Health checks (Terminus) in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the TerminusModule in a NestJS module.

NestJS

import { Module } from '@nestjs/common';
import { [1] } from '@nestjs/terminus';

@Module({
  imports: [[1]],
})
export class AppModule {}

Drag options to blanks, or click blank then click option'

AHealthCheckModule

BHealthModule

CTerminusModule

DHealthTerminus

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect module names like HealthModule or HealthCheckModule.

Forgetting to import the TerminusModule.

The TerminusModule is the official NestJS module for health checks and must be imported to use Terminus features.

2fill in blank

medium

Complete the code to create a health check endpoint using Terminus in a controller.

NestJS

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

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

  @Get()
  @HealthCheck()
  check() {
    return this.health.[1]([]);
  }
}

Drag options to blanks, or click blank then click option'

Aexecute

Brun

Cperform

Dcheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like run() or execute() which do not exist on HealthCheckService.

Forgetting to call the method as a function.

The HealthCheckService provides a method called check() to run all registered health indicators.

3fill in blank

hard

Fix the error in the health indicator registration by completing the code.

NestJS

import { Injectable } from '@nestjs/common';
import { HealthIndicator, HealthIndicatorResult, HealthCheckError } from '@nestjs/terminus';

@Injectable()
export class DiskHealthIndicator extends HealthIndicator {
  async isHealthy(key: string): Promise<HealthIndicatorResult> {
    const diskFree = 100; // pretend value
    const isHealthy = diskFree > 50;
    if (!isHealthy) {
      throw new HealthCheckError('Disk space too low', [1]);
    }
    return this.getStatus(key, isHealthy, { diskFree });
  }
}

Drag options to blanks, or click blank then click option'

Athis.getStatus(key, isHealthy, { diskFree })

Bthis.getStatus(key, false, { diskFree })

Cthis.getStatus(key, true, { diskFree })

Dthis.getStatus(key, isHealthy)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing incomplete or incorrect status objects to HealthCheckError.

Omitting the third argument with details.

The HealthCheckError requires the health indicator result object, which is returned by getStatus with the current health state and details.

4fill in blank

hard

Fill both blanks to add a memory health indicator to the health check controller.

NestJS

import { Controller, Get } from '@nestjs/common';
import { HealthCheck, HealthCheckService, [1] } from '@nestjs/terminus';

@Controller('health')
export class HealthController {
  constructor(
    private health: HealthCheckService,
    private [2]: MemoryHealthIndicator
  ) {}

  @Get('memory')
  @HealthCheck()
  checkMemory() {
    return this.health.check([
      () => this.[2].checkHeap('memory_heap'),
    ]);
  }
}

Drag options to blanks, or click blank then click option'

AMemoryHealthIndicator

BMemoryIndicator

CMemoryCheck

DMemoryHealth

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names like MemoryIndicator or MemoryCheck.

Mismatching the injected variable name and the class.

MemoryHealthIndicator is the official class to check memory usage in Terminus and must be imported and injected with the same name.

5fill in blank

hard

Fill all three blanks to create a custom health indicator and register it in the health check controller.

NestJS

import { Injectable } from '@nestjs/common';
import { HealthIndicator, HealthIndicatorResult, HealthCheckError } from '@nestjs/terminus';

@Injectable()
export class [1] extends HealthIndicator {
  async isHealthy(key: string): Promise<HealthIndicatorResult> {
    const serviceUp = true; // pretend check
    const isHealthy = serviceUp;
    if (!isHealthy) {
      throw new HealthCheckError('Service down', this.getStatus(key, isHealthy));
    }
    return this.getStatus(key, isHealthy);
  }
}

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

@Controller('health')
export class HealthController {
  constructor(
    private health: HealthCheckService,
    private [2]: [1]
  ) {}

  @Get('custom')
  @HealthCheck()
  checkCustom() {
    return this.health.check([
      () => this.[2].isHealthy('custom_service'),
    ]);
  }
}

Drag options to blanks, or click blank then click option'

ACustomHealthIndicator

BcustomHealthIndicator

CCustomIndicator

DcustomIndicator

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent names between class and injected variable.

Calling wrong methods or using wrong variable names.

The custom health indicator class is named CustomHealthIndicator, and the injected variable uses camelCase customHealthIndicator. The method isHealthy is called on the injected instance.

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