State pattern in LLD - System Design Exercise

Design: State Pattern Implementation

Design and implement the state pattern for a single context object with multiple states. Out of scope are concurrency handling, persistence, and UI integration.

Functional Requirements

FR1: Design a system that allows an object to change its behavior when its internal state changes.

FR2: The system should encapsulate state-specific behavior in separate state objects.

FR3: Clients should interact with the context object without knowing the current state.

FR4: Support adding new states without modifying existing code.

FR5: Ensure that state transitions are handled cleanly and maintainably.

Non-Functional Requirements

NFR1: The system should be easy to extend with new states.

NFR2: State transitions should be explicit and controlled.

NFR3: The design should avoid large conditional statements for state behavior.

NFR4: The system should be maintainable and testable.

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Context object holding a reference to a state object

State interface or abstract class defining state-specific methods

Concrete state classes implementing the state interface

State transition logic within states or context

Design Patterns

State pattern for encapsulating state behavior

Strategy pattern for interchangeable behaviors

Factory pattern for creating state instances

Observer pattern if external components need state change notifications

Reference Architecture

  +----------------+
  |    Context     |
  |----------------|
  | - state: State |
  | + request()    |
  +--------+-------+
           |
           v
  +----------------+      +----------------+      +----------------+
  |   State (I)    |<-----| ConcreteStateA |      | ConcreteStateB |
  | + handle()     |      | + handle()     |      | + handle()     |
  +----------------+      +----------------+      +----------------+

Components

Context

Class in any OOP language

Holds current state and delegates requests to it

State Interface

Interface or abstract class

Defines methods for state-specific behavior

Concrete States

Classes implementing State interface

Implement behavior for each specific state and handle transitions

Request Flow

1. Client calls a method on the Context object.

2. Context delegates the call to the current State object.

3. State object executes behavior specific to that state.

4. State object may change the Context's state to another State object.

5. Next client call will be handled by the new State object.

Database Schema

Not applicable for this design pattern as it focuses on object behavior and state management within application memory.

Scaling Discussion

Bottlenecks

Adding many states can increase the number of classes and complexity.

State transitions can become hard to track if scattered across states.

If states share common data, synchronization might be needed in concurrent environments.

Solutions

Use a centralized state transition manager or context to control transitions.

Group related states or use hierarchical state machines to reduce complexity.

Document state transitions clearly and use diagrams for maintainability.

For concurrency, use thread-safe state objects or synchronization mechanisms.

Interview Tips

Time: Spend 10 minutes explaining the problem and requirements, 15 minutes designing the pattern with diagrams and code sketches, and 5 minutes discussing scaling and extensions.

Explain how the State pattern helps avoid large conditional statements.

Show how behavior changes by switching state objects.

Discuss how new states can be added without modifying existing code.

Mention how state transitions are managed and encapsulated.

Highlight maintainability and testability benefits.