Proxy pattern in LLD - Architecture Diagram

System Overview - Proxy pattern

The Proxy pattern provides a placeholder or surrogate for another object to control access to it. It is used to add additional functionality like access control, lazy loading, or logging without changing the original object's code.

Key requirements include controlling access to the real object, possibly delaying its creation, and adding extra behavior transparently.

Architecture Diagram

User
  |
  v
Proxy
  |
  v
Real Subject
  |
  v
Resource

Components

User

client

Initiates requests to access the resource

Proxy

proxy

Controls access to the Real Subject, adds extra behavior like access control or caching

Real Subject

service

The actual object that performs the core operations

Resource

resource

Underlying data or service used by the Real Subject

Request Flow - 7 Hops

User → Proxy

Proxy → Proxy

Proxy → Real Subject

Real Subject → Resource

Resource → Real Subject

Real Subject → Proxy

Proxy → User

Failure Scenario

Component Fails:Proxy

Impact:User requests cannot be processed or access control fails, potentially exposing Real Subject directly or blocking access

Mitigation:Implement fallback mechanisms or redundant proxies; ensure Real Subject has its own access controls

Architecture Quiz - 3 Questions

What is the main role of the Proxy in the Proxy pattern?

A To store the underlying resource data

B To replace the User in sending requests

C To control access to the Real Subject and add extra behavior

D To directly process all user requests without forwarding

Design Principle

The Proxy pattern demonstrates how to add a control layer between a client and a service to manage access, add caching, or delay expensive operations, improving modularity and flexibility without changing the core service.

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