## 2 Preliminaries
### Data Distribution Service Middleware  
A data distributed service is commonly regarded as the ideal middleware for unmanned aerial formation control due to its ability to handle data as a result of real-time mission-critical and publication-subscription systems. It also has a pliable established policy setting for quality of service (QoS) that may be used for a variety of applications. DDS middleware is deemed in our investigation for UAV formation control because it gives almost all performance measures. The DDS middleware is built on the concept of a subscription-publishing, by postulating some of these UAVs acting as publishers and others as subscribers collaborating with each other. A publish-subscribe structure en courages flexible and dynamic loose coupling between the data architecture. DDS-based systems can easily be adapted and applied to different conditions and criteria. In addition, a publish-subscribe structure is where multiple subscribers and publishers share highly typed information across a common topic. A reliable quality of service model manages the communication. These QoS regulations might be used separately or as a set, in order to influence different aspects of communications, such as reliability, durability, and data persistence. These QoS regulations sets are a feature of the DDS not available in all other middleware approaches for communication. They guarantee high reliability, performance, and low-latency integration of DDS data transmission. On the other hand, The DDS middleware has some drawbacks, such as that QoS policies are only enforced in a tight DDS framework. In addition, DDS requires double bandwidth compared to MQTT middleware. Furthermore, Web applications are not supported by DDS. To solve this, the OMG designed a web-enabled DDS standard. A web application can be constructed utilizing this interface that interacts directly with the DDS system via a gateway. The following are a customizable set of quality of service settings criteria that must be completed prior to the interaction between both the UAV subscriber and the UAV publisher:  
* **Durability**: Indicates whether or not the publisher formerly transferred information to a new subscriber.
* **Reliability**: Can decide whether the DDS should retransmit information that has been lost over the middleware. Reliability can be configured in two ways: RELIABLE (retransmit missing information) or BEST EFFORT (do not retransmit missing information). With this QoS, the DDS middleware can prevent the formation system from losing the information caused by the DoS attacks.
* **History**: Indicates whether or not data receipted or transmitted by a publisher would be kept for a subscriber. History can be configured in two ways: KEEP ALL and KEEP LAST. The first option does not store an infinite amount of data.
* **Deadline**: Sets the maximum period for specimens to arrive for subscribers.
* **Ownership**: Can decide whether or not a subscriber would receive new samples that were collected from different publishers at the same time. There are two types of ownership: EXCLUSIVE and SHARED.
* **Presentation**: Manages how the middleware presents the information received by the subscriber.
* **Time-based Filter**: Confirms the minimum time duration between the current information and the future information received by the subscriber.
* **Latency budget**: Indicates the time allowed by the middleware to deliver the information.
* **Lifespan**: Determines for how long the middleware will accept the validity of the information sent by the publisher.
* **Resource Limits**: Sets the memory space that a publisher or subscriber could assign for storing information in cache memory.