diagram showing notifyObservers() → update() loop Concrete bindings (realizations):
  • Use case “Real-time Card Update” in task management app → Bind Subject = Card, Observer = Watcher (User)
  • Use case “Live Inventory Tracking” in e-commerce → Bind Subject = ProductStock, Observer = WarehouseDashboard
  • Use case “Stock Price Ticker” in trading platform → Bind Subject = TickerSymbol, Observer = TradingScreen Benefit: One Observer mechanism reused across domains; change notification policy (e.g., add batching) in one place.
• Strategy Mechanism (Pluggable Algorithm)
• Parameterized collaboration: «mechanism» Strategy
  • Roles: Context, Strategy interface, ConcreteStrategyA/B
  • Behavioral: Context delegates to strategy.execute() Realizations:
  • Use case “Calculate Shipping Cost” → Bind Context = Order, Strategy = USPSRateStrategy / FedExRateStrategy / FlatRateStrategy
  • Use case “Apply Discount” → Bind Context = Cart, Strategy = PercentageDiscount / BuyXGetY / LoyaltyPointsDiscount
  • Use case “Sort Task List” → Bind Context = Board, Strategy = DueDateSort / PrioritySort / CustomOrderSort Practical: Enables runtime switching of algorithms without changing core classes.
• Transaction Mechanism (ACID Boundary)
  • Parameterized collaboration: «mechanism» TransactionBoundary
  • Roles: TransactionCoordinator, Participant, commit()/rollback()
  • Behavioral: Sequence with prepare/commit/rollback phases Realizations:
  • Use case “Transfer Funds Between Accounts” → Bind Coordinator = TransferService, Participants = DebitAccountOp, CreditAccountOp
  • Use case “Place Order with Payment & Inventory Reserve” → Bind Participants = ReserveStock, AuthorizePayment, CreateOrder
  • Use case “Update Patient Record & Log Audit” → Ensures atomicity of PHI update + audit trail Benefit: Consistent transaction semantics across distributed services.
• Adapter Mechanism (Legacy Integration)
  • Parameterized collaboration: «mechanism» Adapter
  • Roles: Target, Adaptee, Adapter Realizations (strangler fig pattern):
  • Use case “Process Invoice” migrating from legacy ERP → Bind Target = NewInvoiceService, Adaptee = LegacyERP, Adapter = ERPInvoiceAdapter
  • Use case “Authenticate User” with legacy LDAP → Bind Target = AuthService, Adaptee = OldLDAPServer Practical: Gradually replaces legacy calls while new use cases use modern interfaces.
• Circuit Breaker Mechanism (Resilience Pattern)
  • Parameterized collaboration: «mechanism» CircuitBreaker
  • Roles: ProtectedCall, CircuitBreakerState (Closed, Open, HalfOpen)
  • Behavioral: State machine + fallback logic Realizations:
  • Use case “Fetch External Product Catalog” → Bind ProtectedCall = CatalogAPIClient
  • Use case “Check Payment Gateway Status” → Prevents cascading failures during outages
  • Use case “Real-time Currency Conversion” → Fallback to cached rates when service down Benefit: Models resilience explicitly in architecture views.
• Command Mechanism (Undoable Operations)
  • Parameterized collaboration: «mechanism» Command
  • Roles: Command, Receiver, Invoker, execute()/undo() Realizations:
  • Use case “Move Card Between Lists” in task app → Bind Command = MoveCardCommand, Receiver = Board
  • Use case “Apply Batch Discount” → Supports undo across multiple items Practical: Enables undo/redo stack in UI-heavy applications.
• Composite Mechanism (Tree Structure)
  • Parameterized collaboration: «mechanism» Composite
  • Roles: Component, Composite, Leaf Realizations:
  • Use case “Manage Task Hierarchy” (sub-tasks) → Bind Composite = TaskFolder, Leaf = SimpleTask
  • Use case “Build Bill of Materials” in manufacturing → Parts contain sub-parts recursively Benefit: Uniform treatment of whole-part hierarchies.
• Realizing Complex Operations
  • Use case “Calculate Order Total with Discounts & Taxes” → Collaboration: OrderCalculation
  • References Strategy mechanism for discount rules
  • References Observer for notifying price changes
  • Sequence diagram shows flow across Cart, DiscountEngine, TaxCalculator → Forward engineered to method in Order class.

In Visual Paradigm:

• Create collaboration as a composite structure or collaboration diagram.
• Define template parameters (formal roles) in the collaboration properties.
• Bind actual elements using binding relationships or role mappings.
• Draw realization arrow from collaboration to use case or operation.
• Use mechanisms package for reusable patterns; apply via drag-and-drop binding.
• Semantic backplane ensures bindings stay consistent across views.

By modeling design patterns as parameterized collaborations and mechanisms, teams create a reusable architectural vocabulary that directly realizes use cases and complex operations—making systems easier to understand, extend, and maintain while keeping models tightly coupled to code in Agile iterations.

This prepares you for the hands-on implementation workflows: forward, reverse, and round-trip engineering in the next sections.