Explanation
SLB and Auto Scaling are two Alibaba Cloud services that can work together to provide high availability, scalability, and performance for your applications. SLB is a load balancing service that distributes incoming traffic across multiple ECS instances. Auto Scaling is a service that automatically adjusts the number of ECS instances based on your business needs. When you use SLB and Auto Scaling together, you need to follow some best practices and precautions to ensure the optimal performance and reliability of your system. One of the precautions is that SLB instances must have Health Check enabled, or they cannot be used together with Auto Scaling1. Health Check is a feature of SLB that monitors the health status of the backend ECS instances.
If an ECS instance becomes unhealthy or fails, SLB automatically stops sending traffic to that instance and redirects the traffic to other healthy instances. This ensures that your application can continue to serve user requests without interruption. Health Check is also essential for Auto Scaling, because Auto Scaling relies on the health status reported by SLB to decide whether to add or remove ECS instances. If an ECS instance is unhealthy, Auto Scaling will automatically replace it with a new one. If the traffic increases and the average health status of the ECS instances decreases, Auto Scaling will automatically add more instances. If the traffic decreases and the average health status of the ECS instances increases, Auto Scaling will automatically remove some instances. Therefore, Health Check is a prerequisite for using SLB and Auto Scaling together. If you disable Health Check, SLB and Auto Scaling will not be able to communicate with each other and coordinate their actions. This may result in performance degradation, service interruption, or resource waste.
The other options are incorrect, because they are not necessary or relevant for using SLB and Auto Scaling together. The backend ECS instances under the same SLB instance can have different operating systems, configurations, and scaling groups, as long as they can provide the same service and respond to the same requests. SLB and Auto Scaling do not impose any restrictions on these aspects of the ECS instances.
References: Associate SLB instances with or disassociate SLB instances from a scaling group