Matching Over Linked Data Streams in the Internet of Things A new system increases efficiency by integrating Linked Data streams Physical-Cyber-Social Computing generated from data collectors and disseminating matched data to relevant data consumers, based on user queries. The data structure, called TP-automata, suits the needs of high-performance Linked Data stream dissemination. Here, the authors use a real-world dataset generated in a Smart Building IoT Project to evaluate the system. Results show that the system can disseminate Linked Data Streams at one million triples per second with 100,000 registered user queries, which is several orders of magnitude faster than existing techniques. Yongrui Qin and he Internet is a global system of see, hear, and sense the real world. Erics- Quan Z. Sheng networks interconnecting com- son predicts that the number of Internet- University of Adelaide puters using the standard Internet connected things will reach 50 billion T 1 protocol suite. It has significant impact by 2020. Edward Curry on the world, serving billions of users Connecting all the things that peo- National University of Ireland, worldwide. Millions of private, public, ple care about becomes possible in Galway academic, business, and government the IoT, and this leads to vast scales of networks — from local to global in scope — real-time data. By exploiting such data, all contribute to the Internet’s formation. cities will become smarter and more It’s a network of networks, and each net- efficient. Some promising IoT appli- work connects various computers. Hence, cations in future smart cities include the traditional Internet has a focus on aiding resource-management issues,2 computers and can be called the Internet effectively managing street parking for of Computers. In contrast, the Internet of reducing traffic congestion and fuel con- Things (IoT) aims to connect everyday sumption,3 efficiently distributing drink- objects — such as coats, shoes, watches, ing water, tracking and recovering stolen ovens, washing machines, bikes, cars, property,1 and so on. humans, plants, animals, and changing Making IoT’s potential a reality environments — to the Internet to enable requires that we manage and process communications and interactions.1 IoT’s data efficiently and effectively. Given ultimate goal is to enable computers to the scale of data generated in IoT, topics 2 Published by the IEEE Computer Society 1089-7801/15/$31.00 © 2015 IEEE IEEE INTERNET COMPUTING Matching Over Linked Data Streams in the Internet of Things Related Work on Linked Data Stream Dissemination ecent work in data summaries on Linked Data1 trans- thematic event processing4 apply semantic matching. These R forms Resource Description Framework (RDF) triples techniques also return false-negative matching results, which into numerical space. Then data summaries are built upon aren’t allowed in our system. numerical data instead of strings, as summarizing numbers is Moreover, existing work on pattern matching, such as more efficient than summarizing strings. To transform triples stream reasoning5 and Linked Data stream processing,6 doesn’t into numbers, you apply hash functions on the individual com- support large-scale query evaluation, instead focusing on evalu- ponents (s, p, o) of triples. Thus, you can consider a derived ation of a single query or a small number of parallel queries triple of numbers as a 3D point. In this way, you can map a set over the streaming Linked Data. Therefore, the issue of sup- of RDF triples into a set of points in a 3D space. To facilitate porting pattern matching over a large number of BGPs against query processing over data summaries, one group of research- Linked Data streams remains open. ers adopted a spatial index named QTree1 (evolved from stan- dard R-tree2) as the basic index. Data summaries are designed References mainly for indexing various Linked Data sources and are used 1. A. Harth et al., “Data Summaries for On-Demand Queries over Linked for identifying relevant sources for a given query. Data,” Proc. Conf. World Wide Web, 2010, pp. 411–420. However, data summaries aren’t suitable for our Linked Data 2. A. Guttman, “R-Trees: A Dynamic Index Structure for Spatial Searching,” stream dissemination system. First, techniques on data summa- Proc. Sigmod, 1984, pp. 47–57. ries, such as QTree, don’t consider variables in the Basic Graph 3. S. Hasan and E. Curry, “Approximate Semantic Matching of Events for the Patterns (BGPs); these techniques only consider RDF triples with Internet of Things,” ACM Trans. Internet Technology, vol. 14, no. 1, 2014, arti- concrete strings. Further, because data summaries are concise cle no. 2. and imprecise representations of data sources,1 they just pro- 4. S. Hasan and E. Curry, “Thematic Event Processing,” Proc. 15th Int’l Conf. vide match estimation. Hence, query evaluation on them would Middleware, 2014, pp. 109–120. return false negative results, which isn’t allowed in our system. 5. D. Anicic et al., “EP-SPARQL: A Unified Language for Event Processing and Researchers also have studied semantic matching, which Stream Reasoning,” Proc. Conf. World Wide Web, 2011, pp. 635–644. aims to match semantically related RDF triples against BGPs. 6. D.L. Phuoc et al., “A Native and Adaptive Approach for Unified Process- This approach might provide false positive match results, but ing of Linked Streams and Linked Data,” Proc. 10th Int’l Conf. Semantic Web, not false negative. Both approximate event matching3 and 2011, pp. 370–388. such as distributed processing, real-time data interconnected things (instead of humans) act stream analytics, and event processing are as the main data producers, as well as the main critical. We might need to revisit these areas data consumers. Computers will be able to learn to improve existing technologies for applica- and gain information and knowledge to solve tions at the IoT scale.4,5 In this context, semantic real-world problems directly with the data fed technologies such as Linked Data, which aim to from things. As an ultimate goal, computers facilitate machine-to-machine communications, enabled by IoT technologies will be able to sense play an increasingly important role.6 Linked and react to the real world for humans. Data is part of a growing trend toward highly To move toward this goal, we must efficiently distributed systems, with thousands or poten- retrieve the most-relevant data from IoT environ- tially millions of independent sources providing ments (see, for example, the process of convert- structured data. In collecting all of this data, ing data into information in Figure 1). Hence, we one challenge is how to efficiently disseminate propose an efficient data stream dissemination the data to relevant data consumers. system for semantic IoT by leveraging semantic Thus, here we focus on the study of IoT from technologies, such as Linked Data. Our system a data perspective. As Figure 1 shows, data are efficiently retrieves relevant data from the deluge processed differently in IoT than in traditional of IoT data, which can then facilitate the extrac- Internet environments (such as the Internet of tion of useful information (for others’ work in Computers). In the Internet of Computers, the this area, see the related sidebar). The system first main data producers and consumers are human integrates data generated from various data col- beings. However, in the IoT, the main actors lectors. Then it transforms all the data to Linked become things, where things are the majority Data streams in Resource Description Framework of data producers and consumers. Therefore, (RDF) format (see www.w3.org/RDF). Meanwhile, in the context of the Internet, addressable and data consumers can register their interest in the maY/June 2015 3 Physical-Cyber-Social Computing Internet of Computers Internet of Things We believe that this research aligns well Physical world with the vision of physical-cyber-social (PCS) computing (see http://wiki.knoesis.org/index. php/PCS). It deals with data from both the phys- ical and cyber worlds. After being disseminated to relevant data consumers, these consumers People Things can integrate such data with information and generate Web & Web of data generate knowledge from the social world to provide bet- ter understanding, correlation, and contextu- ally relevant abstractions to humans. Linked Data Stream Dissemination System People Things To disseminate high-quality information and pro- gain Information gain vide high-performance matching services to data consumers (or subscribers), we aim to design a system that won’t return false-negative match results. Therefore, we investigate pattern matching here. Pattern matching performs individual com- ponent matching between RDF triples and BGPs. People Things It doesn’t consider semantic relatedness between discover Knowledge discover an RDF triple and a BGP. It might return false- positive matching results but not false-negative ones. Recent work on pattern matching includes Linked Data stream processing7 and stream rea- soning.8 However, because these solutions are mainly designed for optimizations of individual People Things query evaluations, they aren’t quite suitable for propose Solutions propose processing a large number of concurrent queries. An example of pattern matching is that pat- tern (?s, :is, :Student) will match triple (:James, :is, :Student) but won’t match (:James, :is, :PhD- Student). Other types of matching include match estimation and semantic matching, both of which might return false-negative results. Again, take Figure 1. Internet of Computers versus the Internet of Things pattern (?s, :is, :Student) as an example. In match (IoT). In the Internet of Computers, the main data producers and estimation, the main task is to estimate which consumers are human beings. However, in the IoT, the main actors dataset matches pattern (?s, :is, :Student) the best become things, where things are the majority of data producers by using some summarization techniques among and consumers.