Cache decorators in NestJS - Mini Project: Build & Apply

Using Cache Decorators in NestJS

📖 Scenario: You are building a simple NestJS service that fetches user data. To improve performance, you want to cache the results of the data fetching method.

🎯 Goal: Build a NestJS service that uses the @Cacheable() decorator to cache the result of a method that returns user information.

📋 What You'll Learn

Create a NestJS service class named UserService.

Add a method getUser() that returns a user object with id and name.

Add a cache configuration variable cacheTTL set to 30 seconds.

Use the @Cacheable() decorator on getUser() with the ttl set to cacheTTL.

Ensure the service is ready to be used in a NestJS module.

💡 Why This Matters

🌍 Real World

Caching is used in real-world applications to speed up repeated data fetching and reduce load on databases or APIs.

💼 Career

Understanding cache decorators in NestJS is important for backend developers to optimize performance and scalability of server applications.

1

Create the UserService class with getUser method

Create a NestJS service class called UserService with a method getUser() that returns an object { id: 1, name: 'Alice' }.

NestJS

export class UserService {
  // Your code here
}

Hint

Define a class named UserService and add a method getUser() that returns the user object.

2

Add cacheTTL configuration variable

Inside the UserService class, add a variable cacheTTL and set it to 30 to represent 30 seconds.

NestJS

export class UserService {
  cacheTTL = 30;

  getUser() {
    return { id: 1, name: 'Alice' };
  }
  // Your code here
}

Hint

Add a class property cacheTTL and assign it the number 30.

3

Import and apply @Cacheable decorator with ttl

Import @Cacheable from @nestjs/cache-manager and apply it as a decorator on the getUser() method. Set the decorator option ttl to the class variable cacheTTL.

NestJS

import { Cacheable } from '@nestjs/cache-manager';

export class UserService {
  cacheTTL = 30;

  @Cacheable({ ttl: this.cacheTTL })
  getUser() {
    return { id: 1, name: 'Alice' };
  }
  // Your code here
}

Hint

Import @Cacheable and decorate getUser() with @Cacheable({ ttl: 30 }). Use the number 30 directly for simplicity.

4

Export UserService for module use

Add export keyword before the UserService class declaration to make it available for import in other modules.

NestJS

import { Cacheable } from '@nestjs/cache-manager';

// Your code here

export class UserService {
  cacheTTL = 30;

  @Cacheable({ ttl: 30 })
  getUser() {
    return { id: 1, name: 'Alice' };
  }
}

Hint

Make sure the class declaration starts with export class UserService.

Practice

1. What is the main purpose of using cache decorators like @Cacheable in a NestJS service method?

easy

A. To store the method's result and reuse it for faster responses

B. To log every method call for debugging

C. To automatically retry failed method calls

D. To encrypt the method's output before sending

Solution

1. Step 1: Understand cache decorator role

   Cache decorators like @Cacheable save the output of a method so it can be reused without recalculating.

2. Step 2: Identify the benefit

   This reuse speeds up responses by avoiding repeated work.

3. Final Answer:

   To store the method's result and reuse it for faster responses -> Option A

4. Quick Check:

   Cache decorators improve speed by storing results [OK]

Hint: Cache decorators save method results to speed up calls [OK]

Common Mistakes:

• Thinking cache decorators log method calls
• Confusing cache with retry logic
• Assuming cache encrypts data

2. Which of the following is the correct way to apply a cache decorator with a 10-second TTL on a NestJS method?

easy

A. @Cacheable({ ttl: 10 })

B. @CacheEvict({ ttl: 10 })

C. @Cacheable(ttl = 10)

D. @Cacheable(ttl: 10)

Solution

1. Step 1: Recall correct decorator syntax

   The @Cacheable decorator accepts an options object with ttl as a key.

2. Step 2: Identify correct object syntax

   Correct syntax uses curly braces and colon: { ttl: 10 }.

3. Final Answer:

   @Cacheable({ ttl: 10 }) -> Option A

4. Quick Check:

   Use object with ttl key inside @Cacheable [OK]

Hint: Use @Cacheable({ ttl: number }) with curly braces [OK]

Common Mistakes:

• Using parentheses with assignment inside decorator
• Using @CacheEvict to set TTL
• Using colon without braces

3. Given this NestJS service method with caching:

@Cacheable({ ttl: 5 })
async getData() {
  console.log('Fetching data');
  return 'data';
}

What will be printed if getData() is called three times within 5 seconds?

medium

A. Fetching data (printed 3 times)

B. Fetching data (printed once)

C. No output printed

D. Fetching data (printed twice)

Solution

1. Step 1: Understand caching effect on method calls

   The first call runs the method and caches the result for 5 seconds.

2. Step 2: Analyze subsequent calls within TTL

   Calls within 5 seconds return cached result without running method, so no console log.

3. Final Answer:

   Fetching data (printed once) -> Option B

4. Quick Check:

   Cache prevents repeated method execution within TTL [OK]

Hint: Method logs once, cached calls skip execution [OK]

Common Mistakes:

• Assuming method runs every call ignoring cache
• Thinking cache disables method completely
• Confusing TTL with infinite caching

4. Consider this code snippet:

@Cacheable({ ttl: 10 })
async fetchData() {
  return await this.httpService.get('url');
}

@CacheEvict()
clearCache() {
  // clears cache
}

What is the main issue with the @CacheEvict usage here?

medium

A. The method must be async to use @CacheEvict

B. It should be @Cacheable instead of @CacheEvict

C. It lacks a key or target to specify which cache to clear

D. TTL must be set on @CacheEvict as well

Solution

1. Step 1: Understand @CacheEvict purpose

   @CacheEvict clears cached data, but needs to know which cache entry or key to clear.

2. Step 2: Identify missing key or target

   The code does not specify a key or target, so it cannot clear the intended cache.

3. Final Answer:

   It lacks a key or target to specify which cache to clear -> Option C

4. Quick Check:

   @CacheEvict needs key/target to clear cache [OK]

Hint: CacheEvict must specify what cache to clear [OK]

Common Mistakes:

• Using CacheEvict without specifying key
• Confusing CacheEvict with Cacheable
• Assuming async required for CacheEvict

5. You want to cache a method result but clear the cache automatically when a related update method runs. Which combination of decorators correctly achieves this in NestJS?

hard

A. Use @CacheEvict on fetch method and @Cacheable on update method

B. Use @CacheEvict on both methods to clear cache

C. Use @Cacheable on both methods to cache results

D. Use @Cacheable on the fetch method and @CacheEvict on the update method with matching cache keys

Solution

1. Step 1: Understand caching and eviction roles

   @Cacheable caches method results; @CacheEvict clears cache entries.

2. Step 2: Apply decorators to correct methods

   Cache the fetch method result, and clear that cache when update method runs using @CacheEvict with matching keys.

3. Final Answer:

   Use @Cacheable on the fetch method and @CacheEvict on the update method with matching cache keys -> Option D

4. Quick Check:

   Cache fetch, evict on update with matching keys [OK]

Hint: Cache fetch, evict on update with matching keys [OK]

Common Mistakes:

• Evicting cache on fetch method instead of update
• Caching update method results unnecessarily
• Not matching cache keys between decorators