Adapter pattern in LLD - Scalability & System Analysis

Scalability Analysis - Adapter pattern

Growth Table: Adapter Pattern

Users	Changes in System
100 users	Single adapter instance handles requests; simple direct calls; low latency.
10,000 users	Multiple adapter instances needed; increased load on adaptee; possible latency increase.
1,000,000 users	Adapters become bottleneck; need load balancing; adaptee service may need scaling; caching considered.
100,000,000 users	Adapters fully distributed; sharding of adaptee services; asynchronous communication; heavy caching and CDN usage.

First Bottleneck

The adaptee service behind the adapter is the first bottleneck. As all adapter calls translate to adaptee calls, the adaptee's capacity limits overall throughput.

Scaling Solutions

• Horizontal scaling: Add more adapter instances behind a load balancer to distribute client requests.
• Adaptee scaling: Scale the adaptee service horizontally or vertically to handle increased load.
• Caching: Cache adaptee responses in adapters or a shared cache to reduce repeated calls.
• Asynchronous processing: Use message queues to decouple adapters from adaptee for heavy workloads.
• Sharding: Partition adaptee data/services to reduce load per instance.

Back-of-Envelope Cost Analysis

Assuming each adapter instance handles ~2000 concurrent requests:

• At 10,000 users: ~5 adapter instances needed.
• At 1,000,000 users: ~500 adapter instances; adaptee must handle ~1M QPS.
• Storage depends on caching size; e.g., 1MB cache per adapter instance = 500MB at 500 instances.
• Network bandwidth: 1M QPS * average 1KB request/response = ~1GB/s bandwidth needed.

Interview Tip

Start by explaining the adapter's role as a translator between incompatible interfaces. Then discuss how scaling affects both the adapter and adaptee. Identify the adaptee as the bottleneck and propose solutions like horizontal scaling, caching, and asynchronous processing. Use concrete numbers to show understanding of capacity limits.

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 load before scaling vertically or sharding.

Key Result

The adaptee service behind the adapter is the first bottleneck as user load grows; scaling requires distributing adapter instances, scaling adaptee services, and adding caching or asynchronous processing.

Practice

1. What is the main purpose of the Adapter pattern in system design?

easy

A. To improve the performance of a system by caching results

B. To allow incompatible interfaces to work together by converting one interface to another

C. To store data persistently in a database

D. To create multiple instances of a class

Solution

1. Step 1: Understand the Adapter pattern role

   The Adapter pattern acts as a bridge between two incompatible interfaces, allowing them to communicate.

2. Step 2: Compare with other options

   The other options describe different design goals unrelated to interface compatibility.

3. Final Answer:

   To allow incompatible interfaces to work together by converting one interface to another -> Option B

4. Quick Check:

   Adapter pattern = interface compatibility [OK]

Hint: Adapter = bridge between incompatible interfaces [OK]

Common Mistakes:

• Confusing Adapter with Singleton or Factory patterns
• Thinking Adapter improves performance directly
• Assuming Adapter stores data

2. Which of the following is the correct way to declare an Adapter class in a typical object-oriented language?

easy

A. class Adapter implements Adaptee { private TargetInterface target; }

B. class Adapter extends Adaptee implements TargetInterface { }

C. class Adapter extends TargetInterface { private Adaptee adaptee; }

D. class Adapter implements TargetInterface { private Adaptee adaptee; }

Solution

1. Step 1: Identify Adapter pattern structure

   The Adapter class implements the TargetInterface and holds a reference to the Adaptee to delegate calls.

2. Step 2: Analyze options

   class Adapter implements TargetInterface { private Adaptee adaptee; } correctly shows Adapter implementing TargetInterface and containing Adaptee. The other options misuse inheritance or interfaces.

3. Final Answer:

   class Adapter implements TargetInterface { private Adaptee adaptee; } -> Option D

4. Quick Check:

   Adapter implements target and wraps adaptee [OK]

Hint: Adapter implements target interface, holds adaptee instance [OK]

Common Mistakes:

• Using inheritance incorrectly instead of composition
• Confusing which interface to implement
• Swapping adaptee and target roles

3. Given this Adapter pattern code snippet, what will be the output?

class Adaptee {
  String specificRequest() { return "Adaptee response"; }
}

interface Target {
  String request();
}

class Adapter implements Target {
  private Adaptee adaptee = new Adaptee();
  public String request() {
    return adaptee.specificRequest();
  }
}

public class Main {
  public static void main(String[] args) {
    Target target = new Adapter();
    System.out.println(target.request());
  }
}

medium

A. Adaptee response

B. Target response

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Trace method calls in Adapter

   The Adapter's request() calls adaptee.specificRequest(), which returns "Adaptee response".

2. Step 2: Check main method output

   Main creates Adapter as Target and prints target.request(), so output is "Adaptee response".

3. Final Answer:

   Adaptee response -> Option A

4. Quick Check:

   Adapter delegates request() to adaptee.specificRequest() [OK]

Hint: Adapter.request() returns adaptee.specificRequest() result [OK]

Common Mistakes:

• Expecting 'Target response' instead of adaptee's
• Assuming compilation or runtime errors without cause
• Confusing method names

4. Identify the error in this Adapter pattern implementation:

interface Target {
  void request();
}

class Adaptee {
  void specificRequest() { System.out.println("Adaptee called"); }
}

class Adapter implements Target {
  Adaptee adaptee = new Adaptee();
  public void request() {
    specificRequest();
  }
}

medium

A. Adapter calls specificRequest() without referencing adaptee instance

B. Adaptee should implement Target interface

C. Adapter should extend Adaptee instead of implementing Target

D. Target interface method signature is incorrect

Solution

1. Step 1: Review Adapter's request() method

   Adapter calls specificRequest() directly, but specificRequest() belongs to Adaptee instance, so it should call adaptee.specificRequest().

2. Step 2: Validate other options

   Adaptee does not need to implement Target. Adapter implements Target correctly. Target interface method is correct.

3. Final Answer:

   Adapter calls specificRequest() without referencing adaptee instance -> Option A

4. Quick Check:

   Adapter must call adaptee.specificRequest() [OK]

Hint: Call adaptee methods via adaptee instance inside Adapter [OK]

Common Mistakes:

• Calling adaptee methods without instance prefix
• Misusing inheritance instead of composition
• Changing Target interface unnecessarily

5. You need to integrate a legacy payment system with a new e-commerce platform. The legacy system has a method processLegacyPayment(), but the platform expects processPayment(). How should you apply the Adapter pattern to solve this?

hard

A. Create a new payment system and discard the legacy system

B. Modify the legacy system to rename processLegacyPayment() to processPayment()

C. Create an Adapter class implementing the platform's payment interface and inside it call processLegacyPayment() of the legacy system

D. Use inheritance to make the legacy system extend the platform's payment interface

Solution

1. Step 1: Understand integration requirement

   The platform expects processPayment(), but legacy system only has processLegacyPayment().

2. Step 2: Apply Adapter pattern

   Create an Adapter that implements the platform interface and inside calls legacy's processLegacyPayment() to adapt calls without changing legacy code.

3. Step 3: Evaluate other options

   Modifying the legacy code is risky. Discarding the legacy system is costly. Using inheritance is not possible if the legacy system is not designed for it.

4. Final Answer:

   Create an Adapter class implementing the platform's payment interface and inside it call processLegacyPayment() of the legacy system -> Option C

5. Quick Check:

   Adapter wraps legacy method to match platform interface [OK]

Hint: Adapter wraps legacy calls to match new interface [OK]

Common Mistakes:

• Changing legacy code directly
• Ignoring Adapter pattern benefits
• Trying inheritance when not supported