.NET Observability Knowledge Base

You are implementing observability for a .NET application. Read the relevant reference docs below based on what you're building. Always use DI-first patterns — inject ITracer, ILogger<T>, etc. rather than using static accessors.

Observability Philosophy

Good observability answers three questions: What happened? (logs), Where did it happen? (traces), How much? (metrics). Every signal should be correlated — a single request should be traceable across logs, spans, and metrics via trace context. See 12-observability-philosophy.md for the full framework.

Quick Decision: Which Signal Type?

• Need to understand a specific request's journey? → Distributed Tracing (Elastic APM or OpenTelemetry)
• Need to debug unexpected behavior or errors? → Structured Logging (Serilog → Seq / Elasticsearch)
• Need to track rates, durations, or resource usage? → Metrics (OpenTelemetry Meters or Elastic APM)
• Need all three correlated? → APM + Serilog with trace context enrichment (see doc 09)

Reference Documentation

Read the relevant docs based on your task:

Logging Foundation (start here for logging)

• ${CLAUDE_PLUGIN_ROOT}/docs/01-structured-logging.md — Message templates, semantic logging, enrichers, log levels, destructuring
• ${CLAUDE_PLUGIN_ROOT}/docs/02-serilog-configuration.md — appsettings.json config, UseSerilog(), minimum level overrides, IServiceCollection registration
• ${CLAUDE_PLUGIN_ROOT}/docs/03-serilog-sinks.md — Console, File, Seq, Elasticsearch sinks, async wrappers, formatting

Log Destinations

• ${CLAUDE_PLUGIN_ROOT}/docs/04-seq.md — Seq sink setup, querying, signals, dashboards, API keys, health checks
• ${CLAUDE_PLUGIN_ROOT}/docs/05-elastic-logging.md — Elasticsearch sink, ECS formatting, index naming, ILM, data streams

APM & Tracing

• ${CLAUDE_PLUGIN_ROOT}/docs/06-elastic-apm.md — Native Elastic APM agent, ITracer (DI-first), transactions, spans, distributed tracing, W3C propagation
• ${CLAUDE_PLUGIN_ROOT}/docs/07-opentelemetry.md — OTel .NET SDK, ActivitySource, Activity spans, Meter/Counter/Histogram, OTLP exporters
• ${CLAUDE_PLUGIN_ROOT}/docs/08-otel-elastic-integration.md — OTel-to-Elastic export, feature comparison, migration path, config mapping

Correlation & Conventions

• ${CLAUDE_PLUGIN_ROOT}/docs/09-apm-correlation.md — Log-trace correlation, trace.id injection, Serilog enrichers, cross-service trace visualization
• ${CLAUDE_PLUGIN_ROOT}/docs/10-naming-conventions.md — Transaction naming, span naming, metric naming, PascalCase properties, avoiding high-cardinality

Patterns & Philosophy

• ${CLAUDE_PLUGIN_ROOT}/docs/11-instrumentation-patterns.md — Middleware, action filters, health check exclusion, sampling, IHttpClientFactory, EF Core, error capture
• ${CLAUDE_PLUGIN_ROOT}/docs/12-observability-philosophy.md — Three pillars, anti-patterns, alert hygiene, signal selection

Critical Rules

1. Never use string interpolation in message templates — Use Log.Information("Processing order {OrderId}", orderId) not Log.Information($"Processing order {orderId}"). Interpolation defeats structured logging.
2. Always null-check CurrentTransaction — _tracer.CurrentTransaction can be null when no transaction is active (background threads, startup code). Always guard: _tracer.CurrentTransaction?.StartSpan(...).
3. Always null-check Activity.Current — Activity.Current is null when no listener is registered or no parent activity exists. Always guard before accessing properties.
4. Always close spans/transactions in try/finally or using blocks — Unclosed spans leak memory and corrupt trace trees. Use try/finally { span.End(); } or the using pattern.
5. Always capture exceptions on spans before ending them — Call span.CaptureException(ex) or activity?.SetStatus(ActivityStatusCode.Error) in the catch block, before the finally block ends the span.
6. Never log sensitive data — PII, passwords, tokens, connection strings must never appear in log properties. Use [LogMasked] attributes or custom destructuring policies.
7. Always propagate trace context across service boundaries — HTTP calls automatically propagate W3C traceparent headers when using IHttpClientFactory. For message queues, manually inject/extract trace context.
8. Avoid high-cardinality transaction/metric names — Never include user IDs, order IDs, or GUIDs in transaction names. Use route templates (GET /api/orders/{id}) not resolved paths (GET /api/orders/12345).
9. Use DI-first, static as fallback — Inject ITracer and ILogger<T> via constructor injection. Only use Agent.Tracer or Serilog.Log.Logger in non-DI contexts (startup, static helpers).
10. Always set minimum level overrides for noisy namespaces — Suppress Microsoft.AspNetCore to Warning, System.Net.Http to Warning, Microsoft.EntityFrameworkCore.Database.Command to Warning (or Information if you need query logging).
11. Use LogContext.PushProperty for request-scoped enrichment — Don't repeat the same property on every log call. Push it once at the middleware/filter level.
12. Never fire-and-forget without trace context — Background work (Task.Run, IHostedService) loses ambient trace context. Explicitly capture and restore Activity.Current or start a new transaction linked to the parent.