Cache interceptor in NestJS - Mini Project: Build & Apply

Cache Interceptor in NestJS

📖 Scenario: You are building a simple NestJS API that returns user data. To improve performance, you want to add caching so repeated requests return faster.

🎯 Goal: Build a NestJS cache interceptor that caches the response of a controller method.

📋 What You'll Learn

Create a cache interceptor class that implements NestInterceptor

Add a cache time-to-live (TTL) configuration variable

Use the cache manager to store and retrieve cached responses

Apply the cache interceptor to a controller method

💡 Why This Matters

🌍 Real World

Caching API responses improves performance and reduces server load in real-world web applications.

💼 Career

Understanding interceptors and caching is important for backend developers working with NestJS or similar frameworks.

1

Create the Cache Interceptor Class

Create a class called CacheInterceptor that implements NestInterceptor from @nestjs/common. Import Injectable and decorate the class with it.

NestJS

import { Injectable, NestInterceptor } from '@nestjs/common';

// Your code here

Hint

Use implements NestInterceptor and decorate the class with @Injectable().

2

Add Cache TTL Configuration

Inside the CacheInterceptor class, add a private readonly property called ttl and set it to 30 (seconds).

NestJS

import { Injectable, NestInterceptor } from '@nestjs/common';

@Injectable()
export class CacheInterceptor implements NestInterceptor {
  private readonly ttl = 30;

  intercept(context, next) {
    // Your caching logic will go here
  }
}

// Your code here

Hint

Declare private readonly ttl = 30; inside the class but outside the intercept method.

3

Implement Cache Logic in Intercept Method

In the intercept method, import ExecutionContext and CallHandler from @nestjs/common. Use next.handle() to get the response stream. For now, just return next.handle() without caching.

NestJS

import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';

@Injectable()
export class CacheInterceptor implements NestInterceptor {
  private readonly ttl = 30;

  intercept(context: ExecutionContext, next: CallHandler) {
    // Return the response stream
    return next.handle();
  }
}

// Your code here

Hint

Make sure to type the parameters as ExecutionContext and CallHandler, and return next.handle().

4

Apply Cache Interceptor to Controller Method

Create a controller class called UserController with a method getUser that returns a string 'User data'. Import and apply the CacheInterceptor to the getUser method using the @UseInterceptors(CacheInterceptor) decorator.

NestJS

import { Controller, Get, UseInterceptors } from '@nestjs/common';

@Controller('user')
export class UserController {
  @UseInterceptors(CacheInterceptor)
  @Get()
  getUser() {
    // Return user data string
  }
}

// Your code here

Hint

Use @UseInterceptors(CacheInterceptor) above the getUser method and return the exact string 'User data'.

Practice

1. What is the main purpose of the CacheInterceptor in NestJS?

easy

A. To handle authentication and authorization

B. To store responses and speed up repeated requests

C. To log all incoming requests

D. To format response data before sending

Solution

1. Step 1: Understand the role of CacheInterceptor

   The CacheInterceptor caches HTTP responses to avoid recomputing them on repeated requests.

2. Step 2: Compare with other options

   Logging, authentication, and formatting are handled by other interceptors or middleware, not CacheInterceptor.

3. Final Answer:

   To store responses and speed up repeated requests -> Option B

4. Quick Check:

   CacheInterceptor caches responses = C [OK]

Hint: CacheInterceptor caches responses to speed up requests [OK]

Common Mistakes:

• Confusing CacheInterceptor with logging interceptors
• Thinking it handles security tasks
• Assuming it formats data

2. Which of the following is the correct way to apply the CacheInterceptor to a controller method in NestJS?

easy

A. Add CacheInterceptor in the constructor of the controller

B. @Cacheable() decorator on the method

C. @UseInterceptors(CacheInterceptor) above the method

D. Use @CacheInterceptor() as a method parameter

Solution

1. Step 1: Recall how to apply interceptors in NestJS

   Interceptors are applied using the @UseInterceptors() decorator above methods or controllers.

2. Step 2: Check each option

   Only @UseInterceptors(CacheInterceptor) above the method uses the correct decorator syntax. Options B, C, and D are invalid or do not exist in NestJS.

3. Final Answer:

   @UseInterceptors(CacheInterceptor) above the method -> Option C

4. Quick Check:

   UseInterceptors decorator = A [OK]

Hint: Use @UseInterceptors(CacheInterceptor) to apply caching [OK]

Common Mistakes:

• Using non-existent @Cacheable decorator
• Trying to inject interceptor in constructor
• Passing interceptor as method parameter

3. Given this code snippet, what will be the output on the second request to /hello?

import { Controller, Get, UseInterceptors } from '@nestjs/common';
import { CacheInterceptor } from '@nestjs/cache-manager';

@Controller()
export class HelloController {
  private count = 0;

  @UseInterceptors(CacheInterceptor)
  @Get('hello')
  getHello() {
    this.count++;
    return `Hello count: ${this.count}`;
  }
}

medium

A. "Hello count: 1" first request, "Hello count: 2" second request

B. Error because count is private

C. "Hello count: 0" on both requests

D. "Hello count: 1" on first and second request

Solution

1. Step 1: Understand CacheInterceptor behavior

   CacheInterceptor caches the response of the first request and returns it for subsequent requests without calling the method again.

2. Step 2: Analyze the count variable increment

   On the first request, count increments to 1 and returns "Hello count: 1". On the second request, the cached response is returned, so count does not increment again.

3. Final Answer:

   "Hello count: 1" on first and second request -> Option D

4. Quick Check:

   Cache returns cached response = B [OK]

Hint: CacheInterceptor returns cached response, method not re-executed [OK]

Common Mistakes:

• Assuming count increments on every request
• Thinking private variables cause errors here
• Ignoring caching effect on method execution

4. Identify the error in this NestJS CacheInterceptor usage:

import { Controller, Get, UseInterceptors } from '@nestjs/common';
import { CacheInterceptor } from '@nestjs/cache-manager';

@Controller()
export class TestController {
  @UseInterceptors(CacheInterceptor())
  @Get('test')
  test() {
    return 'Test response';
  }
}

medium

A. CacheInterceptor should not be called as a function

B. Missing CacheModule import in the controller

C. Method test must be async to use CacheInterceptor

D. CacheInterceptor must be applied globally, not per method

Solution

1. Step 1: Check how CacheInterceptor is applied

   CacheInterceptor is a class, not a function, so it should be passed without parentheses to @UseInterceptors().

2. Step 2: Verify other options

   CacheModule import is required but not shown here (not an error in this snippet). Method does not need to be async. CacheInterceptor can be applied per method.

3. Final Answer:

   CacheInterceptor should not be called as a function -> Option A

4. Quick Check:

   Use class name without () = A [OK]

Hint: Pass CacheInterceptor class, not call it as function [OK]

Common Mistakes:

• Calling CacheInterceptor with parentheses
• Assuming method must be async
• Thinking CacheModule import error here

5. You want to customize the cache key for a route in NestJS using CacheInterceptor. Which approach correctly achieves this?

hard

A. Extend CacheInterceptor and override the trackBy method to return a custom key

B. Pass a custom key string directly to @UseInterceptors(CacheInterceptor)

C. Set a custom key in the controller constructor

D. Use @CacheKey('customKey') decorator on the method

Solution

1. Step 1: Understand how to customize cache keys

   CacheInterceptor allows customization by extending it and overriding the trackBy method to define how keys are generated.

2. Step 2: Evaluate other options

   Passing a string to @UseInterceptors() is invalid. Setting keys in constructor or using @CacheKey decorator does not exist in NestJS.

3. Final Answer:

   Extend CacheInterceptor and override the trackBy method to return a custom key -> Option A

4. Quick Check:

   Override trackBy method = D [OK]

Hint: Override trackBy method in extended CacheInterceptor [OK]

Common Mistakes:

• Trying to pass key directly to decorator
• Using non-existent @CacheKey decorator
• Setting keys in constructor instead of method