What is Server Load Balancer

Last Updated: Sep 27, 2017

Alibaba Cloud Server Load Balancer is a traffic distribution control service. It distributes the incoming application traffic among multiple ECS instances according to a scheduling algorithm and listening rules.

By setting a virtual IP address, the Server Load Balancer service virtualizes the ECS instances located in the same region into a high-performing and highly available application service pool. Client requests are distributed to the cloud server pool according to the defined listening rules. This increases the fault tolerance of your applications.

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 resolves the single point of failure (SPOF) problem and improves the overall service capability.

In addition to standard load balancing capabilities, TCP and HTTP listening can defend DDoS attacks, enhancing the protection capability of the application servers.


The Server Load Balancer service consists of three parts: a Server Load Balancer instance, one or more listeners, and multiple backend servers.

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.


  • Server Load Balancer instance: If you want to use the Server Load Balancer service, you must create a Server Load Balancer instance. You can add multiple listeners and backend servers to a Server Load Balancer instance.

  • Listener: Before using Server Load Balancer, you must add at least one listener to the Server Load Balancer instance, which defines how the client requests are forwarded to the backend servers.

  • Backend server: The ECS instances added to the Server Load Balancer instances are the backend servers used to process the distributed requests. You can add the ECS instances separately to the backend server pool, or add them in a batch through a VServer group or master-slave server group.

    By default, the backend servers are maintained in the Server Load Balancer instance dimension. All listeners can only forward requests to the same ECS instances with the same backend port configured in the listeners. With the VServer group function, you are allowed to maintain the backend servers in the listener dimension. You can create different VServer groups for different listeners, that is, the listeners in a Server Load Balancer instance can forward requests to different backend servers through different ports.

    Additionally, the layer-7 load balancing service supports the configuration of domain names or URL forwarding rules. The listener can forward the requests from different domain names or URLs to different VServer groups.


  • High availability

    • Designed to work in full-redundancy mode without SPOF. Server Load Balancer supports local and cross-region disaster tolerance when used together with DNS, delivering service availability of up to 99.95%.

    • Server Load Balancer can flexibly scale its service, based on the application load without interrupting external services during traffic fluctuation.

  • Low cost

    Server Load Balancer is 60% more cost-efficient than traditional hardware load-balancing systems. By providing free access to private network instances without generating any O&M cost, the service completely removes the need to purchase expensive load-balancing equipment.

  • Security

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

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