# A Survey on Data Storage and Information Discovery in the WSANs-Based Edge Computing Systems
## 3 Analysis of the Data-Storage and Information-Discovery Schemes Proposed for WSANs
### Schemes Based on the Event-Driven Model
By making the clearing grids form a closed source-centered balloon that packages all the cleared grids, ballooning achieves the aim that any contaminated grid is not adjacent to any cleared one, since they are separated by the clearing grids inside the balloon. As the closed balloon grows larger and larger, the latest actuator will be discovered once it is covered by the clearing grids. The ballooning strategy is illustrated in Figure 4.  
Although using the ballooning strategy can ensure that the actuators can be discovered within a latency bound, there is still big space to improve its energy efficiency because of the inflation of the balloon in many directions. The worst case is that the actuator is discovered at the boundary of the network field so that the discovery message has to be broadcasted almost every node in the network during the inflation of the balloon. Moreover, in ballooning, no method is presented to solve the problem how to make the balloon stop inflating at other directions when the latest actuator is discovered at one direction. In 2012, [PERSON] et al. proposed another location-searching protocol, namely MLS (Mobility Location Service) [55]. In MLS, the network field is also divided into grids as in [54], and the only actuator moves in a Random Waypoint Model [56] with no pause time (in the Random Waypoint Model, an actuator repeats to do the following two steps: (1) choose a destination randomly; and (2) move to the destination with a constant speed \(v\) along a straight line.). Each time when the actuator reach a destination (\(x_{0}\), \(y_{0}\)), it disseminates an update package, which includes: (1) the current timestamp \(t_{0}\): (2) the current location (\(x_{0}\), \(y_{0}\)); (3) the destination (\(x_{1}\), \(y_{1}\)); and (4) the moving time \(\tau\), to all the nodes, which act as the location servers during the moving time \(\tau\), in the grids of the same column as the actuator is currently located in. The moving time \(\tau\) can be calculated as follows [55]:  
\[\tau=\frac{\sqrt{\left(x_{1}-x_{0}\right)^{2}+\left(y_{1}-y_{0}\right)^{2}}}{v}. \tag{3}\]  
When a sensor node (the source) detects an event, it forwards the event report at both west and east directions. One of the location servers must receive the event report finally because of the intersection of the rows and the columns. Then, the location server who receives the event report  
Figure 4: The ballooning strategy (the keepers are the sensor nodes who received the searching messages generated by the source; the white grids represent the cleared grids, the deep blue grids are the contaminated grids, and the light blue grids represent the clearing grids) [54]. estimates the current location (\(x^{\prime}\), \(y^{\prime}\)) of the actuator according to Equation (4) [55], where \(t\) is the timestamp when the location server who received the event report begins to estimate the location of the actuator, and then forwards the event report to the actuator using geographic routing protocol [57]. This procedure is shown in Figure 5:  
\[\left\{ \begin{array}{l}x^{\prime}=x_{0}+\frac{t-t_{0}}{\tau}\times\left(x_{1 }-x_{0}\ \right)\\ y^{\prime}=y_{0}+\frac{t-t_{0}}{\tau}\times\left(y_{1}-y_{0}\ \right)\end{array} \right.. \tag{4}\]  
Although the simulation results in [55] show that MLS performs better than some of the existing schemes about location services in WSANs on energy efficiency and scalability, it cannot cover up the obvious drawback of MLS that it requires perfect time synchronization, which is hard to be achieved in WSANs. From the above-mentioned schemes, we can find that it is very important to find out an energy-efficient and delay-bounded strategy to deliver the events detected by the sensor nodes to the actuators or the Sink (for some WSANs which include the Sink) during the procedure of data storage and information discovery in WSANs. In fact, to achieve a QoS (Quality of service)-support routing, another metric should also be considered: the delivery ratio. In 2013, [PERSON] et al.