Stefan Tai

The next step in Web services

How three specifications support creating robust service compositions.

A Domain-Specific Language for Web APIs and Services Mashups

Distributed programming has shifted from private networks to the public Internet and from using private and controlled services to increasingly using publicly available heterogeneous Web services (e.g., REST, SOAP, RSS, and Atom). This move enables the creation of innovative end-user-oriented composed services with user interfaces. These services mashupsare typically point solutions to specific (specialized) problems; however, what is missing is a programming model that facilitates and accelerates creation and deployment of mashups of diverseservices. In this paper we describe a domain-specific language that unifies the most common service models and facilitates service composition and integration into end-user-oriented Web applications. We demonstrate our approach with an implementation that leverages the Ruby on Rails framework.

Thema: Byzantine-fault-tolerant middleware for Web-service applications

Distributed applications composed of collections of Web services may call for diverse levels of reliability in different parts of the system. Byzantine fault tolerance (BFT) is a general strategy that has recently been shown to be practical for the development of certain classes of survivable, client-server, distributed applications; however, little research has been done on incorporating it into selective parts of multi-tier, distributed applications like Web services that have heterogeneous reliability requirements. To understand the impacts of combining BFT and Web services, we have created Thema, a new BFT middleware system that extends the BFT and Web services technologies to provide a structured way to build Byzantine-fault-tolerant, survivable Web services that application developers can use like other Web services. From a reliability perspective, our enhancements are also novel in that they allow Byzantine-fault-tolerant services: (1) to support the multi-tiered requirements of Web services, and (2) to provide standardized Web services support for their own clients (through WSDL interfaces and SOAP communication). In this paper we study key architectural implications of combining BFT with Web services and provide a performance evaluation of Thema using the TPC-W benchmark.

Composition of coordinated web services

The Web services architecture defines separate specifications for the composition and the coordination of Web services. BPEL is a language for creating service compositions in the form of business processes, whereas the WS-Coordination framework defines coordination protocols for distributed activities. In this paper, we investigate the combination of these two aspects to compose coordinated Web services. We argue for a policy-based approach to address this problem and introduce a new model and middleware that enables the flexible integration of diverse coordination types into (existing) process-based Web services compositions.

Do Clouds Compute? A Framework for Estimating the Value of Cloud Computing

On-demand provisioning of scalable and reliable compute services, along with a cost model that charges consumers based on actual service usage, has been an objective in distributed computing research and industry for a while. Cloud Computing promises to deliver on this objective: consumers are able to rent infrastructure in the Cloud as needed, deploy applications and store data, and access them via Web protocols on a pay-per-use basis. The acceptance of Cloud Computing, however, depends on the ability for Cloud Computing providers and consumers to implement a model for business value co-creation. Therefore, a systematic approach to measure costs and benefits of Cloud Computing is needed. In this paper, we discuss the need for valuation of Cloud Computing, identify key components, and structure these components in a framework. The framework assists decision makers in estimating Cloud Computing costs and to compare these costs to conventional IT solutions. We demonstrate by means of representative use cases how our framework can be applied to real world scenarios.

GlueQoS: middleware to sweeten quality-of-service policy interactions

A holy grail of component-based software engineering is write-once, reuse everywhere. However, in modern distributed, component-based systems supporting emerging application areas such as service-oriented e-business (where Web services are viewed as components) and peer-to-peer computing, this is difficult. Non-functional requirements (related to quality-of-service (QoS) issues such as security, reliability, and performance) vary with deployment context, and sometimes even at run-time, complicating the task of re-using components. In this paper, we present a middleware-based approach to managing dynamically changing QoS requirements of components. Policies are used to advertise non-functional capabilities and vary at run-time with operating conditions. We also provide middleware enhancements to match, interpret, and mediate QoS requirements of clients and servers at deployment time and/or runtime.

Eventual consistency: How soon is eventual? An evaluation of Amazon S3's consistency behavior

Over the last few years, Cloud storage systems and so-called NoSQL datastores have found widespread adoption. In contrast to traditional databases, these storage systems typically sacrifice consistency in favor of latency and availability as mandated by the CAP theorem, so that they only guarantee eventual consistency. Existing approaches to benchmark these storage systems typically omit the consistency dimension or did not investigate eventuality of consistency guarantees. In this work we present a novel approach to benchmark staleness in distributed datastores and use the approach to evaluate Amazons Simple Storage Service (S3). We report on our unexpected findings.

MetaStorage: A Federated Cloud Storage System to Manage Consistency-Latency Tradeoffs

Cost and scalability benefits of Cloud storage services are apparent. However, selecting a single storage service provider limits availability and scalability to the selected provider and may further cause a vendor lock-in effect. In this paper, we present MetaStorage, a federated Cloud storage system that can integrate diverse Cloud storage providers. MetaStorage is a highly available and scalable distributed hash table that replicates data on top of diverse storage services. MetaStorage reuses mechanisms from Amazons Dynamo for cross-provider replication and hence introduces a novel approach to manage consistency-latency tradeoffs by extending the traditional quorum (N,R,W) configurations to an (N_P,R,W) scheme that includes different providers as an additional dimension. With MetaStorage, new means to control consistency-latency tradeoffs are introduced.

Transaction policies for service-oriented computing

Service-oriented computing is emerging as a distributed computing model where autonomous services interact with each other using standard Internet technology. In addition to the application-specific functions that services provide (different) services may also support (different) sets of protocols and formats addressing extra-functional concerns such as transaction processing and reliable messaging. This raises the need for services to complement their functional service descriptions with descriptions of extra-functional capabilities, requirements, and/or preferences, which must be matched and enforced for service interactions. In this paper, we address the problem of transactional coordination in service-oriented computing. We argue for the use of declarative policy assertions to advertise and match support for different transaction styles (direct transaction processing, queued transaction processing, and compensation-based transaction processing), and introduce the concept of and system support for transaction coupling modes as the policy-based contracts guiding transactional business process execution. We focus on concrete, protocol-specific policies that apply to relevant Web services specifications. Using transaction policies and our middleware system, we are able to support a reliable SOC environment.

(MC2)2: criteria, requirements and a software prototype for Cloud infrastructure decisions

Cloud computing is a disruptive technology, representing a new model for information technology solution engineering and management that promises to introduce significant cost savings and other benefits. The adoption of Cloud computing requires a detailed comparison of infrastructure alternatives, taking a number of aspects into careful consideration. Existing methods of evaluation, however, limit decision‐making to the relative costs of Cloud computing, but do not take a broader range of criteria into account. The Multi‐Criteria Comparison Method for Cloud Computing ( (MC2)2 ) offers a multi‐criteria‐based decision framework that can be applied to Cloud computing. The framework allows organizations to create evaluation methods that help to determine what infrastructure best suits their needs by evaluating and ranking infrastructure alternatives using multiple criteria derived from a comprehensive criteria catalog. Therefore, (MC2)2 offers a way to differentiate infrastructures not only by costs but also in terms of benefits, opportunities and risks. (MC2)2 can be adapted to facilitate a wide array of decision‐making scenarios within the domain of information technology infrastructures, depending on the criteria selected to support the framework. The software prototype presented in this paper creates an interface to decision‐makers that makes the framework more usable for Cloud infrastructure decisions in practice. Copyright