3118800
Developing flexible and high-performance Web servers with frameworks and patterns
The goal of this paper is to illustrate how frameworks and patterns address complexities that arise in the design and implementation of high-performance distributed software systems. These complexities are both inherent (e.g., latency reduction and throughput preservation), and accidental (e.g., the continuous reinvention of key concepts and components). This paper explains how complexities occurring in the development of high-performance Web servers can be alleviated with the use of design patterns and object-oriented application frameworks. These techniques were applied to the development our high-performance adaptive Web server framework, JAWS. JAWS exemplifies how a framework can remain flexible without sacrificing performance. 1 Applying Patterns and Frameworks to Web Servers Developers of Web servers strive to build fast, scalable, and configurable systems. This paper describes some common pitfalls encountered by these developers and how to avoid these pitfalls. Common pitfalls include (1) coping with tedious and error-prone low-level programming details, (2) lack of portability, and (3) the complexity of navigating the wide range of server design alternatives. By carefully utilizing patterns and frameworks, these hazards can be avoided, by allowing developers to leverage reuse of design and code. 1.1 Common Pitfalls of Developing Web Server Software Web servers perform the following tasks: connection establishment, service initialization, event demultiplexing, event handler dispatching, interprocess communication, memory management and file caching, static and dynamic component configuration, concurrency, synchronization, and persistence. In most Web servers, these tasks are implemented in an ad hoc manner using low-level native OS application programming interfaces (APIs), such as Win32 or UNIX/POSIX, which are written in C. Unfortunately, native OS APIs are not an effective way to develop Web servers or other types of communication middleware and applications [1]. The following are common pitfalls associated with the use of native OS APIs: Excessive low-level details: Building Web servers with native OS APIs requires developers to have intimate knowledge of low-level OS details. Developers must carefully track which error codes are returned by each system call and handle these OS-specific problems in their servers. Such details divert attention from the broader, more strategic issues, such as protocol semantics and server structure. For example, UNIX developers who use the wait system call must distinguish between return errors due to no child processes being present and errors from signal interrupts. In the latter case, the wait must be reissued. Reinvention of incompatible programming abstractions: A common remedy for the excessive level of detail with OS APIs is to define higher-level programming abstractions. For instance, many Web servers create a file cache to avoid accessing the filesystem for each client request. However, these types of abstractions are often rediscovered and reinvented independently by each developer or project. This ad hoc devel-
8854332
The design of the TAO real-time object request broker
Many real-time application domains can benefit from flexible and open distributed architectures, such as those defined by the CORBA specification. CORBA is an architecture for distributed object computing being standardized by the OMG. Although CORBA is well-suited for conventional request/response applications, CORBA implementations are not yet suited for real-time applications due to the lack of key quality of service (QoS) features and performance optimizations. This paper makes three contributions to the design of real-time CORBA systems. First, the paper describes the design of TAO, which is our high-performance, real-time CORBA 2.0-compliant implementation that runs on a range of OS platforms with real-time features including VxWorks, Chorus, Solaris 2.x, and Windows NT. Second, it presents TAO's real-time scheduling service that can provide QoS guarantees for deterministic real-time CORBA applications. Finally, the paper presents performance measurements that demonstrate the effects of priority inversion and non-determinism in conventional CORBA implementations and how these hazards are avoided in TAO.
2701566
Evaluating policies and mechanisms to support distributed real-time applications with CORBA
To be an effective platform for performance-sensitive real-time systems, commodity-off-the-shelf (COTS) distributed object computing (DOC) middleware must support application quality of service (QoS) requirements end-to-end. However, conventional COTS DOC middleware does not provide this support, which makes it unsuited for applications with stringent latency, determinism, and priority preservation requirements. It is essential, therefore, to develop standards-based, COTS DOC middleware that permits the specification, allocation, and enforcement of application QoS requirements end-to-end. 
 
The real-time CORBA and messaging specifications in the CORBA 2.4 standard are important steps towards defining standards-based, COTS DOC middleware that can deliver end-to-end QoS support at multiple levels in distributed and embedded real-time systems. These specifications still lack sufficient detail, however, to portably configure and control processor, communication, and memory resources for applications with stringent QoS requirements. 
 
This paper provides four contributions to research on real-time DOC middleware. First, we illustrate how the CORBA 2.4 real-time and messaging specifications provide a starting point to address the needs of an important class of applications with stringent real-time requirements. Second, we illustrate how the CORBA 2.4 specifications are not sufficient to solve all the issues within this application domain. Third, we describe how we have implemented portions of these specifications, as well as several enhancements, using TAO, which is our open-source real-time CORBA ORB. Finally, we evaluate the performance of TAO empirically to illustrate how its features address the QoS requirements for certain classes of real-time applications. Copyright © 2001 John Wiley & Sons, Ltd.
10426622
Object-Oriented Components for High-speed Network Programming
This paper makes two contributions to the development and evaluation of object-oriented communication software. First, it reports performance results from benchmarking several network programming mechanisms (such as sockets and CORBA) on Ethernet and ATM networks. These results illustrate that developers of bandwidth-intensive and delay-sensitive applications (such as interactive medical imaging or teleconferencing) must evaluate their performance requirements and the efficiency of their communication infrastructure carefully before adopting a distributed object solution. Second, the paper describes the software architecture and design principles of the ACE object-oriented network programming components. These components encapsulate UNIX and Windows NT network programming interfaces (such as sockets, TLI, and named pipes) with C++ wrappers. Developers of object-oriented communication software have traditionally had to choose between high-performance, lower-level interfaces provided by sockets or TLI or less efficient, higher-level interfaces provided by communication frameworks like CORBA or DCE. ACE represents a midpoint in the solution space by improving the correctness, programming simplicity, portability, and reusability of performance-sensitive communication software.
6974153
Integrating publisher/subscriber services in component middleware for distributed real-time and embedded systems
Although component-based software development has widespread acceptance in the enterprise business and desktop application domains, developers of distributed real-time and embedded (DRE) systems have encountered limitations with the available component middleware platforms, such as the CORBA Component Model (CCM) and the Java 2 Enterprise Edition (J2EE). These limitations often preclude developers of DRE systems from fully exploiting the benefits of component software. In particular, component middleware platforms lack standards-based publisher/subscriber communication mechanisms that support key quality-of-service (QoS) requirements, such as low latency, bounded jitter, and end-to-end operation priority propagation. QoS-enabled publisher/subscriber services are available in object middleware platforms, such as Real-time CORBA, but such services have not been integrated into component middleware due to a number of development and configuration challenges.This paper provides three contributions to the integration of publisher/subscriber services in component middleware. First, we outline key challenges associated with integrating publisher/subscriber services into component middleware. Second, we describe a methodology for resolving these challenges based on software patterns. Third, we describe a pattern-oriented component middleware platform that we have developed to integrate publisher/subscriber services into component middleware applications.
6993185
Metric-driven analysis and feedback systems for enabling empirically guided software development
The authors summarize the goals of metric-driven analysis and feedback systems and describe a prototype system, Amadeus, which defines abstract interfaces and embodies architectural principles for these types of systems. Metric-driven analysis and feedback systems enable developers to define empirically guided processes for software development and maintenance. The authors provide an overview of the Amadeus system operation, including an example of the empirically guided process, a description of the system characteristics, an explanation of the system conceptual operation, and a summary of the users' view of the system. The centerpiece of the system is a pro-active server, which interprets scripts and coordinates event monitoring and agent activation. Amadeus provides an extensible framework for adding new empirically based analysis techniques.<<ETX>>
13868346
Should UML be used for declarative programming?
Widely used in object-oriented programming, the Unified Modelling Language (UML) is largely ignored by the declarative programming community. this paper attempts to introduce the UML and provide reasons for its adoption by declarative programmers as both a design methodology and as a novel application area.
602812
Autonomous behavior-based exploration of office environments
Besides safe motion control the gain of environmental knowledge is a key for a reliable home or office service robot. When being set into a completely unknown environment the robot has to be able to derive a certain abstract internal representation of this world without any user interaction. This knowledge enables the robot to know how to get from its actual place in one room to a target position in another room as a prerequisite for transportation tasks for example. In this context, the combination of a behavior-based motion control system and an abstract topological map based on geometric representations of rooms seems promising. As the concept of motion and exploration behaviors facilitates to compete with noisy sensor information and geometrically imprecise maps, it has been used to develop completely autonomous exploration strategies for deriving topological representations of common indoor environments. The only prescribed world knowledge is the fact that these environments are composed of rectangular entities (rooms) which are connected by openings (doors). The developed system has successfully been tested in simulation and reality.
14582007
Adapting Big-Step Semantics to Small-Step Style: Coinductive Interpretations and "Higher-Order" Derivations
We adapt Kahn-style (“big-step”) natural semantics to take on desirable aspects of small-step and denotational semantics forms, more precisely: (i) the ability to express divergent computations; (ii) the ability to reason about the (length of a) computation of a derivation; and (Hi) the ability to compute upon and reason about higher-order values. To accomplish these results, we extend the classical, inductive interpretation of natural semantics with coinduction mechanisms and use “negative” rules to express divergence. A simple reformatting of the syntax of derivations allows a simple description of the “length” of a derivation. Finally, the receding of closure values into denotational-semantics-like functions lets one embed derivations within closures that embed within derivations; in this sense, the semantics becomes “higher order.” Examples are given to support the definitional developments.
1350141
Polymorphic lambda calculus and subtyping
We present a denotational model for F, the extension of second-order lambda calculus with subtyping defined in Cardelli and Wegner (ACM Comput. Surveys 17(4) (1985) 471-522.) Types are interpreted as arbitrary cpos and elements of types as natural transformations. We prove the soundness of our model with respect to the equational theory of F (Cardelli et al. (Internat. Conf. on Theoretical Aspects of Computer Software, Lecture Notes in Computer Science, vol. 526, Springer, Berlin, 1991, pp. 750-770)) and show coherence. Our model is of independent interest, because it integrates ad hoc and parametric polymorphism in an elegant fashion, admits nontrivial records and record update operations, and formalizes an "order faithfulness" criterion for well-behaved multiple subtyping.
43800980
Type Inference for Action Semantics
In a series of papers [11,12,13,14,15,25,26], Mosses and Watt defme action semantics, a metalanguage for high level, domain-independent formulation of denotational semantics definitions. Action semantics was designed to support readability, abstraction, modularity, and modifiability of language definitions [13,15]. In our recent work [5], we studied a combinator-based version of action semantics. We developed a typing system for actions based on types and k/nds. The types within a kind are partially ordered to reflect subtyping. We then defined a category-sorted algebra model [18,20] for the system where actions are natural transformations over interpretation functors (which map the type names and their ordering to predomains and coercion functions). Thus, an action is a family of continuous functions that behave consistently with respect to the subtyping relation. Coercion functions-even nonembeddings-disappear. The typing system and its interpretation support an ML-style type inference algorithm for action expressions. Type inference must respect the subtyping laws, so we encounter the problems discovered by Mitchell [10], Fuh and Mishra [6], and Wand [24] regarding constraints sets. But action notation and its model are formulated so that the major problems regarding constraints are "restricted away." In particular, the monotonic behavior of actions with respect to the subtyping ordering keeps the cardinality of constraints sets small and makes constraints on record types unnecessary, even in the presence of record union and concatenation operations. This paper presents our variant of action semantics and its typing system. We give the type inference algorithms, state properties that action semantics expressions satisfy, give soundness and completeness properties and explain why they hold for type inference on action semantics expressions. We also present a small example.