Health checks (Terminus) in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of health checks in a NestJS application using Terminus?

Health checks help monitor if the application and its dependencies are working correctly. Terminus provides easy ways to create these checks so the system can report its status.

beginner

How do you add a simple HTTP health check endpoint using Terminus in NestJS?

You create a controller with the @HealthCheck() decorator and use the HealthCheckService to run built-in checks like HTTP or database checks. This endpoint returns the status of the app.

intermediate

What is a 'readiness' health check and why is it important?

A readiness check tells if the app is ready to handle requests. It usually checks if dependencies like databases are connected. This helps load balancers send traffic only to ready instances.

intermediate

Explain how Terminus integrates with NestJS dependency injection for custom health indicators.

You create a custom health indicator class that extends HealthIndicator. Then you inject it into your health controller to include custom checks in the health endpoint.

intermediate

What is the difference between liveness and readiness probes in Terminus health checks?

Liveness probes check if the app is alive (not stuck or crashed). Readiness probes check if the app is ready to serve traffic (all dependencies are working). Both help Kubernetes manage app lifecycle.

Which decorator is used to mark a health check endpoint in NestJS Terminus?

A @CheckHealth()

B @GetHealth()

C @HealthCheck()

D @Health()

The @HealthCheck() decorator marks a method as a health check endpoint in Terminus.

What does a readiness probe typically check?

A If the app dependencies are connected and ready

B If the app process is running

C If the server CPU is below 50%

D If the app has received any requests

Readiness probes check if dependencies like databases are connected and the app can serve traffic.

How do you add a custom health check in Terminus?

A Create a custom HealthIndicator class and inject it

B Override the default health controller

C Use @CustomHealth() decorator

D Modify the main.ts file

You create a custom HealthIndicator class implementing HealthIndicator interface and inject it into the health controller.

Which service runs the health checks in a Terminus controller?

A HealthService

B HealthCheckService

C TerminusService

D CheckService

HealthCheckService runs the registered health indicators and returns the combined status.

What HTTP status code does a healthy Terminus endpoint usually return?

A 302 Redirect

B 404 Not Found

C 500 Internal Server Error

D 200 OK

A healthy status returns HTTP 200 OK to indicate the app is working properly.

Describe how you would implement a basic health check endpoint in a NestJS app using Terminus.

Think about the steps from module setup to controller method.

Explain the difference between liveness and readiness probes and why both are useful in a cloud environment.

Consider how Kubernetes uses these probes.

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