The W3C Linked Data Platform (LDP) candidate recom-

mendation defines a standard HTTP-based protocol for read/write Linked

Data. The W3C R2RML recommendation defines a language to map re-

lational databases (RDBs) and RDF. This paper presents morph-LDP,

a novel system that combines these two W3C standardization initiatives

to expose relational data as read/write Linked Data for LDP-aware ap-

plications, whilst allowing legacy applications to continue using their

relational databases

Corcho, Oscar

Esteban Gutiérrez, Miguel

García Castro, Raúl

Mihindukulasooriya, Nandana Sampath

Priyatna, Freddy

The W3C Linked Data Platform (LDP) candidate recom-mendation defines a standard HTTP-based protocol for read/write Linked Data. The W3C R2RML recommendation defines a language to map re-lational databases (RDBs) and RDF. This paper presents morph-LDP, a novel system that combines these two W3C standardization initiatives to expose relational data as read/write Linked Data for LDP-aware ap-plications, whilst allowing legacy applications to continue using their relational databases

Archivo Digital UPM

morph-LDP: an R2RML-based Linked Data Platform implementation

Nandana Mihindukulasooriya

Freddy Priyatna

Oscar Corcho

Raúl García-Castro

Miguel Esteban-Gutiérrez

Crossref

morph-LDP: An R2RML-Based Linked Data Platform Implementation

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

morph-LDP: An R2RML-based Linked Data
Platform implementation
Nandana Mihindukulasooriya1,2, Freddy Priyatna2, Oscar Corcho2,
Rau´l Garc´ıa-Castro1,2, and Miguel Esteban-Gutie´rrez1,2
1 Center for Open Middleware
2 Ontology Engineering Group, Facultad de Informa´tica
Universidad Polite´cnica de Madrid, Spain
{nmihindu,fpriyatna,ocorcho,rgarcia,mesteban}@fi.upm.es
Abstract. The W3C Linked Data Platform (LDP) candidate recom-
mendation defines a standard HTTP-based protocol for read/write Linked
Data. The W3C R2RML recommendation defines a language to map re-
lational databases (RDBs) and RDF. This paper presents morph-LDP,
a novel system that combines these two W3C standardization initiatives
to expose relational data as read/write Linked Data for LDP-aware ap-
plications, whilst allowing legacy applications to continue using their
relational databases.
1 Introduction
The W3C Linked Data Platform (LDP) candidate recommendation is focused
on supporting read/write Linked Data by providing a standard HTTP-based
RESTful protocol. An obvious way of generating an LDP implementation is
by storing RDF data in a triple store and internally doing SPARQL SELECT
and UPDATE queries. However, there are contexts in which organizations are
interested in continuing with their existing relational databases instead of trans-
forming data into an RDF format and managing it from a triple store. Hence,
the work that has been done in the past at W3C on the definition of the R2RML
language, as well as the implementations available that support such language
(e.g., Ontop [1], morph-RDB [2], D2R [3]), may be useful as an alternative im-
plementation for LDP, as shown in Figure 1.
In this demo we show how we can make use of the W3C R2RML recommen-
dation as the underlying support for providing read/write Linked Data access to
relational databases. Database administrators only need to generate a R2RML
mapping document and morph-LDP exposes the relational data as Linked Data.
Such Linked Data is not only dereferenceable and available through the HTTP
GET operation, but can also be updated using write operations such as PUT,
POST, PATCH, and DELETE.
2 Background
The Linked Data Platform (LDP)[4] is an initiative by the W3C LDP WG
to provide a standard protocol and a set of best practices enabling read/write
2 morph-LDP: An R2RML-based Linked Data Platform implementation
Fig. 1. The morph-LDP use case
Linked Data scenarios. The LDP specification extends the HTTP protocol with
a set of new constraints, HTTP headers, and Link headers that are useful for
read/write Linked Data applications. The two main types of concepts defined in
LDP include:
– A Linked Data Platform Resource (LDPR) which is any HTTP resource that
conforms to additional constraints defined in the LDP specification.
– A Linked Data Platform Container (LDPC) which is a specialization of an
LDPR that acts as a collection resource that helps organizing LDPRs and
creating new LDPRs as its members.
R2RML [5] is a W3C recommendation for the definition of a mapping lan-
guage from relational databases to RDF. An R2RML mapping document con-
tains a set of TriplesMap, used to specify the rules to generate RDF triples from
database rows/values. A TriplesMap consists of a LogicalTable (either a base ta-
ble or a SQL view), a SubjectMap (to generate the subject component of RDF
triples), and a set of PredicateObjectMaps. A PredicateObjectMap is composed
of a set of PredicateMaps and ObjectMaps (to generate the predicate and object
components of RDF triples, respectively).
3 Related Works
There is some previous work on the provision of update functionalities to R2RML-
based systems (e.g., [6]). However, so far Linked Data generated by such an ap-
proach cannot be dereferenced using HTTP GET but rather have to be queried
through a SPARQL endpoint. This hinders the usage of the follow-your-nose
approach3 to traverse links found in Linked Data because it requires out-of-
band information such as the SPARQL endpoint in addition to the link URI.
In contrast, Linked Data generated by morph-LDP can be dereferenced and
3 http://patterns.dataincubator.org/book/follow-your-nose.html
morph-LDP: An R2RML-based Linked Data Platform implementation 3
updated using URIs conforming to the HTTP-based LDP protocol. Moreover,
morph-LDP does not require the clients to be SPARQL-aware, hence making
the morph-LDP approach more attractive to those Web developers who are new
to Semantic Web technologies. Finally, the usage of morph-RDB [2] underneath
makes the query evaluation process more efficient.
Another approach consists in mapping relational data to Web resources fol-
lowing REST principles and making those resources accessible via the HTTP
protocol. Some systems that follow this approach are: HTTP database connector
(HDBC) [7], sqlREST4, and restSQL5. Nevertheless, none of those systems pro-
vides RDF representations. HDBC generates data according to the Atom format,
while both sqlREST and restSQL generate XML outputs using W3C XLink and
custom SQL Resources. While following the same approach, morph-LDP pro-
vides RDF representations (Turtle, RDF/XML, N-Triples and JSON-LD utiliz-
ing HTTP content negotiation). In addition, another advantage of morph-LDP
is the use of a standard mapping language.
4 morph-LDP
4.1 Mapping LDP components with R2RML
In this section, we explain how the concepts of LDP are mapped to R2RML
so that the two standards can be combined to provide read/write Linked Data
access to relational data.
The second Linked Data principle6 mandates the use of HTTP URIs so
that people can look up those names. By inspecting the rules specified in Sub-
jectMaps, these URIs can be mapped to Linked Data resources (LDPR) enabling
read/write access through the LDP protocol.
PredicateObjectMaps allow generating the information about a specific re-
source, so that when someone looks up a URI, morph-LDP can provide useful
information using RDF following the third Linked Data principle. LDP not only
allows to lookup those resources but also to update them using HTTP write op-
erations.
In R2RML, TriplesMaps are used to generate RDF triples out of the rows
of logical tables. In LDP, LDP Containers are used as collection resources to
organize LDPRs. Thus logical tables and their corresponding TripleMaps can be
mapped to LDP Containers.
4.2 Implementation
Our implementation7 is based on the result of two existing projects: morph-
RDB and LDP4j. morph-RDB[2]8 is a Scala-based RDB2RDF engine that
4 http://sqlrest.sourceforge.net/
5 http://restsql.org/
6 http://www.w3.org/DesignIssues/LinkedData.html
7 https://github.com/fpriyatna/morph-LDP
8 https://github.com/fpriyatna/morph
4 morph-LDP: An R2RML-based Linked Data Platform implementation
complies with the R2RML specification. Its query translator component is based
on the algorithm defined in [8] with some optimizations, such as self-join, left-
outer join, and subquery elimination. morph-RDB has been successfully applied
in several projects: Re´pener [9], BizkaiSense9, and Integrate10. LDP4j11 is a
Java-based middleware for the development of LDP-aware applications. LDP4j
is being developed in the context of the ALM iStack project, where the LDP
middleware is used to integrate Application Lifecycle Management tools [10].
morph-LDP extends morph-RDB with an LDP layer provided by LDP4j.
The LDP layer extracts the metadata from the HTTP request and send it as
an input for morph-RDB. morph-RDB has two modes of operation: API mode
and SPARQL query mode and handles the transformation between RDF and
relational data. Figure 4.2 illustrates the process when morph-LDP receives an
HTTP Request when using the SPARQL query mode.
Fig. 2. An example of morph-LDP in action, using the SPARQL query mode.
5 An Example Scenario
Our demo scenario uses a relational database which manages information of
members of a research group. morph-LDP exposes this data as Linked Data
that can be managed using the LDP protocol. In the demo, we showcase the
following user stories:
As a Linked Data application developer, I want to:
– retrieve the list of members of the research group (retrieve an LDP Con-
tainer).
– retrieve details of a certain member of the group (retrieve an LDPR).
– update the details of a certain member of the group (update an LDPR).
– create a new member record of the group (create a new LDPR).
9 http://www.tecnologico.deusto.es/projects/bizkaisense/
10 http://www.fp7-integrate.eu
11 http://ldp4j.org
morph-LDP: An R2RML-based Linked Data Platform implementation 5
– delete an outgoing member record of the group (delete a LDPR).
A video of the demo, together with the HTTP Requests/Responses, and
SPARQL/SQL queries, can be found in the morph-LDP page12.
6 Conclusion
W3C LDP is in the final stages of the standardization process and we believe
that integrating it with relational databases using W3C R2RML will help a wider
adoption in the industry. In this paper, we presented how LDP and R2RML can
be combined to expose relational data as read/write Linked Data. Nevertheless,
there is still some work to be done on providing support for Quality-of-Service
requirements such as secure access and transactions. In addition to that, cur-
rently morph-LDP supports only simple R2RML mappings (no SQL view, no
multiple mappings to a class/property) and we are working on giving support
for more complex mappings.
Acknowledgments: The authors have been supported by the PlanetData (FP7-
257641) , myBigData (TIN2010-17060), and ALM iStack (Center for Open Mid-
dleware) projects.
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