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Implementing Health Checks with Custom Diagnostics in .NET Core

Suzan Kara

Production monitoring necessitates visibility into downstream service availability, database connectivity, and application health.NET Core offers an integrated Health Checks framework that exposes HTTP endpoints to report status to load balancers (like Azure App Gateway or NGINX) or container orchestrators (like Kubernetes).

Step 1: Install Required NuGet Packages

Add the health checks UI and database diagnostics packages to your project:

Bash

dotnet add package Microsoft.Extensions.Diagnostics.HealthChecks
dotnet add package AspNetCore.Diagnostics.HealthChecks.EntityFrameworkCore

Step 2: Implement a Custom Health Check

Beyond checking database connections, you might want to verify custom dependencies, such as external API reachability, disk space availability, or custom memory thresholds. Implement IHealthCheck:

using Microsoft.Extensions.Diagnostics.HealthChecks;

public class ExternalApiHealthCheck : IHealthCheck
{
    private readonly HttpClient _httpClient;

    public ExternalApiHealthCheck(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<HealthCheckResult> CheckHealthAsync(
        HealthContext context,
        CancellationToken cancellationToken = default)
    {
        try
        {
            // Ping an external dependency or internal microservice endpoint
            var response = await _httpClient.GetAsync("https://api.github.com/status", cancellationToken);

            if (response.IsSuccessStatusCode)
            {
                return HealthCheckResult.Healthy("External API is fully operational.");
            }

            return HealthCheckResult.Degraded("External API returned a non-success status code.");
        }
        fatch (Exception ex)
        {
            return HealthCheckResult.Unhealthy("External API is unreachable.", ex);
        }
    }
}

Step 3: Register Health Checks in Program.cs

Configure your built-in and custom health checks in the dependency injection container, segmenting them by operational tiers (e.g., Liveness vs. Readiness probes).

using Microsoft.AspNetCore.Diagnostics.HealthChecks;
using Microsoft.Extensions.Diagnostics.HealthChecks;

var builder = WebApplication.CreateBuilder(args);

// Register Health Checks services
builder.Services.AddHealthChecks()
    // 1. Built-in EF Core database check
    .AddDbContextCheck<AppDbContext>("database", failureStatus: HealthStatus.Unhealthy)
    // 2. Custom registered HTTP dependency check
    .AddCheck<ExternalApiHealthCheck>("external-github-api", failureStatus: HealthStatus.Degraded);

builder.Services.AddHttpClient<ExternalApiHealthCheck>();
builder.Services.AddControllers();

var app = builder.Build();

// Map comprehensive endpoint reporting JSON details
app.MapHealthChecks("/health/detailed", new HealthCheckOptions
    {
        ResponseWriter = async (context, report) =>
        {
            context.Response.ContentType = "application/json";
            var response = new
            {
                Status = report.Status.ToString(),
                Checks = report.Entries.Select(e => new
                {
                    Component = e.Key,
                    Status = e.Value.Status.ToString(),
                    Description = e.Value.Description,
                    Duration = e.Value.Duration
                }),
                TotalDuration = report.TotalDuration
            };
            await context.Response.WriteAsJsonAsync(response);
        }
    });

// Map lightweight liveness endpoint for Kubernetes probes
app.MapHealthChecks("/health/liveness", new HealthCheckOptions
    {
        Predicate = r => r.Tags.Contains("liveness")
    });

app.Run();

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