Cache decorators in NestJS - Step-by-Step Execution

Concept Flow - Cache decorators

Method call

↓

Check cache for key

↓

Return cached

↓

Return value

↓

Response sent

When a method with a cache decorator is called, it first checks if the result is cached. If yes, it returns cached data. Otherwise, it runs the method, caches the result, then returns it.

Execution Sample

NestJS

@Cacheable()
getData() {
  return fetchFromDb();
}

A method decorated to cache its result after fetching data once.

Execution Table

Step	Action	Cache Key Exists?	Method Executed?	Cache Updated?	Returned Value
1	Call getData()	No	Yes	Yes	Data from DB
2	Call getData() again	Yes	No	No	Data from DB
3	Call getData() third time	Yes	No	No	Data from DB

💡 After first call caches data, subsequent calls return cached data without executing method.

Variable Tracker

Variable	Start	After 1	After 2	After 3
cache	{}	{"getData": "Data from DB"}	{"getData": "Data from DB"}	{"getData": "Data from DB"}
methodResult	undefined	"Data from DB"	"Data from DB"	"Data from DB"

Key Moments - 2 Insights

Why does the method not run on the second call?

Because the cache already has the key from the first call, so the decorator returns cached data directly (see execution_table step 2).

What happens if the cache is empty?

The method runs normally, its result is stored in cache, then returned (see execution_table step 1).

Visual Quiz - 3 Questions

Look at the execution table, what is returned on the third call?

A Data fetched fresh from DB

B Undefined

C Cached Data

D Error

💡 Hint

Check the 'Returned Value' column at step 3 in the execution_table.

At which step does the cache get updated?

A Step 1

B Step 2

C Step 3

D Never

💡 Hint

Look at the 'Cache Updated?' column in the execution_table.

If the cache was cleared before the third call, what would happen?

A Throws an error

B Method executes and cache updates again

C Returns cached data anyway

D Returns undefined

💡 Hint

Refer to the behavior in step 1 where cache is empty and method runs.

Concept Snapshot

Cache decorators in NestJS:
- Use @Cacheable() on methods
- On call, check cache for key
- If cached, return cached result
- Else, run method, cache result, return it
- Speeds up repeated calls by avoiding work

Full Transcript

Cache decorators in NestJS help store method results so repeated calls return cached data instead of running the method again. When a decorated method is called, it first checks if the cache has the result. If yes, it returns cached data immediately. If not, it runs the method, stores the result in cache, then returns it. This saves time and resources for repeated calls. The execution table shows the first call runs the method and caches the result. Later calls return cached data without running the method again. Variables track the cache content and method results over calls.

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