Overview

Introduction

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 process describes how a backend server group forwards traffic:

A client sends a request to your application. The listeners added to your load balancer use the protocols and ports you have configured forward the request to the associated backend server group.
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.
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.

Shared load balancers have only one type of backend server group, where you can only add cloud servers.

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

Figure 1 Backend server group architecture

**Table 1** Backend server group types
Backend Server Group Type	Backend Server Type	Example	Reference
Hybrid	ECSs, BMSs, or supplementary network interfaces 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 servers or supplementary network interfaces in VPC1. In backend server group B, you can add IP addresses in VPC2 as backend servers.	Adding Backend Servers Adding Supplementary Network Interfaces Adding IP Addresses as Backend Servers
IP as a backend server	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.	Adding IP Addresses 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.

Key Functions

You can configure the key functions listed in Table 2 for each backend server group to ensure service stability.

**Table 2** Key functions
Key Function	Description	Detail
Health Check	Specifies whether to enable the health check option. Health checks determine whether backend servers are healthy. If a backend server is detected unhealthy, it will not receive requests from the associated load balancer, improving your service reliability.	Health Check
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 backend servers in this mode. When the slow start duration elapses, the load balancer sends full share of requests to backend servers and exits the slow start mode. NOTE: Slow start is only available for HTTP and HTTPS backend server groups of dedicated load balancers.	Slow Start (Dedicated Load Balancers)