Dianfu Ma

Towards a formal semantics for the AADL behavior annex

AADL is an Architecture Description Language which describes embedded real-time systems. Behavior annex is an extension of the dispatch mechanism of AADL execution model. This paper proposes a formal semantics for the AADL behavior annex using Timed Abstract State Machine (TASM). Firstly, the semantics of AADL default execution model is given, and then we formally define some aspects semantics of behavior annex. A prototype of real-time behavior modeling and verification is proposed, and finally, a case study will be given to validate the feasibility.

From AADL to Timed Abstract State Machines: A verified model transformation

Architecture Analysis and Design Language (AADL) is an architecture description language standard for embedded real-time systems widely used in the avionics and aerospace industry to model safety-critical applications. To verify and analyze the AADL models, model transformation technologies are often used to automatically extract a formal specification suitable for analysis and verification. In this process, it remains a challenge to prove that the model transformation preserves the semantics of the initial AADL model or, at least, some of the specific properties or requirements it needs to satisfy. This paper presents a machine checked semantics-preserving transformation of a subset of AADL (including periodic threads, data port communications, mode changes, and the AADL behavior annex) into Timed Abstract State Machines (TASM). The AADL standard itself lacks at present a formal semantics to make this translation validation possible. Our contribution is to bridge this gap by providing two formal semantics for the subset of AADL. The execution semantics provided by the AADL standard is formalized as Timed Transition Systems (TTS). This formalization gives a reference expression of AADL semantics which can be compared with the TASM-based translation (for verification purpose). Finally, the verified transformation is mechanized in the theorem prover Coq.

An adaptive heuristic approach for distributed QoS-based service composition

QoS-based service selection becomes a commonly accepted procedure to support rapid and dynamic web service composition. In this paper, we study the problem of QoS-based service selection in distributed QoS management environments where QoS values of alternative services are maintained by distributed QoS registries. A distributed heuristic approach is proposed to solve the problem efficiently with a high approximation ratio, and enable adaptability in distributed cross-organization environments with data privacy protection and a low cost of communication. The proposed approach consists of four stages in which variable elimination is used to reduce the size of the problem; constraint decomposition allows performing service selection independently on each QoS registry; supplementary service selection and concentrated optimization improve the approximation ratio. Performance analyses and simulation experiments show that the proposed approach performs efficiently with close-to-optimal results and fits well to distributed QoS management environments.

Adaptive QoS-Aware Service Process Reconfiguration

During the execution of a QoS-aware service process, Web Services (WSs) may become faulty and cause the whole process to violate the predefined QoS constraints. Service processes need to adapt to the runtime faults so as to support reliable service-based applications. In this paper, we study the problem of runtime service process reconfiguration under end-to-end QoS constraints. An adaptive QoS-aware service process reconfiguration approach is proposed to solve the problem efficiently with low reconfiguration costs. In our approach, supplementary services are selected during service composition and then used as a backup source for efficient recovery, a region-based reconfiguration algorithm is proposed to minimize the reconfiguration cost by identifying a limited reconfiguration region that includes a small number of services. Experimental results show that, by using our approach, a high percentage of faulty service processes can be successfully reconfigured with high efficiency and significant cost savings.

MIAPS: A web-based system for remotely accessing and presenting medical images

MIAPS (medical image access and presentation system) is a web-based system designed for remotely accessing and presenting DICOM image. MIAPS is accessed with web browser through the Internet. MIAPS provides four features: DICOM image retrieval, maintenance, presentation and output. MIAPS does not intent to replace sophisticated commercial and open source packages, but it provides a web-based solution for teleradiology and medical image sharing. The system has been evaluated by 39 hospitals in China for 10 months.

Two Formal Semantics of a Subset of the AADL

The analysis and verification of an AADL model usually requires its transformation into the meta-model of this model-checker or that schedulability analysis tool. However, one challenging problem is to prove that the transformation into the target model of computation (MoC) preserves the semantics of the original AADL model or at least some of its properties. Moreover, the AADL standard lacks a formal semantics to make the validation of this translation possible. Albeit some of the related works give informal explanations on the model transformations they apply to interpret or compile AADL, the formal proof of semantics preservation remains in most cases altogether impossible. Our contribution is to bridge this gap by providing two formal semantics for a synchronous subset of AADL, which includes periodic threads and data port communications. Its operational semantics is formalized as a TTS (Timed Transition System). This formalization is one prerequisite to the formal proof of semantics preservation for our model transformation from AADL sources to our target verification formalism: TASM (Timed Abstract State Machine). In this paper, an abstract syntax of (our subset of) AADL is given, together with the abstract syntax of TASM. The translation is formalized by a family of semantics functions, which associates each AADL construct to a TASM fragment. Then, the proof of simulation equivalence between the TTSs of the AADL and the TASM models is formalized and mechanized using the proof assistant Coq.

Collaborative Visualization of Large Scale Datasets Using Web Services

Visualization and collaboration of large scale data sets on Internet is still one of biggest challenges in scientific visualization. A distributed, real-time, collaborative system for large scale data like seismic model can be a valuable tool to support scientific research. In this paper, we present a new approach for Web-based synchronized collaborative visualization of large scale data using Web services and rich Web clients which supports collaborative visualization in Web browsers. We use WS-resources in WSRF (Web services resource framework) to maintain states collaborative server. On client side, we design an Ajax-based (asynchronous JavaScript and XML) application using standard Web technologies. A collaborative demonstration of 3D seismic model which is 2GB size is presented and experimental results are showed finally.

A verified transformation: from polychronous programs to a variant of clocked guarded actions

SIGNAL belongs to the synchronous languages family. Such languages are widely used in the design of safety-critical real-time systems such as avionics, space systems, and nuclear power plants. This paper reports a key step of a verified SIGNAL compiler prototype, that is the transformation from a subset of SIGNAL to S-CGA (a variant of clocked guarded actions) and the proof of semantics preservation. Compared with the existing SIGNAL compiler, we use clocked guarded actions as the intermediate representation, to integrate more synchronous programs into our verified compiler prototype in the future. However, in contrast to the SIGNAL language, clocked guarded actions can evaluate a variable even if its clock does not hold. Thus, we propose a variant of clocked guarded actions, namely S-CGA, which constrains variable accesses as done by SIGNAL. To conform with the revised semantics of clocked guarded actions, we also do some adjustments on the existing translation rules from SIGNAL to clocked guarded actions. Finally, the verified transformation is mechanized in the theorem prover Coq.

A Comparative Study of FIACRE and TASM to Define AADL Real Time Concepts

This paper presents some real-time concepts as they arefound in the AADL language and proposes their expressionin two formalisms suitable for formal analysis: FIACREwhich is based on Timed Transition Systems and TASMwhich extends Abstract State Machines with resource consumption mechanisms.

A multi-resource load balancing algorithm for cloud cache systems

With the advent of cloud computing model, distributed caches have become the cornerstone for building scalable applications. Popular systems like Facebook [1] or Twitter use Memcached [5], a highly scalable distributed object cache, to speed up applications by avoiding database accesses. Distributed object caches assign objects to cache instances based on a hashing function, and objects are not moved from a cache instance to another unless more instances are added to the cache and objects are redistributed. This may lead to situations where some cache instances are overloaded when some of the objects they store are frequently accessed, while other cache instances are less frequently used. In this paper we propose a multi-resource load balancing algorithm for distributed cache systems. The algorithm aims at balancing both CPU and Memory resources among cache instances by redistributing stored data. Considering the possible conflict of balancing multiple resources at the same time, we give CPU and Memory resources weighted priorities based on the runtime load distributions. A scarcer resource is given a higher weight than a less scarce resource when load balancing. The system imbalance degree is evaluated based on monitoring information, and the utility load of a node, a unit for resource consumption. Besides, since continuous rebalance of the system may affect the QoS of applications utilizing the cache system, our data selection policy ensures that each data migration minimizes the system imbalance degree and hence, the total reconfiguration cost can be minimized. An extensive simulation is conducted to compare our policy with other policies. Our policy shows a significant improvement in time efficiency and decrease in reconfiguration cost.