Soichiro Hidaka

Bidirectionalizing graph transformations

Bidirectional transformations provide a novel mechanism for synchronizing and maintaining the consistency of information between input and output. Despite many promising results on bidirectional transformations, these have been limited to the context of relational or XML (tree-like) databases. We challenge the problem of bidirectional transformations within the context of graphs, by proposing a formal definition of a well-behaved bidirectional semantics for UnCAL, i.e., a graph algebra for the known UnQL graph query language. The key to our successful formalization is full utilization of both the recursive and bulk semantics of structural recursion on graphs. We carefully refine the existing forward evaluation of structural recursion so that it can produce sufficient trace information for later backward evaluation. We use the trace information for backward evaluation to reflect in-place updates and deletions on the view to the source, and adopt the universal resolving algorithm for inverse computation and the narrowing technique to tackle the difficult problem with insertion. We prove our bidirectional evaluation is well-behaved. Our current implementation is available online and confirms the usefulness of our approach with nontrivial applications.

Maintaining invariant traceability through bidirectional transformations

Following the “convention over configuration” paradigm, model-driven development (MDD) generates code to implement the “default” behaviour that has been specified by a template separate from the input model, reducing the decision effort of developers. For flexibility, users of MDD are allowed to customise the model and the generated code in parallel. A synchronisation of changed model or code is maintained by reflecting them on the other end of the code generation, as long as the traceability is unchanged. However, such invariant traceability between corresponding model and code elements can be violated either when (a) users of MDD protect custom changes from the generated code, or when (b) developers of MDD change the template for generating the default behaviour. A mismatch between user and template code is inevitable as they evolve for their own purposes. In this paper, we propose a two-layered invariant traceability framework that reduces the number of mismatches through bidirectional transformations. On top of existing vertical (model↔code) synchronisations between a model and the template code, a horizontal (code↔code) synchronisation between user and template code is supported, aligning the changes in both directions. Our blinkit tool is evaluated using the data set available from the CVS repositories of a MDD project: Eclipse MDT/GMF.

Feature-based classification of bidirectional transformation approaches

Bidirectional model transformation is a key technology in model-driven engineering (MDE), when two models that can change over time have to be kept constantly consistent with each other. While several model transformation tools include at least a partial support to bidirectionality, it is not clear how these bidirectional capabilities relate to each other and to similar classical problems in computer science, from the view update problem in databases to bidirectional graph transformations. This paper tries to clarify and visualize the space of design choices for bidirectional transformations from an MDE point of view, in the form of a feature model. The selected list of existing approaches are characterized by mapping them to the feature model. Then, the feature model is used to highlight some unexplored research lines in bidirectional transformations.

A compositional approach to bidirectional model transformation

Bidirectional model transformation plays an important role in maintaining consistency between two models, and has many potential applications in software development, including model synchronization, round-trip engineering, software evolution, multiple-view software development, and reverse engineering. However, unclear bidirectional semantics, domain-specific bidirectionalization method, and lack of systematic development framework are known problems that prevent it from being practically used. In this paper, we propose a novel compositional framework for bidirectional model transformation based on an existing graph querying language UnQL, so that one can develop various useful bidirectional model transformation by combination of a fixed number of primitive bidirectional model transformations. We have implemented a prototype system, and the experimental results show promise of the new approach.

Toward bidirectionalization of ATL with GRoundTram

ATL is a language for describing model transformations currently in uni-direction. In our previous work we have shown that transformations of graph structures given in some form can be bidirectionalized and have implemented a system called GRoundTram system for bidirectional graph transformations. We say a transformation t is bidirectionalized when we obtain a backward transformation t′ so that the pair (t, t′) of transformations satisfies certain well-behavedness properties. Bidirectional model transformation is used to reflect the changes in the target model back to the source model, and vice versa. In this paper, as a first step toward realizing practical bidirectional model transformations, we present bidirectionalization of core part of the ATL by encoding it in the UnQL language, which is used as a transformation language in the GRound-Tram system. We give the algorithm for the encoding, based on which we have implemented the system for bidirectionalizing the core ATL in OCaml language.

Graph-transformation verification using monadic second-order logic

This paper presents a new approach to solving the problem of verification of graph transformation, by proposing a new static verification algorithm for the Core UnCAL, the query algebra for graph-structured databases proposed by Bunemann et al. Given a graph transformation annotated with schema information, our algorithm statically verifies that any graph satisfying the input schema is converted by the transformation to a graph satisfying the output schema. We tackle the problem by first reformulating the semantics of UnCAL into monadic second-order logic (MSO). The logic-based foundation allows to express the schema satisfaction of transformations as the validity of MSO formulas over graph structures. Then by exploiting the two established properties of UnCAL called bisimulation-genericity and compactness, we reduce the problem to the validity of MSO over trees, which has a sound and complete decision procedure. The algorithm has been efficiently implemented; all the graph transformations in this paper and the system web page can be verified within several seconds.

Structural recursion for querying ordered graphs

Structural recursion, in the form of, for example, folds on lists and catamorphisms on algebraic data structures including trees, plays an important role in functional programming, by providing a systematic way for constructing and manipulating functional programs. It is, however, a challenge to define structural recursions for graph data structures, the most ubiquitous sort of data in computing. This is because unlike lists and trees, graphs are essentially not inductive and cannot be formalized as an initial algebra in general. In this paper, we borrow from the database community the idea of structural recursion on how to restrict recursions on infinite unordered regular trees so that they preserve the finiteness property and become terminating, which are desirable properties for query languages. We propose a new graph transformation language called lambdaFG for transforming and querying ordered graphs, based on the well-defined bisimulation relation on ordered graphs with special epsilon-edges. The language lambdaFG is a higher order graph transformation language that extends the simply typed lambda calculus with graph constructors and more powerful structural recursions, which is extended for transformations on the sibling dimension. It not only gives a general framework for manipulating graphs and reasoning about them, but also provides a solution to the open problem of how to define a structural recursion on ordered graphs, with the help of the bisimilarity for ordered graphs with epsilon-edges.

Optimization for iterative queries on MapReduce

We propose OptIQ, a query optimization approach for iterative queries in distributed environment. OptIQ removes redundant computations among different iterations by extending the traditional techniques of view materialization and incremental view evaluation. First, OptIQ decomposes iterative queries into invariant and variant views, and materializes the former view. Redundant computations are removed by reusing the materialized view among iterations. Second, OptIQ incrementally evaluates the variant view, so that redundant computations are removed by skipping the evaluation on converged tuples in the variant view. We verify the effectiveness of OptIQ through the queries of PageRank and k-means clustering on real datasets. The results show that OptIQ achieves high efficiency, up to five times faster than is possible without removing the redundant computations among iterations.

Towards co-evolution in model-driven development via bidirectional higher-order transformation

In model-Driven development (MDD), metamodels, models, and model transformations are interdependent. A change in one artifact must be reflected in all other related artifacts. Regardless of their dependencies, (meta)models and transformations can evolve autonomously rendering referenced artifacts invalid. Coupling the evolution of models to their corresponding metamodels tries to prevent such mismatches, but is currently limited to one-way adaptations and does not take model transformations into account. To eliminate these short-comings, we combine first-class transformation models with bidirectional transformations (BX). Our generic approach integrates BX into well-established Eclipse-based MDD tools, thereby neither being restricted to a specific modeling nor model transformation language.

A relative cost model for XQuery

XQuery is a functional query language for XML. We propose a relative XQuery cost model that is able to estimate the performance gain during source level transformation. This research facilitates the evaluation of various rewriting techniques without introducing real engines. The cost model consists of simple recursive functions based on functional language constructs. They are determined using formal semantics and other known efficient algorithms. Analytic comparison of costs between expressions before and after transformation is possible in an engine-independent manner. The relativity of the model allows uninterpreted components within, which do not affect the mathematical proof of the comparison. Moreover, it can be tailored to reflect engine specific evaluation strategies such as the order of evaluation of operands.