## 5 Multi-Replication in Data Centric Storage
### Double Rulings for Information Brokerage in Sensor Networks  
Previous works employ structured grid-based replication. This work [35] is a much more complex approach that uses circumferences to replicate the data. The basic idea is: a producer generates a big circumference between itself and the home node, storing the information in all the nodes along the path. A consumer for that event type does exactly the same, so that, at some point, this two big circumferences crosses each other (at least in the home node). Thus, the node located in the crossing can reply to the consumer with the data.  
It is obvious that all the production curves related to a given event type have at least a common node, which is the home node. However, the paper demonstrates that there is a second point where all the production circumferences for a given event type collide. Therefore, there are at least two nodes that store all the events of a given type, meaning two global replication nodes (these are always two in contrast with multi-replication GHT and ToW that can adapt that number).  
Another contribution of this paper is that it defines several ways to retrieve the data depending on the application's interest:  
* **GHT retrieval**: A consumer accesses one of the two home nodes (the closest one) and retrieve all the information.
* **Distance-Sensitive retrieval**: It defines a mechanism to assure an upper bound distance when a consumer wants to retrieve the data from a particular producer, then the consumer has to find the cross between its consumption circumference and the suitable production circumference.
* **Aggregated data retrieval**: The consumer uses a closed curve that separates both home nodes, collects all relevant data and computes the aggregates.
* **Double rulings retrieval**: It defines the full power data retrieval rule. That is, the consumer travels along any great circle and is able to retrieve all the data stored in the sensornet.  
The authors compares its proposal in front of GHT without replication [9] and GHT with replication [30; 31] being the network depth 1 (\(d\)=1). Table 2 shows the average producer and consumer costs in hop numbers for the three approaches. The authors conclude that double rulings clearly outperforms GHT without and with replication. However, this may be not true in all the cases, since it very much depends on the number of producers' events and consumers' queries. For instance in a scenario with many producers' events and just few consumers' queries, GHT without replication would provide better results than Double Rulings just by following the data provided in the paper.  
\begin{table}
\begin{tabular}{|c|c|c|} \hline  & Producer Cost & Consumer Cost \\ \hline GHT without replication & 44.9 & 45.6 \\ \hline GHT with replication (\(d=1\)) & 172.4 & 17.4 \\ \hline Double Rulings & 77.5 & 29.0 \\ \hline \end{tabular}
\end{table}
Table 2: Average producer and consumer costs in hop numbers for GHT, multi-replication GHT and Double Rulings.  
The main drawback of this paper is the complexity of implementing the proposed routing. Forwarding messages following a curve is far from being the standard routing mechanism and surely adds complexity to the implementation. Furthermore, as it has been already mentioned, Double Rulings can only employ 2 global replicas, which could be not enough in some scenarios, whereas other solutions like ToW or GHT with multiple replicas could adapt the number of replicas to the existing traffic load.