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.