TTL configuration in NestJS - Step-by-Step Execution

Concept Flow - TTL configuration

Start NestJS app

↓

Configure CacheModule

↓

Set TTL value in options

↓

Cache stores data with expiration

↓

Request data

↓

Check cache

This flow shows how NestJS CacheModule uses TTL (time-to-live) to expire cached data automatically after a set time.

Execution Sample

NestJS

import { CacheModule } from '@nestjs/common';

CacheModule.register({
  ttl: 5, // seconds
});

This code sets up NestJS cache with a TTL of 5 seconds, meaning cached data expires after 5 seconds.

Execution Table

Step	Action	TTL Setting	Cache State	Result
1	App starts and CacheModule registers	5 seconds	Empty	Cache ready with TTL=5s
2	Data requested first time	5 seconds	Empty	Cache miss, fetch data
3	Data cached with TTL=5s	5 seconds	Data stored with expiry timestamp	Data cached
4	Data requested within 5s	5 seconds	Data valid	Cache hit, return cached data
5	Wait 6 seconds (TTL expired)	5 seconds	Data expired	Cache miss, fetch fresh data
6	Data cached again with TTL=5s	5 seconds	New data stored with expiry	Data cached again

💡 Execution stops because TTL causes cached data to expire after 5 seconds, forcing fresh fetch.

Variable Tracker

Variable	Start	After Step 3	After Step 5	Final
cacheData	null	{value: 'data', expiresAt: t+5s}	null (expired)	{value: 'new data', expiresAt: t+5s}
ttl	5	5	5	5

Key Moments - 2 Insights

Why does the cache miss happen after 5 seconds even if data was cached before?

Because TTL is set to 5 seconds, cached data expires after that time (see execution_table step 5), so cache miss forces fresh data fetch.

What happens if TTL is not set in CacheModule?

Without TTL, cached data never expires automatically, so cache hit always returns old data unless manually cleared.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache state at step 4?

A Cache empty

B Data valid and cached

C Data expired

D Cache disabled

💡 Hint

Refer to execution_table row with Step 4 showing cache state as 'Data valid'

At which step does the cached data expire due to TTL?

A Step 3

B Step 2

C Step 5

D Step 6

💡 Hint

Check execution_table row Step 5 where cache state is 'Data expired'

If TTL was set to 10 seconds instead of 5, how would step 5 change?

A Cache miss would happen later than step 5

B Cache miss would happen earlier than step 5

C Cache miss would happen exactly at step 5

D Cache miss would never happen

💡 Hint

TTL controls expiration time; longer TTL delays cache miss (see variable_tracker ttl value)

Concept Snapshot

NestJS CacheModule TTL configuration:
- Use CacheModule.register({ ttl: seconds }) to set expiration
- Cached data expires after TTL seconds automatically
- Cache hit returns data if not expired
- Cache miss fetches fresh data and caches it
- TTL helps keep cache fresh without manual clearing

Full Transcript

This visual execution shows how TTL (time-to-live) works in NestJS CacheModule. When the app starts, CacheModule is configured with a TTL value in seconds. When data is requested the first time, cache is empty, so data is fetched fresh and stored with an expiration timestamp based on TTL. Subsequent requests within TTL return cached data (cache hit). After TTL seconds pass, cached data expires, causing a cache miss on next request, which fetches fresh data again and caches it with a new expiration. Variables like cacheData track stored data and expiry time. Key moments include understanding why cache misses happen after TTL expires and what happens if TTL is not set. The quizzes test understanding of cache state at different steps and TTL effects. This helps beginners see how TTL keeps cache data fresh automatically.