What is Server Load Balancer

Last Updated: Dec 22, 2017

Alibaba Cloud Server Load Balancer is a traffic distribution control service that distributes the incoming traffic among multiple Elastic Compute Service (ECS) instances according to the configured forwarding rules. It expands the service capabilities of the application and increases the availability of the application.

By setting a virtual service IP address, Server Load Balancer virtualizes the ECS instances located in the same region into a high-performance and highly-available application service pool. Client requests are distributed to the ECS instances in the cloud server pool according to the defined forwarding rules.

Server Load Balancer checks the health status of the ECS instances in the cloud server pool and automatically isolates any ECS instances with an abnormal status. This eliminates the single point of failure (SPOF) of an ECS instance and improves the overall service capability. Additionally, Server Load Balancer also provides the capability of defending DDoS attacks, which enhances security of the application.


Server Load Balancer consists of the following components:

  • Server Load Balancer instances: A Server Load Balancer instance is a running load balancing service that receives and distributes the incoming traffic to the backend servers.

    To use the Server Load Balancer service, you must create a Server Load Balancer instance with at least one listener and two ECS instances configured.

  • Listeners: A listener checks the client requests and forwards the requests to the backend servers. It also performs health check on the backend servers.

  • Backend servers: Backend servers are the ECS instances added to a Server Load Balancer instance to process the distributed requests. You can group the ECS instances hosting different applications or functioning different roles into different server groups.

    As shown in the following figure, after the Server Load Balancer instance receives a client request, the listener forwards the request to the corresponding backend ECS instances according to the configured listening rules.



  • High availability

    Server Load Balancer is designed to work in the full-redundancy mode without SPOF. Server Load Balancer supports local and cross-region disaster tolerance. When Server Load Balancer is used together with DNS, the service availability is up to 99.95%.

  • Scalability

    You can scale your service based on the application load, without interrupting services continuity.

  • Low cost

    Compared with the traditional hardware load balancing system, Server Load Balancer reduces the cost by 60%.

  • Security

    Combined with Alibaba Cloud Security, Server Load Balancer can defend against up to 5 Gbps DDoS attacks, such as HTTP flood and SYN flood attacks.

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