Volume 56, Issue 3 - Special Issue: Architecture of Web Application / René Peinl

Distributed information systems predominantly have client-server architectures, as does the Web itself. In this article, we review the evolution of the interface of client-server distributed systems, from Messaging and RPC systems that predate the Web, to RESTful Web APIs. We highlight the often overlooked importance of the client-server interface in Web applications, and we reference historic and current systems to discuss the roles of “Web Service” technologies and Service-Oriented Architectures. Considering the future, we point out four directions in which we can see Web APIs moving, including the incorporation of hypermedia and semantics.

Managing a Service Oriented Architecture (SOA) requires a well-defined model of what a service actually is, and a structure within which services can be published, documented, and consumed. Without such a definition and organizational structure, it is hard to reap the benefits of a SOA. This paper presents a case study that is closely aligned with how the Web is organized as a SOA, but adds some structure so that service producers and service consumers can be supported in their goals. Using this approach, it is possible to realize the architectural benefits of a RESTful architecture, but can still make sure that the published services follow a set of guidelines and constraints that the SOA is based on.

The migration of existing applications to the cloud enables enterprises to preserve their previous investments and—at the same time—to benefit from the properties of the cloud. This article presents a semi-automated approach for migrating existing enterprise applications to the cloud. Thereby, information about the application is gathered in the source environment, the application is extracted, transformed, and cloud-enabled. This makes the application ready for provisioning in the target cloud. Cloud-enabling an application preserves its business functionality and does not change the fundamental way the application was built.

Today's Web applications are often compositions of distributed yet interconnected services that offer features and data through defined interfaces via standardized protocols. Providing a set of best practices for organizing and utilizing distributed capabilities, the service-oriented architecture design pattern largely contributed to this trend. To react on emerging customer requirements, using agile methodology for Web application development fits well in this context. While it allows promptly responding to change by adjusting the Web application architecture, security must be applied as a holistic approach throughout the entire Web application's lifecycle. There is a need for a flexible, expressive and easy-to-use way to model a Web application's architecture with a strong emphasis on security. This article discusses our work on extending the WebComposition Architecture Model towards a semantically enriched description of a Web application's architecture. For enabling systematic exploitation of such architecture descriptions, we utilize W3C's WebID identity mechanism, the WAC authorization method, and fine-grained filters. We explain how WebID can be applied to allow Web services to mutually authenticate and exchange data, e. g., interface definitions and service parameters, in a controlled way.

Modern web applications are concurrently used by many users and provide increasingly interactive features. Multi-core processors, highly distributed backend architectures, and new web technologies force a reconsideration of approaches for concurrent programming in order to fulfil scalability demands and to implement modern web application features. We provide a survey on different concepts and techniques of concurrency inside web architectures and guide through viable concurrency alternatives for architects and developers.

Software as a service is becoming an increasingly common software delivery model but causes a challenge to service providers related to the design of both applications and data management. Multi-tenant databases allow them to reduce operating costs by leveraging economies of scale. Load balancing of multi-tenant databases with shared-nothing architectures can be done in different tiers of the software stack. In this article, different approaches for this purpose are discussed and the proxy server H2 Proxy is presented. H2 Proxy can be deployed in different tiers and routes tenant requests to database servers corresponding to a tenant placement policy which is changeable at runtime. This article gives an overview of its implementation and demonstrates its efficient routing according to test results.

The typical identity management (IdM) techniques used in web-based applications are about to change from application-specific means for identification, authentication and authorization towards the support of standardized, secure and privacy friendly mechanisms for Single Sign-On (SSO). In this paper we outline the different phases of this evolution, which started with the introduction of standardized interfaces for authentication and authorization and allowed to shift these sensitive tasks from the application towards the web application server. In a second phase the interfaces were extended to support authentication and authorization in distributed systems and feature SSO-techniques. The third phase adds identification and aims at providing more security for distributed authentication infrastructures and finally there is a trend towards providing more privacy friendly mechanisms for identity management in the future.

In this thesis, we discuss approaches that allow robots to learn motor skills. Motor skills can often be represented by motor primitives, which encode elemental motions. To date, there have been a number of successful applications of learning motor primitives employing imitation learning. However, many interesting motor learning problems are high-dimensional reinforcement learning problems which are often beyond the reach of current reinforcement learning methods. This thesis contributes to the state of the art in reinforcement learning applied to robotics both in terms of novel algorithms and applications. We show how motor primitives can be employed to learn motor skills on three different levels. All proposed approaches have been extensively validated with tasks such as Ball-in-a-Cup, darts, table tennis, ball throwing, or ball bouncing both in simulation, and on real robots.