Arturo Crespo

Routing indices for peer-to-peer systems

Finding information in a peer-to-peer system currently requires either a costly and vulnerable central index, or flooding the network with queries. We introduce the concept of routing indices (RIs), which allow nodes to forward queries to neighbors that are more likely to have answers. If a node cannot answer a query, it forwards the query to a subset of its neighbors, based on its local RI, rather than by selecting neighbors at random or by flooding the network by forwarding the query to all neighbors. We present three RI schemes: the compound, the hop-count, and the exponential routing indices. We evaluate their performance via simulations, and find that RIs can improve performance by one or two orders of magnitude vs. a flooding-based system, and by up to 100% vs. a random forwarding system. We also discuss the tradeoffs between the different RI schemes and highlight the effects of key design variables on system performance.

Semantic overlay networks for p2p systems

In a peer-to-peer (P2P) system, nodes typically connect to a small set of random nodes (their neighbors), and queries are propagated along these connections. Such query flooding tends to be very expensive. We propose that node connections be influenced by content, so that for example, nodes having many Jazz files will connect to other similar nodes. Thus, semantically related nodes form a Semantic Overlay Network (SON). Queries are routed to the appropriate SONs, increasing the chances that matching files will be found quickly, and reducing the search load on nodes that have unrelated content. We have evaluated SONs by using an actual snapshot of music-sharing clients. Our results show that SONs can significantly improve query performance while at the same time allowing users to decide what content to put in their computers and to whom to connect.

WebWriter: a browser-based editor for constructing Web applications

Constructing server-based Web applications requires creating both Web pages and programs that generate Web pages. This requires a knowledge of the Hypertext Markup Language (HTML), the Common Gateway Interface (CGI) protocol, and a programming language, such as C++, Python, or Perl. While this is not a barrier for programmers, it is for non-programmers. This paper describes WebWriter, an integrated system for constructing Web applications that supports the creation of Web pages by non-programmers. WebWriter includes a direct manipulation Web page editor, the WebWriter Editor, which runs in a Web browser as a CGI service, and the WebWriter Page Generator, which creates new pages as an application runs. As in HyperCard, users create a Web application as a stack of pages, where each page can contain output regions that are filled in at runtime by a script. This paper describes the WebWriter system, issues of server-based authoring tools, and some example applications.

Query merging: improving query subscription processing in a multicast environment

This paper introduces techniques for reducing data dissemination costs of query subscriptions in a multicast environment. The reduction is achieved by merging queries with overlapping, but not necessarily equal, answers. The paper formalizes the query-merging problem and introduces a general framework and cost model for evaluating merging. We prove that the problem is NP-hard and propose exhaustive algorithms and three heuristic algorithms: the pair merging algorithm, the directed search algorithm, and the clustering algorithm. We develop a simulator, which uses geographical queries as a representative example for evaluating the different heuristics and show that the performance of our heuristics is close to optimal.

Archival storage for digital libraries

We propose an architecture for Digital Library Repositories that assures long-term archival storage of digital objects. The architecture is formed by a federation of independent but collaborating sites, each managing a collection of digital objects. The architecture is based on the following key components: use of signatures as object handles, no deletions of digital objects, functional layering of services, the presence of an awareness service in all layers, and use of disposable auxiliary structures. Long-term persistence of digital objects is achieved by creating replicas at several sites.

Implementing a Reliable Digital Object Archive

An Archival Repository reliably stores digital objects for long periods of time (decades or centuries). The archival nature of the system requires new techniques for storing, indexing, and replicating digital objects. In this paper we discuss the specialized indexing needs of a write-once archive. We also present a reliability algorithm for effectively replicating sets of related objects. We describe a data import utility for archival repositories. Finally, we discuss and evaluate a prototype repository we have built, the Stanford Archival Vault (SAV).

Peer-to-peer research at Stanford

n this paper we present recent and ongoing research projects of the Peers research group at Stanford University.

Cost-driven design for archival repositories

Designing an archival repository is a complex task because there aremany alternative configurations, each with different reliability levels and costs. In this paper we study the costs involved in an Archival Repository and we introduce a design framework for evaluating alternatives and choosing the best configuration in terms of reliability and cost. We also present a new version of our simulation tool, ArchSim/C that aids in the decision process. The design framework and the usage of ArchSim/C are illustrated with a case study of a hypothetical (yet realistic) archival repository shared between two universities.

Efficient Query Subscription Processing in a Multicast Environment

This paper introduces techniques for reducing data dissemination costs of query subscriptions. The reduction is achieved by merging queries with overlapping, but not necessarily equal, answers. The paper formalizes the query-merging problem and introduces a general cost model for it. We prove that the problem is NP-hard and propose exhaustive algorithms and three heuristic algorithms: the Pair Merging Algorithm, the Directed Search Algorithm and the Clustering Algorithm. We develop a simulator for evaluating the different heuristics and show that the performance of our heuristics is close to optimal.

Responsive interaction for a large Web application: the meteor shower architecture in the WebWriter II Editor

Traditional server-based web applications allow access to server-hosted resources, but often exhibit poor responsiveness due to server load and network delays. Client-side web applications, on the other hand, provide excellent interactivity at the expense of limited access to server resources. The WebWriter II Editor, a direct manipulation HTML editor that runs in a web browser, uses both server-side and client-side processing in order to achieve the advantages of both. In particular, this editor downloads the document data structure to the browser and performs all operations locally. The user interface is based on HTML frames and includes individual frames for previewing the document and displaying general and specific control panels. All editing is done by JavaScript code residing in roughly twenty HTML pages that are downloaded into these frames as needed. Such a client-server architecture, based on frames, client-side data structures, and multiple JavaScript-enhanced HTML pages appears promising for a wide variety of applications. This paper describes this architecture, the Meteor Shower Application Architecture, and its use in the WebWriter II Editor.