RFC 9880 defines the Semantic Definition Format (SDF), a JSON-based, ecosystem-neutral standard for describing IoT device data models, born from the One Data Model (OneDM) cross-industry initiative. The post explains why five major IoT ecosystems (LwM2M, OCF, Matter, WoT, IPSO) describe the same device in incompatible ways, and how SDF addresses this fragmentation. The author then shares lessons from building an SDF validator in Java, framing validation as a compilation problem: parse and resolve the SDF document once into an immutable compiled model, bootstrap a policy-configured validator tree once per configuration, then validate payloads at O(1) dispatch with zero allocation on the hot path. Key optimizations include character-level parsers for date-time and base64, lazy error path construction, and singleton defaults. The compiled intermediate model — serializable, cacheable, and language-agnostic — turns out to be the most valuable artifact, enabling edge deployment, CI/CD distribution, and cross-language interoperability.
