Why Cache decorators in NestJS? - Purpose & Use Cases

The Big Idea

What if you could speed up your app with just one simple tag above your functions?

The Scenario

Imagine you have a web app that fetches user data from a database every time someone visits their profile page.

Each visit triggers the same slow database query again and again.

The Problem

Manually adding caching means writing extra code everywhere to check if data is already saved.

This makes your code messy, hard to maintain, and easy to forget.

Also, you risk serving outdated data or missing cache updates.

The Solution

Cache decorators let you add caching with a simple tag above your functions.

This keeps your code clean and automatically stores and retrieves data from cache.

You don't have to write repetitive cache logic yourself.

Before vs After

✗ Before

async getUser(id) {
  const cached = await cache.get(id);
  if (cached) return cached;
  const user = await db.findUser(id);
  await cache.set(id, user);
  return user;
}

✓ After

@Cacheable()
async getUser(id) {
  return await db.findUser(id);
}

What It Enables

You can easily speed up your app by caching results without cluttering your code.

Real Life Example

When many users view the same product page, cache decorators serve the stored product info instantly instead of querying the database each time.

Key Takeaways

Manual caching adds repetitive, error-prone code.

Cache decorators simplify caching by adding it declaratively.

This keeps code clean and improves app speed automatically.

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