Dynamic Classes: Modular Asynchronous Evolution of Distributed Concurrent Objects

An Open System Operational Semantics for an Object-Oriented and Component-Based Language

Creol: A type-safe object-oriented model for distributed concurrent systems

Creol is an object-oriented modeling language in which inherently concurrent objects exchange asynchronous method calls. The operational semantics of Creol is written in an actor-based style, formulated in rewriting logic. The operational semantics yields a language interpreter in the Maude system, which can be used to analyze models. Recently, Creol has been applied to the modeling of systems with radio communication, such as sensor systems. With radio communication, messages expire and, if sent simultaneously, they may collide in the air. In order to capture these and other properties of distributed systems, we extended Creol's operational semantics with a notion of time. We exploit the framework of a language interpreter to use a lightweight notion of time, in contrast to that needed for a general purpose specification language. This paper presents a timed extension of Creol, including the semantics and the implementation strategy, and discusses its properties using an extended example. The approach can be generalized to other concurrent object or actor-based systems.

In this paper we investigate the addition of first-class relationships to a prototypical object-oriented programming language (a “middleweight” fragment of Java). We provide language-level constructs to declare relationships between classes and to manipulate relationship instances. We allow relationships to have attributes and provide a novel notion of relationship inheritance. We formalize our language giving both the type system and operational semantics and prove certain key safety properties.KeywordsOperational SemanticType SoundnessMethod BodyRelationship InheritanceSubject ReductionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Programming languages suitable for distributed computation contain constructs that should map well to the structure of the underlying system executing the programs, while being easily usable by the programmers and amenable to computer-aided verification. For object-oriented languages, asynchronous method calls returning futures that will be filled only after the called method has finished its execution have been proposed as a reasonably simple and analyzable programming construct. In this paper, we show how to map from a language with asynchronous method calls and futures to a language with explicit communication primitives and cryptographic operations. Our target language is reasonably similar to common process calculi, and translating it further to e.g. the applied pi calculus requires only known techniques. The translation is valid even for programs executing in open environments, where method calls and futures can be transmitted between the program and the environment.KeywordsOO languagesprocess calculifull abstraction

This thesis proposes a novel way of performing type checking, whose results are incremental, depending on the provided local information. This new way of type checking is called co-contextual, where all context information of expressions, methods, classes, etc., is removed. Instead, we introduce corresponding structures using requirements. Standard type systems are translated to the co-contextual ones systematically using dualism as technique. Type systems play an important role to prevent execution errors from occurring during runtime. They are used to check programs statically for potential errors. Programs are type checked against a given set of rules. Depending on these rules programs are well-typed or not. The set of these rules is called typing rules. Each type rule associates types to the constructs of a program given a certain context. There can be different forms of contexts, depending on the features of the typed programming language. Functional languages use a typing context of variables and their types; object-oriented (OO) languages use additional class tables. Class tables are used for example to ensure that method and class declarations are well-typed. Type checking is performed top-down. While traversing the syntax tree of a program, typing contexts are extended with information on the expressions and their types. In case of OO, class tables are extended with clauses from class declarations, including the class members, i.e., fields, methods, or constructors. Contexts are passed through the nodes of the syntax tree in order to coordinate type checking between them. Therefore, while traversing the syntax tree top-down, the type checker creates dependencies between otherwise independent subexpressions. This way, it inhibits incrementalization and parallelization of type checking. That is, a change to a node of the syntax tree would require to redo the type check of the whole syntax tree. In this thesis a novel formulation of type systems is proposed, in order to remove dependencies between subexpressions. We propose a co-contextual formulation of typing rules that depends only on the local program constructs, e.g., expressions, methods, classes. The co-contextual typing rules have as conclusion a type and sets of requirements. That is, contexts and class tables are replaced by the dual concept of context and class table requirements. In addition, operations on contexts and class tables are replaced by new dual operations on requirements. The co-contextual type checker traverses a syntax tree bottom-up and merges context requirements of independently checked subexpressions. We describe a method for systematically constructing a co-contextual formulation of type rules from a regular context-based formulation and we show how co-contextual type rules give rise to incremental type checking. We derive co-contextual type checkers for functional and OO languages. As a representative of functional languages we consider PCF and extensions of it: records, parametric polymorphism, structural subtyping and let-polymorphism. Also, we investigate featherweight java (FJ) as the basis of OO languages and extensions of it: method overloading and generics. We build a1 co-contextual type checker for FJ enabling key features of OO languages: subtype polymorphism, nominal typing and implementation inheritance. The dualism between the co-contextual and contextual type systems preserves the correctness of the contextual calculus. That is, we prove the correctness of the co-contextual calculus via the equivalence between contextual type rules and their co-contextual formulations. We implemented an incremental type checker for PCF along with a performance evaluation showing that co-contextual type checking has performance comparable to standard context-based type checking, and incrementalization can improve performance significantly. Regarding FJ, we implemented a co-contextual type checker with incrementalization and compared its performance against javac on a number of realistic programs. Our performance evaluation shows significant speedups for the co-contextual type checker with incrementalization in comparison to javac.

An approach is presented for generating a performance prediction model so that both qualitative (logical correctness) and quantitative (timeliness) properties of a real‐time system can be evaluated. The architecture of a system is layered into a functional layer and a physical one. Both architectural layers are developed as executable models: the executable functional model is a Petri net and the executable physical model is a queuing net. The two‐layered executable models are then connected to develop a performance prediction model. A message‐passing pattern is generated from the Petri net using a state space analysis technique. Then, the queuing net model processes these messages preserving the pattern. Once the network delays are obtained from the queuing model, their values are inserted back into the Petri net model. Since the communication service demands are isolated from the executable functional model, the communications network can be specified independently at any preferred level of detail. This enables the executable functional model to be invariant with respect to the executable physical model resulting in additional flexibility in designing a large‐scale information system. This property, together with the synthesis technique, enables both formal and simulation methods to be used together, when each one utilizes a different self‐contained commercial‐off‐the‐shelf software application. © 2002 Wiley Periodicals, Inc. Syst Eng 6: 1–18, 2003

This paper presents ABS, an abstract behavioral specification language for designing executable models of distributed object-oriented systems. The language combines advanced concurrency and synchronization mechanisms for concurrent object groups with a functional language for modeling data. ABS uses asynchronous method calls, interfaces for encapsulation, and cooperative scheduling of method activations inside concurrent objects. This feature combination results in a concurrent object-oriented model which is inherently compositional. We discuss central design issues for ABS and formalize the type system and semantics of Core ABS, a calculus with the main features of ABS. For Core ABS, we prove a subject reduction property which shows that well-typedness is preserved during execution; in particular, “method not understood” errors do not occur at runtime for well-typed ABS models. Finally, we briefly discuss the tool support developed for ABS.

This paper introduces a run-time mechanism for preventing leakage of secure information in distributed systems. We consider a general concurrency language model where concurrent objects interact by asynchronous method calls and futures. The aim is to prevent leakage of secure information to low-level viewers. The approach is based on a notion of security wrappers, where a wrapper encloses an object or a component and controls its interactions with the environment. Our run-time system automatically adds a wrapper to an insecure component.The wrappers are invisible such that a wrapped component and its environment are not aware of it.The security policies of a wrapper are formalized based on a notion of security levels. At run-time, future components will be wrapped upon need, and objects of unsafe classes will be wrapped, using static checking to limit the number of unsafe classes and thereby reducing run-time overhead. We define an operational semantics and sketch a proof of non-interference. A service provider may use wrappers to protect its services in an insecure environment, and vice-versa: a system platform may use wrappers to protect itself from insecure service providers.KeywordsActive objectsFuturesInformation-flow securityNon-interferenceLanguage-based securityDistributed systems

Formal Engineering Methods are a bridge from Formal Methods to industrial applications. In this paper I describe the relation between formal engineering methods and formal methods, and present a specific formal engineering method SOFL (Structured Object-Oriented Formal Language) for developing quality software systems. I explain how SOFL can be applied in practice through examples.

In this paper we consider the problem of fault handling inside an object-oriented language with asynchronous method calls whose results are returned inside futures. We present an extension for those languages where futures are used to return fault notifications and to coordinate error recovery between the caller and callee. This can be exploited to ensure that invariants involving many objects are restored after faults.

An executable domain-specific modeling language for simulating organizational auction-based coordination strategies for crisis response

Abstract. At a fundamental level, functional and object-oriented programming languages are all ‘higher-order’, in the sense that they support computing with values that are themselves pieces of program code encapsulated with a local environment. In functional languages these ‘active’ values are functions, while in object-oriented languages they are objects. Both styles of higher-order language claim to provide good support for writing adaptable programs, but functional and object-oriented languages achieve this adaptability in different ways: functional programs rely on parameterisation at the value, type and module level, while object-oriented languages rely primarily on subtyping and implementation inheritance. Here we compare these two approaches, mainly in terms of the features and properties of their type systems, and consider the benefits and disadvantages of unifying (or merging) the two paradigms by adding object-oriented features to ML as a base language. We argue that while some of the simpler aspects of object-oriented languages are compatible with ML, adding a full- edged class-based object system to ML leads to an excessively complex type system and relatively little expressive gain, especially if we aim to preserve that mostly functional style of programming that is a major advantage of ML.

Dynamic type-checking and object-oriented programming often go hand-in-hand; scripting languages such as Python, Ruby, and JavaScript all embrace object-oriented (OO) programming. When scripts written in such languages grow and evolve into large programs, the lack of a static type discipline reduces maintainability. A programmer may thus wish to migrate parts of such scripts to a sister language with a static type system. Unfortunately, existing type systems neither support the flexible OO composition mechanisms found in scripting languages nor accommodate sound interoperation with untyped code. In this paper, we present the design of a gradual typing system that supports sound interaction between statically- and dynamically-typed units of class-based code. The type system uses row polymorphism for classes and thus supports mixin-based OO composition. To protect migration of mixins from typed to untyped components, the system employs a novel form of contracts that partially seal classes. The design comes with a theorem that guarantees the soundness of the type system even in the presence of untyped components.

Using type systems, this paper treats heap slicing which is a technique transforming a program into a new one that produces the same result while working on a heap sliced into independent regions. Heap slicing is a common approach to handle the problem of modifying the heap layout without changing the program semantics. Heap slicing has applications in the areas of performance optimization and security.Towards solving the problem of heap slicing, this paper introduces three type systems. The first type system does a pointer analysis and annotates program points with pointer information. This type system is an augmentation of a previously developed type system by the author. The second type system does a region analysis and refines the result of the first type system by augmenting the pointer information with region information. The region information approximately specifies at each program point for each memory cell the region where the cell exists. The third type system uses the information gathered by the region type system to do the principal transformation of heap slicing.The paper also presents two operational semantics; one for single-region heap scenario and the other for multi-regions heap scenario. These semantics are used to prove the soundness of the type systems.Keywordsheap slicingtype systemssemantics of programming languagesoperational semanticsregion analysispointer analysis