Lijun Mei

A Tale of Clouds: Paradigm Comparisons and Some Thoughts on Research Issues

Cloud computing is an emerging computing paradigm. It aims to share data, calculations, and services transparently among users of a massive grid. Although the industry has started selling cloud-computing products, research challenges in various areas, such as UI design, task decomposition, task distribution, and task coordination, are still unclear. Therefore, we study the methods to reason and model cloud computing as a step toward identifying fundamental research questions in this paradigm. In this paper, we compare cloud computing with service computing and pervasive computing. Both the industry and research community have actively examined these three computing paradigms. We draw a qualitative comparison among them based on the classic model of computer architecture. We finally evaluate the comparison results and draw up a series of research questions in cloud computing for future exploration.

Data flow testing of service-oriented workflow applications

WS-BPEL applications are a kind of service-oriented application. They use XPath extensively to integrate loosely-coupled workflow steps. However, XPath may extract wrong data from the XML messages received, resulting in erroneous results in the integrated process. Surprisingly, although XPath plays a key role in workflow integration, inadequate researches have been conducted to address the important issues in software testing. This paper tackles the problem. It also demonstrates a novel transformation strategy to construct artifacts. We use the mathematical definitions of XPath constructs as rewriting rules, and propose a data structure called XPath rewriting graph (XRG), which not only models how an XPath is conceptually rewritten but also tracks individual rewritings progressively. We treat the mathematical variables in the applied rewriting rules as if they were program variables, and use them to analyze how information may be rewritten in an XPath conceptually. We thus develop an algorithm to construct XRGs and a novel family of data flow testing criteria to test WS-BPEL applications. Experiment results show that our testing approach is promising.

Test case prioritization for regression testing of service-oriented business applications

Regression testing assures the quality of modified service-oriented business applications against unintended changes. However, a typical regression test suite is large in size. Earlier execution of those test cases that may detect failures is attractive. Many existing prioritization techniques order test cases according to their respective coverage of program statements in a previous version of the application. On the other hand, industrial service-oriented business applications are typically written in orchestration languages such as WS-BPEL and integrated with workflow steps and web services via XPath and WSDL. Faults in these artifacts may cause the application to extract wrong data from messages, leading to failures in service compositions. Surprisingly, current regression testing research hardly considers these artifacts. We propose a multilevel coverage model to capture the business process, XPath, and WSDL from the perspective of regression testing. We develop a family of test case prioritization techniques atop the model. Empirical results show that our techniques can achieve significantly higher rates of fault detection than existing techniques.

Modeling and testing of cloud applications

What is a cloud application precisely? In this paper, we formulate a computing cloud as a kind of graph, a computing resource such as services or intellectual property access rights as an attribute of a graph node, and the use of a resource as a predicate on an edge of the graph. We also propose to model cloud computation semantically as a set of paths in a subgraph of the cloud such that every edge contains a predicate that is evaluated to be true. Finally, we present algorithms to compose cloud computations and a family of model-based testing criteria to support the testing of cloud applications.

An Adaptive Service Selection Approach to Service Composition

In service computing, the behavior of a service may evolve. When an organization develops a service-oriented application in which certain services are provided by external partners, the organization should address the problem of uninformed behavior evolution of external services. This paper proposes an adaptive framework that bars problematic external services to be used in the service-oriented application of an organization. We use dynamic WSDL information in public service registries to approximate a snapshot of a network of services, and apply link analysis on the snapshot to identify services that are popularly used by different service consumers at the moment. As such, service composition can be strategically formed using the highly referenced services. We evaluate our proposal through a simulation study. The results show that, in terms of the number of failures experienced by service consumers, our proposal significantly outperforms the random approach in selecting reliable services to form service compositions.

Data flow testing of service choreography

Service computing has increasingly been adopted by the industry, developing business applications by means of orchestration and choreography. Choreography specifies how services collaborate with one another by defining, say, the message exchange, rather than via the process flow as in the case of orchestration. Messages sent from one service to another may require the use of different XPaths to manipulate or extract message contents. Mismatches in XML manipulations through XPaths (such as to relate incoming and outgoing messages in choreography specifications) may result in failures. In this paper, we propose to associate XPath Rewriting Graphs (XRGs), a structure that relates XPath and XML schema, with actions of choreography applications that are skeletally modeled as labeled transition systems. We develop the notion of XRG patterns to capture how different XRGs are related even though they may refer to different XML schemas or their tags. By applying XRG patterns, we successfully identify new data flow associations in choreography applications and develop new data flow testing criteria. Finally, we report an empirical case study that evaluates our techniques. The result shows our techniques are promising in detecting failures in choreography applications.

XML-manipulating test case prioritization for XML-manipulating services

A web service may evolve autonomously, making peer web services in the same service composition uncertain as to whether the evolved behaviors are compatible with its original collaborative agreement. Although peer services may wish to conduct regression testing to verify the agreed collaboration, the source code of the former service may be inaccessible to them. Owing to the black-box nature of peer services, traditional code-based approaches to regression testing are inapplicable. In addition, traditional techniques assume that a regression test suite for verifying a web service is available. The location to store a regression test suite is also a problem. On the other hand, we note that the rich interface specifications of a web service provide peer services with a means to formulate black-box testing strategies. In this paper, we provide a strategy for black-box service-oriented testing. We also formulate new test case prioritization strategies using tags embedded in XML messages to reorder regression test cases, and reveal how the test cases use the interface specifications of web services. We experimentally evaluate the effectiveness of these black-box strategies in revealing regression faults in modified WS-BPEL programs. The results show that the new techniques can have a high chance of outperforming random ordering. Moreover, our experiment shows that prioritizing test cases based on WSDL tag coverage can achieve a smaller variance than that based on the number of tags in XML messages in regression test cases, even though their overall fault detection rates are similar.

Tag-Based Techniques for Black-Box Test Case Prioritization for Service Testing

A web service may evolve autonomously, making peer web services in the same service composition uncertain as to whether the evolved behaviors may still be compatible to its originally collaborative agreement. Although peer services may wish to conduct regression testing to verify the original collaboration, the source code of the former service can be inaccessible to them. Traditional code-based regression testing strategies are inapplicable. The rich interface specifications of a web service, however, provide peer services with a means to formulate black-box testing strategies. In this paper, we formulate new test case prioritization strategies using tags embedded in XML messages to reorder regression test cases, and reveal how the test cases use the interface specifications of services. We evaluate experimentally their effectiveness on revealing regression faults in modified WS-BPEL programs. The results show that the new techniques can have a high probability of outperforming random ordering.

A Subsumption Hierarchy of Test Case Prioritization for Composite Services

Many composite workflow services utilize non-imperative XML technologies such as WSDL, XPath, XML schema, and XML messages. Regression testing should assure the services against regression faults that appear in both the workflows and these artifacts. In this paper, we propose a refinement-oriented level-exploration strategy and a multilevel coverage model that captures progressively the coverage of different types of artifacts by the test cases. We show that by using them, the test case prioritization techniques initialized on top of existing greedy-based test case prioritization strategy form a subsumption hierarchy such that a technique can produce more test suite permutations than a technique that subsumes it. Our experimental study of a model instance shows that a technique generally achieves a higher fault detection rate than a subsumed technique, which validates that the proposed hierarchy and model have the potential to improve the cost-effectiveness of test case prioritization techniques.

Preemptive Regression Testingof Workflow-Based Web Services

An external web service may evolve without prior notification. In the course of the regression testing of a workflow-based web service, existing test case prioritization techniques may only verify the latest service composition using the not-yet-executed test cases, overlooking high-priority test cases that have already been applied to the service composition before the evolution. In this paper, we propose Preemptive Regression Testing (PRT), an adaptive testing approach to addressing this challenge. Whenever a change in the coverage of any service artifact is detected, PRT recursively preempts the current session of regression test and creates a sub-session of the current test session to assure such lately identified changes in coverage by adjusting the execution priority of the test cases in the test suite. Then, the sub-session will resume the execution from the suspended position. PRT terminates only when each test case in the test suite has been executed at least once without any preemption activated in between any test case executions. The experimental result confirms that testing workflow-based web service in the face of such changes is very challenging; and one of the PRT-enriched techniques shows its potential to overcome the challenge.