Backend Server Group Overview

What Is a Backend Server Group?

A backend server group is a logical collection of one or more backend servers to receive massive concurrent requests at the same time. A backend server can be an ECS, BMS, supplementary network interface, or IP address.

The following table describes how a backend server group forwards traffic.

**Table 1** Traffic distribution process
Step 1	A client sends a request to your application. The listeners added to your load balancer use the protocols and ports you have configured to forward the request to the associated backend server group.
Step 2	Healthy backend servers in the backend server group receive the request based on the load balancing algorithm, handle the request, and return a result to the client.
Step 3	In this way, massive concurrent requests can be processed at the same time, improving the availability of your applications.

For dedicated load balancers, the backend server group type can be Hybrid or IP as a backend server. You can add an ECS, BMS, supplementary network interface, or IP address to a hybrid backend server group. If you set the type to IP as a backend server, you can only add IP addresses as backend servers.

Figure 1 shows the architecture of different types of backend server groups.

Figure 1 Backend server group architecture

**Table 2** Backend server group types
Backend Server Group Type	Backend Server Type	Example
Hybrid	ECSs, supplementary network interfaces, or BMSs that are in the same VPC as the load balancer Cloud servers in other VPCs or on-premises servers if IP as a backend is enabled for the load balancer	As shown in Figure 1: In backend server group A, you can add ECSs or supplementary network interfaces in VPC1. In backend server group B, you can add IP addresses in VPC2 as backend servers.
IP as a backend server	IP addresses of cloud servers in other VPCs or on-premises servers if IP as a backend is enabled for the load balancer	As shown in Figure 1, IP addresses can be added to backend server group C as backend servers.

Advantages

Backend server groups can bring the following benefits:

Reduced costs and easier management: You can add or remove backend servers as traffic changes over the time. This can help avoid low resource utilization and makes it easy to manage backend servers.
Higher reliability: Traffic is routed only to healthy backend servers in the backend server group.

Controlling Traffic Distribution

You can configure the key functions listed in Table 3 for each backend server group to ensure service stability.

**Table 3** Key functions
Key Function	Description	Detail
Load Balancing Algorithm	The load balancer distributes traffic based on the load balancing algorithm you have configured for the backend server group.	Load Balancing Algorithms
Sticky Session	Specifies whether to enable the sticky session option. If you enable this option, all requests from a client during one session are sent to the same backend server.	Sticky Session
Slow Start	Specifies whether to enable slow start. After you enable it, the load balancer linearly increases the proportion of requests to new backend servers in the backend server group. When the slow start duration elapses, the load balancer sends full share of requests to these backend servers and exits the slow start mode.	Slow Start