Health checks (Terminus) in NestJS - Performance & Optimization

Performance: Health checks (Terminus)

MEDIUM IMPACT

Health checks impact server responsiveness and uptime monitoring, indirectly affecting user experience by ensuring service availability.

Implementing health checks to monitor service status

NestJS

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

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

  @HealthCheck()
  check() {
    return this.health.check([]); // lightweight async checks
  }
}

Uses Terminus async health checks that do not block event loop and integrate with NestJS lifecycle.

📈 Performance GainNon-blocking, responds within milliseconds, minimal CPU usage

Implementing health checks to monitor service status

NestJS

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

@Controller('health')
export class HealthController {
  @Get()
  check() {
    // Synchronous heavy operation
    const result = heavySyncCheck();
    return { status: result ? 'ok' : 'fail' };
  }
}

function heavySyncCheck() {
  // Simulate CPU intensive task
  for (let i = 0; i < 1e8; i++) {}
  return true;
}

Blocking synchronous operations delay response and block event loop, reducing server responsiveness.

📉 Performance CostBlocks event loop for 100+ ms per request, increasing response time and reducing throughput

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous heavy health check	0	0	0	[X] Bad
Asynchronous Terminus health check	0	0	0	[OK] Good

Rendering Pipeline

Health check requests are handled by the server without rendering UI, but inefficient checks can block the Node.js event loop, delaying all other requests.

→Request Handling

→Event Loop

→Response Sending

⚠️ BottleneckEvent Loop blocking due to synchronous or heavy operations

Optimization Tips

1Avoid synchronous or CPU-intensive tasks in health check endpoints.

2Use asynchronous health check libraries like Terminus to keep event loop free.

3Keep health checks lightweight to ensure fast server responsiveness.

Performance Quiz - 3 Questions

What is the main performance risk of implementing health checks with synchronous heavy operations in NestJS?

A Increasing DOM reflows and layout shifts

B Blocking the Node.js event loop and delaying all requests

C Adding large assets to the client bundle

D Causing CSS selector complexity

DevTools: Network

How to check: Open DevTools Network panel, trigger health check endpoint, observe response time and status code.

What to look for: Fast response times (under 50ms) and HTTP 200 status indicate good performance; slow or hanging responses indicate blocking.

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