=Paper=
{{Paper
|id=Vol-1268/paper11
|storemode=property
|title=LDP4j: A framework for the development of interoperable read-write Linked Data applications
|pdfUrl=https://ceur-ws.org/Vol-1268/paper11.pdf
|volume=Vol-1268
|dblpUrl=https://dblp.org/rec/conf/semweb/GutierrezMG14
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==LDP4j: A framework for the development of interoperable read-write Linked Data applications==
https://ceur-ws.org/Vol-1268/paper11.pdf
LDP4j: A framework for the development of
interoperable read-write Linked Data applications
Miguel Esteban-Gutiérrez, Nandana Mihindukulasooriya, and
Raúl García-Castro
Center for Open Middleware
Ontology Engineering Group, Escuela Técnica Superior de Ingenieros Informáticos
Universidad Politécnica de Madrid, Spain
{mesteban,nmihindu,rgarcia}@fi.upm.es
Abstract. Enterprises are increasingly using a wide range of heteroge-
neous information systems for executing and governing their business
activities. Even if the adoption of service orientation has improved loose
coupling and reusability, applications are still isolated data silos whose
integration requires complex transformations and mediations.
However, by leveraging Linked Data principles those data silos can now
be seamlessly integrated, and this opens the door to new data-driven
approaches for Enterprise Application Integration (EAI). In this paper
we present LDP4j, an open souce Java-based framework for the develop-
ment of interoperable read-write Linked Data applications, based on the
W3C Linked Data Platform (LDP) specification.
Keywords: Enterprise Application Integration, Linked Data Platform,
LDP4j
1 Introduction
Nowadays every organization uses several information systems to manage their
information and integrating those applications is a key requirement for efficiently
exciting the business processes of organizations. Enterprise Application Integra-
tion (EAI) techniques, which propose solutions to this problem, have evolved over
time from ad hoc one-to-one integrations to approaches such as Service-Oriented
Architectures (SOA) and Enterprise Service Buses (ESBs), using either SOAP-
based or RESTful web services. Application integration using read-write Linked
Data is a novel approach that is getting traction in the industry1 .
Application connection (interfaces) and data integration are the two main
problems in application and data integration; there are three main challenges:
syntactic heterogeneity, structural heterogeneity, and semantic heterogeneity [1].
The usage of standard data exchange formats such as XML or JSON solves the
syntactic heterogeneity problem in the current EAI techniques and the structural
heterogeneity problem is solved by complex schema transformations. However,
1
https://jazz.net/story/about/about-jazz-platform.jsp
ISWC 2014 Developers Workshop Copyright held by the authors 61
2 Interoperable read-write Linked Data applications with LDP4j
because the explicit semantics of data is not clearly expressed, traditional ap-
proaches struggle with semantic heterogeneity.
Nevertheless, Semantic Web technologies provide better solutions to the data
integration problem. The Linked Data principles help creating a global data
space [2] with typed links between data from different sources [3], hence break-
ing isolated data silos. RDF provides a simple and flexible data model that is
well-suited for data integration and the conceptualization of domain models can
be expressed in terms of RDF Schema and OWL ontologies. Machine-readable
structured data with explicit formal semantics that are expressed using stan-
dards makes merging, integrating, processing, and analyzing data possible with-
out needing out-of-band knowledge or proprietary tools. Links to related entities
in data make it possible to start from a piece of data and traverse through differ-
ent sources with a follow-your-nose approach2 in order to discover more entities
and get context information.
The Linked Data Platform (LDP) specification3 provides a standard pro-
tocol for read-write Linked Data and a set of best practices, based on HTTP
access to web resources that describe their state using the RDF data model.
The standarization of this protocol represents a step forward in the Linked Data
community as it lays the ground for the development of interoperable read-write
Linked Data applications.
As a consequence of this, LDP provides a base for application integration
using read-write Linked Data; however, this approach requires having tools and
libraries that provide support for developing read-write Linked Data applications
that support the LDP protocol.
At the time of this writing, support for this specification is being included in
existing Semantic Web commercial tools as well as in green-field and brown-field
open source projects4 . Unfortunately, in these cases LDP support is provided as
a remote data access mechanism, not an application integeration mechanism.
The LDP4j framework is one effort to fill this gap by providing a library
and a set of tools that facilitate the development for read-write Linked Data
applications so that they can follow this novel approach of application integration
using Linked Data.
This paper presents the LDP4j framework, discusses lessons learned, chal-
lenges, and future work. The paper is organized as follows. Section 2 discusses
the requirements beyond LDP for application integration scenarios. Section 3
introduces the LDP4j framework. Finally, Section 4 draws some conclusions.
2 Interoperable read-write Linked Data Applications:
Beyond the Linked Data Platform
Applications that support the LDP protocol can expose all or part of their data
using one or more vocabularies and can consume Linked Data from other ap-
2
http://patterns.dataincubator.org/book/follow-your-nose.html
3
http://www.w3.org/TR/ldp/
4
See https://www.w3.org/wiki/LDP_Implementations for a list of LDP implementations.
ISWC 2014 Developers Workshop Copyright held by the authors 62
Interoperable read-write Linked Data applications with LDP4j 3
plications by following links and traversing through data. This opens the door
to a novel approach for integrating applications [4]. However, one of the lessons
learned while developing LDP4j was that as a middleware provider LDP support
is not enough to get adopted as a viable approach in industry. In order to inte-
grate heterogeneous applications using read-write Linked Data in a production
environment, several quality requirements have to be fulfilled and this section
discusses some of these requirements5 .
Most enterprise applications have security requirements that generally in-
clude authentication, authorization, accounting, integrity, confidentiality, and
non-repudiation. The LDP protocol must not just be integrated with current
security solutions for web applications (i.e., WebID [5]) but also explore the
specificities that stem from the way in which the data is made available.
Furthermore, applications need support for business transactions [6] to en-
sure consistency. Depending on the level of consistency required, strong ACID
properties [7] or other alternatives such as BASE [8] must be guaranteed. Several
RESTful transaction models have been proposed for web applications in the last
few years with some limitations [9] and a transaction model for LDP should be
built using them as the base.
Finally, data validation is a vital step for ensuring the quality of data in
applications and expressive schema languages and related tools are essential
for effective data validation. In constrant with relational databases and XML,
RDF is built upon the Open World Assumption and the Non-unique Name
Assumption, and this makes data validation challenging, as modelling languages
(RDF Schema and OWL) are more more suited for inferring than for validation.
Thus, there is a need for new standards and tooling support6 .
3 LDP4j
LDP4j7 is a Java-based framework for the development of interoperable read-
write Linked Data applications based on the LDP specification. This frame-
work provides the components required by clients and servers for handling their
communication, hidding the complexity of the protocol details from application
developers and letting them focus on implementing their application-specific
business logic (see section 3.1). In addition, the framework plans extensions to
the LDP specification by providing additional features aimed at enhancing the
interoperability between LDP-based read-write Linked Data applications (see
section 3.2). The LDP adapter for the Bugzilla [10] bug tracker and morph-LDP
[11] are two example applications built using the LDP4j framework.
3.1 LDP Support
In order to facilitate the development of LDP-based applications, LDP4j pro-
vides different components that developers may use for the development of such
5
For an extended discussion of the requirements please refer to [4].
6
http://www.w3.org/2014/data-shapes/charter
7
http://www.ldp4j.org/
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4 Interoperable read-write Linked Data applications with LDP4j
applications; in particular: an extensible LDP Server and an extensible LDP
Client. The following sections present both components.
LDP4j Server Component. The purpose of the LDP4j Server Component (or
LDP Server for short) is to provide the means for publishing application-specific
LDP containers and resources, abstracting developers from the particularities of
the LDP protocol and letting them focus on the particular business logic behind
the containers and resources themselves.
The LDP Server API is divided into: (1) the server API, which defines
entities related to different aspects of the LDP protocol that are of interest
for the LDP Server (i.e., resources, content, and format); (2) the server SPI,
which defines APIs that provide extensions to the types supported by the server
API ; (3) the server frontend, which provides developers with a frontend that
allows the interaction with the different services provided by the LDP Server, if
required; and (4) the developer API, which defines the low-level interfaces and
annotations that have to be used to create server-side LDP applications using
LDP4j. This latter part of the API provides and extensibility layer that enables
the developer to specify the behaviour of the LDP4j Server when dealing with
LDP containers and resources.
In order to develop basic LDP containers, developers must implement the
org.ldp4j.server.core.ILinkedDataPlatformContainer interface that provides the
means for handling the creation of LDP resources given an RDF serialization as
well as for retrieving a RDF summary of contents of one or more of the resources
managed by the container. Each container must be identified by an application
unique identifier. In addition, every created resource must be identified by a
container unique identifier.
On the other hand, the development of LDP resources requires implementing
the org.ldp4j.server.core.ILinkedDataPlatformResourceHandler, which provides
the means for retrieving the contents of existing resources and well as for up-
dating their contents given an RDF serialization. Resource handlers must be
identified by an application unique identifier, which should match that of the
container used for the creation of the resources handled by the handler.
If the application supports resource deletion, developers will have to ex-
tend their resource handler classes in one of two ways: implementing also the
org.ldp4j.server.core.Deletable interface, or adding a method with the same sig-
nature as the delete method defined in the latter interface, annotated with
org.ldp4j.server.core.Delete.
LDP4j Client Component. While the LDP4j Server Component is meant
to help developers in publishing contents via the LDP protocol, the purpose of
the LDP4j Client Component is to let developers build applications capable of
consuming those contents by exploiting the LDP protocol. Again, the LDP4j
Client Component hides the specificities of the LDP protocol to the developer
so that he only has to decide how to use those contents.
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Interoperable read-write Linked Data applications with LDP4j 5
The LDP Client API provides the frontend to the LDP Client that developers
have to use for consuming LDP contents. The frontend consists of a facade that
allows creating proxies to containers and resources given their URLs. For the
time being, these proxies provide the client-side functionalities that match those
offered by the current version of the LDP Server API.
3.2 Extensions to LDP
Supporting the whole LDP specification is the first step in the development
roadmap. However, in order to fully realize the vision of interoperable read-write
Linked Data applications it is necessary to make explicit any application-specific
domain knowledge required for interacting with the particular application in a
sensible way. Further LDP4j work is geared towards the development of a set of
extensions for enriching the LDP protocol along this line:
– Vocabulary support. Enable the publication and discovery of the vocab-
ularies used by a read-write Linked Data application together with the re-
strictions that apply. As a side effect, this would relief application developers
from the burden of input data validation as this could be transparently done
by middleware.
– Co-reference support. Provide middleware services for dealing with the
coreference problem 8 , following the lines already identified in [12].
– Transaction support. Provide middleware services for providing trans-
action support for Linked Data applications using a RESTful transaction
model.
Finally, support for other enterprise requirements aimed at facilitating the
adoption and uptake of the framework are planned, in particular the provision of
Linked Data-specific access control, authorization, and accounting mechanisms.
4 Conclusions
The usage of Linked Data as a means for integrating applications has several
advantages over traditional EAI approaches. However, using Linked Data for this
purpose requires having mechanisms not just for reading, but for writing data.
The LDP specification defines a protocol for read-write Linked Data applications
and its standardization represents a big step towards the industrial adoption of
Linked Data technologies.
However, the interoperability of applications supporting the LDP protocol is
limited to domain-independent concerns (i.e., exchange formats, communication
patterns, failure signaling, etc.). Thus, their integration requires out-of-band
domain knowledge in order to use them properly while interacting with them
8
When information about a certain entity —which is identified in different manners—
is spread across different sources.
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6 Interoperable read-write Linked Data applications with LDP4j
via the LDP protocol. As a result, LDP is not enough for Linked Data-based
EAI: it is necessary to put domain-knowledge into the game.
In this paper we have presented LDP4j, a framework for the development of
read-write domain-aware Linked Data applications. The framework implements
and extends the LDP protocol with features for exposing the domain knowledge
that dictates how to interact with the application. This framework represents the
next big step for realizing the vision of Linked Data-based Enterprise Application
Integration.
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