Health checks (Terminus) in NestJS

Introduction

Health checks help your app tell if it is working well. Terminus makes it easy to add these checks in NestJS.

You want to know if your NestJS app is running fine.

You need to check if your database or other services are connected.

You want to let monitoring tools see your app's health automatically.

You want to restart or fix your app if it stops working properly.

Syntax

NestJS

import { Module, Controller, Get } from '@nestjs/common';
import { TerminusModule } from '@nestjs/terminus';
import { HealthCheckService, HttpHealthIndicator } from '@nestjs/terminus';

@Controller('health')
export class HealthController {
  constructor(
    private health: HealthCheckService,
    private http: HttpHealthIndicator,
  ) {}

  @Get()
  check() {
    return this.health.check([
      () => this.http.pingCheck('nestjs-docs', 'https://docs.nestjs.com'),
    ]);
  }
}

@Module({
  imports: [TerminusModule],
  controllers: [HealthController],
  providers: [HealthCheckService, HttpHealthIndicator],
})
export class AppModule {}

Use TerminusModule to enable health checks in your app.

Health checks are functions that return if a part of your app is healthy.

Examples

Check if Google website is reachable.

NestJS

return this.health.check([
  () => this.http.pingCheck('google', 'https://google.com'),
]);

Check if your database connection is alive.

NestJS

return this.health.check([
  () => this.db.pingCheck('database'),
]);

Custom health check using your own service logic.

NestJS

@Get('custom')
checkCustom() {
  return this.health.check([
    async () => ({
      customService: await this.customService.isHealthy(),
    }),
  ]);
}

Sample Program

This NestJS app has a health endpoint at /health. It checks if the NestJS docs website is reachable. If yes, it returns a healthy status.

NestJS

import { Module, Controller, Get } from '@nestjs/common';
import { TerminusModule, HealthCheckService, HttpHealthIndicator } from '@nestjs/terminus';

@Controller('health')
export class HealthController {
  constructor(
    private health: HealthCheckService,
    private http: HttpHealthIndicator,
  ) {}

  @Get()
  check() {
    return this.health.check([
      () => this.http.pingCheck('nestjs-docs', 'https://docs.nestjs.com'),
    ]);
  }
}

@Module({
  imports: [TerminusModule],
  controllers: [HealthController],
  providers: [HealthCheckService, HttpHealthIndicator],
})
export class AppModule {}

Important Notes

Health checks should be fast and not block your app.

You can add many checks for databases, APIs, or custom services.

Use the @Get() route to expose the health check URL.

Summary

Terminus helps add health checks in NestJS easily.

Health checks tell if your app and its parts are working well.

Use HealthCheckService and indicators like HttpHealthIndicator to create checks.

Practice

1. What is the main purpose of using Terminus in a NestJS application?

easy

A. To handle user authentication and authorization

B. To add health checks that monitor if the app and its parts are working well

C. To manage database migrations automatically

D. To optimize the app's performance by caching responses

Solution

1. Step 1: Understand Terminus role in NestJS

   Terminus is a tool designed to add health checks to monitor app status.

2. Step 2: Compare options with Terminus purpose

   Only To add health checks that monitor if the app and its parts are working well describes health checks, which is Terminus's main use.

3. Final Answer:

   To add health checks that monitor if the app and its parts are working well -> Option B

4. Quick Check:

   Terminus = Health checks [OK]

Hint: Terminus is about health checks, not auth or caching [OK]

Common Mistakes:

• Confusing Terminus with authentication libraries
• Thinking Terminus manages database tasks
• Assuming Terminus improves performance directly

2. Which of the following is the correct way to inject the HealthCheckService in a NestJS Terminus health check controller?

easy

A. constructor(private healthCheckService) {}

B. constructor(public healthCheckService: HealthCheckService) {}

C. constructor(private readonly healthCheckService: HealthCheckService) {}

D. constructor(healthCheckService: HealthCheckService) {}

Solution

1. Step 1: Recall NestJS dependency injection syntax

   Use constructor with private readonly and type for proper injection.

2. Step 2: Check each option's correctness

   constructor(private readonly healthCheckService: HealthCheckService) {} uses private readonly and type, which is correct. Others miss readonly, type, or access modifier.

3. Final Answer:

   constructor(private readonly healthCheckService: HealthCheckService) {} -> Option C

4. Quick Check:

   Inject with private readonly and type [OK]

Hint: Use 'private readonly' with type for injection [OK]

Common Mistakes:

• Omitting access modifier (private/public)
• Not specifying the type HealthCheckService
• Using public instead of private readonly

3. Given this NestJS Terminus health check code snippet, what will be the HTTP response status when the database is healthy?

async check() {
  return this.healthCheckService.check([
    () => this.dbHealthIndicator.pingCheck('database')
  ]);
}

medium

A. 200 OK with status 'ok' and details about database

B. 500 Internal Server Error with failure details

C. 404 Not Found because no route is defined

D. 302 Redirect to another URL

Solution

1. Step 1: Understand healthCheckService.check behavior

   It returns 200 OK with status 'ok' if all checks pass.

2. Step 2: Analyze the database pingCheck result

   If database is healthy, pingCheck passes, so overall status is 'ok'.

3. Final Answer:

   200 OK with status 'ok' and details about database -> Option A

4. Quick Check:

   Healthy check = 200 OK [OK]

Hint: Healthy checks return 200 OK with status 'ok' [OK]

Common Mistakes:

• Assuming failure status on healthy database
• Confusing route existence with health check response
• Expecting redirects from health checks

4. Identify the error in this NestJS Terminus health check controller code:

@Controller('health')
export class HealthController {
  constructor(private readonly healthCheckService: HealthCheckService) {}

  @Get()
  async check() {
    return this.healthCheckService.check([
      this.httpHealthIndicator.pingCheck('nestjs-docs', 'https://docs.nestjs.com')
    ]);
  }
}

medium

A. Missing injection of HttpHealthIndicator in constructor

B. Wrong HTTP method; should use @Post instead of @Get

C. HealthCheckService is not imported

D. Controller decorator is missing a path

Solution

1. Step 1: Check usage of httpHealthIndicator

   httpHealthIndicator is used but not injected or declared anywhere.

2. Step 2: Verify constructor parameters

   Only healthCheckService is injected; httpHealthIndicator is missing.

3. Final Answer:

   Missing injection of HttpHealthIndicator in constructor -> Option A

4. Quick Check:

   Inject all indicators used [OK]

Hint: Inject all health indicators you use [OK]

Common Mistakes:

• Forgetting to inject HttpHealthIndicator
• Using wrong HTTP method for health checks
• Assuming decorators need no path

5. You want to create a custom health check in NestJS Terminus that checks both database connectivity and an external API availability. Which approach correctly combines these checks in one health endpoint?

hard

A. Call dbHealthIndicator.pingCheck inside httpHealthIndicator.pingCheck callback

B. Create two separate controllers, one for database and one for API, each with its own healthCheckService

C. Use only healthCheckService.check with dbHealthIndicator.pingCheck and ignore API check

D. Use healthCheckService.check with an array of functions calling dbHealthIndicator.pingCheck and httpHealthIndicator.pingCheck

Solution

1. Step 1: Understand combining multiple checks

   healthCheckService.check accepts an array of async functions for multiple checks.

2. Step 2: Verify correct usage of indicators

   Use dbHealthIndicator.pingCheck and httpHealthIndicator.pingCheck as separate functions in the array.

3. Step 3: Evaluate other options

   Separate controllers or nesting checks is incorrect; ignoring API check misses requirement.

4. Final Answer:

   Use healthCheckService.check with an array of functions calling dbHealthIndicator.pingCheck and httpHealthIndicator.pingCheck -> Option D

5. Quick Check:

   Combine checks in array for one endpoint [OK]

Hint: Pass all checks as functions array to healthCheckService.check [OK]

Common Mistakes:

• Splitting checks into multiple controllers unnecessarily
• Nesting one check inside another incorrectly
• Ignoring one of the required checks