Backend Server Group Overview

Updated on 2025-01-20 GMT+08:00

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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 a cloud server, 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 cloud servers, supplementary network interfaces, or IP addresses 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	Cloud servers and 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 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
Forwarding Mode	Specifies the forwarding mode used by the load balancer to distribute traffic. There are two options: Load balancing and Active/Standby. Load balancing: Multiple backend servers can be added to this type of backend server group. The load balancer distributes requests across these backend servers based on the load balancing algorithm configured for this backend server group. Active/Standby: Only two backend servers can be added to the backend server group, one acting as the active server and the other as the standby server. The load balancer routes the traffic to the active server if it works normally. If the active server becomes unhealthy, the load balancer then routes the traffic to the standby server.	Creating a Backend Server Group
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
Forward to Same Port	Specifies whether to enable the forward to same port option. After you enable it, you do not need to specify a backend port when you add a backend server. The listener routes the requests to the backend server over the same port as the frontend port. NOTE: This option is available only for TCP, UDP, or QUIC backend server groups associated with a dedicated load balancer.	Creating a Backend Server Group

Backend Server Group and Listener Protocols

You can associate a backend server group with different dedicated load balancers under the same enterprise project or different listeners.

The backend protocol of the new backend server group must match the frontend protocol of the listener as described in Table 4.

**Table 4** The frontend and backend protocol
Load Balancer Specification	Frontend Protocol	Backend Protocol
Network load balancing	TCP	TCP
Network load balancing	UDP	UDP QUIC
Network load balancing	TLS	TLS TCP
Application load balancing	HTTP	HTTP
Application load balancing	HTTPS	HTTP HTTPS gRPC
Application load balancing	QUIC	HTTP HTTPS gRPC

NOTE:

TLS, gRPC, and QUIC will be available in more regions. You can see which regions support them on the console.

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