# Resource Indexing and Querying in Large Connected Environments
## 3 Related Work
### Summary Table  
In Table 1, we present a comparison between the different approaches based on three criteria.  
The first column indicates whether the approach considers limited device capacities. Approaches that explicitly address device capacity constraints are marked with a \"yes\", while those that do not discuss or consider capacity are marked with a \"no\".  
The second column lists the different types of queries implemented in each paper.  
The third column assesses the level of urgency associated with each query type. The urgency level is defined by the need to return a response, not the time taken to reach the device. Three levels of urgency are used to classify queries:  
* One-path queries use a single path to reach the device, without considering whether the query will return a response. One-path queries have the lowest priority but consume the least amount of resources;
* Retry queries have moderate priority. If no response is returned, there is a probability of finding another path to reach the target destination (e.g., trying to avoid missing data while gathering data). When a blocked path is encountered, another path is searched, starting from the previous node. Since this is a moderate-priority query, multiple paths may be tested, but not all of them, to reduce energy consumption;  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Device Capacities** & **Types of Queries** & **Indexing Coverage** \\ \hline
**GHT/DIFS**[24; 25] & No & One-path & Only Covered zones \\ \hline
**GH-Indexing**[26] & No & One-path & Not explained \\ \hline
**Mod. DP-means**[27] & No & Sequential (One-path) & Only covered zones \\ \hline
**ECH**[28]/**EGF-Tree**[29] & No & One-path & Only covered zones \\ \hline
**DSIS**[30] & Yes (capacity is the number of sensors for WSN) & Guaranteed (Success/Fail) & Not explained \\ \hline
**BCCF-Tree**[31] & Yes (container) & One-path & Not explained \\ \hline
**Multi-index**[33] & No & One-path & Not explained \\ \hline
**A-DBSCAN**[32] & No & One-path & Only covered zones \\ \hline
**Our approach** & yes & One-path, Retry, Guaranteed & Covered and uncovered zones \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison table of indexing approaches for IoT resources.  
* Guaranteed queries will try all possible paths to reach the destination. Consequently, the system will undoubtedly generate a response (if the network allows), but this has the highest resource consumption. The last column evaluates the indexing coverage criteria. We specify whether the approach considers both covered zones (zones containing at least one device) and uncovered zones (zones without any devices). In the first column, one can see that GHT, DIFS, GH-Indexing, DP-means, ECH, EGF, Multi-index, and DBSCAN do not consider device capacities, while in DSIS, the capacity is considered as the number of sensors inside a WSN, and in a BCCF-tree, the capacity is defined by the capacity of containers. On the other hand, most papers used One-path queries, especially methods that rely on clustering the devices, while the DSIS uses guaranteed queries. Finally, all the approaches did not include uncovered zones in their study. In our approach, we consider devices with a limited capacity, which is not the case in most of the existing approaches. We also consider several types of queries (One-path, retry, guaranteed), with each giving different levels of quality of service, in terms of energy consumption and the probability of obtaining an answer. Finally, our approach considers covered and uncovered zones, which is not the case with most of the other approaches.