Hub Module

ID: hub_module | Severity: Medium (default)

A "Hub Module" is a central point in the dependency graph, characterized by both high Fan-in (many dependents) and high Fan-out (many dependencies), while containing relatively little internal logic.

Why this is a smell

Hub modules represent dangerous "single points of failure" in your architecture. Because they sit at the center of many paths, they become extremely fragile. A minor change in a hub module can trigger a massive ripple effect across the entire codebase, making them difficult and risky to refactor.

Examples

Bad

A module that merely re-exports or coordinates many unrelated services and is itself used by the entire application.

// app-hub.ts
import { AuthService } from './auth';
import { ApiService } from './api';
import { LoggerService } from './logger';
import { ConfigService } from './config';
// ... 10 more imports

export class AppHub {
  constructor(
    public auth: AuthService,
    public api: ApiService,
    public logger: LoggerService
    // ... 10 more dependencies
  ) {}
}

Good

Break down the hub into specific, focused coordinators or use dependency injection at the consumer level to avoid a central hub.

// auth-coordinator.ts (Focused on auth-related coordination)
import { AuthService } from './auth';
import { SessionStore } from './session';

export class AuthCoordinator {
  constructor(
    private auth: AuthService,
    private session: SessionStore
  ) {}
}

Configuration

rules:
  hub_module:
    severity: medium
    min_fan_in: 5
    min_fan_out: 5
    max_complexity: 5

ESLint Rule

This detector is available as an ESLint rule for real-time feedback in your editor.

// eslint.config.js
export default [
  {
    rules: {
      '@archlinter/no-hub-modules': 'warn',
    },
  },
];

See ESLint Integration for setup instructions.

How to fix

Break the hub! Identify the different paths of data or control passing through the hub and extract them into separate, more focused modules.