# ADR-018 — Audit Logging & DPA Compliance

**Status:** Accepted
**Date:** 2026-05-11
**Companion guide:** docs/guides/audit-and-compliance.md

## Context

DPA compliance mandates audit logging for every personal-data access event:
VIEW/CREATE/UPDATE/DELETE/EXPORT/PERMISSION_CHANGE, with immutable storage,
GDPR-deletable path, centralized aggregation, and strict "what NOT to log"
boundaries. The interface decisions from ADR-014 (R31-R51) carry over but
audit needs its own channel — observability data is sampled and short-retention,
audit data is lossless and long-retention with privileged erasure.

## Decision (12 points)

1. **`AuditLogProtocol` in `core-shared`** — must-have universal surface.
   Features call `ctx.auditLog?.record(entry)` without importing the optional package.
2. **`AuditEntry` type with closed action enum** — VIEW/CREATE/UPDATE/DELETE/
   EXPORT/PERMISSION_CHANGE; new actions require explicit type bump. No
   payload/body/oldValue/newValue fields — type enforces "what NOT to log".
3. **`@repo/core-audit` as 5th optional package** — joins realtime, events,
   trpc, ui. Scaffolded via `pnpm turbo gen core-package audit`.
4. **Four impls + Recording test double**: NoopAuditLog, PayloadAuditLog
   (local cache), StdoutJsonAuditLog (operator ships via Vector/Fluent Bit),
   MultiSinkAuditLog (fan-out), RecordingAuditLog (core-testing).
5. **Append-only Payload collection** — `update: () => false` access rule
   is the compliance backbone; erasure path uses `overrideAccess: true`.
6. **GDPR erasure** — sha256-salted pseudonymization (`erased-{hash[0:16]}`)
   or hard delete. AUDIT_PSEUDONYM_SALT env REQUIRED in production; bind-time
   validation fails fast.
7. **Erasure trigger surface** — admin tRPC procedure (`audit.eraseSubject`),
   Payload `afterDelete` hook factory (`createAuditErasureHook`), auth
   integration via printed generator next-steps (NOT auto-installed).
8. **OTel correlation bridge** — `currentTraceId()` helper in core-shared;
   `TraceIdEnrichingAuditLog` decorator at bind time auto-populates
   `AuditEntry.correlationId` from active OTel span. Explicit caller wins.
9. **VIEW capture via BOTH patterns** — use-case `record()` calls (developer
   decides per-read-path) AND `createAuditAfterReadHook` factory (opt-in
   per-collection automatic capture). Fire-and-forget for hooks.
10. **IP/UA explicit at call sites** — no AsyncLocalStorage. Callers use
    `truncateIp(raw)` (/24 IPv4, /48 IPv6) and pass into `record({ from: { ... } })`.
    Sentinels for non-HTTP context: `"system"` / `"background-job"`.
11. **Multi-tenancy: tenant field required** — `AuditEntry.scope.tenant`
    non-optional; single-tenant projects pass `"default"`. Forces multi-tenant
    thinking from day one.
12. **Six-phase delivery** matching established cadence.

## Alternatives considered

- **Vendor-coupled SDK (Datadog/Grafana direct)** — rejected; couples to vendor.
- **Payload-only sink** — fails compliance (hostile-actor immutability).
- **Aggregator-only sink** — fails dev ergonomics. Fan-out is the balance.
- **AsyncLocalStorage for request context** — rejected per user preference;
  explicit > implicit.
- **Optional tenant field** — rejected; DPA-aligned scope discipline benefits
  from forcing the question on every call.

## Consequences

**Positive:**

- DPA-compliant baseline ships with the optional package.
- Vendor-neutral via stdout JSON + log shipper; any aggregator works.
- OTel correlation gives compliance auditors one-click pivot to traces.
- Type-enforced exclusion of "what not to log" prevents categories of mistakes.

**Negative:**

- Boilerplate at every record() call site (IP/UA explicit).
- core-audit ↔ auth coupling for the user-collection hook is awkward
  (manual install via generator next-steps).
- StdoutJsonAuditLog's eraseSubject is best-effort (tombstone only; past
  stdout lines can't be retroactively removed).

## Relationship to other ADRs

- ADR-014 (instrumentation interfaces): audit is a parallel channel, not a
  signal flowing through OTel. The correlationId field is the bridge.
- ADR-015 (events/jobs): no overlap; audit is observational, events are reactive.
- ADR-017 (OTel migration): provides currentTraceId() helper.