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.