Features in detail
Built for professionals. Open source for everyone.
Kalenda isn't the hundredth appointment booking tool. It's the uncompromising, Swiss answer to US cloud booking software. Every feature is transparent, auditable and AGPL-3.0 open source.
Calendar integration without data silos
Kalenda connects directly to any CalDAV-capable backend: OpenDesk and Open-Xchange from ZenDiS, Nextcloud, iCloud, Posteo, mailbox.org, Synology, FastMail, FreeIPA or self-hosted Radicale. No middleware, no extra account, no data copy on a third-party server.
- Bidirectional synchronisation in both directions
- Existing free days and appointments respected
- Multiple calendars per provider (work + private)
- Bookings appear instantly in the native calendar client
iFrame embed with URL parameters
One line of HTML is enough: <iframe src="https://booking.cloud-services-ag.com/embed/martin?lang=en"></iframe>. Language, theme and booking topic can be passed via URL parameters — separately for DE and EN. Your marketing site stays in your control.
- Locale parameter: ?lang=de or ?lang=en
- Theme: ?theme=light, ?theme=dark or auto
- Topic: ?topic_de=… and ?topic_en=… (bilingual)
- UTM parameters automatically forwarded to Plausible
GDPR + Swiss DPA by design
No Google Analytics, no Google reCAPTCHA, no Google Fonts, no US server. Self-hosted Plausible for aggregate analytics, Altcha (proof-of-work) for spam protection, fonts served from our own infrastructure. Data stays in Switzerland or the EU — you decide where the backend sits.
- No US subprocessors
- No third-party cookies
- Privacy policy ships in DE and EN
- Swiss processing register (DPA Art. 12) template included
Multilingual — structurally, not just on the surface
Kalenda uses next-intl with localised routes. The German privacy policy lives at /de/datenschutz, the English version at /en/privacy — both linked correctly via hreflang. That way Google finds the right version for the right audience.
- DE and EN out of the box
- More languages via JSON files
- Localised URL slugs (e.g. /datenschutz vs /privacy)
- Server-side language negotiation
Exclusive slot locking
Race conditions are hell in booking systems. Kalenda uses Valkey-based distributed locking with `SET NX PX` — the first click wins, the second gets a clean error message. Double-bookings are technically impossible, even with thousands of concurrent requests.
- Valkey (BSD-3, Linux Foundation fork of Redis OSS)
- Per-slot lock with TTL — no deadlocks possible
- In-memory fallback for single-instance setups
- Per-IP rate limiting bundled
Altcha instead of reCAPTCHA
Google reCAPTCHA tracks your visitors. Altcha solves it differently: the visitor's browser runs a proof-of-work computation — invisible, no cookies, no third-party tracking. Bots are still reliably blocked.
- BSD-3 licensed, fully open source
- No third-party calls
- Works without JavaScript fingerprinting
- Server-side verifiable on the backend
Cloud-native deployable
Kalenda ships with a Helm chart, ArgoCD examples and Keeper Secrets Manager integration — everything our own production infrastructure uses. You can run the same setup on your K3s, K8s or OpenShift.
- Helm chart at charts/kalenda
- ArgoCD multi-source Application pattern
- Single-tenant per customer (namespace + DB)
- Next.js standalone output → Docker image < 200 MB
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