# Ontology schema

File layout and field definitions for domains, tables, joins, and metrics.

The format follows the [Open Knowledge Format (OKF)](https://github.com/GoogleCloudPlatform/knowledge-catalog/tree/main/okf): knowledge is stored as an open, portable file tree that people and agents can read directly and review in git. Cassis provides the analytics-specific profile documented here, with validated structures for domains, tables, columns, joins, and metrics.

-   [Layout](/reference/schema/layout/): The tree, required fields, canonical form, identifier case.
-   [Domains](/reference/schema/domains/): Markdown domains, frontmatter, the generated navigation, domain paths.
-   [Tables and joins](/reference/schema/tables/): Physical and virtual tables, columns, and the join graph.
-   [Metrics](/reference/schema/metrics/): Definitions, expressions, filters, and precomputed metrics.

Two complete example trees are yours to copy: a [minimal one](https://github.com/GetCassis/cassis-ontology-examples/tree/main/examples/minimal) as a skeleton, and a [fully authored one](https://github.com/GetCassis/cassis-demo-stallora/tree/main/cassis) for a fictional marketplace.