=Paper=
{{Paper
|id=None
|storemode=property
|title=Lenses for View Synchronization in Metamodel-Based Multi-View Modeling
|pdfUrl=https://ceur-ws.org/Vol-648/paper6.pdf
|volume=Vol-648
}}
==Lenses for View Synchronization in Metamodel-Based Multi-View Modeling
==
https://ceur-ws.org/Vol-648/paper6.pdf
Lenses for View Synchronization in
Metamodel-Based Multi-View Modeling
Arif Wider
Humboldt-Universität zu Berlin
Unter den Linden 6, D-10099 Berlin, Germany
wider@informatik.hu-berlin.de
Abstract. When using multiple views to describe a system, the underly-
ing models of these views have to be kept consistent, which is called model
synchronization. Manually implemented model synchronizations that are
not simple bijections are hard to maintain and to reason about. Special
languages for expressing bidirectional transformations can help in this re-
spect, but existing languages applicable in model-driven engineering are
often restricted to bijections or complex to use. I adapt lenses, a promis-
ing term-rewriting-based approach to bidirectional transformations, to
model synchronization. This allows for flexible view synchronization that
can be integrated with existing metamodel-based technologies.
Introduction
Modeling a system using multiple views is a common means nowadays to break
down the complexity of the system description. A prominent example for that
is the architecture of the UML, providing multiple diagram types which serve
as aspect-specific views on a system. Using multiple views to describe a system
imposes the problem of inter-view consistency.
In model-driven engineering (MDE) different views on a system can be im-
plemented as different domain-specific languages (DSLs). This way, the system
description consists of an ensemble of models described using these languages.
This is called multi-view modeling or domain-specific multimodeling [11, 10]. In
multimodeling inter-view consistency is achieved by synchronizing these models.
Naively implemented model synchronizations, i.e., pairs of forward and back-
ward transformations described in a general-purpose language, can be hard to
maintain and to reason about because consistency of forward and backward
transformations has to be ensured and transformations can be arbitrarily com-
plex. Special languages for describing bidirectional transformations provide no-
tations to describe consistency relations between models. From this notation a
forward and a backward transformation can be automatically inferred so that
the consistency of these transformations is ensured by construction. This is easy
if the relation is a bijection but gets hard if it is neither surjective nor injec-
tive. Unfortunately, as the idea of a view is to hide information which is not
aspect-specific, bijections hardly occur in a multi-view setting.
Bidirectional transformations are researched for a long time, e.g, in the graph
transformation community using Triple Graph Grammars (TGGs) [18, 6]. With
QVT Relational 1 , there is even a standard by the OMG for describing bidirec-
tional transformations of metamodel-based models. Nevertheless, languages for
describing bidirectional model transformations are still not widely used in MDE.
Concerning QVT, Stevens points out semantic issues that could be one reason
for this limited acceptance [19]. Another practical issue could be the weak sit-
uation regarding tool support for QVT Relations: Although QVT specification
was completed in 2008, there are only few implementations and even those are
not maintained regularly.
Lenses [8, 7] is a combinator-based approach to bidirectional transformations:
Foster et al. provide small, well-unterstood bidirectional transformations (called
lenses) and a set of combinators that allow more complex transformations to
be composed from those smaller ones. This greatly improves extensibility and
comprehensibility. Furthermore, a type system guarantees that composed lenses
preserve certain properties of their sub-lenses. This combinator-based approach
is possible because lenses are restricted to the asymmetric case where one of the
two models to be synchronized is an abstraction of the other, i.e., the relation
is at least surjective. This way, the problem of model synchronization resembles
the view update problem that has been studied in the database community for
decades [4].
In contrast to less restricted symmetric approaches like TGGs and QVT that
were designed for transformations of graphs and models, respectively, lenses were
designed for synchronization of tree-like data and were mainly implemented for
string transformations [3], e.g., synchronization of XML-data. This poses some
conceptual challenges, when attempting to use lenses for model synchronization
in a metamodel-based setting.
Related Work
There are several approaches using bidirectional transformations in the con-
text of MDE, but most of them use symmetric bidirectional transformations
and therefore lack the combinator-based nature that can be achieved using an
asymmetric approach (e.g., the AToM3 Framework [1]). Among them are also
some, that integrate with existing metamodel-based technologies, e.g., the Tefkat
transformation engine [16] that is closely connected with QVT and integrates
with the Eclipse Modeling Framework (EMF)2 . Recently, Hettel et al. presented
an asymmetric approach for using the SQL-like Tefkat language for round-trip
engineering [12].
An approach quite similar to lenses which is also used for view synchroniza-
tion is the work of Hu et al. [14, 17]. Somehow similar to my approach, Garcia
proposed to use their work in a metamodel-based context [9]. However, in their
approach, changes in a model have to be explicitly marked to be synchronized.
This prevents agnostic integration with existing metamodel-based technologies.
1
http://www.omg.org/spec/QVT/1.0/
2
http://www.eclipse.org/modeling/emf/
Furthermore, there are some approaches to bidirectional transformations that
are heavily inspired by lenses or extend lenses, but are not used for view synchro-
nization: Hidaka combines lenses with a query language but not in a metamodel-
based context [13].
Probably closest to my work is the work of Xiong [22, 21, 5], who integrated
concepts of lenses into his work on bidirectional transformations of metamodel-
based models, but he proposes an update-based approach in contrast to the
state-based approach of lenses and his work does not focus on integration with
existing metamodel-based technologies.
Approach
My approach is to use lenses for view synchronization in metamodel-based multi-
view modeling environments. In order to achieve this, I want to show that
1. the advantages of lenses, especially their composability, can be leveraged
when describing bidirectional transformations of metamodel-based models,
2. that in conjunction with a synchronization architecture that incorporates a
common model, lenses can be beneficially used for view synchronization and
3. that this approach allows for straightforward integration with existing meta-
model-based technologies.
Lenses for Bidirectional Model Transformations In order to use lenses
for model synchronization, the concepts of lenses have to be bridged from the
grammarware technological space that lenses originate from to the modelware
technological space [20]. For this, the following challenges are to be solved:
– Typing: In the original lens framework typing is mainly used for ensuring
that certain lens properties are preserved when composing lenses. In MDE,
transformations usually transform models conforming to one metamodel so
that they conform to another metamodel. Therefore, typing of input and
output data of lenses is highly desirable for model transformations.
– Ordered data: Originally, lenses work either on unordered tree-like data
or certain keys have to be defined to be able to synchronize changes regard-
ing order. With metamodel-based models, i.e., in an object-oriented setting,
there is the object identity as an implicit key. This can be used to propagate
changes in order and other complicated changes back to the original model.
– References: Models in general are graphs because they can contain ref-
erences, whereas lenses were designed for synchronizing tree-like data. A
pragmatic solution could be to make use of the containment hierarchy that
is provided by many metamodeling frameworks anyway.
A Lens-Based Model Synchronization Architecture While the restric-
tion to asymmetric synchronization does not seem to be flexible enough for the
general MDE setting, it fits well to view synchronization: Lenses can be used to
asymmetrically synchronize view-models with a common model. This common
model can be a shared abstraction, i.e., it only contains those information that
is represented in more than one view. In the database community, this approach
to view synchronization was already proposed by Atzeni & Torlone in 1996 [2].
Another approach is to synchronize views with a shared complete model of the
system containing the information of all models to be synchronized. In both cases
the changes made in one view-model are propagated to the other view-models
through the common model.
Technological Integration As stated before, it is my goal to provide a so-
lution that can be integrated with existing metamodel-based technologies, in
particular, with the Eclipse Modeling Framework (EMF). As EMF is a Java-
based framework, I decided to implement lenses for model transformations as an
internal DSL in the Scala 3 programming language. Scala code compiles to JVM
bytecode and Scala provides great interoperability with Java-based frameworks.
Moreover, Scala combines functional and object-oriented concepts, which fits to
the task of adapting lenses that come from a functional background to be used in
a metamodel-based, i.e., object-oriented setting. Finally, Scala has static typing
and it is my goal to achieve compile-time type checking for transformations in
as many situations as possible. Therefore, I make use of heterogeneously typed
lists [15], that were originally developed for the Haskell programming language.
My solution will be deliverable as a Scala library. As a consequence, no further
tools than the Scala compiler and a Scala IDE plug-in should be needed to inte-
grate the solution into existing projects and tool chains. Hopefully, this results
in higher user acceptance compared to solutions like QVT that always depend
on up-to-date tool support.
Evaluation and Expected Contributions
The evaluation of my approach is tightly coupled with the ongoing development
of a domain-specific workbench for the development of optical nanostructures,
which is subject of a cooperation with a group of physicists. This workbench
provides different DSLs for describing different aspects of experiments in nanos-
tructure development and is being implemented with EMF-based technologies.
This project serves as a comprehensive case study for my solution. In particular,
it has to be evaluated if a reasonably sized set of basic lenses and lens combina-
tors can be provided that enable to accomplish common view synchronization
tasks in a concise way.
As a result of my work, the following contributions can be expected:
– An extended formal lens framework, adapted for an object-oriented setting.
– A language for bidirectional model transformations implemented as an in-
ternal DSL in Scala, deliverable as a Scala library.
– A lens-based view synchronization architecture that integrates with EMF-
based technologies and can be used in multi-view modeling environments.
3
http://www.scala-lang.org
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