Content negotiation

OPDA’s persistent namespace is https://opda.org.uk/pdtf/* (per ADR-0006). These IRIs are identifiers first — graph-node names a consumer cites and compares. Dereferenceability is optional and aspirational, not required for the standard to be valid: a consumer never has to fetch an IRI to use the ontology (the council ranked resolution “aspirational, not required”).

When resolution is offered, it is served directly from opda-controlled hosting — opda.org.uk DNS + origin under OPDA’s control. There is no W3C PICG redirect: ADR-0006 dropped the w3id.org/opda → openpropdata.org.uk redirect chain when the base moved to opda.org.uk, because opda now owns the domain and serves RDF from it directly. The serving model below describes that planned behaviour; DNS for the per-term endpoints is not yet configured.

Accept-header content-negotiation flow

When served, consumers fetch resources via standard HTTP with Accept: headers; the origin returns TTL, JSON-LD, RDF/XML, or HTML depending on the header.

sequenceDiagram autonumber accTitle: Accept-header content-negotiation flow accDescr: Shows the four Accept-header response paths the deployment supports (Turtle, JSON-LD, RDF/XML, HTML) and which source the opda.org.uk origin returns for each. participant C as Client participant S as opda.org.uk alt Accept text/turtle C->>S: GET .../pdtf/Property
Accept: text/turtle S-->>C: 200 OK
Content-Type: text/turtle
(opda-property.ttl) else Accept application/ld+json C->>S: GET .../pdtf/Property
Accept: application/ld+json S-->>C: 200 OK
Content-Type: application/ld+json
(TTL transformed via shared @context) else Accept application/rdf+xml C->>S: GET .../pdtf/Property
Accept: application/rdf+xml S-->>C: 200 OK
Content-Type: application/rdf+xml
(TTL re-serialised via rdflib) else Accept text/html or */* C->>S: GET .../pdtf/Property
Accept: text/html S-->>C: 302 Found
Location: /docs/manual/concept/property/Property end

The HTML branch is a 302 Found to the Concept-tier page (not a permanent redirect), so the human-readable target stays editable as the manual evolves.

Accept-header routing

The origin responds to the Accept header per the matrix below:

Accept value	Response format	Source
`text/turtle`	Turtle (RDF 1.1)	source TTLs in `source/03-standards/ontology/` or derived profile
`application/ld+json`	JSON-LD 1.1 with shared `@context`	TTL transformed via shared context (see jsonld-context.md)
`application/rdf+xml`	RDF/XML	TTL re-serialised via rdflib
`text/html`, `application/xhtml+xml`, `/` (no preference)	HTML — redirect to Concept-tier page	Concept tier at `docs/manual/concept/<module>/<entity>.md` rendered as Astro page

Per-resource availability is documented in the format matrix.

JSON-LD context

A single canonical @context applies to every JSON-LD response, regardless of which resource the consumer requests. The context is documented in jsonld-context.md and is served at https://opda.org.uk/pdtf/context.jsonld.

Per ADR-0013, the canonical context preserves:

@vocab → https://opda.org.uk/pdtf/ (so all unqualified terms resolve to OPDA’s flat term namespace)
Standard ontology prefixes: dct:, dpv:, owl:, rdf:, rdfs:, skos:, sh:, dash:, xsd:, vann:, prov:
OPDA-specific predicate type-coercions so a JSON consumer can treat opda:hasSpecialCategoryData as a boolean, opda:formVersion as a string, etc., without explicit @type annotations on every literal.

Per-resource format availability

See format-matrix.md for the per-resource table:

Foundation namespace https://opda.org.uk/pdtf/
Release snapshot https://opda.org.uk/pdtf/harness/release/1.0.0/ (immutable; the owl:versionIRI target, bumps on every release)
BASPI5 profile https://opda.org.uk/pdtf/shape/profiles/baspi5
Per-entity dereference https://opda.org.uk/pdtf/<EntityLocalName> (e.g. https://opda.org.uk/pdtf/Property)

Caching

TTL / JSON-LD / RDF/XML responses: Cache-Control: public, max-age=3600 for release-snapshot URIs (immutable per release); shorter for the foundation namespace and per-entity dereferences (revalidate against ETag).
The HTML-branch 302 to the Concept-tier page is uncached on the consumer side by spec; the origin RDF responses are cacheable per the above.

Source ADR

ADR-0006 — w3id.org/opda ontology namespace — namespace scheme + opda-direct serving (the original w3id/PICG redirect was dropped when the base moved to opda.org.uk).
ADR-0013 — Overlay profile emission — derived-profile composition feeding content negotiation.