Proxy pattern in LLD - Scalability & System Analysis

Scalability Analysis - Proxy pattern

Growth Table: Proxy Pattern Scaling

Users/Requests	What Changes?
100 users	Single proxy instance handles requests; minimal latency added; backend service load manageable.
10,000 users	Proxy starts to see higher concurrent connections; CPU and memory usage increase; backend load rises; caching at proxy beneficial.
1,000,000 users	Single proxy becomes bottleneck; needs horizontal scaling; caching and rate limiting essential; backend services require load balancing.
100,000,000 users	Multiple proxy clusters distributed geographically; global load balancing; advanced caching layers; backend sharding and microservices needed.

First Bottleneck

The proxy server itself is the first bottleneck as user requests grow. It handles all incoming traffic and forwards it to backend services. At high load, CPU, memory, and network bandwidth on the proxy limit throughput.

Scaling Solutions

• Horizontal Scaling: Add more proxy instances behind a load balancer to distribute traffic evenly.
• Caching: Implement caching at the proxy to reduce backend calls for repeated requests.
• Rate Limiting: Protect backend services by limiting request rates at the proxy.
• Geographical Distribution: Deploy proxy clusters closer to users to reduce latency and balance load.
• Backend Scaling: Use load balancers and sharding for backend services to handle increased traffic.

Back-of-Envelope Cost Analysis

Assuming each user sends 1 request per second:

• At 1,000 users: 1,000 RPS; one proxy server can handle ~3,000 RPS comfortably.
• At 10,000 users: 10,000 RPS; need ~4 proxy servers (10,000/3,000 ≈ 4) for load balancing.
• At 1,000,000 users: 1,000,000 RPS; requires ~334 proxy servers; caching reduces backend load.
• Bandwidth: Each request ~10KB; at 1M RPS -> 10GB/s bandwidth needed at proxy layer.
• Storage: Proxy caching requires memory proportional to cache size; e.g., 100GB RAM cluster for effective caching.

Interview Tip

Start by explaining the proxy role and its load. Discuss bottlenecks clearly, then propose scaling solutions step-by-step: horizontal scaling, caching, rate limiting, and backend scaling. Use numbers to justify your choices and show understanding of trade-offs.

Self Check

Your database handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Add read replicas and implement caching to reduce direct database queries before scaling vertically or sharding.

Key Result

The proxy server becomes the first bottleneck as user requests grow; horizontal scaling and caching at the proxy layer are key to maintaining performance.

Practice

1.

What is the main purpose of the Proxy pattern in system design?

easy

A. To replace the real object permanently with a new one

B. To control access to an object by using a placeholder

C. To directly modify the real object's data

D. To create multiple instances of an object

Solution

1. Step 1: Understand the Proxy pattern role

   The Proxy pattern acts as a stand-in or placeholder to control access to another object.

2. Step 2: Compare options with the pattern definition

   Only To control access to an object by using a placeholder correctly describes controlling access using a placeholder, which matches the Proxy pattern.

3. Final Answer:

   To control access to an object by using a placeholder -> Option B

4. Quick Check:

   Proxy pattern = control access placeholder [OK]

Hint: Proxy controls access via placeholder, not object creation [OK]

Common Mistakes:

• Confusing Proxy with Singleton pattern
• Thinking Proxy modifies the real object directly
• Assuming Proxy replaces the real object permanently

2.

Which of the following is the correct way to declare a proxy class that implements the same interface as the real object?

interface Image {
    void display();
}

// Which proxy class declaration is correct?

easy

A. interface ImageProxy implements Image { void display(); }

B. class ImageProxy extends Image { void display() { /* control access */ } }

C. class ImageProxy { void display() { /* control access */ } }

D. class ImageProxy implements Image { void display() { /* control access */ } }

Solution

1. Step 1: Recall interface implementation syntax

   In many languages, a class implements an interface using the 'implements' keyword, not 'extends'.

2. Step 2: Check each option's correctness

   class ImageProxy implements Image { void display() { /* control access */ } } correctly declares a class implementing the interface. class ImageProxy extends Image { void display() { /* control access */ } } wrongly uses 'extends' for an interface. class ImageProxy { void display() { /* control access */ } } misses interface implementation. interface ImageProxy implements Image { void display(); } wrongly declares an interface implementing another interface.

3. Final Answer:

   class ImageProxy implements Image { void display() { /* control access */ } } -> Option D

4. Quick Check:

   Proxy class implements interface = class ImageProxy implements Image { void display() { /* control access */ } } [OK]

Hint: Proxy class must implement interface, not extend it [OK]

Common Mistakes:

• Using 'extends' instead of 'implements' for interfaces
• Not implementing the interface at all
• Declaring proxy as an interface instead of a class

3.

Given the following code snippet, what will be the output?

class RealImage {
    void display() { System.out.println("Displaying real image"); }
}

class ProxyImage {
    RealImage realImage;
    void display() {
        if (realImage == null) {
            realImage = new RealImage();
        }
        realImage.display();
    }
}

ProxyImage proxy = new ProxyImage();
proxy.display();
proxy.display();

medium

A. Displaying real image
Displaying real image

B. Displaying real image

C. No output

D. NullPointerException

Solution

1. Step 1: Analyze the first call to display()

   Since realImage is null initially, it is created and then display() prints "Displaying real image".

2. Step 2: Analyze the second call to display()

   realImage is no longer null, so display() is called directly again, printing the same message.

3. Final Answer:

   Displaying real image Displaying real image -> Option A

4. Quick Check:

   Lazy loading calls display twice [OK]

Hint: Proxy lazy loads real object, so display prints twice [OK]

Common Mistakes:

• Assuming display prints only once
• Expecting NullPointerException due to uninitialized realImage
• Thinking proxy suppresses output

4.

Identify the error in the following proxy implementation and select the fix:

class Proxy {
    RealObject real;
    void request() {
        real.request();
    }
}

medium

A. Remove the proxy and call RealObject directly

B. Change method name from request() to execute()

C. Initialize 'real' before calling request()

D. Make 'real' a static variable

Solution

1. Step 1: Identify the problem in the code

   The proxy calls real.request() but 'real' is never initialized, causing a NullPointerException.

2. Step 2: Determine the correct fix

   Initializing 'real' before calling request() ensures the real object exists and avoids errors.

3. Final Answer:

   Initialize 'real' before calling request() -> Option C

4. Quick Check:

   Uninitialized real causes error; initialize it [OK]

Hint: Always initialize real object before proxy calls it [OK]

Common Mistakes:

• Ignoring null initialization error
• Changing method names unnecessarily
• Removing proxy defeats pattern purpose

5.

You are designing a system where loading a large image file is expensive. You want to use the Proxy pattern to improve performance by loading the image only when needed. Which of the following design choices best applies the Proxy pattern for this scenario?

hard

A. Create a proxy class that holds the image filename and loads the real image only when display() is called

B. Load the real image immediately when the proxy is created to avoid delays

C. Use a proxy that duplicates the image data to speed up access

D. Replace the real image with a smaller thumbnail permanently

Solution

1. Step 1: Understand the problem context

   Loading large images is expensive, so delaying loading until necessary improves performance.

2. Step 2: Match design choices to Proxy pattern benefits

   Create a proxy class that holds the image filename and loads the real image only when display() is called uses lazy loading by deferring real image loading until display() is called, which fits Proxy pattern use.

3. Final Answer:

   Create a proxy class that holds the image filename and loads the real image only when display() is called -> Option A

4. Quick Check:

   Lazy loading with proxy = Create a proxy class that holds the image filename and loads the real image only when display() is called [OK]

Hint: Proxy delays expensive load until needed (lazy loading) [OK]

Common Mistakes:

• Loading real object immediately defeats proxy benefit
• Duplicating data wastes resources
• Replacing with thumbnail changes object behavior