Memento pattern in LLD - Architecture Diagram

System Overview - Memento pattern

The Memento pattern helps save and restore an object's state without exposing its internal details. It is useful when you want to implement undo or rollback features in applications.

Key requirements include capturing the object's state, storing it safely, and restoring it on demand without breaking encapsulation.

Architecture Diagram

User
  |
  v
Originator <--> Memento
  |
  v
Caretaker

Components

User

actor

Initiates commands to save or restore state

Originator

service

Creates a memento capturing its current state and restores state from a memento

Memento

data_object

Stores the internal state of the Originator without exposing details

Caretaker

manager

Keeps track of mementos and manages saving/restoring without inspecting state

Request Flow - 6 Hops

User → Originator

Originator → Memento

Originator → Caretaker

User → Caretaker

Caretaker → Originator

Originator → Originator

Failure Scenario

Component Fails:Caretaker

Impact:Loss of saved mementos means undo or restore operations fail

Mitigation:Implement persistent storage or backup for mementos to prevent data loss

Architecture Quiz - 3 Questions

Which component is responsible for storing the internal state snapshot without exposing details?

A Originator

B Memento

C Caretaker

D User

Design Principle

The Memento pattern separates state capture and storage from the object itself, preserving encapsulation while enabling undo functionality. It clearly divides responsibilities among Originator (state owner), Memento (state snapshot), and Caretaker (state manager).

Practice

1. What is the main purpose of the Memento pattern in system design?

easy

A. To create multiple instances of an object efficiently

B. To convert one interface to another compatible interface

C. To manage concurrent access to shared resources

D. To save and restore an object's state without exposing its internal details

Solution

1. Step 1: Understand the role of Memento pattern

   The Memento pattern is designed to capture and externalize an object's internal state so that it can be restored later without exposing the object's implementation details.

2. Step 2: Compare with other design patterns

   Other options describe different patterns: A is about object creation (Factory), C is about synchronization (Mutex), D is about interface compatibility (Adapter).

3. Final Answer:

   To save and restore an object's state without exposing its internal details -> Option D

4. Quick Check:

   Memento = Save & Restore State [OK]

Hint: Memento = save state secretly, no details shown [OK]

Common Mistakes:

• Confusing Memento with Factory or Adapter patterns
• Thinking it manages concurrency
• Assuming it changes object interfaces

2. Which of the following correctly represents the key components of the Memento pattern?

easy

A. Subject, Observer, ConcreteObserver

B. Originator, Memento, Caretaker

C. Client, Proxy, RealSubject

D. Component, Decorator, ConcreteComponent

Solution

1. Step 1: Identify components of Memento pattern

   The Memento pattern consists of three main parts: Originator (the object whose state is saved), Memento (the object storing the state), and Caretaker (manages mementos).

2. Step 2: Eliminate other patterns

   Options B, C, and D correspond to Observer, Proxy, and Decorator patterns respectively, which are unrelated to Memento.

3. Final Answer:

   Originator, Memento, Caretaker -> Option B

4. Quick Check:

   Components = Originator + Memento + Caretaker [OK]

Hint: Remember 3 parts: Originator, Memento, Caretaker [OK]

Common Mistakes:

• Mixing Memento components with Observer or Proxy
• Forgetting the Caretaker role
• Confusing Memento with Decorator pattern

3. Consider this simplified Python code using the Memento pattern:

class Memento:
    def __init__(self, state):
        self._state = state

class Originator:
    def __init__(self):
        self._state = ""
    def set_state(self, state):
        self._state = state
    def save(self):
        return Memento(self._state)
    def restore(self, memento):
        self._state = memento._state

originator = Originator()
originator.set_state("State1")
memento = originator.save()
originator.set_state("State2")
originator.restore(memento)
print(originator._state)

What will be printed?

medium

A. None

B. State2

C. State1

D. Error

Solution

1. Step 1: Trace state changes in Originator

   Initially, Originator's state is set to "State1". Then a Memento is saved capturing "State1". Next, state changes to "State2".

2. Step 2: Restore state from Memento

   Calling restore with the saved Memento sets the state back to "State1". The print statement outputs the restored state.

3. Final Answer:

   State1 -> Option C

4. Quick Check:

   Restore resets state to saved value [OK]

Hint: Restore sets state back to saved snapshot [OK]

Common Mistakes:

• Assuming print shows latest state before restore
• Confusing save and restore methods
• Expecting error due to private variable access

4. In the following code snippet, what is the main issue that breaks the Memento pattern?

class Originator:
    def __init__(self):
        self._state = ""
    def set_state(self, state):
        self._state = state
    def save(self):
        return self._state  # returns state directly
    def restore(self, memento):
        self._state = memento

originator = Originator()
originator.set_state("State1")
memento = originator.save()
originator.set_state("State2")
originator.restore(memento)
print(originator._state)

medium

A. The save method returns state directly, exposing internal details

B. The restore method does not update the state

C. The Originator class lacks a Memento class

D. The set_state method is missing

Solution

1. Step 1: Analyze the save method

   The save method returns the internal state directly instead of encapsulating it in a Memento object, exposing internal details.

2. Step 2: Understand Memento pattern principle

   The pattern requires hiding the internal state inside a Memento object to prevent external access. Returning raw state breaks encapsulation.

3. Final Answer:

   The save method returns state directly, exposing internal details -> Option A

4. Quick Check:

   Save must hide state in Memento [OK]

Hint: Save must return Memento, not raw state [OK]

Common Mistakes:

• Thinking restore method is faulty
• Believing Memento class is mandatory in code
• Ignoring encapsulation principle

5. You are designing a text editor with undo functionality using the Memento pattern. Which approach best balances memory usage and undo capability?

hard

A. Store a Memento only after significant changes or at checkpoints

B. Store a Memento after every single character change

C. Store all changes as raw text snapshots without Memento objects

D. Do not store any state; rely on user to retype

Solution

1. Step 1: Consider memory and undo tradeoff

   Storing a Memento after every character change (Store a Memento after every single character change) uses excessive memory and is inefficient.

2. Step 2: Evaluate checkpoint strategy

   Storing Mementos after significant changes or checkpoints (Store a Memento only after significant changes or at checkpoints) reduces memory use while allowing meaningful undo steps.

3. Step 3: Assess other options

   Store all changes as raw text snapshots without Memento objects wastes memory by storing raw snapshots without encapsulation; Do not store any state; rely on user to retype removes undo capability.

4. Final Answer:

   Store a Memento only after significant changes or at checkpoints -> Option A

5. Quick Check:

   Checkpoint Mementos balance memory and undo [OK]

Hint: Save states at checkpoints, not every keystroke [OK]

Common Mistakes:

• Saving state too frequently causing memory bloat
• Ignoring encapsulation by storing raw snapshots
• Not implementing undo at all