International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August - 01 September 2012, Melbourne, Australia 
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temporal criteria and then receive near real-time notifications 
containing the data matches the criteria. The remainder of this 
section introduces works related to this paper. Then in section 
2, we present the proposed system including the system 
architecture and details of each component. Section 3 shows 
and discusses the preliminary experimental results. Finally, 
section 4 offers conclusions and future work. 
1.2 Related Works 
Publish/Subscrihe is a communication model decoupling 
publishers and subscribers. Subscribers first register their event 
of interest, and asynchronously get notifications of events 
generated by publishers. Unlike the point-to-point synchronous 
communication model, the asynchronous publish/subscribe 
model is more suitable for large-scale distributed applications. 
For example, publish/subscribe model has been widely applied 
in web blogging with RSS 1 (RDF Site Summary) and Atonr 
technologies. Eugster et al. (2003) wrote a well-cited summary 
paper about publish/subscribe systems. 
Besides the publish/subscribe system, there are other types of 
applications aim on processing streaming data or events, such as 
complex event processing (CEP), data stream management 
system (DSMS) (similar to event stream processing (ESP)), and 
simple event processing. The original designs of these 
applications were different. However, as the evolving of these 
applications, their functionalities become similar. 
Based on their original designs, we can differentiate these 
applications by the degree of query complexity they handle. In 
general, publish/subscribe systems handled the simplest queries 
and simple event processing added filtering functionality on the 
basic publish/subscribe. While publish/subscribe and simple 
event processing focused on individual events, DSMS and CEP 
processed multiple data streams. 
There have been many publish/subscribe systems and stream 
processing systems. For example, Birman et al. (1987), Powell 
(1996), Skeen (1998), TIBCO (1999), Siena (Carzaniga et al. 
2001), JEDI (Cugola et al. 2001), and Hermes (Pietzuch 2004) 
are the existing works on publish/subscribe system. NiagaraCQ 
(Chen et al. 2000), TelegraphCQ (Madden and Franklin 2002; 
Chandrasekaran et al. 2003), COUGAR (Bonnet et al. 2001), 
PLACE (Mokbel et al. 2005), Tapestry (Terry et al. 1992), 
Cayuga (Brenna et al. 2007), StreamBase (2011), Oracle CEP 
(2009), Esper (2012), IBM System S (Gedik et al. 2008), and 
Microsoft Streamlnsight (2012) are the researches related to 
data stream processing system. 
At the current stage, publish/subscribe systems and DSMS are 
similar in terms of their high level architectures and 
functionalities. They both allow users to register queries, and 
allow data providers to push data to the system through an input 
adaptor. Then they both have a continuous query engine to 
match the new coming data with the predefined queries. Finally, 
they both have an output adaptor to disseminate notifications to 
users. 
However, as mentioned before, one of the major reasons that 
publish/subscribe system is difficult to be applied for sensor 
webs is that most of current sensor web services only support 1 2 
1 RSS 2.0 Specification (http://www.rssboard.org/rss- 
specification) 
2 Atom wiki (http://www.intertwingly.net/wiki/pie/FrontPage) 
pull-based communication model. In other words, there is no 
suitable sensor data source for a traditional publish/subscribe 
system as they require data to be pushed from data sources. 
Therefore, in order to address this issue, this paper proposes a 
solution that modifies the input adaptor module in a 
publish/subscribe system. Instead of only accept pushed data, 
the input adaptor generates requests that pull data from data 
sources, and then it pushes the new data to the next module in 
the publish/subscribe system. Hence, we this proposed solution 
as a hybrid pull-push system. 
2. PROPOSED SOLUTION 
This paper focuses on the input adaptor module in a 
publish/subscribe system to retrieve sensor data from pull-based 
sensor web services in a timely manner. Other modules of a 
publish/subscribe system, e.g., continuous query engine, are out 
of the scope of this paper. 
2.1 System Architecture 
The proposed input adaptor has three major components, 
namely (1) query aggregator, (2) adaptive feeder, and (3) sensor 
data cache. With the queries users submit, the query aggregator 
first aggregates queries in order to avoid redundant requests. 
Then the adaptive feeder tries to get new data with the 
aggregated queries in a timely manner. Finally, the sensor data 
cache is where the system keeps the sensor data according to the 
query criteria. The workflow and architecture is shown in 
Figure 1. 
In this paper, we use OGC SOS as the sensor data source as it is 
one of the most popular open standards to share sensor data 
online. In addition, OGC SOS also only supports pull-based 
interaction model, which matches the major issue we mentioned 
earlier. 
Figure 1. System architecture and workflow 
In the following two subsections, we present the details of the 
query aggregator and the adaptive feeder. 
2.2 Query Aggregator 
Before presenting the functionality of the query aggregator, we 
need to discuss what a query is in the sensor web context. Since 
sensors measure a specific physical phenomenon (e.g., wind 
speed) at a certain geographical location and time point, each 
sensor reading contains at least the following five elements, 
namely, a physical phenomenon identifier, a measurement 
value, a unit of measurement, a geographical location, and a