1.2.2 Open Loop Systems
Open loop control systems are those that do not provide feedback of the actual state of the system in order to adjust the system.
1.2.2.1. Control Theory
The open loop control system is a non-feedback system in which the control input to the system is determined using only the current state of the system and a model of the system. There is no feedback to determine if the system is achieving the desired output based on the reference input or set point. The system does not observe itself to correct itself and, as such, is more prone to errors and cannot compensate for disturbances to the system.
Figure 3. Open Loop Control System
1.2.2.2. Open Loop Dynamic IaaS
When a scale out triggering event occurs, the infrastructure expands its capacity through the appropriate bursting mode, either auto scaling or cloud bursting.
There is no feedback in the system from the usage of the new capacity to tightly control the amount of resources added or decommissioned from the service based on real world service utilization. A model of the service determines the appropriate scaling activities.
For example, a basic model of this service shows that for every 100 active sessions, one virtual machine in the web tier is required to provide a 100ms transaction time. Capacity planning data indicates that 1000 active sessions must be supported during weekdays and 250 active sessions on weekends.
On weekdays the environment scales to 10 virtual machines in the web tier (1000 sessions/100 sessions per virtual machine), and on weekends it scales to three virtual machines in the web tier (250 sessions).
The model describes the number of virtual machines per 100 sessions, but it does not account for rogue sessions that might consume significantly more resources than the typical session. It also does not account for transient spikes in resource consumption that might occur, which makes the 100 sessions per virtual machine model incorrect. In this scenario, the open loop control method does not account for the real-world state of the system, and the end-user experience degrades.
Figure 4. Open Loop Dynamic IaaS
