Pin the observability foundation. Two signals are in scope: structured
logs and distributed traces.
Logs: pino + nestjs-pino, JSON line-delimited on stdout, with a fixed
envelope (level, time, service, version, env, trace_id, span_id,
session_id, user_id_hash, audience, msg, ...). Pino redact strips a
reviewable allowlist of sensitive paths (Authorization/Cookie headers,
*.password, *.access_token, *.refresh_token, ...). user_id_hash uses a
per-environment salt (LOG_USER_ID_SALT) so the same userId is not
correlatable across environments.
Tracing: OpenTelemetry SDK for Node + auto-instrumentations (HTTP,
Express, NestJS, pg, ioredis, Prisma). The SPA also runs OTel-Web with
an HTTP interceptor propagating traceparent on outbound calls; the same
trace_id is the correlation identifier from the user click to the DB
query. No separate X-Correlation-ID.
Request-scoped context lives in nestjs-cls; the Pino formatter and the
future audit-log writer pull from CLS - no per-call threading.
Sampling is 100% at the application; tail sampling is performed at the
local OpenTelemetry Collector (deferred to the phase-3 infrastructure
ADR, where the on-prem backend - Grafana stack, ELK, or other - will be
chosen). Output: stdout for logs, OTLP/HTTP for traces, both consumed
by the local collector. The application stays vendor-neutral.
Audit logs are explicitly out of scope of this ADR - they share the
trace_id but use a separate writer and a separate sink (next ADR).
decisions/README.md index updated. CLAUDE.md gains an explicit
'Observability' summary pointing to ADR-0012.
Julien Gautier