# Scheduling the Exchange of Context Information for Time-Triggered Adaptive Systems
## 2 Related Work
### Agreement Protocols for Time-Triggered Systems  
The idea of the agreement protocol considered in this work does not include security. We limit our discussion of the agreement regarding adaptation services for time-triggered systems. We discuss the related work based on three contexts: _Who is adapting? To what are we adapting? When do we adapt?_  
In [24], it is argued that an agreement protocol is required for a distributed system to maintain a consistent view of the order of non-sparse events. An agreement protocol requires the exchange of context information between processing elements. Each processing element acquires a different local view of the state information of other processing elements. After this, every processing element possesses the same information as every other. The processing elements apply a deterministic algorithm to this consistent information to reach the same conclusion about assigning the event to an active interval of the sparse time base.  
A system-wide, fault-tolerant state agreement protocol for NoCs in a time-triggered MPSoC is proposed in [13; 14]. The works separate the network of context information exchange from the application network to minimize energy consumption. It addresses the adaptation trigger in response to slack and failure scenarios (_to what we adapt_). The work is based on the premise that synchronous adaptable events have a predictable offset between the potential occurrence and the start of an agreement protocol. Therefore, the agreement protocol is scheduled to fit the adaptable event's volatility. However, the work does not present a suitable solution for when to run the agreement protocol for adaptable events that occur asynchronously, a challenge for _when to adapt_.  
Another overhead introduced in [13] is that all processing elements participate in each run of the agreement protocol. Certain adaptable events do not require a global adaptation by all processing elements. Therefore, running the agreement protocol for unaffected processing elements creates communication overheads and results in unnecessary computational costs. Hence, the question of who should participate in information exchange within the agreement protocol must be addressed.  
In contrast, we propose a solution that triggers adaptation services only for the affected processing elements, thereby minimizing unnecessary computation and communication overhead from unaffected processing elements. Therefore, we solve the challenge of _who should adapt_. Also, we develop an algorithm to schedule the exchange of context within the schedules computed for adaptation. This approach contrasts with the periodic triggering, which has a loose coupling with the adaptation schedules and a high communication cost. Our proposed solution reduces the computation and communication overhead, addressing the challenge of _when to adapt_. Furthermore, given the question of _what to adapt_, our approach uses a metascheduler to compute schedules required for adaptation to slack, failure, and environmental or user-defined scenarios.  
Although our agreement protocol does not focus on security, similar coordination challenges have been studied in secure communication systems. For example, the work in [25] proposes secure multi-destination data transmission using relay selection and interference to protect against eavesdropping. While it addresses coordination and communication overhead, it does not consider globally scheduled context exchange or provide timing guarantees. In contrast, our approach schedules context exchange as part of the global system schedule, ensuring predictable timing, reduced overhead, and reliable adaptation, without targeting security.