1---
2title: Use Proper Module Sharing Patterns
3impact: CRITICAL
4impactDescription: Prevents duplicate instances, memory leaks, and state inconsistency
5tags: architecture, modules, sharing, exports
6---
7 
8## Use Proper Module Sharing Patterns
9 
10NestJS modules are singletons by default. When a service is properly exported from a module and that module is imported elsewhere, the same instance is shared. However, providing a service in multiple modules creates separate instances, leading to memory waste, state inconsistency, and confusing behavior. Always encapsulate services in dedicated modules, export them explicitly, and import the module where needed.
11 
12**Incorrect (service provided in multiple modules):**
13 
14```typescript
15// StorageService provided directly in multiple modules - WRONG
16// storage.service.ts
17@Injectable()
18export class StorageService {
19  private cache = new Map(); // Each instance has separate state!
20 
21  store(key: string, value: any) {
22    this.cache.set(key, value);
23  }
24}
25 
26// app.module.ts
27@Module({
28  providers: [StorageService], // Instance #1
29  controllers: [AppController],
30})
31export class AppModule {}
32 
33// videos.module.ts
34@Module({
35  providers: [StorageService], // Instance #2 - different from AppModule!
36  controllers: [VideosController],
37})
38export class VideosModule {}
39 
40// Problems:
41// 1. Two separate StorageService instances exist
42// 2. cache.set() in VideosModule doesn't affect AppModule's cache
43// 3. Memory wasted on duplicate instances
44// 4. Debugging nightmares when state doesn't sync
45```
46 
47**Correct (dedicated module with exports):**
48 
49```typescript
50// storage/storage.module.ts
51@Module({
52  providers: [StorageService],
53  exports: [StorageService], // Make available to importers
54})
55export class StorageModule {}
56 
57// videos/videos.module.ts
58@Module({
59  imports: [StorageModule], // Import the module, not the service
60  controllers: [VideosController],
61  providers: [VideosService],
62})
63export class VideosModule {}
64 
65// channels/channels.module.ts
66@Module({
67  imports: [StorageModule], // Same instance shared
68  controllers: [ChannelsController],
69  providers: [ChannelsService],
70})
71export class ChannelsModule {}
72 
73// app.module.ts
74@Module({
75  imports: [
76    StorageModule, // Only if AppModule itself needs StorageService
77    VideosModule,
78    ChannelsModule,
79  ],
80})
81export class AppModule {}
82 
83// Now all modules share the SAME StorageService instance
84```
85 
86**When to use @Global() (sparingly):**
87 
88```typescript
89// ONLY for truly cross-cutting concerns
90@Global()
91@Module({
92  providers: [ConfigService, LoggerService],
93  exports: [ConfigService, LoggerService],
94})
95export class CoreModule {}
96 
97// Import once in AppModule
98@Module({
99  imports: [CoreModule], // Registered globally, available everywhere
100})
101export class AppModule {}
102 
103// Other modules don't need to import CoreModule
104@Module({
105  controllers: [UsersController],
106  providers: [UsersService], // Can inject ConfigService without importing
107})
108export class UsersModule {}
109 
110// WARNING: Don't make everything global!
111// - Hides dependencies (can't see what a module needs from imports)
112// - Makes testing harder
113// - Reserve for: config, logging, database connections
114```
115 
116**Module re-exporting pattern:**
117 
118```typescript
119// common.module.ts - shared utilities
120@Module({
121  providers: [DateService, ValidationService],
122  exports: [DateService, ValidationService],
123})
124export class CommonModule {}
125 
126// core.module.ts - re-exports common for convenience
127@Module({
128  imports: [CommonModule, DatabaseModule],
129  exports: [CommonModule, DatabaseModule], // Re-export for consumers
130})
131export class CoreModule {}
132 
133// feature.module.ts - imports CoreModule, gets both
134@Module({
135  imports: [CoreModule], // Gets CommonModule + DatabaseModule
136  controllers: [FeatureController],
137})
138export class FeatureModule {}
139```
140 
141Reference: [NestJS Modules](https://docs.nestjs.com/modules#shared-modules)
142