Load Balancing Algorithms

Updated on 2022-01-25 GMT+08:00

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ELB supports the following load balancing algorithms:

Weighted round robin: Requests are distributed across backend servers in sequence based on their weights. Backend servers with higher weights receive proportionately more requests, whereas equal-weighted servers receive the same number of requests. This algorithm is often used for short connections, such as HTTP connections.
Weighted least connections: In addition to the number of active connections to each backend server, a weight is assigned to each backend server based on their processing capability. This algorithm is often used for persistent connections, such as database connections.
Source IP hash: The source IP address of each request is calculated using the hash algorithm to obtain a unique hash key, and all backend servers are numbered. The generated key allocates the client to a particular server. This allows requests from different clients to be routed based on source IP addresses and ensures that a client is directed to the same server that it was using previously. This algorithm works well for TCP connections of load balancers that do not use cookies.

Server Weight

Each backend server can be given a numeral value from 0 to 100 to indicate the proportion of requests to receive. Requests will not be routed to the backend server whose weight is 0, when its health check result is meaningless. The following algorithms allow you to set the server weight:

Weighted round robin: If none of the servers have a weight of 0, the load balancer routes requests to these servers using the round robin algorithm based on their weights. When the weights of backend servers are the same, the weights do not take effect, and the load balancer distributes requests using the round robin algorithm.
Weighted least connections: If none of the servers have a weight of 0, the load balancer calculates each server's workload using the formula: Overhead = Number of current connections/Server weight. The load balancer routes requests to the backend server with the lowest overhead in each request distribution.
Source IP hash: The server weight does not take effect. Requests from same IP address are scheduled to the same backend server.

To set the server weight, perform the following operations:

Log in to the management console.
In the upper left corner of the page, click and select the desired region and project.
Click Service List. Under Network, click Elastic Load Balance.
Locate the target load balancer and click its name.
Click Backend Server Groups, locate the target backend server group and then the target server, and click the number in the Weight column to set the server weight.
Click OK.