When preparing for system design interviews, mastering design patterns is essential. These patterns provide tried-and-true solutions for common software design problems, ensuring that your system is scalable, maintainable, and efficient. Below are some of the most important design patterns that you should be familiar with for system design interviews:
1. Singleton Pattern
Purpose: Ensures a class has only one instance and provides a global point of access to it.
Use Case: When you need a single, shared resource throughout the lifetime of the application, such as a database connection pool, a logging service, or a configuration manager.
Example: A logging utility in a distributed system to ensure consistent logging across services.
Pros:
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Easy to implement.
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Reduces memory consumption when a single instance is shared.
Cons:
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Can be difficult to test.
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Introduces global state, which might lead to hidden dependencies.
2. Factory Method Pattern
Purpose: Defines an interface for creating objects, but allows subclasses to alter the type of objects that will be created.
Use Case: When you need to create instances of a class, but the exact type of the object is determined by external conditions or needs to be delayed.
Example: A document processing system where different document types (PDF, Word, etc.) are created by different factories.
Pros:
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Flexibility to choose different subclasses.
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Encapsulates object creation.
Cons:
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May introduce unnecessary complexity.
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Tight coupling between the client and the factory.
3. Observer Pattern
Purpose: Allows a subject to notify its observers when a change occurs. This is useful in scenarios where multiple objects need to be updated in response to a single event.
Use Case: Often used in event-driven systems where you want different components to react to state changes in a core system. Examples include message queues, real-time notifications, and stock price monitoring.
Example: A stock price tracker where multiple clients are observing a particular stock’s price.
Pros:
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Decouples subject and observer.
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Allows dynamic subscription to events.
Cons:
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Can lead to performance issues if there are many observers.
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Managing circular dependencies can be tricky.
4. Decorator Pattern
Purpose: Allows behavior to be added to an object dynamically, without affecting the behavior of other objects from the same class.
Use Case: When you want to add responsibilities to an object dynamically. For example, adding features like logging, caching, or compression to objects in a scalable and flexible way.
Example: In an e-commerce system, the pricing of an item can be dynamically decorated with discounts, taxes, or shipping fees.
Pros:
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Allows flexibility in adding functionality.
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Avoids subclassing.
Cons:
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Can lead to many small classes that are hard to manage.
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Might make the code harder to follow.
5. Strategy Pattern
Purpose: Enables selecting an algorithm at runtime. It defines a family of algorithms, encapsulates each one, and makes them interchangeable.
Use Case: When you need to switch between different algorithms or behaviors dynamically without altering the clients that use them.
Example: Payment processing in a shopping cart where users can choose between multiple payment methods (Credit Card, PayPal, etc.).
Pros:
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Simplifies code by delegating responsibility to different strategies.
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Increases code flexibility and extensibility.
Cons:
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Increases the number of classes.
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Not ideal when the strategy selection is very complex.
6. Adapter Pattern
Purpose: Converts one interface to another, so that incompatible classes can work together.
Use Case: When integrating new code into an existing codebase, but the new code doesn’t follow the expected interface.
Example: A payment gateway that needs to integrate with multiple third-party systems that expose different APIs.
Pros:
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Promotes reusability of existing code.
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Makes incompatible interfaces compatible.
Cons:
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Increases the number of classes and objects.
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Can lead to confusion if used too frequently.
7. Proxy Pattern
Purpose: Provides a surrogate or placeholder for another object. This is typically used when access to the original object needs to be controlled, such as when access is expensive, sensitive, or needs to be delayed.
Use Case: When implementing a system with resource-heavy operations that should be deferred or only performed under certain conditions.
Example: A caching layer in a web service that prevents expensive calls to the database by storing previous results.
Pros:
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Reduces resource usage.
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Controls access to the real object.
Cons:
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Additional layer of abstraction can lead to complexity.
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Can introduce latency in some cases.
8. Command Pattern
Purpose: Encapsulates a request as an object, thereby allowing for parameterization of clients with queues, requests, and operations.
Use Case: When implementing undo/redo functionality or in a system where actions are queued for execution at a later time.
Example: In a remote control system, the commands to turn on/off different appliances are encapsulated into command objects.
Pros:
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Provides a uniform interface for different types of requests.
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Supports undo/redo functionality.
Cons:
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Adds complexity to the system.
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Can lead to bloated systems if overused.
9. State Pattern
Purpose: Allows an object to change its behavior when its internal state changes. The object will appear to change its class.
Use Case: When an object’s behavior is influenced by its internal state, and it must transition between different states during its lifecycle.
Example: A state machine for an order processing system (Order Placed → Shipped → Delivered → Paid).
Pros:
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Organizes state transitions in one place.
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Can simplify the system when states change frequently.
Cons:
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Introduces a lot of classes if there are many states.
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State transitions can become hard to manage.
10. Chain of Responsibility Pattern
Purpose: Passes a request along a chain of handlers. Each handler processes the request or passes it to the next handler in the chain.
Use Case: When multiple objects can handle a request, but the handler isn’t known in advance.
Example: Logging systems where different levels of logs (debug, info, error) are handled by different loggers.
Pros:
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Decouples sender and receiver.
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Allows flexible chaining of handlers.
Cons:
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Can lead to long chains and delays.
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Hard to debug when multiple handlers are involved.
Conclusion
Mastering these design patterns will not only help you during system design interviews but also improve your ability to design scalable, maintainable, and efficient software. Understanding when and how to apply each of these patterns can make the difference between building a well-architected system and a complex, hard-to-maintain one.