Health checks (Terminus) in NestJS - Step-by-Step Execution

Concept Flow - Health checks (Terminus)

Start NestJS App

↓

Import Terminus Module

↓

Define Health Indicator(s)

↓

Create Health Controller

↓

Add /health Endpoint

↓

Request /health

↓

Run Health Checks

↓

Return JSON Status

↓

Client Receives Health Status

The app imports Terminus, defines health checks, exposes a /health endpoint, runs checks on request, and returns status.

Execution Sample

NestJS

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

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

  @Get()
  async check() {
    return this.health.check([
      () => this.db.pingCheck('database'),
    ]);
  }
}

This code sets up a health check endpoint that verifies database connectivity.

Execution Table

Step	Action	Evaluation	Result
1	App starts and imports TerminusModule	TerminusModule loaded	Terminus features ready
2	HealthController created with HealthCheckService	HealthCheckService injected	Ready to run checks
3	GET /health endpoint called	Triggers check() method	Health checks start
4	Run db.pingCheck('database')	Database connection tested	Returns 'up' or 'down'
5	HealthCheckService aggregates results	Combines all checks	Creates JSON status
6	Return JSON response	Status sent to client	{"status":"ok","info":{"database":{"status":"up"}},"error":{},"details":{"database":{"status":"up"}}}
7	Client receives response	Reads health status	Knows app is healthy
8	If any check fails	Status changes to 'error'	Client alerted of failure

💡 Execution stops after sending health status response to client

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
healthCheckResult	undefined	{"database":"up"}	{"status":"ok","info":{"database":{"status":"up"}}}	{"status":"ok","info":{"database":{"status":"up"}},"error":{},"details":{"database":{"status":"up"}}}

Key Moments - 3 Insights

Why does the health check return a JSON object instead of just 'ok' or 'fail'?

The health check returns detailed JSON (see execution_table step 6) to provide info on each check, not just overall status.

What happens if the database ping check fails?

If db.pingCheck fails (execution_table step 8), the status changes to 'error' and the client is informed of the failure.

Why do we inject HealthCheckService into the controller?

HealthCheckService runs the checks and aggregates results (step 2 and 5), so injecting it allows the controller to trigger health checks.

Visual Quiz - 3 Questions

Look at the execution table, what is the health check result after step 4?

A undefined

B {"status":"ok"}

C {"database":"up"}

D {"status":"error"}

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 4 in the execution_table.

At which step does the client receive the health status response?

A Step 7

B Step 5

C Step 6

D Step 8

💡 Hint

Look for the step mentioning 'Client receives response' in the execution_table.

If the database ping check fails, how does the status change in the response?

A Status remains 'ok'

B Status changes to 'error'

C Status becomes 'unknown'

D No status is returned

💡 Hint

See execution_table step 8 about failure handling.

Concept Snapshot

Health checks with Terminus in NestJS:
- Import TerminusModule in your app
- Inject HealthCheckService in a controller
- Define health indicators like db.pingCheck
- Create a GET /health endpoint that runs checks
- Returns detailed JSON status showing each check
- Status 'ok' means healthy; 'error' means failure

Full Transcript

This visual execution trace shows how NestJS apps use Terminus for health checks. The app imports TerminusModule and creates a HealthController with HealthCheckService injected. When a client calls GET /health, the controller runs health indicators like database ping checks. The results are combined into a JSON response showing overall status and details. If all checks pass, status is 'ok'; if any fail, status is 'error'. The client receives this JSON to know if the app is healthy. Variables like healthCheckResult update step-by-step as checks run. Key moments include understanding why detailed JSON is returned, what happens on failure, and why HealthCheckService is injected. The quiz tests understanding of these steps and results.

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