Computational logic systems can offer an attractive environment for developing Internet applications. They share many of the important characteristics of popular network programming tools, including dynamic memory management, well-behaved structure and pointer manipulation, robustness, and compilation to architecture-independent bytecode. However, in addition, computational logic systems offer some unique features such as very powerful symbolic processing capabilities, constraint solving, dynamic databases, search facilities, grammars, sophisticated meta-programming, and well understood semantics. Such features can often make it very easy to code simple applications.

This special issue concerned with applications is the third of its kind in a journal sponsored by the Association for Logic Programming. The first appeared in 1990, and showed the potential for logic programming to be extended. The second issue highlighted some papers from the Practical Applications of Prolog conference that had been held. This third time, the applications are concerned with the Internet and reflect the profound impact that the Internet has had on the computing landscape.

We discuss from a practical point of view a number of issues involved in writing distributed Internet and WWW applications using LP/CLP systems. We describe PiLLoW, a public-domain Internet and WWW programming library for LP/CLP systems that we have designed to simplify the process of writing such applications. PiLLoW provides facilities for accessing documents and code on the WWW; parsing, manipulating and generating HTML and XML structured documents and data; producing HTML forms; writing form handlers and CGI-scripts; and processing HTML/XML templates. An important contribution of PiLLoW is to model HTML/XML code (and, thus, the content of WWW pages) as terms. The PiLLoW library has been developed in the context of the Ciao Prolog system, but it has been adapted to a number of popular LP/CLP systems, supporting most of its functionality. We also describe the use of concurrency and a high-level model of client-server interaction, Ciao Prolog's active modules, in the context of WWW programming. We propose a solution for client-side downloading and execution of Prolog code, using generic browsers. Finally, we also provide an overview of related work on the topic.

This paper presents the multi-threading and internet message communication capabilities of Qu-Prolog. Message addresses are symbolic and the communications package provides high-level support that completely hides details of IP addresses and port numbers as well as the underlying TCP/IP transport layer. The combination of the multi-threads and the high level inter-thread message communications provide simple, powerful support for implementing internet distributed intelligent applications.

Most cities in Germany regularly publish a booklet called the Mietspiegel. It basically contains a verbal description of an expert system. It allows the calculation of the estimated fair rent for a flat. By hand, one may need a weekend to do this task. With our computerized version, the Munich Rent Advisor, the user just fills in a form in a few minutes, and the rent is calculated immediately. We also extended the functionality and applicability of the Mietspiegel so that the user need not answer all questions on the form. The key to computing with partial information using high-level programming was to use constraint logic programming. We rely on the Internet, and more specifically the World Wide Web, to provide this service to a broad user group, the citizens of Munich and the people who are planning to move to Munich. To process the answers from the questionnaire and return its result, we wrote a small simple stable special-purpose web server directly in ECLiPSe. More than 10,000 people have used our service in the last three years. This article describes the experiences in implementing and using the Munich Rent Advisor. Our results suggest that logic programming with constraints can be an important ingredient in intelligent internet systems.

LogicWeb mobile code consists of Prolog-like rules embedded in Web pages, thereby adding logic programming behaviour to those pages. Since LogicWeb programs are downloaded from foreign hosts and executed locally, there is a need to protect the client from buggy or malicious code. A security model is crucial for making LogicWeb mobile code safe to execute. This paper presents such a model, which supports programs of varying trust levels by using different resource access policies. The implementation of the model derives from an extended operational semantics for the LogicWeb language, which provides a precise meaning of safety.

We describe a scheme for moving living code between a set of distributed processes coordinated with unification based Linda operations, and its application to building a comprehensive Logic programming based Internet programming framework. Mobile threads are implemented by capturing first order continuations in a compact data structure sent over the network. Code is fetched lazily from its original base turned into a server as the continuation executes at the remote site. Our code migration techniques, in combination with a dynamic recompilation scheme, ensure that heavily used code moves up smoothly on a speed hierarchy while volatile dynamic code is kept in a quickly updatable form. Among the examples, we describe how to build programmable client and server components (Web servers, in particular) and mobile agents.