GDPR — Technical Controls

GDPR (and its UK / EEA siblings) is a legal framework with a heavy technical surface. This skill stays on the engineering side: what to build, what to log, what to expose, what to delete. For DACH-specific Impressum/AGB/AVV requirements, see dach-compliance. For data-protection legal questions, talk to a DPO.

The terms used here line up with the regulation but are paraphrased. When in doubt, the regulation text is the source of truth.

When to invoke

• Building or reviewing a product handling EU/EEA/UK resident data
• Responding to a Subject Access Request (SAR) or deletion request
• Preparing for a Datenschutz / DPA audit
• Adding a new third-party processor (analytics, support, email, AI, CDN)
• After a suspected personal-data breach
• A user asked "where is my data?"

Step 1 — Data inventory

You cannot protect data you cannot enumerate. Build and maintain a record of:

• What personal data fields you store (users.email, orders.shipping_address.street, …)
• Where each lives (Postgres table, S3 bucket, log file, third-party processor)
• Purpose of each (legal basis: consent / contract / legitimate interest / legal obligation)
• Retention period — when it should be deleted
• Who can access it (roles, audit log)
• Third parties (processors) receiving copies, and the lawful basis for each transfer

A reasonable starting format:

Field                    | Location                 | Purpose / basis        | Retention | Processors
-------------------------|--------------------------|------------------------|-----------|-----------
user.email               | postgres.users           | account (contract)     | until-deletion + 30d backup decay | Mailgun (transactional)
user.password_hash       | postgres.users           | auth (contract)        | until-deletion | -
order.shipping_address   | postgres.order_addresses | fulfillment (contract) | 10y (tax)  | DHL (delivery)
user.ip                  | nginx access logs        | abuse (legit interest) | 90d        | -
user.session_cookie      | redis, nginx logs        | session (contract)     | session+30d | -

This is the foundation for SARs, deletion requests, breach scoping, and processor-agreement enumeration. Keep it in a private repo or wiki — version-controlled, not in a one-off spreadsheet.

Step 2 — Subject Access Request (SAR) endpoint

A user can ask for a copy of all their personal data. You have one month to respond (extendable to three with reason).

Don't build this manually per request. Build a SAR endpoint or admin tool that produces the export programmatically.

// Sketch — admin tool that produces a SAR bundle
async function generateSAR(userId: string) {
  const bundle: Record<string, unknown> = {};

  bundle.profile = await db.users.findUnique({ where: { id: userId }, select: safeProfileFields });
  bundle.orders = await db.orders.findMany({ where: { userId }, select: orderFieldsForSAR });
  bundle.addresses = await db.addresses.findMany({ where: { userId }});
  bundle.support_tickets = await db.tickets.findMany({ where: { userEmail: bundle.profile.email }});
  bundle.audit_log = await db.authEvents.findMany({ where: { userId }, take: 1000 });
  bundle.consent_history = await db.consents.findMany({ where: { userId }});

  // Files / media — include URLs to time-limited download links
  bundle.uploads = await listUserUploads(userId);

  return bundle;   // serialize to JSON or ZIP
}

Patterns:

• Authenticate the request rigorously — the SAR endpoint is itself a takeover vector. Require login + recent re-auth + email-confirm-link before issuing the export.
• Time-limited download — the export contains the same PII you protect everywhere else. Serve via a single-use URL, valid for 24h.
• Document what's NOT in the export — purely operational metadata (server logs, monitoring data) is generally not required to include but mention it transparently.
• Right to portability is a related right — machine-readable format. JSON is fine; offer CSV for tabular fields.

Step 3 — Deletion endpoint

A user can ask to be deleted (Article 17). You generally have one month.

Hard problem: most apps have data scattered across services, with foreign keys, with cached copies in third parties.

async function deleteUser(userId: string) {
  // 1. Soft-mark for deletion, prevent further logins immediately
  await db.users.update({ where: { id: userId }, data: { deletedAt: new Date(), status: 'deleting' }});
  await invalidateAllSessions(userId);

  // 2. Anonymize records that must be retained for legal reasons (tax invoices etc.)
  await db.invoices.updateMany({
    where: { userId },
    data: { userId: null, customerName: 'DELETED USER', customerEmail: null }
  });

  // 3. Hard-delete records with no retention obligation
  await db.preferences.deleteMany({ where: { userId }});
  await db.sessions.deleteMany({ where: { userId }});
  await db.notifications.deleteMany({ where: { userId }});

  // 4. Files
  await deleteAllUserUploads(userId);

  // 5. Third-party processors — propagate via their APIs
  await mailgunApi.removeContact(user.email);
  await stripeApi.customers.delete(user.stripeCustomerId);
  await intercomApi.users.archive(user.intercomId);

  // 6. Backups — flag for purge on next cycle
  await db.backupPurgeQueue.create({ userId, scheduledFor: nextBackupCycle });

  // 7. Finally, remove the user record itself
  await db.users.delete({ where: { id: userId }});
}

Patterns:

• Retention exceptions exist — tax law, fraud prevention, ongoing-litigation hold. Anonymize rather than delete in those cases. Document the exception per field.
• Backups are the awkward part. Most teams cannot delete from immutable backup archives without restoring everything; the accepted approach is to keep a "delete on restore" pipeline so any restore re-applies the deletion list. Document this in your privacy policy.
• Propagate to every processor in your inventory. This is why the inventory exists.
• Soft-mark first, then delete — gives you a window to abort if the request was forged.

Step 4 — Anonymization vs pseudonymization

• Pseudonymization — the identifier is replaced with a token, but the mapping exists somewhere (e.g. user_842 instead of [email protected]). Re-identification is possible if you have the mapping. Still considered personal data under GDPR.
• Anonymization — re-identification is not reasonably possible. The data is no longer personal data and outside GDPR scope.

True anonymization is harder than it sounds. Removing names doesn't anonymize if a single record can be uniquely identified (e.g. "person aged 47, working in Eisenstadt, with a Tesla" might be one person).

Use pseudonymization in development / staging / analytics. Reserve true anonymization for data leaving the EU regime entirely.

Step 5 — Log scrubbing

Logs are often the worst PII leak. The app sanitizes inputs; the logger doesn't know about that.

// Log middleware — strip known sensitive headers before recording
function safeReqLog(req) {
  return {
    method: req.method,
    path: req.path,
    ip: hashIp(req.ip),                      // hash, not raw
    userAgent: req.headers['user-agent'],
    requestId: req.headers['x-request-id'],
    // Explicitly NOT: authorization, cookie, body
  };
}

// JSON loggers — strip in serializer
const REDACT_KEYS = new Set(['password', 'token', 'cookie', 'authorization', 'email', 'phone']);
function redactingReplacer(key, value) {
  return REDACT_KEYS.has(key.toLowerCase()) ? '[REDACTED]' : value;
}

Rules:

• No bodies in access logs. Bodies leak passwords on login routes, payment data on checkout, PII on profile updates.
• Hash or truncate IPs — full IPv4 is borderline PII per several DPAs. xxx.xxx.xxx.0 or HMAC-with-rotating-key is safer.
• Redact known-PII fields in structured logs by key name.
• Log retention bounded — see log-strategy. 30 days hot is plenty for most operational needs; longer needs justification.
• Audit logs (auth events etc.) get separate, longer retention because they support investigations — but still strip body content.

Step 6 — Breach notification (72 hours)

Article 33 requires notifying the supervisory authority within 72 hours of becoming aware of a personal-data breach. Article 34 requires notifying affected data subjects if the breach is likely to result in "high risk to the rights and freedoms" of individuals.

Engineering preparations:

• Define "becoming aware" in your IR runbook — typically confirmation, not first suspicion. See incident-response.
• Pre-draft the notification format — supervisory authorities (Datenschutzbehörde, Bundesbeauftragte) publish templates. Pre-fill the org info; per-incident fields go in at incident time.
• Inventory lets you scope fast — "what data was affected" must be answerable in hours, not days. The inventory from step 1 is what makes this possible.
• Notification can be staged — first notice in 72h with what you know, follow-ups as you learn more. Better than missing the deadline waiting for full clarity.

Step 7 — Sub-processors (DPA / AVV)

Every third party that processes personal data on your behalf — analytics, email, CRM, support, AI, CDN, hosting — is a sub-processor and requires a Data Processing Agreement (DPA, German: AVV) before going live.

Engineering responsibilities:

• Maintain the sub-processor list — public-facing in most jurisdictions. Update before you wire in a new service.
• Vet new processors — security posture, GDPR commitments, sub-sub-processors, location, certifications (ISO 27001, SOC 2). Document the review.
• Implement consent / contractual basis — analytics typically needs consent (see DACH cookie consent in dach-compliance); transactional email is usually contract-based and does not.

Step 8 — Cross-border transfers

EU personal data leaving the EEA needs a transfer mechanism: adequacy decision, Standard Contractual Clauses (SCCs), or a derogation. The post-Schrems-II / Data Privacy Framework landscape changes; check current status before relying on US transfers.

Engineering knobs:

• EU-region defaults for cloud services (AWS Frankfurt, GCP europe-, Azure West Europe, Cloudflare EU-data-localization)
• Data residency on managed Postgres / Mongo etc. — pick EU region at provisioning, document
• CDN / edge caches may replicate globally — consider an EU-only edge for sensitive paths

Quick checklist

Engineering preparation for a GDPR audit / SAR / breach:

• [ ] Data inventory exists, is version-controlled, is up to date
• [ ] SAR endpoint or admin tool produces an export programmatically
• [ ] Deletion endpoint exists; covers DB, files, third parties, backups (via flag)
• [ ] Logs do not contain bodies or unredacted PII
• [ ] IP addresses in logs are hashed or truncated
• [ ] Sub-processor list is current; DPAs/AVVs in place before integration
• [ ] EU-region defaults for storage / compute
• [ ] Breach notification template pre-drafted, owner assigned
• [ ] Privacy policy mentions retention periods and backup-purge approach
• [ ] Anonymization vs pseudonymization is intentional, documented per use

What this skill will not do

• Provide legal advice — talk to a DPO / data-protection lawyer for borderline cases
• Help process data without a lawful basis
• Recommend skipping the SAR or deletion endpoints "until we get bigger"