## 1 Introduction  
The wireless sensor network (WSN) contains numerous sensor nodes deployed densely over the monitored area. Such nodes are low-cost and can communicate with each other via wireless links to send their collected data to the sink node. However, energy conservation is a crucial challenge in designing WSNs due to the nodes' energy constraints. At the same time, since the sensor nodes are usually deployed in hazardous or inaccessible areas, it could be impossible or inconvenient to recharge their batteries. Therefore, the network lifetime and energy saving have great scope in the research field [1,2]. Communication has been proven to be the major source of energy consumption in WSNs [3]. Consequently, many existing routing techniques were used to attempt to find the shortest path for data transmission to minimize energy consumption. However, the shortest path approach did not prolong the network's lifetime. The network's energy usage must be balanced for energy-efficient routing [4, 5, 6]. Although many researchers based their research on the design of routing protocols in WSNs where a single sink is often chosen to prolong the network lifetime [7, 8, 9, 10, 11], the WSNs with a single sink still suffers from many problems. The main problem is that the sensor nodes located close to the sink node become depleted more quickly than the other distant nodes of the network ones as they need to relay more traffic in multi-hop WSNs. Therefore, the energy-hole problem emerged due to the unbalanced energy consumption, which will inevitably affect the lifetime of the whole network [12]. As a result, it may be infeasible to use WSN with a single sink. Therefore, to avoid or bypass the energy hole formation problem, multi-sink strategies for avoiding such problems have been developed [5, 6, 13, 14, 15]. The main idea of such strategies is that multiple sink nodes are located at fixed positions in the monitored field to collect the sensed nodes' data. In addition, energy balancing can be achieved among sensor nodes, and thus the energy-hole problem can be effectively mitigated with multi-sink topology; relatively speaking, it is better than a single sink in a number of ways. Firstly, the network throughput can be improved under a multi-sink environment, whereas if any sink node fails for any reason, the data will be transmitted through other sinks. As the use of multiple sinks decreases, the average distance between sensor nodes and sink nodes, data transmission latency decreases, and the energy consumption is reduced. Last, but not least, deploying multiple sink nodes in the network may help alleviate the traffic congestion issue to some degree [12, 16, 17]. However, the optimum selection of sink nodes is uncertain since the sensor nodes must choose the best sink for data transfer. Another challenging issue is that environmental conditions influence the WSN performance since it is commonly installed in hostile environments, exposing sensor nodes and sink nodes to failure. Thus, the network seldom works. For instance, sensor and sink nodes are more likely to fail at high temperatures and might be entirely burnt out. Moreover, their short-circuits probability would be raised at extremely high humidity [7,15,18]. Thus, reducing network performance consequences such as temperature and humidity is important. Therefore, the data flow should be prevented from transmitting through dangerous areas (for example, areas located in a harsh environment). If the data flow crosses a risky zone, such as a fire zone, data delivery will be cut off due to burned relay nodes. Consequently, environmental awareness should be considered when designing the routing algorithm. Although WSNs are a potential solution for a wide range of real-time applications, such as industrial automation, the sensed data must be delivered to the sink node in real-time (deadline). In this scenario, timely data transfer guarantees that the appropriate actions are made, whereas late data delivery affects the action's efficacy [8].