All Products
Search
Document Center

Server Load Balancer:ALB server group overview

Last Updated:Jul 25, 2024

Application Load Balancer (ALB) distributes client requests to one or more backend servers in server groups. ALB checks the availability of backend servers by performing health checks. You must specify a server group when you add a listener to ALB. After you create a listener, the listener uses the protocol and the port that you specify to check for connection requests and forward the requests to the associated server group.

ALB server group types

For more information about how to create an ALB server group and add backend servers, see Create and manage a server group.

Server group type

Backend server type

Description

References

Server

Allows you to specify Elastic Compute Service (ECS) instances, elastic network interfaces (ENIs), and elastic container instances as backend servers.

The backend servers and the server group must belong to the same virtual private cloud (VPC). The backend servers are used to receive requests distributed by ALB.

For more information about how to specify ECS instances as backend servers, see the following topics:

IP

Allows you to specify IP addresses as backend servers.

  • If you enable the remote IP address feature, IP addresses that fall into the following CIDR blocks are supported:

    • 10.0.0.0/8

    • 100.64.0.0/10

    • 172.16.0.0/12

    • 192.168.0.0/16

  • If you do not enable the remote IP address feature, only IP addresses that fall into the CIDR block of the VPC in which the server group is created are supported.

For more information about how to add backend servers across regions, see the following topics:

Function Compute

Allows you to specify functions of Function Compute as backend servers.

You must activate Function Compute. The functions that you want to add must belong to the same region as the ALB instance.

Specify a function from Function Compute as a backend server of ALB

Important

After a backend server of an ALB instance is released or after the private IP address of a backend server is modified, ALB does not update the status of the backend server. When you release or modify an ALB backend server, we recommend that you remove the backend server from the ALB server group to ensure that your business is not affected.

Scheduling algorithms

This section describes the scheduling algorithms that are supported by ALB. For more information, see SLB scheduling algorithms.

  • Weighted Round-robin: Backend servers that have higher weights receive more requests than backend servers that have lower weights.

  • Weighted Least Connections: Requests are forwarded based on the weights and number of connections to backend servers. If two backend servers have the same weight, the backend server that has fewer connections receives more requests.

  • Consistent Hash: Requests from the same source IP address are forwarded to the same backend server.

    Hash Factor: Select a hash factor.

    • Source IP: Requests from the same source IP address are forwarded to the same backend server.

    • URL Parameter: Requests for the same URL are forwarded to the same backend server. If you select this operation, configure the Specified URL parameter.

Backend protocols and health check protocols

The following table describes the server groups and the health check protocols that are supported by different ALB listeners.

Listener protocol

Backend protocol of server group

Server group type

Health check protocol

HTTP

HTTP and HTTPS

Server type, IP type, and Function Compute type

Note

If the backend server group type is Function Compute, you do not need to specify a backend or health check protocol.

HTTP, HTTPS, TCP, and gRPC

Note

Standard and Web Application Firewall (WAF)-enabled ALB instances support HTTPS health checks. Basic ALB instances do not support HTTPS health checks.

HTTPS

HTTP, HTTPS, and gRPC

Note
  • If you select gRPC for the backend servers, make sure that the listeners use HTTPS and have HTTP/2.0 enabled.

  • You can associate only server groups that use HTTPS or gRPC with HTTPS listeners of basic ALB instances.

QUIC

HTTP

Health checks

You can configure health checks to monitor the conditions of server groups. This helps you evaluate the availability of backend servers in server groups.

ALB allows you to configure health checks for server groups. By default, health checks are enabled for all server groups.

  • When health checks are enabled, ALB automatically routes requests to healthy backend servers and probes the availability of all backend servers at a specified interval. A backend server must pass health checks for a specific number of times (N times) before the backend server is declared healthy. You can specify N based on your business requirements. This prevents health check errors caused by network jitter.

    • If a backend server fails health checks for a specific number of times, the backend server is declared unhealthy. In this case, ALB automatically stops distributing requests to the backend server.

    • After the backend server recovers, ALB automatically distributes requests to the backend server.

    • Health checks use non-persistent connections. The connections are closed after health checks are completed.

Note

Health checks are not performed on backend servers whose weights are 0.

For more information, see Health checks.

Session persistence

By default, ALB distributes requests to different backend servers. After you enable the session persistence feature, requests from the same client are forwarded to the same backend server. This allows the backend server to maintain status information and provide continuous service to the client.

  • Session persistence disabled: Requests from the same client may be distributed to different backend servers of ALB. In some scenarios, such as scenarios in which you log on to a backend server to obtain interaction information, you may need to log on to the backend server multiple times.

  • Session persistence enabled: Requests from the same client are distributed to the same backend server of ALB. In some scenarios, such as scenarios in which you log on to a backend server to obtain interaction information, you do not need to log on to the backend server multiple times.

Server groups of the Function Compute type do not support session persistence.

For more information, see Configure session persistence.

Persistent connections on backend servers

If persistent connections are enabled, ALB maintains a certain number of connections to backend servers. Requests are preferentially distributed to idle TCP persistent connections to reduce the number of TCP handshakes. This reduces the loads on backend servers.

Server groups of the Function Compute type do not support persistent connections.

For more information, see Create a server group.