Cache decorators in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @Cacheable() decorator in NestJS?

The @Cacheable() decorator marks a method so that its result is stored in cache. When the method is called again with the same arguments, the cached result is returned instead of running the method again, improving performance.

intermediate

How does the @CacheEvict() decorator work in NestJS?

The @CacheEvict() decorator is used to remove specific cache entries when a method is called. This helps keep the cache updated by deleting stale data after changes like updates or deletes.

intermediate

Explain the role of the @CachePut() decorator in NestJS.

The @CachePut() decorator updates the cache with the method's result every time the method is called, regardless of whether the cache already has a value. It is useful for refreshing cache data.

beginner

What is the default cache store used by NestJS cache decorators?

By default, NestJS uses an in-memory cache store for caching. This means cached data is stored in the server's memory and is lost when the server restarts.

intermediate

Why is it important to use cache decorators with proper cache keys?

Proper cache keys ensure that cached data is correctly identified and retrieved. Without unique keys, different method calls might overwrite each other's cache or return wrong data.

Which NestJS cache decorator stores the method result only if it is not already cached?

A @Cacheable()

B @CacheEvict()

C @CachePut()

D @CacheClear()

@Cacheable() caches the result only if it is not already cached. @CachePut() always updates the cache.

What does the @CacheEvict() decorator do?

A Updates cache without checking

B Stores method result in cache

C Removes cache entries

D Disables caching for a method

@CacheEvict() removes cache entries to keep cache data fresh.

If you want to refresh cache data every time a method runs, which decorator should you use?

A @CachePut()

B @CacheEvict()

C @Cacheable()

D @CacheDisable()

@CachePut() updates the cache every time the method is called.

What happens to the default in-memory cache when the NestJS server restarts?

A Cache persists on disk

B Cache is cleared

C Cache is backed up automatically

D Cache is sent to clients

In-memory cache is lost when the server restarts.

Why should cache keys be unique for different method calls?

A To speed up method execution

B To encrypt cached data

C To disable caching

D To prevent cache overwriting

Unique keys prevent different cached results from overwriting each other.

Describe how you would use cache decorators in NestJS to improve performance of a data fetching method.

Think about storing, updating, and clearing cache for data methods.

Explain the difference between @Cacheable(), @CachePut(), and @CacheEvict() decorators in NestJS.

Focus on when each decorator affects the cache.

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