Handle Larger Traffic with Load Balancer – SLB Overview

What is Server Load Balancer?

Server Load Balancer (SLB) is a traffic distribution and control service that distributes inbound traffic among several backend servers, namely ECS instances, based on configured forwarding rules. SLB expands the serving capacity of applications and enhances their availability.

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By setting a virtual service address, SLB virtualizes added ECS instances into a server pool that offers a high level of performance and high availability. SLB distributes client requests to ECS instances in the server pool based on forwarding rules.

SLB also checks the health status of added ECS instances, and automatically isolates unhealthy instances to eliminate Single Point of Failures (SPOF), improving the overall serving capacity of your application. Additionally, working with Alibaba Anti-, SLB can defend against DDoS attacks.


SLB consists of SLB instances, listeners, and backend servers.

  • SLB instances

An SLB is a running load balancing service that distributes incoming traffic to backend servers. To use the SLB service, you must create an SLB instance, and then configure the instance with at least one listener and two backend servers.

  • Listeners

A listener checks client requests and forwards the requests to backend servers based on the configured rules. It also performs health checks on backend servers.

  • Backend servers

Backend servers are the ECS instances added to an SLB instance to process the distributed requests. You can add ECS instances to the default server group, a VServer group, or an active/standby server group for better management.


  • High availability

SLB is designed to work in full-redundancy mode to avoid SPOFs. It supports local and cross-region disaster tolerance. When SLB is used together with DNS, the service availability is up to 99.95%.

You can scale your service based on application loads, without interrupting service continuity.

  • Scalability

You can increase or decrease the number of backend servers as needed to expand the serving capacity of your applications.

  • Cost-effectiveness

Compared with traditional load balancing hardware, SLB can reduce costs by up to 60%.

Combined with Alibaba Cloud Security, SLB can defend against up to 5 Gbit/s DDoS attacks, such as HTTP flood and SYN flood attacks.

  • High concurrency

The SLB cluster supports hundreds of millions of concurrent connections and a single SLB instance supports tens of millions of these connections.

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