# APIs & Integration > Selected Engineer ApS work demonstrating APIs & Integration — the achievements that prove it. - [Designed an organization context‑switching system with client localStorage and server‑side cookie mirroring, letting users act as managed organizations while enforcing least‑privilege authorization.](https://platform.engineer.company/portfolio/designed-an-organization-context-switching-system-with-client-59/) - [Generated the API contract outward from the database — OpenAPI, a 44,076‑line typed TypeScript client, 61 mock handlers and the limits the UI enforces — with a guard at every hop that fails on drift.](https://platform.engineer.company/portfolio/built-a-request-schema-validation-contract-with-automated-61/) - [Built email as a platform capability — three providers with failover, delivery webhooks, send and delivery logging, templating and campaigns — behind a startup check that will not boot without one.](https://platform.engineer.company/portfolio/built-email-as-a-platform-capability-with-failover-62/) - [Hardened the application with nonce‑based CSP, HSTS, SameSite cookies, least‑privilege database roles and server‑side entitlement re‑checks.](https://platform.engineer.company/portfolio/hardened-the-application-with-nonce-based-csp-hsts-77/) - [Built the payments and entitlements layer — Stripe alongside Apple and Google in‑app purchase — gating the directory, search and export through an 11‑table access model checked on the server.](https://platform.engineer.company/portfolio/built-the-payments-and-entitlements-layer-96/) - [Built fail‑closed abuse controls — 22 Redis‑backed rate limiters, Cloudflare Turnstile, request idempotency and an origin lock — so the platform sheds bots and floods instead of trusting its callers.](https://platform.engineer.company/portfolio/built-fail-closed-abuse-controls-and-rate-limiting-100/) - [Reconciled a 20‑record DNS zone declaratively against the Cloudflare API with separate audit and BIND‑export entry points, and turned the CDN proxy back off on privacy grounds after building it.](https://platform.engineer.company/portfolio/reconciled-a-dns-zone-declaratively-118/) - [Automated the provisioning of a second server on a second cloud provider, creating the firewall before the machine so it is born behind one, with both providers' firewalls written directly against their REST APIs to avoid a third‑party collection.](https://platform.engineer.company/portfolio/provisioned-a-second-server-from-code-123/) - [Wrote a parser that reads the real 7,308‑line C header and verifies every call site, every enum constant and that every pointer‑owning class is final, after a hand‑written placeholder header let calls to three removed functions compile, link and crash.](https://platform.engineer.company/portfolio/wrote-a-parser-that-verifies-every-ffi-call-site-157/) - [Reached the half of the messaging core the application had never used — backup transfer, disappearing messages, message editing and resending, verified invitations, proxies and encryption policy — driving every test against the real library with no mocks.](https://platform.engineer.company/portfolio/reached-the-unused-half-of-the-messaging-core-161/)