RFC 2002: OpenTelemetry Backend Observability System

Status: Implemented
Date: 2025-10-20
Authors: Alexandru Olariu
Related Components:
- sites/api.arolariu.ro/src/Common/Telemetry/
- sites/api.arolariu.ro/src/Core/Domain/General/Extensions/

Abstract

This RFC documents the OpenTelemetry (OTel) observability implementation in the api.arolariu.ro .NET modular monolith. The system provides comprehensive distributed tracing, structured logging, and performance metrics collection with Azure Application Insights integration. The implementation leverages OpenTelemetry SDK for standardized observability while maintaining zero-allocation performance through source generators and automatic instrumentation.

1. Motivation

1.1 Problem Statement

Modern distributed systems require comprehensive observability to:

Diagnose Performance Issues: Identify bottlenecks across service layers (API → Orchestration → Foundation → Database)
Correlate Distributed Operations: Track requests through multiple layers of the modular monolith
Monitor System Health: Collect real-time metrics on throughput, latency, and error rates
Debug Production Issues: Access structured logs with trace context correlation
Ensure SLA Compliance: Measure and alert on performance degradation

Without standardized observability, debugging production issues requires manual log correlation and lacks visibility into cross-layer performance characteristics.

1.2 Design Goals

Standard Compliance: Use OpenTelemetry specification for vendor-neutral observability
Zero-Allocation Logging: Leverage source generators for compile-time logging optimization
Automatic Instrumentation: Capture HTTP, database, and dependency calls without manual intervention
Azure Integration: Export telemetry to Azure Application Insights for centralized monitoring
Development Visibility: Provide console export during debugging for immediate feedback
Modular Architecture: Align tracing with DDD domain boundaries (Common, Core, Auth, Invoices)
Secure Authentication: Use Managed Identity for production Azure integration

2. Technical Design

2.1 Architecture Overview

The observability system consists of four pillars:

┌─────────────────────────────────────────────────────────────────┐
│                    Application Layer                            │
│  (Controllers, Endpoints, Middleware)                           │
└───────────────────────────┬─────────────────────────────────────┘
                            │
                ┌───────────┴───────────┐
                │                       │
                ▼                       ▼
          ┌──────────────┐      ┌──────────────┐
          │   Tracing    │      │   Logging    │
          │  (Spans &    │      │ (Structured  │
          │  Activities) │      │   Logs)      │
          └──────┬───────┘      └──────┬───────┘
                │                     │
                │      ┌──────────────┴──────────┐
                │      │                         │
                │      ▼                         ▼
                │  ┌──────────┐          ┌─────────────┐
                │  │ Metering │          │ Application │
                │  │ (Metrics │          │  Insights   │
                │  │  & KPIs) │          │  Telemetry  │
                │  └────┬─────┘          │  (Legacy)   │
                │       │                └─────────────┘
                │       │
                └───────┴─────────────┐
                                      ▼
                              ┌─────────────────┐
                              │ Azure Monitor   │
                              │ (OTel Exporter) │
                              └─────────────────┘
                                      │
                              ┌────────┴────────┐
                              │  Console Export │
                              │  (Debug Only)   │
                              └─────────────────┘

2.2 Core Components

2.2.1 Tracing Infrastructure

Location: Common/Telemetry/Tracing/

Purpose: Distributed request tracing with automatic instrumentation

Key Classes:

TracingExtensions: Configures OpenTelemetry tracing with ASP.NET Core, HTTP Client, and EF Core instrumentation
ActivityGenerators: Centralized activity sources for domain-specific tracing

Activity Sources (aligned with DDD domains):

public static class ActivityGenerators
{
  // Infrastructure layer tracing
  public static readonly ActivitySource CommonPackageTracing =
    new("arolariu.Backend.Common");

  // Application core tracing
  public static readonly ActivitySource CorePackageTracing =
    new("arolariu.Backend.Core");

  // Authentication domain tracing
  public static readonly ActivitySource AuthPackageTracing =
    new("arolariu.Backend.Auth");

  // Invoice domain tracing
  public static readonly ActivitySource InvoicePackageTracing =
    new("arolariu.Backend.Domain.Invoices");
}

Automatic Instrumentation:

ASP.NET Core: HTTP request spans with route, status code, duration
HTTP Client: Outbound dependency tracking with correlation headers
Entity Framework Core: Database query spans with SQL execution time

2.2.2 Logging Infrastructure

Location: Common/Telemetry/Logging/

Purpose: High-performance structured logging with compile-time optimization

Key Classes:

LoggingExtensions: Configures OpenTelemetry logging with Azure Monitor integration
Log: Source-generated logging methods (zero-allocation)

Source-Generated Logging Pattern:

public static partial class Log
{
  [LoggerMessage(0, LogLevel.Critical,
    "The option {propertyName} is missing from configuration AND Key Vault: {keyVaultName}")]
  public static partial void LogOptionValueIsCompletelyMissing(
    this ILogger logger, string propertyName, string keyVaultName);

  [LoggerMessage(1, LogLevel.Information,
    "The option {propertyName} was loaded from Key Vault: {keyVaultName}.")]
  public static partial void LogOptionValueFromKeyVault(
    this ILogger logger, string propertyName, string keyVaultName);

  [LoggerMessage(2, LogLevel.Information,
    "The option {propertyName} was loaded from configuration file.")]
  public static partial void LogOptionValueFromConfiguration(
    this ILogger logger, string propertyName);
}

Benefits:

Zero Allocation: No boxing, string interpolation, or closure allocation
Compile-Time Validation: Type-safe parameters with compiler checks
Trace Correlation: Automatic log-trace correlation via Activity context

2.2.3 Metering Infrastructure

Location: Common/Telemetry/Metering/

Purpose: Performance metrics and KPI collection

Key Classes:

MeteringExtensions: Configures OpenTelemetry metrics with automatic instrumentation

Automatic Metrics Collection:

ASP.NET Core Metrics:
- http.server.request.duration: Request latency histogram
- http.server.active_requests: Concurrent request counter
- http.server.request.body.size: Request size distribution
- http.server.response.body.size: Response size distribution
HTTP Client Metrics:
- http.client.request.duration: Dependency call latency
- http.client.active_requests: Concurrent outbound requests

2.2.4 Azure Monitor Export Integration

Location: Implemented directly in:

Common/Telemetry/Logging/LoggingExtensions.cs
Common/Telemetry/Metering/MeteringExtensions.cs
Common/Telemetry/Tracing/TracingExtensions.cs

Purpose: Export OpenTelemetry logs, metrics, and traces to Azure Monitor/Application Insights using Azure.Monitor.OpenTelemetry.Exporter.

Features:

Azure Monitor exporters configured per signal (logs/metrics/traces)
DefaultAzureCredential + managed identity support via AZURE_CLIENT_ID
Debug-time console exporters for local diagnostics

2.3 Configuration & Integration

Startup Configuration (Core/Domain/General/Extensions/WebApplicationBuilderExtensions.cs):

public static void AddGeneralDomainConfiguration(this WebApplicationBuilder builder)
{
  // ... other services

  builder.AddOTelLogging();      // OpenTelemetry logging
  builder.AddOTelMetering();     // OpenTelemetry metrics
  builder.AddOTelTracing();      // OpenTelemetry tracing
}

Authentication:

Development: Uses DefaultAzureCredential() (Visual Studio, Azure CLI, etc.)
Production: Uses Managed Identity via AZURE_CLIENT_ID environment variable

Connection String Resolution:

// Resolved via IOptionsManager from:
// 1. appsettings.json (local development)
// 2. Azure Key Vault (production)
string instrumentationKey = optionsManager
  .GetRequiredService<IOptionsManager>()
  .GetApplicationOptions()
  .ApplicationInsightsEndpoint;

3. Implementation Examples

3.1 Manual Activity Creation (Processing Layer)

public partial class InvoiceProcessingService : IInvoiceProcessingService
{
  public async Task CreateInvoice(Invoice invoice, Guid? userIdentifier = null) =>
    await TryCatchAsync(async () =>
    {
      // Create activity for distributed tracing
      using var activity = InvoicePackageTracing.StartActivity(nameof(CreateInvoice));

      // Activity automatically captures:
      // - Start/end timestamps
      // - Parent-child relationships
      // - Trace/span IDs for correlation

      await invoiceOrchestrationService
        .CreateInvoiceObject(invoice)
        .ConfigureAwait(false);

      // Activity disposed automatically; duration recorded
    }).ConfigureAwait(false);
}

3.2 Activity with Tags (Endpoint Layer)

internal static async Task<IResult> CreateNewInvoiceAsync(
  [FromServices] IInvoiceProcessingService invoiceProcessingService,
  [FromBody] CreateInvoiceDto invoiceDto,
  ClaimsPrincipal principal)
{
  try
  {
    // Server activity with explicit kind
    using var activity = InvoicePackageTracing.StartActivity(
      nameof(CreateNewInvoiceAsync),
      ActivityKind.Server);

    // Add custom tags for filtering/analysis
    activity?.SetTag("invoice.merchant_id", invoiceDto.MerchantId);
    activity?.SetTag("invoice.total_amount", invoiceDto.TotalAmount);

    var invoice = invoiceDto.ToInvoice();
    await invoiceProcessingService.CreateInvoice(invoice).ConfigureAwait(false);

    activity?.SetStatus(ActivityStatusCode.Ok);
    return TypedResults.Created($"/rest/v1/invoices/{invoice.id}", invoice);
  }
  catch (Exception ex)
  {
    // Exception automatically captured by OTel instrumentation
    return TypedResults.Problem(detail: ex.Message, statusCode: 500);
  }
}

3.3 High-Performance Logging

public class ConfigurationManager
{
  private readonly ILogger<ConfigurationManager> _logger;

  public string LoadOptionValue(string propertyName, string keyVaultName)
  {
    // 1. Try configuration file
    var configValue = configuration[propertyName];
    if (configValue != null)
    {
      // Zero-allocation logging
      _logger.LogOptionValueFromConfiguration(propertyName);
      return configValue;
    }

    // 2. Try Key Vault
    var secretValue = keyVaultClient.GetSecret(propertyName);
    if (secretValue != null)
    {
      // Structured log with trace correlation
      _logger.LogOptionValueFromKeyVault(propertyName, keyVaultName);
      return secretValue;
    }

    // 3. Critical failure
    _logger.LogOptionValueIsCompletelyMissing(propertyName, keyVaultName);
    throw new InvalidOperationException($"Missing configuration: {propertyName}");
  }
}

3.4 Automatic Instrumentation (No Code Changes)

// HTTP request automatically traced:
public class InvoiceController : ControllerBase
{
  [HttpGet("{id}")]
  public async Task<IActionResult> GetInvoice(Guid id)
  {
    // ASP.NET Core instrumentation automatically creates:
    // - Span with http.method, http.route, http.status_code
    // - Parent-child relationship with upstream requests
    // - Correlation headers (traceparent, tracestate)

    var invoice = await dbContext.Invoices.FindAsync(id);

    // EF Core instrumentation automatically traces:
    // - SQL query execution time
    // - Database connection details
    // - Query parameters (if configured)

    return Ok(invoice);
  }
}

4. Trade-offs and Alternatives

4.1 Considered Alternatives

Alternative 1: Manual Logging with Serilog
- Rejected: Lacks automatic instrumentation, no standardized tracing, manual correlation required
Alternative 2: Application Insights SDK Only
- Rejected: Vendor lock-in, no OpenTelemetry standard compliance, limited cross-platform support
Alternative 3: Custom Tracing Solution
- Rejected: Reinventing the wheel, no community support, high maintenance burden

4.2 Trade-offs

Pros:

✅ Standard Compliance: OpenTelemetry is CNCF standard with broad adoption
✅ Zero-Allocation Logging: Source generators eliminate runtime overhead
✅ Automatic Instrumentation: 90% of telemetry requires no code changes
✅ Vendor Flexibility: Can switch exporters without application changes
✅ Rich Ecosystem: Extensive instrumentation libraries for .NET
✅ Azure Integration: Native support for Application Insights export

Cons:

❌ Learning Curve: Developers must understand Activity/Span concepts
❌ Configuration Complexity: Multiple extension methods for full setup
❌ Package Dependencies: Requires 7 OpenTelemetry NuGet packages
❌ Dual Telemetry: Running both OTel and Application Insights SDK (temporary)

5. Performance Considerations

5.1 Performance Impact

Logging:

Source Generators: Zero-allocation logging (< 50ns overhead per log call)
Filtering: Logs filtered at source based on log level
Async Export: Background export prevents blocking application threads

Tracing:

Sampling: Production uses head-based sampling (1-10% of requests)
Activity Overhead: ~100ns per activity creation (negligible)
Propagation: W3C Trace Context headers add ~200 bytes per request

Metering:

Aggregation: Metrics aggregated in-memory before export
Export Interval: 60-second batching reduces network overhead
Cardinality: Limited to high-value metrics to prevent explosion

5.2 Optimization Strategies

Conditional Compilation: Console exporters only enabled in DEBUG builds
Activity Filtering: Use ShouldListenTo to disable low-value activities
Log Level Configuration: Production runs at Information level or higher
Managed Identity Caching: Credential caching reduces authentication overhead
Batch Export: Telemetry exported in batches (100 items or 30 seconds)

6. Security Considerations

Authentication:

Managed Identity: Production uses Azure Managed Identity (no secrets in code)
Credential Hierarchy: DefaultAzureCredential provides fallback chain for development

Data Protection:

PII Redaction: No automatic PII capture in logs or traces
Secret Masking: Connection strings not logged; only referenced by Key Vault URL
Scope Isolation: Activity sources isolated by domain boundary

Compliance:

GDPR: User identifiers only logged with explicit consent tracking
LGPD: Telemetry data stored in Azure Brazil South region (configurable)
Retention: Application Insights data retained per configured retention policy (default: 90 days)

7. Testing Strategy

7.1 Unit Tests

Approach: Test telemetry configuration without actual export

[Fact]
public void AddOTelTracing_ShouldRegisterActivitySources()
{
  // Arrange
  var builder = WebApplication.CreateBuilder();

  // Act
  builder.AddOTelTracing();

  // Assert
  var serviceProvider = builder.Services.BuildServiceProvider();
  var tracerProvider = serviceProvider.GetService<TracerProvider>();
  Assert.NotNull(tracerProvider);
}

7.2 Integration Tests

Approach: Verify telemetry end-to-end with in-memory exporter

[Fact]
public async Task CreateInvoice_ShouldCreateActivitySpan()
{
  // Arrange
  var activities = new List<Activity>();
  using var listener = new ActivityListener
  {
    ShouldListenTo = source => source.Name.StartsWith("arolariu.Backend"),
    Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllData,
    ActivityStarted = activity => activities.Add(activity)
  };
  ActivitySource.AddActivityListener(listener);

  // Act
  await invoiceProcessingService.CreateInvoice(testInvoice);

  // Assert
  Assert.Contains(activities, a => a.DisplayName == nameof(CreateInvoice));
}

7.3 Observability Tests

Approach: Validate telemetry appears in Azure Application Insights

Manual validation using Azure Portal Application Map
Query Kusto logs for expected trace patterns
Alert testing for critical scenarios

8. Migration Guide

8.1 Breaking Changes

None - This is the initial observability implementation.

8.2 Migration from Legacy Logging

For developers adding new features:

Replace manual logging:

// Before
logger.LogInformation($"Option {propName} loaded from config");

// After
logger.LogOptionValueFromConfiguration(propName);

Add activity tracing:

// Add to service methods
using var activity = InvoicePackageTracing.StartActivity(nameof(MethodName));

Use appropriate activity source:
- Common infrastructure: CommonPackageTracing
- Core application: CorePackageTracing
- Authentication: AuthPackageTracing
- Invoice domain: InvoicePackageTracing

9. Documentation Requirements

XML documentation on all telemetry classes
README in Common/Telemetry/ explaining usage patterns
Architecture diagram showing telemetry flow
Azure Application Insights dashboard setup guide
Runbook for common troubleshooting scenarios using telemetry

10. Future Work

10.1 Planned Enhancements

Custom Metrics: Business KPIs (invoices processed/hour, average processing time)
Span Events: Fine-grained operation markers within long-running activities
Baggage Propagation: Cross-service context propagation for user preferences
Auto-Instrumentation: Eliminate manual activity creation via source generators
OpenTelemetry Collector: Deploy OTEL Collector for advanced filtering/routing
Distributed Context: Propagate business context (tenant ID, user roles) across boundaries

10.2 Deprecation Plan

Phase Out Application Insights SDK: Once OTel feature parity achieved, remove legacy SDK
Timeline: Q2 2026 (after validating OTel custom events and metrics)

11. References

12. Appendices

Appendix A: NuGet Dependencies

<PackageReference Include="OpenTelemetry" Version="1.13.1" />
<PackageReference Include="OpenTelemetry.Api" Version="1.13.1" />
<PackageReference Include="OpenTelemetry.Exporter.Console" Version="1.13.1" />
<PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.13.1" />
<PackageReference Include="OpenTelemetry.Instrumentation.AspNetCore" Version="1.12.0" />
<PackageReference Include="OpenTelemetry.Instrumentation.EntityFrameworkCore" Version="1.0.0-beta.11" />
<PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.12.0" />
<PackageReference Include="Azure.Monitor.OpenTelemetry.Exporter" Version="1.4.0" />

Appendix B: Activity Naming Conventions

Pattern: Use method name as activity display name

// ✅ Correct
using var activity = InvoicePackageTracing.StartActivity(nameof(CreateInvoice));

// ❌ Incorrect
using var activity = InvoicePackageTracing.StartActivity("create_invoice");

Benefits:

Consistent naming across codebase
Refactoring-safe (renames propagate automatically)
IDE autocomplete support

Appendix C: Azure Application Insights Query Examples

Find slow invoice creation requests:

dependencies
| where name == "CreateInvoice"
| where duration > 5000 // > 5 seconds
| project timestamp, duration, success, resultCode
| order by duration desc

Trace request through all layers:

union traces, requests, dependencies
| where operation_Id == "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"
| project timestamp, itemType, name, message, duration
| order by timestamp asc

Document Version: 1.0.0 Last Updated: 2025-10-20 Reviewed By: Alexandru Olariu Status: Implemented

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Abstract​

1. Motivation​

1.1 Problem Statement​

1.2 Design Goals​

2. Technical Design​

2.1 Architecture Overview​

2.2 Core Components​

2.2.1 Tracing Infrastructure​

2.2.2 Logging Infrastructure​

2.2.3 Metering Infrastructure​

2.2.4 Azure Monitor Export Integration​

2.3 Configuration & Integration​

3. Implementation Examples​

3.1 Manual Activity Creation (Processing Layer)​

3.2 Activity with Tags (Endpoint Layer)​

3.3 High-Performance Logging​

3.4 Automatic Instrumentation (No Code Changes)​

4. Trade-offs and Alternatives​

4.1 Considered Alternatives​

4.2 Trade-offs​

5. Performance Considerations​

5.1 Performance Impact​

5.2 Optimization Strategies​

6. Security Considerations​

7. Testing Strategy​

7.1 Unit Tests​

7.2 Integration Tests​

7.3 Observability Tests​

8. Migration Guide​

8.1 Breaking Changes​

8.2 Migration from Legacy Logging​

9. Documentation Requirements​

10. Future Work​

10.1 Planned Enhancements​

10.2 Deprecation Plan​

11. References​

12. Appendices​

Appendix A: NuGet Dependencies​

Appendix B: Activity Naming Conventions​

Appendix C: Azure Application Insights Query Examples​

Abstract

1. Motivation

1.1 Problem Statement

1.2 Design Goals

2. Technical Design

2.1 Architecture Overview

2.2 Core Components

2.2.1 Tracing Infrastructure

2.2.2 Logging Infrastructure

2.2.3 Metering Infrastructure

2.2.4 Azure Monitor Export Integration

2.3 Configuration & Integration

3. Implementation Examples

3.1 Manual Activity Creation (Processing Layer)

3.2 Activity with Tags (Endpoint Layer)

3.3 High-Performance Logging

3.4 Automatic Instrumentation (No Code Changes)

4. Trade-offs and Alternatives

4.1 Considered Alternatives

4.2 Trade-offs

5. Performance Considerations

5.1 Performance Impact

5.2 Optimization Strategies

6. Security Considerations

7. Testing Strategy

7.1 Unit Tests

7.2 Integration Tests

7.3 Observability Tests

8. Migration Guide

8.1 Breaking Changes

8.2 Migration from Legacy Logging

9. Documentation Requirements

10. Future Work

10.1 Planned Enhancements

10.2 Deprecation Plan

11. References

12. Appendices

Appendix A: NuGet Dependencies

Appendix B: Activity Naming Conventions

Appendix C: Azure Application Insights Query Examples