Ontology And Semantic Graph Architecture

Status: design baseline, 24 July 2026.

If RDF, RDFS, SKOS, OWL, SHACL, DCAT, PROV or the open-world distinction are new to you, first read Knowledge graphs and stable identifiers, The semantic web and ontologies and Validation, provenance and catalogue standards.

This document defines how OKF bundles and the OKF Explorer should grow from typed navigation graphs into evidence-bearing semantic knowledge graphs. It also records the boundary between semantic meaning, validation, inference and graph presentation.

The immediate UI change groups a focus graph by relationship predicate and direction. It does not make a bundle an ontology and it does not infer facts. It establishes the interaction surface on which richer RDFS, SKOS, OWL, SHACL, DCAT and PROV metadata can later operate.

Design Goals

1. Make a selected record and its relationships readable before exposing the whole graph.
2. Give every class and property a stable identifier where the source supports one.
3. Keep asserted, normalized, inferred and model-derived statements visibly distinguishable.
4. Validate closed-world publication requirements without confusing them with open-world semantic reasoning.
5. Keep ontology semantics independent from Explorer layout preferences.
6. Preserve evidence and provenance for every generated or inferred edge.
7. Let older label-only OKF bundles continue to work.
8. Avoid requiring a full description-logic reasoner in a static browser.

The Five Separate Layers

The implementation must not collapse these layers into one object.

PROV-O spans the instance, inference and validation layers because it records who or what produced a statement and from which evidence. DCAT 3 supplies domain terms for catalogues, datasets, data services and distributions.

Why This Separation Matters

RDF, RDFS and OWL use an open-world model. A missing statement is generally unknown, not false. SHACL and JSON Schema can apply closed-world publication requirements such as "every published dataset record must have one title". Explorer layout is neither: putting a property group on the left side of the canvas has no semantic consequence.

The following conclusions are therefore invalid:

• a missing dcterms:license proves that no licence exists;
• a SHACL validation failure proves an OWL ontology is inconsistent;
• an Explorer group order declares an RDF property hierarchy;
• a high confidence value is automatically a relationship strength;
• two records shown close together are semantically equivalent.

Standards Baseline

RDF And JSON-LD

The bundle-wiki profile already publishes JSON-LD and uses stable absolute identifiers. RDF remains the common graph model. JSON-LD remains the practical web serialization and compatibility bridge for the current Explorer.

RDF 1.2 work is active, but the current OKF profile should not claim RDF 1.2 conformance while the relevant documents remain Candidate Recommendation or Working Draft. The profile can track RDF 1.2 changes without making them a runtime dependency.

RDFS

Use RDFS for lightweight, explainable vocabulary structure:

• rdfs:Class;
• rdf:Property;
• rdfs:subClassOf;
• rdfs:subPropertyOf;
• rdfs:domain;
• rdfs:range;
• rdfs:label and rdfs:comment.

Domain and range statements are inference rules, not input-form validation. For example, an edge using dcterms:publisher can entail a type for its subject or object. It does not by itself reject an input row.

SKOS

Use SKOS for controlled concepts that are not naturally OWL classes:

• topics;
• tags promoted into governed concept schemes;
• licence families;
• format families;
• geography and organisational classifications;
• preferred, alternative and hidden labels;
• broader, narrower and related concept navigation.

Do not turn every source tag into an OWL class. A raw tag can remain a literal keyword until a governed concept mapping exists.

For Explorer facets, publish governed concepts in the canonical YAML-LD with skos:inScheme and skos:broader/skos:narrower, then compile a bounded analysis.hierarchies navigation projection into the runtime descriptor. Display-only grouping heuristics may improve a legacy bundle's preview, but must never be serialized back as semantic assertions without review.

OWL 2

Start with an explicitly bounded OWL 2 RL-style inference profile. Candidate features include:

• equivalent and inverse properties;
• equivalent classes;
• transitive or symmetric properties where the domain definition justifies them;
• property and class hierarchy closure;
• carefully reviewed property chains;
• identity only when a strong mapping supports it.

Do not infer owl:sameAs from similar labels, shared URLs or model confidence. Prefer weaker mapping predicates such as SKOS exact/close match when identity has not been established.

Browser execution should consume precomputed, bounded entailments. The bundle builder should record the rule set, ontology versions and source assertions used to produce each materialized edge.

SHACL

SHACL 1.0 is the stable W3C Recommendation baseline for graph validation. SHACL 1.2 Core is a Working Draft dated 23 July 2026 and can be evaluated in an experimental profile, but must not yet be presented as the stable publication contract.

Use SHACL to validate:

• required bundle and concept metadata;
• expected node kinds and cardinalities;
• datatype and controlled-value constraints;
• evidence requirements for inferred statements;
• consistency between descriptor, manifest and semantic projections;
• presentation references to real predicates and facets.

Validation results should be publishable as data and link back to the exact shape, focus node, path, severity and source artifact.

DCAT And PROV

Use DCAT 3 where records describe catalogues, datasets, data services, distributions and catalogue records. Keep the existing OKF standards crosswalk where the source cannot support a complete DCAT assertion.

Use PROV-O to distinguish:

• source entities;
• harvesting, normalization, mapping and inference activities;
• responsible agents;
• primary sources and derivations;
• generation timestamps;
• qualified attribution and influence where a simple edge is insufficient.

Proposed Semantic Extension

The following is a design proposal, not a stable schema:

{
  "extensions": {
    "okf-semantic-model.v1": {
      "status": "experimental",
      "contexts": ["context/okf-bundle-v1.jsonld"],
      "vocabularies": [
        {
          "id": "https://example.org/vocabulary/catalogue",
          "version": "2026-07-24",
          "entrypoint": "semantic/vocabulary.ttl",
          "integrity": "sha256-..."
        }
      ],
      "ontologies": [],
      "shapes": [
        {
          "id": "https://example.org/shapes/publication",
          "conforms_to": "https://www.w3.org/TR/shacl/",
          "entrypoint": "semantic/shapes.ttl"
        }
      ],
      "inference": {
        "profile": "owl2-rl-bounded",
        "materialized_entrypoint": "semantic/entailed-relationships.json",
        "rule_manifest": "semantic/inference-manifest.json"
      },
      "relationship_types": [
        {
          "predicate": "http://purl.org/dc/terms/publisher",
          "label": "published by",
          "inverse_label": "publishes",
          "domain": "http://www.w3.org/ns/dcat#Dataset",
          "range": "http://xmlns.com/foaf/0.1/Agent"
        }
      ],
      "validation_report": "semantic/validation-report.ttl"
    }
  }
}

This extension should be descriptor-level and integrity-bound. Large artifacts remain external entrypoints so startup does not require loading an ontology or all entailments.

Relationship Record Contract

The current Explorer accepts a human kind or label. Semantic bundles should also provide a predicate identifier:

{
  "id": "relationship:dataset-1-publisher",
  "source": "dataset/dataset-1",
  "target": "publisher/ons",
  "kind": "published by",
  "predicate": "http://purl.org/dc/terms/publisher",
  "assertion_status": "normalized",
  "evidence_type": "source-metadata",
  "evidence": ["source/catalogue-row-1"],
  "confidence_score": 1.0,
  "observed_at": "2026-07-24T00:00:00Z"
}

Recommended fields:

confidence_score, strength and count are not interchangeable:

• confidence estimates whether an assertion is correct;
• strength measures a domain-defined magnitude;
• count records multiplicity in an aggregate.

The Explorer renders line width only when one explicit numeric count, strength, weight or evidence-count metric covers every displayed edge and has a non-constant range. It must name that metric and range. Confidence, missing values and constant values do not change line width automatically.

Relationship-Type Registry

Each governed predicate should have one registry entry containing:

• canonical IRI;
• preferred and inverse labels;
• description;
• RDFS super-properties;
• expected domain and range;
• OWL characteristics, if any;
• assertion/evidence policy;
• supported weight metric and unit, if meaningful;
• source vocabulary and version;
• deprecation and replacement metadata.

The registry lets the Explorer group relationships by semantic property even when two bundles use different display labels. Legacy bundles without a predicate continue to group by normalized label and direction.

Explorer Graph Contract

Auto Mode

Auto mode retains the existing deterministic compact graph for small focus graphs. A dense focus graph switches to semantic relationship regions when it has at least twelve visible relationships and at least two relationship groups.

Relationship Mode

Relationship mode groups by predicate and direction relative to the focus:

• outgoing;
• incoming;
• lateral context.

The ordered groups occupy deterministic regions:

1. left list;
2. top staircase;
3. bottom staircase;
4. right list;
5. additional inner lanes in a stable sequence.

When the complete left list fits without collisions, it reserves persistent above-left node labels. This keeps its text column away from relationships converging on the focus node. Larger or conflicting lists and labels in the other regions still use the shared collision planner and cycle when required.

Users can:

• show or hide a relationship group;
• expand a group and show or hide individual members;
• reorder groups by drag and drop;
• reorder with explicit earlier/later buttons;
• switch between Auto and By relationship;
• reset the relationship workspace.

The order and visibility state use repeatable URL parameters:

graph.layout=relationships
graph.group=outgoing%3Ahttp%3A%2F%2Fpurl.org%2Fdc%2Fterms%2Fpublisher
graph.hide=incoming%3Ahttp%3A%2F%2Fwww.w3.org%2Fns%2Fdcat%23record
graph.hideEdge=dataset%2F1%3Epublisher%2Fons%3Apublished+by

The URL parameters are presentation state. They do not alter bundle semantics.

Labels And Inspection

• The node-type key lists only types in the currently displayed graph.
• Node and eligible edge labels share non-overlapping display layers.
• Persistent focus labels are placed with lookahead so they do not consume the only viable position for a boundary node.
• Label layers cycle every two seconds and can be paused.
• Every relationship remains available in the relationship drawer even when direct graph labels are bounded.

Ontology-Oriented Views

The relationship layout is the first semantic control, not the final ontology UI. Candidate future coordinated views are:

Each view should reduce the same active context and preserve addressable URL state. No view should require loading the complete corpus before the first useful screen.

Research-Informed Layout Decisions

The graph controls combine several established directions rather than relying on an unconstrained force graph:

• Semantic substrates place nodes into meaningful, non-overlapping regions and let users control which links are visible.
• Focus-plus-context exploration starts from a selected item, keeps useful context and expands on demand.
• Layered graph drawing is appropriate for class, property and dependency hierarchies.
• Ontology visualization research supports coordinated tree, graph, indented list, treemap and focus views because no single projection answers every ontology task.

Auto layout remains useful for small graphs. Semantic regions become preferable when the node-link diagram's labels and crossings stop supporting comparison.

Conflicts And Decisions

Meaning Versus Layout

Conflict: authors may want ontology terms to control visual placement.

Decision: ontology metadata may identify relationship groups and hierarchies, but region order belongs to the presentation profile or user URL state.

Open World Versus Publication Completeness

Conflict: OWL/RDFS permit unknown values while public data contracts need required fields.

Decision: use SHACL/JSON Schema for publication validation and OWL/RDFS for meaning and entailment. Report the two outcomes separately.

Browser Reasoning Versus Static Reliability

Conflict: client-side reasoning can be interactive but expensive, non-deterministic across engines and difficult to audit.

Decision: materialize bounded entailments during the build, publish the rule and provenance manifests, and use the browser for filtering and inspection.

Confidence Versus Relationship Strength

Conflict: both are numeric and can be tempting inputs to line width.

Decision: require a declared metric. Never interpret categorical confidence or an undocumented score as strength.

owl:sameAs Versus Approximate Matching

Conflict: deduplication benefits from identity links, but false identity propagates assertions in both directions.

Decision: reserve owl:sameAs for established identity. Use SKOS mapping terms or an OKF candidate-match assertion with provenance for weaker mappings.

Inferred Edges Versus Source Evidence

Conflict: materialized entailments can look indistinguishable from source assertions.

Decision: carry assertion_status, derivation activity and source evidence. A future Explorer pass should give asserted and inferred edges distinct, accessible visual encodings.

Delivery Roadmap

Phase 1: Predicate-Aware Exploration

• Group focus graphs by predicate and direction.
• Filter the key to visible types.
• Add group/member visibility, ordering and URL persistence.
• Use explicit varying edge metrics only.
• Retain label-only compatibility.

This phase is implemented in the Svelte Explorer.

Phase 2: Governed Vocabulary

• Publish the semantic extension schema. The first bounded implementation is now the okf-explorer-governed-terms.v1 registry and its JSON Schema.
• Add a predicate registry and integrity-bound vocabulary entrypoints. Large descriptors now support integrity-bound terms and term_validation entrypoints; richer RDF vocabulary artifacts remain future work.
• Require stable predicate IRIs for new exemplar packs.
• Add SKOS concept schemes for governed topics, formats and licences.

Phase 3: Validation

• Expand the current SHACL shapes.
• Publish machine-readable validation reports. The first closed-world report now checks recognition, namespace expansion, provenance, kind, bounded use and artifact coverage.
• Add a Validation view and source-linked remediation paths. Explorer now provides searchable term details, source links, validation status and limitations; a cross-bundle remediation workflow remains future work.
• Keep SHACL 1.2 experiments separate until the specification is stable.

Phase 4: Bounded Inference

• Define an OWL 2 RL-style rule manifest.
• Materialize entailments in the builder.
• Record PROV-O derivations.
• Let the Explorer filter asserted, normalized, inferred and model-derived statements.

Phase 5: Ontology Navigation

• Add class, property and concept-scheme views.
• Add hierarchy-aware search and facet reduction.
• Compare ontologies and mappings without asserting equivalence.
• Evaluate the UI against ontology-engineering and data-discovery user tasks.

Acceptance Criteria

1. A bundle without semantic extensions renders exactly as a compatible label-only bundle.
2. A bundle with predicates groups equivalent properties independently of display label.
3. Graph presentation state round-trips through the URL.
4. The key contains only node types present after graph filtering.
5. A group and any one member can be hidden without mutating source data.
6. Reordering changes layout only.
7. Edge width is neutral when no varying metric is supplied.
8. Every entailed edge identifies its rule and source assertions.
9. SHACL failures do not masquerade as ontology inconsistency.
10. Asserted and inferred statements can be distinguished by users and downstream agents.

References

Standards:

• RDF 1.2 Concepts and Abstract Syntax
• RDF 1.2 Schema
• OWL 2 Web Ontology Language Profiles
• SHACL 1.0 Recommendation
• SHACL 1.2 Core Working Draft
• SKOS Simple Knowledge Organization System Reference
• PROV-O: The PROV Ontology
• Data Catalog Vocabulary (DCAT) 3

Graph and ontology visualization:

• Shneiderman and Aris, Network Visualization by Semantic Substrates, IEEE TVCG, 2006.
• van Ham and Perer, Search, Show Context, Expand on Demand, IEEE TVCG, 2009.
• Katifori et al., Ontology Visualization Methods - A Survey, ACM Computing Surveys, 2007.
• Sugiyama, Tagawa and Toda, Methods for Visual Understanding of Hierarchical System Structures, IEEE Transactions on Systems, Man, and Cybernetics, 1981.