Cache decorators in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to apply the cache decorator to the method.

NestJS

@[1]()
getData() {
  return 'data';
}

Drag options to blanks, or click blank then click option'

ACacheKey

BCacheable

CCacheTTL

DCacheInterceptor

Attempts:

3 left

💡 Hint

Common Mistakes

Using @CacheKey instead of @Cacheable

Using @CacheTTL which sets time but does not enable caching

Using @CacheInterceptor which is for interceptors, not decorators

The @Cacheable() decorator marks the method to use caching.

2fill in blank

medium

Complete the code to set a custom cache key for the method.

NestJS

@Cacheable()
@[1]('custom_key')
fetchItems() {
  return ['item1', 'item2'];
}

Drag options to blanks, or click blank then click option'

ACacheTTL

BCacheInterceptor

CCacheKey

DCacheConfig

Attempts:

3 left

💡 Hint

Common Mistakes

Using @CacheTTL which sets time, not key

Using @CacheInterceptor which is unrelated here

Using @CacheConfig which is for global config

The @CacheKey('custom_key') decorator sets a custom key for caching.

3fill in blank

hard

Fix the error in the code to set cache time-to-live (TTL) correctly.

NestJS

@Cacheable()
@[1](3000)
getUsers() {
  return ['user1', 'user2'];
}

Drag options to blanks, or click blank then click option'

ACacheTTL

BCacheKey

CCacheable

DCacheInterceptor

Attempts:

3 left

💡 Hint

Common Mistakes

Using @CacheKey instead of @CacheTTL

Using @Cacheable with a number argument which is invalid

Using @CacheInterceptor which is unrelated

The @CacheTTL(3000) decorator sets the cache expiration time in seconds.

4fill in blank

hard

Fill both blanks to apply caching with a custom key and TTL.

NestJS

@Cacheable()
@[1]('session_data')
@[2](600)
getSession() {
  return { id: 1, user: 'Alice' };
}

Drag options to blanks, or click blank then click option'

ACacheKey

BCacheTTL

CCacheInterceptor

DCacheConfig

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up CacheKey and CacheTTL decorators

Using CacheInterceptor which is not a decorator here

Using CacheConfig which is for global settings

@CacheKey('session_data') sets the cache key, and @CacheTTL(600) sets the cache duration.

5fill in blank

hard

Fill all three blanks to create a cached method with a custom key, TTL, and global cache config.

NestJS

@[1]()
@[2]('profile_cache')
@[3](1200)
fetchProfile() {
  return { name: 'Bob', age: 30 };
}

Drag options to blanks, or click blank then click option'

ACacheConfig

BCacheKey

CCacheTTL

DCacheable

Attempts:

3 left

💡 Hint

Common Mistakes

Using CacheConfig instead of Cacheable to enable caching

Mixing up the order of decorators

Using CacheInterceptor which is unrelated here

@Cacheable() enables caching, @CacheKey('profile_cache') sets the key, and @CacheTTL(1200) sets the cache duration.

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