Proxy pattern in LLD - System Design Exercise

Design: Proxy Pattern Implementation

Design and implement the proxy pattern for a generic service object. Out of scope are specific business logic of the real object and network communication details.

Functional Requirements

FR1: Provide a surrogate or placeholder for another object to control access to it

FR2: Support lazy initialization to delay object creation until needed

FR3: Allow access control to restrict or log requests to the real object

FR4: Enable caching of expensive operations to improve performance

FR5: Maintain the same interface as the real object for seamless substitution

Non-Functional Requirements

NFR1: The proxy should not change the interface of the real object

NFR2: Must handle concurrent access safely if used in multithreaded environments

NFR3: Should add minimal overhead to method calls

NFR4: Support scalability for multiple proxy instances

NFR5: Latency for proxy operations should be under 50ms for typical calls

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Subject interface defining common methods

Real subject implementing actual business logic

Proxy class implementing the same interface and controlling access

Client interacting only with the subject interface

Design Patterns

Decorator pattern for adding responsibilities dynamically

Adapter pattern for interface compatibility

Singleton pattern if proxy manages a single resource

Lazy initialization for deferred object creation

Reference Architecture

Client
  |
  v
Proxy (implements Subject interface)
  |
  v
Real Subject (implements Subject interface)

Components

Subject Interface

Abstract Interface or Base Class

Defines common methods for Real Subject and Proxy to implement

Real Subject

Concrete Class

Implements actual business logic and operations

Proxy

Concrete Class

Controls access to Real Subject, adds lazy loading, caching, or access control

Client

Any

Uses Subject interface to interact with Real Subject via Proxy transparently

Request Flow

1. Client calls method on Proxy object via Subject interface

2. Proxy checks if Real Subject is initialized; if not, creates it (lazy initialization)

3. Proxy optionally checks permissions or logs the request (access control)

4. Proxy forwards the method call to Real Subject

5. Real Subject executes the method and returns result

6. Proxy optionally caches the result before returning to Client

7. Client receives the result as if directly from Real Subject

Database Schema

Not applicable for Proxy pattern design as it focuses on object interaction and control rather than data storage.

Scaling Discussion

Bottlenecks

Proxy object becoming a performance bottleneck if it adds heavy processing

Concurrency issues if multiple clients access the proxy and real subject simultaneously

Memory overhead if many proxy instances hold references to real objects

Latency increase due to additional proxy layer

Complexity in managing lifecycle of real objects behind proxies

Solutions

Optimize proxy logic to minimize overhead and avoid blocking operations

Use thread-safe patterns or synchronization inside proxy for concurrent access

Implement proxy pooling or reuse to reduce memory footprint

Use asynchronous calls or caching to reduce latency impact

Clearly define ownership and lifecycle management policies for real objects

Interview Tips

Time: Spend 10 minutes explaining the problem and proxy pattern concept, 15 minutes designing the components and data flow, and 10 minutes discussing scaling and trade-offs.

Explain the purpose of the proxy pattern as a control layer

Describe different types of proxies and their use cases

Show how proxy and real subject share the same interface

Discuss lazy initialization and access control benefits

Highlight potential bottlenecks and how to mitigate them

Emphasize clean separation of concerns and interface transparency