# 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
### Comprehensive Comparison and Analysis
On the other side, if the query frequencies of the actuators are high, the energy efficiency will be low, especially when the events happen infrequently. Considering the heterogeneous character of WSANs, to make the sensor nodes and the actuators collaborate efficiently on data storage and information discovery, a hierarchical storage model should be more suitable for WSANs. In other words, the sensor nodes are at one level while the actuators are at another level. Secondly, task allocation for the actuators should tend to improve the load-balance performance of the schemes on data storage and information discovery in WSANs. When an event is detected in a WSAN, existing schemes on task allocation require the actuator, which is the closest to the place where the event occurs, to move to the place and deal with the event. As an alternative, choosing actuators randomly to deal with the events may be a better choice from the load-balance point of view. In this way, the distribution of the actuators can be adjusted adaptively according to the data generating rates of the sensor nodes. The final adjusting result should be that the regions with more events happening will attract more actuators. Thus, there will be more nodes with better storage capacity to share the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Systems** & **Energy** & **Real-Time** & **Load** & **Fault** & **Actuator** \\  & **Efficiency** & **Support** & **Balance** & **Tolerance** & **Movement Support** \\ \hline Traditional data-centric WSNs [42] & high & \(\times\) & bad & bad & \(\surd\) \\ Long-term storage WSANs [44] & high & \(\times\) & good & good & \(\surd\) \\ Delay-aware WSNs [45] & high & \(\surd\) & good & good & \(\times\) \\ Ballooning [54] & low & \(\surd\) & good & good & \(\surd\) \\ MLS [55] & low & \(\surd\) & good & good & \(\surd\) \\ LRP-QS [47] & high & \(\surd\) & good & good & \(\times\) \\ REFER (REAL-time, Fault-tolerant & high & \(\surd\) & good & good & \(\times\) \\ and EnEgy-efficient WSAN) [48] & high & \(\surd\) & good & good & \(\times\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performances of different WSANs-based Edge computing systems. storage load in the regions where data-generating rates are high. Moreover, for the sensor nodes that need to store their data on the actuators, choosing actuators randomly to get their data stored will also improve the load balance of the sensor nodes themselves because they should send the data along different routes to reach different actuators in different places randomly. Finally, to achieve the aforementioned multi-objectives for data storage and information discovery, the data generated in WSANs should also be studied. By observation, we find that the data generated in WSANs are different in importance. For example, the outliers are more important than the normal data because the emergency of the outliers implies the abnormal events; the data with small emerging probabilities are more important than those with bigger emerging probabilities because the former contains much more information; the data that meet the interests of the users are more important than those do not meet the interests of the users because only the users are concerned about the data that they are interested in. Moreover, we also find that data with different importance levels (or priorities) have different characteristics and requirements for WSANs. For instance, the data with higher importance levels are generally generated with lower rates, and the total amount of them is relatively small. They have much higher requirements on the real-time performance of the data-storage and information-discovery schemes in WSANs. For the data with lower importance levels, their generating rates are relatively much higher, and they require the data-storage and information-discovery schemes to perform much better on energy efficiency and load balance. As for the data that are not important at all or even invalid [58], the sensor nodes can even drop them to save the resources.