eclipse-xfsc · Sw3eks · Feb 26, 2026
diff --git a/federated-catalogue/src/docs/architecture/chapters/05_building_block_view.adoc b/federated-catalogue/src/docs/architecture/chapters/05_building_block_view.adoc
@@ -74,9 +74,21 @@ The Graph Database can be considered as a kind of search index. The single sourc
 
 Generically returning the Self-Description files containing claims that influence query response is not possible. To get the relevant Self-Description files, the query to the Graph Database can be formulated to return the Gaia-X entity that is the __credentialSubject__ of a Verifiable Credential. Then this can be used as a filter for the Self-Description endpoint, to download the Self-Description file.
 
-Neo4j is used as implementation of the Graph database.
+The graph database backend is **pluggable**. The implementation is selected via the `graphstore.impl` configuration property:
 
-[IMPORTANT] 
+[options="header",cols="1,2,2"]
+|===
+| Value | Backend | Query Language
+| `neo4j` | Neo4j (default) | openCypher
+| `fuseki` | Apache Jena Fuseki | SPARQL-star
+| `dummy` | No-op (for testing) | —
+|===
+
+Both backends implement the same `GraphStore` interface. Core services contain no backend-specific logic.
+
+===== Neo4j backend
+
+[IMPORTANT]
 .Limitation: queries to non-Enterprise Neo4j Graph database return empty record when no results found, rather than empty list.
 ====
 When there is no data in the Graph database, i.e., no claims extracted from Self-Description, there is still a configuration node for the neosemantics modulefootnote:[https://neo4j.com/labs/neosemantics/4.0/config/[Configuring Neo4j to use RDF data]], which enables Neo4j to support the RDF data model, which is required here.
@@ -89,6 +101,10 @@ However this revoke operation is only supported in Neo4j enterprise.footnote:[ht
 We chose not to implement a workaround that involves query rewriting, as this may have harmful side effects.
 ====
 
+===== Fuseki backend
+
+The Fuseki backend uses Apache Jena with RDF-star. Claims are stored as RDF-star triple nodes: `<< subject predicate object >> cred:credentialSubject credentialSubjectUri`. Queries must use SPARQL-star syntax. Results are shuffled unless the query contains an `ORDER BY` clause.
+
 ==== File Store
 The File Store is responsible to persist all file based content submitted to the catalogue. These are Self-Descriptions and Schemas.
 
@@ -249,7 +265,7 @@ This component is used by the _Verification Service_ to extract Claims from Self
 
 ==== Graph Store
 
-The Graph Store is the interface for interacting with the Graph Database. It receives claims (extracted from Self-Descriptions) and adds them to the graph database. The Graph Database only contains claims from active Self-Descriptions and offers an openCypher query interface. External ontologies are not queried when processing requests.
+The Graph Store is the interface for interacting with the Graph Database. It receives claims (extracted from Self-Descriptions) and adds them to the graph database. The Graph Database only contains claims from active Self-Descriptions and offers a query interface (openCypher for the Neo4j backend, SPARQL-star for the Fuseki backend). External ontologies are not queried when processing requests.
 
 [source,java]
 ----

diff --git a/federated-catalogue/src/docs/architecture/chapters/06_runtime_view.adoc b/federated-catalogue/src/docs/architecture/chapters/06_runtime_view.adoc
@@ -771,14 +771,18 @@ When calling `GET /query` a HTML page with a query input form is displayed.
 
 ==== Query the catalogue
 
+The query endpoint accepts an optional `queryLanguage` query parameter to select the query language. The default is `OPENCYPHER` (for the Neo4j backend). When the Fuseki backend is configured, use `queryLanguage=SPARQL` to issue SPARQL-star queries. Queries in a language not supported by the active backend are rejected with HTTP 501.
+
+Only read queries are allowed. Write queries (e.g., `CREATE`/`DELETE` in openCypher, `INSERT`/`DELETE` in SPARQL) are rejected with HTTP 500.
+
 [mermaid, width=2000]
 ....
 sequenceDiagram
     autonumber
     actor User
     participant API
     participant Graph as Graph-DB
-    User ->> + API: POST /query
+    User ->> + API: POST /query?queryLanguage={OPENCYPHER|SPARQL}
     API ->> + Graph: queryData(sdQuery)
     Graph ->> Graph: validateQuery()
     Note right of Graph: Only read queries are allowed

diff --git a/federated-catalogue/src/docs/architecture/chapters/07_deployment_view.adoc b/federated-catalogue/src/docs/architecture/chapters/07_deployment_view.adoc
@@ -13,13 +13,13 @@ ifndef::imagesdir[:imagesdir: ../../images]
 
 == Deployment View
 
-The Federated Catalogue is provided as a container based application. The different components are shown in <<img_deployment_view>>. The main component is the `Federated Catalogue Server`. It requires a PostgreSQL-Serverfootnote:[https://www.postgresql.org/[PostgreSQL: The world's most advanced open source database]] instance to store Self-Description metadata and a Neo4jfootnote:[https://neo4j.com/[Neo4j Graph Data Platform | Graph Database Management System]] instance to store the Self-Description graph and to support the query interface of the catalogue.
+The Federated Catalogue is provided as a container based application. The different components are shown in <<img_deployment_view>>. The main component is the `Federated Catalogue Server`. It requires a PostgreSQL-Serverfootnote:[https://www.postgresql.org/[PostgreSQL: The world's most advanced open source database]] instance to store Self-Description metadata and a graph database instance to store the Self-Description graph and to support the query interface of the catalogue. The graph database backend is pluggable, selected via the `graphstore.impl` configuration property. Supported backends are Neo4jfootnote:[https://neo4j.com/[Neo4j Graph Data Platform | Graph Database Management System]] (openCypher queries) and Apache Jena Fusekifootnote:[https://jena.apache.org/documentation/fuseki2/[Apache Jena Fuseki]] (SPARQL-star queries).
 
 [#img_deployment_view,reftext='Figure {counter:refnum}']
 image::07_deployment_view.png[title="Deployment View"]
 
 In addition, a Keycloak instance is needed to handle authentication. Keycloak, as well as the portal, used to interact with the Federated Catalogue, is considered out of scope for the Federated Catalogue itself.
 
-<<img_deployment_view>> shows the default ports for the components' interaction. The connection settings (hostnames, ports) are configurable. It is recommended to replace the default credentials. To store persistent data, the `Federated Catalogue Server`, `PostgreSQL` as well as `Neo4J` require a volume, which must be mounted into the container.
+<<img_deployment_view>> shows the default ports for the components' interaction. The connection settings (hostnames, ports) are configurable. It is recommended to replace the default credentials. To store persistent data, the `Federated Catalogue Server`, `PostgreSQL` as well as the graph database (Neo4j or Fuseki) require a volume, which must be mounted into the container.
 
 Kubernetesfootnote:[https://kubernetes.io/[Kubernetes]] is the recommended infrastructure for production deployment. For development, Dockerfootnote:[https://www.docker.com/[Docker: Accelerated, Containerized Application Development]] in combination with docker-compose can be used.
diff --git a/federated-catalogue/src/docs/architecture/chapters/08_concepts.adoc b/federated-catalogue/src/docs/architecture/chapters/08_concepts.adoc
@@ -18,7 +18,7 @@ Data is persisted in the following components:
 
 * Federated Catalogue Server: Self-Description and Schema files (Filesystem)
 * Metadata Store: Self-Description and Schema metadata (PostgreSQL)
-* Graph store: Self-Description graph (Neo4j)
+* Graph store: Self-Description graph (Neo4j or Fuseki, depending on `graphstore.impl` configuration)
 
 During the deployment it must be ensured that volumes are mounted to store the persistent data in the mentioned components.
 

diff --git a/federated-catalogue/src/docs/architecture/chapters/13_appendix.adoc b/federated-catalogue/src/docs/architecture/chapters/13_appendix.adoc
@@ -15,6 +15,8 @@ ifndef::imagesdir[:imagesdir: ../../images]
 
 These examples demonstrate how to interact with the xref:05_building_block_view#_graph_database[Graph Database].
 
+NOTE: The examples below use **openCypher** syntax, which applies to the **Neo4j** backend (`graphstore.impl=neo4j`). When using the **Fuseki** backend (`graphstore.impl=fuseki`), equivalent queries must be expressed in **SPARQL-star** syntax via the `POST /query?queryLanguage=SPARQL` endpoint. See <<sparql_star_examples>> for SPARQL-star equivalents.
+
 All of the following examples have the same structure:
 
 Data::
@@ -279,3 +281,44 @@ MATCH (n:ServiceOffering) WHERE n.name = "Elastic Search DB" RETURN n.uri LIMIT
 ----
 [{n.uri=http://w3id.org/gaia-x/indiv#serviceElasticSearch.json}]
 ----
+
+[[sparql_star_examples]]
+==== SPARQL-star Examples (Fuseki backend)
+
+The following examples show equivalent queries for the Fuseki backend using SPARQL-star syntax. These are sent via `POST /query?queryLanguage=SPARQL`.
+
+===== All triples
+
+Retrieve all triples stored in the graph:
+
+[source,sparql]
+----
+SELECT ?s ?p ?o WHERE { ?s ?p ?o } LIMIT 100
+----
+
+===== All claims with credential subject
+
+Retrieve all Self-Description claims together with the credential subject they belong to, using RDF-star syntax:
+
+[source,sparql]
+----
+PREFIX cred: <https://www.w3.org/2018/credentials#>
+SELECT ?s ?p ?o ?cs
+WHERE {
+  << ?s ?p ?o >> cred:credentialSubject ?cs
+}
+LIMIT 100
+----
+
+===== Claims for a specific credential subject
+
+Retrieve claims belonging to a specific Self-Description:
+
+[source,sparql]
+----
+PREFIX cred: <https://www.w3.org/2018/credentials#>
+SELECT ?s ?p ?o
+WHERE {
+  << ?s ?p ?o >> cred:credentialSubject <http://example.org/my-credential-subject>
+}
+----