Circular Dependency Solution in NestJS: Comprehensive Guide

Meta Description: Learn how to solve the cyclic dependency issue between modules and services in NestJS with sample codes and the forwardRef approach.

📘 What Will You Learn in This Guide?

• You will recognize what circular dependency is in NestJS applications.
• You will learn how to solve cyclic dependency at module level with forwardRef.
• You will see how to break service (provider) level dependencies using @Inject(forwardRef()).
• You will discover how to create a more sustainable solution with the Shared Module approach.

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🧠 What is Circular Dependency in NestJS?

Circular dependency is when two classes need each other.
So class A needs class B to work, and class B needs class A to work.
This causes a deadlock when starting the application.

In NestJS this usually occurs in two ways:

• Cyclic module imports:
  OrdersModule imports PaymentModule, which in turn imports OrdersModule.
• Cyclic service injections:
  OrdersService, PaymentService; PaymentService also injects OrdersService.

💡 Example: In an e-commerce application, Order Service calls Payment Service to initiate payment.
When the payment is completed, Payment Service calls Order Service again to update the status of the order.
In this case, a classic circular dependency occurs.

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🔁 Resolving Circular Dependency Between Modules

When two modules import each other directly, NestJS cannot instantiate these modules.
Each module waits for the other to load and the application gets bogged down.

✅ Solution: Using forwardRef()

forwardRef should not immediately resolve the dependency on NestJS,
It says it will resolve it after the other parts of the application are loaded.

1️⃣ forwardRef Implementation in Payment Module

This code delays parsing of OrdersModule and prevents crashing.

// src/odeme/odeme.module.ts
import { Module, forwardRef } from "@nestjs/common";
import { PaymentService } from "./payment.service";
import { OrdersModule } from "../siparisler/orders.module";

@Module({
  imports: [forwardRef(() => OrdersModule)],
  controllers: [PaymentController],
  providers: [PaymentService],
  exports: [PaymentService],
})
export class OdemeModulu {}

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2️⃣ forwardRef Implementation in Order Module

This code ensures that both modules are loaded without waiting for each other.

// src/siparisler/siparisler.module.ts
import { Module, forwardRef } from "@nestjs/common";
import { OrdersService } from "./orders.service";
import { PaymentModule } from "../odeme/payment.module";

@Module({
  imports: [forwardRef(() => PaymentModule)],
  controllers: [OrdersController],
  providers: [OrdersService],
  exports: [OrdersService],
})
export class SiparislerModulu {}

NestJS goes back and resolves these dependencies after all modules are loaded.

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⚙️ Resolving Circular Dependency Between Services

The loop between services (providers) generally occurs when one service mutually injects constructors with another.

✅ Solution: @Inject() + forwardRef() Combination

This combination ensures that NestJS does not resolve the dependency until it is needed.

1️⃣ Application in Order Service

// src/siparisler/orders.service.ts
import { Injectable, Inject, forwardRef } from '@nestjs/common';
import { PaymentService } from '../odeme/payment.service';

@Injectable()
export class OrdersService {
  constructor(
    @Inject(forwardRef(() => PaymentService))
    private readonly paymentService: PaymentService,
  ) {}
}

---

2️⃣ Application in Payment Service

// src/odeme/payment.service.ts
import { Injectable, Inject, forwardRef } from '@nestjs/common';
import { OrdersService } from '../siparisler/orders.service';

@Injectable()
export class PaymentService {
  constructor(
    @Inject(forwardRef(() => OrdersService))
    private readonly ordersService: OrdersService,
  ) {}
}

In this way, both services can use each other without being completely resolved.

🧩 Alternative Approach: Using Shared Module

In some cases it is cleaner to move dependencies to a common management module rather than forwardRef. This separates the responsibilities of the code and simplifies the architecture.

💡 Example: Return Management Module This module can work independently with both order and payment transactions.

// src/iade-yonetimi/iade-yonetimi.service.ts
import { Injectable } from '@nestjs/common';
import { OrdersService } from '../siparisler/orders.service';
import { PaymentService } from '../odeme/payment.service';

@Injectable()
export class IadeYonetimiService {
  constructor(
    private orderService: OrdersService,
    private paymentService: PaymentService
  ) {}

  async iadeyiBaslat(orderId: string) {
    const uygun = await this.orderService.iadeUygunlugunuKontrolEt(orderId);
    if (!uygun) throw new Error('İade için uygun değil');

    const basarili = await this.paymentService.iadeyiIsle(orderId);
    if (basarili) {
      await this.orderService.durumuGuncelle(orderId, 'İade Edildi');
    }
  }
}

With this method, modules become completely independent of each other. The code becomes more maintainable and manageable.

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🔍 Circular Dependency Detection Tool: Madge

Madge is a powerful tool that graphically analyzes the cycles between your modules.

1️⃣ Madge Installation

npm i madge
# veya
yarn add madge

2️⃣ Finding Circular Dependencies

npx madge --circular src/main.ts
# veya
yarn madge --circular src/main.ts

This command lists which files are cyclically linked to each other.

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❓ Frequently Asked Questions (FAQ)

1. Why is circular dependency a bug?

NestJS's DI system resolves dependencies sequentially. A, B; When B waits for A, a crash occurs and the application cannot be started.

2. Is it bad practice to use forwardRef?

No, but it should be a last resort. Remodeling the architecture is often a more permanent solution.

3. Are the module and service level loops the same?

No. The module level occurs during installation, while the service level occurs at constructor injection. Although the solution methods are similar, the scope is different.

4. When should the shared module be used?

If two modules share a common workflow (e.g. returns process), that module maintains the single responsibility principle.

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🏁 Result

In this guide, you learned why the circular dependency error occurs in NestJS and how to solve it with forwardRef, @Inject. You also explored the common module approach to simplify your architecture.

🚀 You can try these techniques in NestJS projects on your GenixNode platform right now!

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