Memento Design pattern - Sequence Diagram

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Understanding the Memento Sequence Diagram

The Memento Design Pattern is a behavioral pattern used to capture and restore the internal state of an object without exposing its internal structure. While class diagrams show the static relationships between the Originator, Memento, and Caretaker, a Sequence Diagram illustrates the dynamic runtime interactions, message exchanges, and exact chronological order required to save and restore state reliably.

Runtime Sequence: Saving and Restoring State

• Phase 1: Snapshot Creation: When an event triggers a checkpoint, the Caretaker requests the Originator to create a snapshot (e.g., via createMemento()).
• Phase 2: Memento Instantiation: The Originator instantiates a new Memento object, passing its current private state fields into the memento constructor.
• Phase 3: State Storage: The newly created memento is returned to the Caretaker, which safely holds or pushes it onto a history stack without inspecting its contents.
• Phase 4: State Restoration (Undo): When an undo operation is requested, the Caretaker retrieves the memento from history and calls restore(memento) on the Originator, which rehydrates its internal fields.

Key Roles Along the Interaction Lifeline

1. Caretaker Lifeline: Initiates save and restore operations, managing checkpoint lifecycles while remaining completely ignorant of the state data inside the mementos.
2. Originator Lifeline: Creates mementos reflecting its current internal state and accepts past mementos to restore previous configurations.
3. Memento Lifeline: Represents the immutable snapshot object created dynamically during the save phase and passed back during the restore phase.

Why Sequence Understanding Matters

• Encapsulation Protection: The sequence demonstrates visually that the Caretaker never interacts directly with the private data of the Memento or Originator.
• Clear Separation of Concerns: Shows the clean separation between history management (Caretaker) and actual business state tracking (Originator).
• Predictable State Transitions: Clarifies the message flows required to build robust undo/redo stacks, transactional rollback checkpoints, and snapshot systems.

Recommended Next Videos

Continue your deep dive into the Memento pattern and behavioral design patterns with these related tutorials from our channel:

• Memento Design Pattern: A Fundamental Overview
• Memento Design Pattern: Class Diagram Explained
• Memento Design Pattern Implementation: Code Walkthrough
• Memento Design Pattern: Key Points

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Memento Design pattern - Sequence Diagram

See also:

Momento Design pattern - Introduction

Momento Design pattern - Real time Example

Memento Design pattern - Class Diagram

Memento Design pattern - Implementation

Momento Design pattern - KeyPoints

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