# NestJS Observability

> Structured logging (Pino) and Prometheus metrics.

- Skill: `ngxtm-devkit/nestjs-observability` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add ngxtm-devkit/nestjs-observability`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ngxtm-devkit/nestjs-observability/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: ngxtm (https://skillmd.com/u/ngxtm-devkit)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ngxtm-devkit/nestjs-observability

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# Observability Standards

## Structured Logging

- **Standard**: Use `nestjs-pino` for high-performance JSON logging.
  - **Why**: Node's built-in `console.log` is blocking and unstructured.
- **Configuration**:
  - **Redaction**: Mandatory masking of sensitive fields (`password`, `token`, `email`).
  - **Context**: Always inject `Logger` and set the context (`LoginService`).

## Tracing (Correlation)

- **Request ID**: Every log line **must** include a `reqId` (Request ID).
  - `nestjs-pino` handles this automatically using `AsyncLocalStorage`.
  - **Propagation**: Pass `x-request-id` to downstream microservices/database queries key to trace flows.

## Metrics

- **Exposure**: Use `@willsoto/nestjs-prometheus` to expose `/metrics` for Prometheus scraping.
- **Key Metrics**:
  1. `http_request_duration_seconds` (Histogram)
  2. `db_query_duration_seconds` (Histogram)
  3. `memory_usage_bytes` (Gauge)

## Health Checks

- **Terminus**: Implement explicit logic for "Liveness" (I'm alive) vs "Readiness" (I can take traffic).
  - **DB Check**: `TypeOrmHealthIndicator` / `PrismaHealthIndicator`.
  - **Memory Check**: Fail if Heap > 300MB (prevent crash loops).