agent-skills/skills/nestjs-best-practices/rules/perf-use-caching.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Use Caching Strategically	HIGH	Dramatically reduces database load and response times	performance, caching, redis, optimization

Use Caching Strategically

Implement caching for expensive operations, frequently accessed data, and external API calls. Use NestJS CacheModule with appropriate TTLs and cache invalidation strategies. Don't cache everything - focus on high-impact areas.

Incorrect (no caching or caching everything):

// No caching for expensive, repeated queries
@Injectable()
export class ProductsService {
  async getPopular(): Promise<Product[]> {
    // Runs complex aggregation query EVERY request
    return this.productsRepo
      .createQueryBuilder('p')
      .leftJoin('p.orders', 'o')
      .select('p.*, COUNT(o.id) as orderCount')
      .groupBy('p.id')
      .orderBy('orderCount', 'DESC')
      .limit(20)
      .getMany();
  }
}

// Cache everything without thought
@Injectable()
export class UsersService {
  @CacheKey('users')
  @CacheTTL(3600)
  @UseInterceptors(CacheInterceptor)
  async findAll(): Promise<User[]> {
    // Caching user list for 1 hour is wrong if data changes frequently
    return this.usersRepo.find();
  }
}

Correct (strategic caching with proper invalidation):

// Setup caching module
@Module({
  imports: [
    CacheModule.registerAsync({
      imports: [ConfigModule],
      inject: [ConfigService],
      useFactory: (config: ConfigService) => ({
        stores: [
          new KeyvRedis(config.get('REDIS_URL')),
        ],
        ttl: 60 * 1000, // Default 60s
      }),
    }),
  ],
})
export class AppModule {}

// Manual caching for granular control
@Injectable()
export class ProductsService {
  constructor(
    @Inject(CACHE_MANAGER) private cache: Cache,
    private productsRepo: ProductRepository,
  ) {}

  async getPopular(): Promise<Product[]> {
    const cacheKey = 'products:popular';

    // Try cache first
    const cached = await this.cache.get<Product[]>(cacheKey);
    if (cached) return cached;

    // Cache miss - fetch and cache
    const products = await this.fetchPopularProducts();
    await this.cache.set(cacheKey, products, 5 * 60 * 1000); // 5 min TTL
    return products;
  }

  // Invalidate cache on changes
  async updateProduct(id: string, dto: UpdateProductDto): Promise<Product> {
    const product = await this.productsRepo.save({ id, ...dto });
    await this.cache.del('products:popular'); // Invalidate
    return product;
  }
}

// Decorator-based caching with auto-interceptor
@Controller('categories')
@UseInterceptors(CacheInterceptor)
export class CategoriesController {
  @Get()
  @CacheTTL(30 * 60 * 1000) // 30 minutes - categories rarely change
  findAll(): Promise<Category[]> {
    return this.categoriesService.findAll();
  }

  @Get(':id')
  @CacheTTL(60 * 1000) // 1 minute
  @CacheKey('category')
  findOne(@Param('id') id: string): Promise<Category> {
    return this.categoriesService.findOne(id);
  }
}

// Event-based cache invalidation
@Injectable()
export class CacheInvalidationService {
  constructor(@Inject(CACHE_MANAGER) private cache: Cache) {}

  @OnEvent('product.created')
  @OnEvent('product.updated')
  @OnEvent('product.deleted')
  async invalidateProductCaches(event: ProductEvent) {
    await Promise.all([
      this.cache.del('products:popular'),
      this.cache.del(`product:${event.productId}`),
    ]);
  }
}

Reference: NestJS Caching