Sessel

Sessel is a CouchApp for CouchDB that generates RDF triples from JSON documents, which then in turn can be exported to various serialization formats, or queried through a SPARQL endpoint.

Installation

Sessel is designed to be replicated or pushed into any existing CouchDB database that stores data that you want to export as RDF. There is more than one way to install it.

Install Sessel using CouchApp or similar tools

Clone this repository and use CouchApp to push Sessel to <your_host>/<your_db>:

git clone git://github.com/agrueneberg/Sessel.git
couchapp push Sessel/ http://<your_host>/<your_db>

Install Sessel using replication

Replicate an existing deployment of Sessel to <your_host>/<your_db.

Export

The generated triples can be exported to various RDF serialization formats by calling the export interface http://<your_host>/<your_db>/_design/sessel/_rewrite/ with one of the following strings added:

• export.nt – Export as N-Triples
• export.ttl – Export as Turtle
• export.rdf – Export as RDF/XML

A graphical export interface can be accessed at http://<your_host>/<your_db>/_design/sessel/export.html.

Changing the base URI

The default base URI is http://host/db_name/. If you prefer a different URI, add a base_uri parameter to the export URL and provide a percent-encoded value, e.g. http://<your_host>/<your_db>/_design/sessel/_rewrite/export.ttl?base_uri=http%3A%2F%2Fexample.com%2Frdf%23.

Changing the prefix

The default prefix of the base URI is sessel. If you prefer a different prefix, add a prefix parameter to the export URL and provide a value, e.g. http://<your_host>/<your_db>/_design/sessel/_rewrite/export.ttl?base_uri=http%3A%2F%2Fexample.com%2Frdf%23&prefix=example.

Data type mapping

The data types of JSON are mapped to the data types of XML as specified in XML Schema Part 2: Datatypes Second Edition.

• string → xsd:string
• array → xsd:string
• object → xsd:string
• null → xsd:string
• number → xsd:integer or xsd:double
• boolean → xsd:boolean

To activate data type mapping, add a type_literals parameter to the export URL and provide a string value true.

SPARQL Endpoint

Sessel bundles Antonio Garrote's fantastic rdfstore-js, an RDF store with SPARQL support written entirely in JavaScript. A graphical query interface can be accessed at http://<your_host>/<your_db>/_design/sessel/sparql.html. Unfortunately, modern browsers only let rdfstore-js store 5 MB worth of triples. If your data set is large it is recommended to use a standalone SPARQL processor such as ARQ to import the generated triples by pointing it to the export interface:

SELECT *
FROM <http://<your_host>/<your_db>/_design/sessel/_rewrite/export.ttl>
WHERE {
    ?s ?p ?o .
}

Document Conversion

Each JSON document is broken down to key-value pairs. Each key-value pair represents a triple, key and value being predicate and object, respectively. The value of the special key-value pair with the key _id ensuring the uniqueness of a document serves as the subject of the generated triple.