Health checks (Terminus) in NestJS - Mini Project: Build & Apply

Health Checks with Terminus in NestJS

📖 Scenario: You are building a simple NestJS server that needs to report its health status. This is important for monitoring and ensuring the server is running well.

🎯 Goal: Create a health check endpoint using Terminus in NestJS that reports the server is healthy.

📋 What You'll Learn

Create a health check controller

Configure Terminus health check module

Implement a basic health check returning 'ok' status

Expose the health check endpoint at '/health'

💡 Why This Matters

🌍 Real World

Health checks help monitor if your server and its dependencies are working well. This is important for uptime and reliability.

💼 Career

Many companies use health checks in their backend services to integrate with monitoring tools and orchestrators like Kubernetes.

1

Setup Health Check Controller

Create a controller called HealthController with a route /health that will later return health status.

NestJS

import { Controller, Get } from '@nestjs/common';

@Controller('health')
export class HealthController {
  // Your code here
}

Hint

Use @Controller('health') to set the route and @Get() for the GET method.

2

Add Terminus Health Module

Import TerminusModule from @nestjs/terminus and add it to the imports array of your main application module called AppModule.

NestJS

import { Controller, Get } from '@nestjs/common';

@Controller('health')
export class HealthController {
  @Get()
  check() {
    return { status: 'ok' };
  }
}

import { Module } from '@nestjs/common';
import { TerminusModule } from '@nestjs/terminus';

@Module({
  controllers: [HealthController],
  imports: [
    // Your code here
  ],
})
export class AppModule {}

Hint

Remember to import TerminusModule and add it to the imports array in @Module.

3

Use Terminus Health Check Service

Inject HealthCheckService and HttpHealthIndicator into HealthController. Use HealthCheckService.check() with HttpHealthIndicator.pingCheck('nestjs-docs', 'https://docs.nestjs.com') inside the check() method to perform a real health check.

NestJS

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

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

  @Get()
  @HealthCheck()
  check() {
    // Your code here
  }
}

import { Module } from '@nestjs/common';
import { TerminusModule } from '@nestjs/terminus';

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

Hint

Use dependency injection in the constructor and call this.health.check() with this.http.pingCheck() inside the check() method.

4

Complete and Test Health Endpoint

Ensure the HealthController and AppModule are correctly set up with Terminus. The health check endpoint should be accessible at /health and return JSON with the status of the ping check.

NestJS

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

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

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

import { Module } from '@nestjs/common';
import { TerminusModule } from '@nestjs/terminus';

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

// Confirm the setup is complete and ready for testing

Hint

Review all previous steps and ensure the health check endpoint is fully implemented and the module imports are correct.

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