Publications

2007
S. Idreos, M. L. Kersten, and S. Manegold, “Updating a cracked database,” in Proceedings of the 27th ACM SIGMOD International Conference on Management of Data, Beijing, China, 2007, pp. 413-424.Abstract
A cracked database is a datastore continuously reorganized based on operations being executed. For each query, the data of interest is physically reclustered to speed-up future access to the same, overlapping or even disjoint data. This way, a cracking DBMS self-organizes and adapts itself to the workload. So far, cracking has been considered for static databases only. In this paper, we introduce several novel algorithms for high-volume insertions, deletions and updates against a cracked database. We show that the nice performance properties of a cracked database can be maintained in a dynamic environment where updates interleave with queries. Our algorithms comply with the cracking philosophy, i.e., a table is informed on pending insertions and deletions, but only when the relevant data is needed for query processing just enough pending update actions are applied. We discuss details of our implementation in the context of an open-source DBMS and we show through a detailed experimental evaluation that our algorithms always manage to keep the cost of querying a cracked datastore with pending updates lower than the non-cracked case.
IKM_SIGMOD07.pdf
S. Idreos, M. L. Kersten, and S. Manegold, “Database Cracking,” in Proceedings of the 3rd International Conference on Innovative Data Systems Research (CIDR), Asilomar, California, 2007, pp. 68-78.Abstract
Database indices provide a non-discriminative navigational infrastructure to localize tuples of interest. Their maintenance cost is taken during database updates. In this paper, we study the complementary approach, addressing index maintenance as part of query processing using continuous physical reorganization, i.e., cracking the database into manageable pieces. The motivation is that by automatically organizing data the way users request it, we can achieve fast access and the much desired self-organized behavior. We present the first mature cracking architecture and report on our implementation of cracking in the context of a full fledged relational system. It led to a minor enhancement to its relational algebra kernel, such that cracking could be piggy-backed without incurring too much processing overhead. Furthermore, we illustrate the ripple effect of dynamic reorganization on the query plans derived by the SQL optimizer. The experiences and results obtained are indicative of a significant reduction in system complexity. We show that the resulting system is able to self-organize based on incoming requests with clear performance benefits. This behavior is visible even when the user focus is randomly shifting to different parts of the data.
IKM_CIDR07.pdf
2006
Z. Kaoudi, I. Miliaraki, M. Magiridou, E. Liarou, S. Idreos, and M. Koubarakis, “Semantic Grid Resource Discovery using DHTs in Atlas,” in Knowledge and Data Management in Grids, Springer, 2006.Abstract

We study the problem of resource discovery in the Semantic Grid. We show how to solve this problem by utilizing Atlas, a P2P system for the distributed storage and retrieval of RDF(S) data. Atlas is currently under development in project OntoGrid funded by FP6. Atlas is built on top of the distributed hash table Bamboo and supports pull and push querying scenarios. It inherits all the nice features of Bamboo (openness, scalability, fault-tolerance, resistance to high churn rates) and extends Bamboo's protocols for storing and querying RDF(S) data. Atlas is being used currently to realize the metadata service of S-OGSA in a fully distributed and scalable way. In this paper, we concentrate on the main features of Atlas and demonstrate its use for Semantic Grid resource discovery in an OntoGrid use case scenario.

SemanticGridchapter.pdf
P. - A. Chirita, S. Idreos, M. Koubarakis, and W. Nejdl, “Designing Semantic Publish/Subscribe Networks Using Super-Peers,” in Semantic Web and Peer-to-Peer, 2006, pp. 159-179.Abstract
Publish/subscribe systems are an alternative to query-based systems in cases where the same information is asked for over and over, and where clients want to get updated answers for the same query over a period of time. Recent publish/subscribe systems such as P2P-DIET have introduced this paradigm in the P2P context. In this chapter we built on the experience gained with P2P-DIET and the Edutella super-peer infrastructure and present a semantic publish/subscribe system supporting metadata and a query language based on RDF. We define formally the basic concepts of our system and present detailed protocols for its operation.
SemanticWebP2P.pdf
S. Idreos, C. Tryfonopoulos, and M. Koubarakis, “Distributed Evaluation of Continuous Equi-join Queries over Large Structured Overlay Networks,” in Proceedings of the 22nd IEEE International Conference in Data Engineering (ICDE), Atlanta, Georgia, 2006, pp. 43.Abstract
We study the problem of continuous relational query processing in Internet-scale overlay networks realized by distributed hash tables. We concentrate on the case of continuous two-way equi-join queries. Joins are hard to evaluate in a distributed continuous query environment because data from more than one relations is needed, and this data is inserted in the network asynchronously. Each time a new tuple is inserted, the network nodes have to cooperate to check if this tuple can contribute to the satisfaction of a query when combined with previously inserted tuples. We propose a series of algorithms that initially index queries at network nodes using hashing. Then, they exploit the values of join attributes in incoming tuples to rewrite the given queries into simpler ones, and reindex them in the network where they might be satisfied by existing or future tuples. We present a detailed experimental evaluation in a simulated environment and we show that our algorithms are scalable, balance the storage and query processing load and keep the network traffic low.
JoinsICDE2006.pdf
E. Liarou, S. Idreos, and M. Koubarakis, “Evaluating Conjunctive Triple Pattern Queries over Large Structured Overlay Networks,” in Proceedings of the 5th International Semantic Web Conference (ISWC), Athens, Georgia, 2006, pp. 399-413.Abstract
We study the problem of evaluating conjunctive queries com- posed of triple patterns over RDF data stored in distributed hash tables. Our goal is to develop algorithms that scale to large amounts of RDF data, distribute the query processing load evenly and incur little network traffic. We present and evaluate two novel query processing algorithms with these possibly conflicting goals in mind. We discuss the various tradeoffs that occur in our setting through a detailed experimental evaluation of the proposed algorithms.
ISWC06.pdf
2005
S. Idreos, “Distributed Evaluation of Continuous Equi-join Queries over Large Structured Overlay Networks,” 2005. MasterThesis.pdf
C. Tryfonopoulos, S. Idreos, and M. Koubarakis, “LibraRing: An Architecture for Distributed Digital Libraries Based on DHTs,” in Proceedings of the 9th European Conference on Research and Advanced Technology for Digital Libraries (ECDL), Vienna, Austria, 2005, pp. 25-36.Abstract
We present a digital library architecture based on distributed hash tables. We discuss the main components of this architecture and the protocols for offering information retrieval and information filtering functionality. We present an experimental evaluation of our proposals.
ecdl05.pdf
C. Tryfonopoulos, S. Idreos, and M. Koubarakis, “Publish/subscribe functionality in IR environments using structured overlay networks,” in Proceedings of the 28th Annual International ACM SIGIR Conference, Salvador, Brazil, 2005, pp. 322-329.Abstract
We study the problem of offering publish/subscribe functionality on top of structured overlay networks using data models and languages from IR. We show how to achieve this by extending the distributed hash table Chord and present a detailed experimental evaluation of our proposals.
sigir05.pdf
E. Liarou, S. Idreos, and M. Koubarakis, “Publish/Subscribe with RDF Data over Large Structured Overlay Networks,” in Proceedings of the 3rd International Workshop on Databases, Information Systems and Peer-to-Peer Computing (DBISP2P), 2005, pp. 135-146.Abstract
We study the problem of evaluating RDF queries over structured overlay networks.We consider the publish/subscribe scenario where nodes subscribewith long-standing queries and receive notifications whenever triples matching their queries are inserted in the network. In this paper we focus on conjunctive multi-predicate queries. We demonstrate that these queries are useful in various modern applications e.g., distributed digital libraries or Grid resource discovery. Conjunctive multipredicate queries are hard to answer since multiple triples are necessary for their evaluation, and these triples will usually be inserted in the network asynchronously. We present and evaluate query processing algorithms that are scalable and distribute the query processing load evenly.
DBISP2005.pdf
2004
S. Idreos and M. Koubarakis, “P2P-DIET: Ad-hoc and Continuous Queries in Peer-to-Peer Networks Using Mobile Agents,” in Proceedings of the 3rd Hellenic Conference in Artificial Intelligence (SETN), Samos, Greece, 2004, pp. 23-32.Abstract
This paper presents P2P-DIET, a resource sharing system that unifies ad-hoc and continuous query processing in super-peer networks using mobile agents. P2P-DIET offers a simple data model for the description of network resources based on attributes with values of type text. It also utilizes very efficient query processing algorithms based on indexing of resource metadata and queries. The capability of location-independent addressing is supported, which enables P2P-DIET clients to connect from anywhere in the network and use dynamic IP addresses. The features of stored notifications and rendezvous guarantee that all important information is delivered to interested clients even if they have been disconnected for some time. P2P-DIET has been developed on top of the Open Source mobile agent system DIET Agents and is currently been demonstrated as a file sharing application.
SETN2004.pdf
S. Idreos, M. Koubarakis, and C. Tryfonopoulos, “P2P-DIET: An Extensible P2P Service that Unifies Ad-hoc and Continuous Querying in Super-Peer Networks,” in Proceedings of the ACM SIGMOD International Conference on Management of Data, Paris, France, 2004, pp. 933-934. SIGMOD2004.pdf
P. - A. Chirita, S. Idreos, M. Koubarakis, and W. Nejdl, “Publish/Subscribe for RDF-based P2P Networks,” in Proceedings of the 1st European Semantic Web Conference (ESWC), Heraklion, Greece, 2004, pp. 182-197.Abstract
Publish/subscribe systems are an alternative to query based systems in cases where the same information is asked for over and over, and where clients want to get updated answers for the same query over a period of time. Recent publish/subscribe systems such as P2P-DIET have introduced this paradigm in the P2P context. In this paper we built on the experience gained with P2P-DIET and the Edutella P2P infrastructure and present the first implementation of a P2P publish/subscribe system supporting metadata and a query language based on RDF. We define formally the basic concepts of our system and present detailed protocols for its operation. Our work utilizes the latest ideas in query processing for RDF data, P2P indexing and routing research.
ESWC2004.pdf
S. Idreos, M. Koubarakis, and C. Tryfonopoulos, “P2P-DIET: One-Time and Continuous Queries in Super-Peer Networks,” in Proceedings of the 9th International Conference on Extending Database Technology (EDBT), 2004, pp. 851-853. EDBTdemo2004.pdf
S. Idreos, C. Tryfonopoulos, M. Koubarakis, and Y. Drougas, “Query Processing in Super-Peer Networks with Languages Based on Information Retrieval: The P2P-DIET Approach,” in Proceedings of the 1st Peer-to-peer Computing and Databases Workshop (P2P-DB), Heraklion, Greece, 2004, pp. 496-505.Abstract
This paper presents P2P-DIET, an implemented resource sharing system that unifies one-time and continuous query processing in super-peer networks P2P-DIET offers a simple data model for the description of network resources based on attributes with values of type text and a query language based on concepts from Information Retrieval The focus of this paper is on the main modelling concepts of P2P-DIET (metadata, advertisements and queries), the routing algorithms (inspired by the publish/subscibe system SIENA) and the scalable indexing of resource metadata and queries.
P2PDB2004.pdf
2003
S. Idreos, “P2P-DIET: A query and notification service based on mobile agents for rapid implementation of peer-to-peer applications,” 2003. DiplomaThesis.pdf
M. Koubarakis, C. Tryfonopoulos, S. Idreos, and Y. Drougas, “Selective information dissemination in P2P networks: problems and solutions,” ACM SIGMOD Record, Special issue on Peer-to-Peer Data Management, vol. 32, no. 3, pp. 71-76, 2003.Abstract
We study the problem of selective dissemination of information in P2P networks. We present our work on data models and languages for textual information dissemination and discuss a related P2P architecture that motivates our efforts. We also survey our results on the computational complexity of three related algorithmic problems (query satisfiability, entailment and filtering) and present efficient algorithms for the most crucial of these problems (filtering). Finally, we discuss the features of P2P-DIET, a super-peer system we have implemented at the Technical University of Crete, that realizes our vision and is able to support both ad-hoc querying and selective information dissemination scenarios in a P2P framework.

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