DiscoveryService

Kind: Class

Source: packages/core/discovery/discovery-service.ts

Part of: Core

DiscoveryService provides runtime access to registered NestJS modules, providers, and controllers. It also creates typed discoverable decorators and retrieves metadata associated with those decorators, enabling convention-based discovery patterns such as plugin registries or handler scanning.

Methods

Where it refuses work

• DiscoveryService stops the work with an early return when methodKey.

Diagram

mermaid
graph LR
  A[DiscoveryService] --> B[ModulesContainer]
  B --> C[Module]
  C --> D[Providers<br/>InstanceWrapper]
  C --> E[Controllers<br/>InstanceWrapper]
  A --> F[createDecorator]
  F --> G[DiscoverableDecorator]
  G --> H[Metadata on classes]
  A --> I[getMetadataByDecorator]
  I --> H

Usage

ts
import { Injectable, OnModuleInit } from '@nestjs/common';
import {
  DiscoveryService,
  InstanceWrapper,
} from '@nestjs/core';

// Create a reusable decorator for classes that implement application jobs.
export const JobHandler = DiscoveryService.createDecorator<{
  queue: string;
}>();

@JobHandler({ queue: 'emails' })
@Injectable()
export class SendEmailJob {
  async execute() {
    // Send email work
  }
}

@Injectable()
export class JobRegistry implements OnModuleInit {
  constructor(private readonly discoveryService: DiscoveryService) {}

  onModuleInit() {
    const providers = this.discoveryService.getProviders();

    const jobHandlers = providers
      .map((provider: InstanceWrapper) => ({
        provider,
        metadata: this.discoveryService.getMetadataByDecorator(
          JobHandler,
          provider,
        ),
      }))
      .filter(({ metadata }) => metadata !== undefined);

    for (const { provider, metadata } of jobHandlers) {
      console.log(
        `Registered ${provider.name} for the "${metadata.queue}" queue.`,
      );
    }
  }
}

AI Coding Instructions

• Use createDecorator<T>() to define typed metadata contracts for discoverable providers or controllers.
• Call discovery methods after module initialization (for example, in onModuleInit) so the NestJS container has been populated.
• Always handle undefined from getMetadataByDecorator(); not every discovered wrapper has the requested decorator.
• Inspect InstanceWrapper.metatype or InstanceWrapper.instance carefully, since wrappers can represent aliases, factories, or unresolved providers.
• Use getProviders(), getControllers(), and getModules() for framework integration and registry-building logic rather than maintaining duplicate manual registries.

Used by

3 references from 3 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

Imported by (3)

• NonAppliedDecorator — integration/discovery/src/decorators/non-applied.decorator.ts:9
• Webhook — integration/discovery/src/decorators/webhook.decorators.ts:3
• WebhooksExplorer — integration/discovery/src/webhooks.explorer.ts:5