This topic provides an overview of Server Load Balancer (SLB). SLB distributes inbound network traffic across multiple Elastic Compute Service (ECS) instances that act as backend servers based on forwarding rules. You can use SLB to improve the responsiveness and availability of your applications.


After you add ECS instances that reside in the same region to an SLB instance, SLB uses virtual IP addresses (VIPs) to virtualize these ECS instances into backend servers in a high-performance server pool that ensures high availability. Client requests are distributed to the ECS instances based on forwarding rules.

SLB checks the health status of the ECS instances and automatically removes unhealthy ones from the server pool to eliminate single points of failure (SPOFs). This enhances the resilience of your applications. You can also use SLB to defend your applications against distributed denial of service (DDoS) attacks.


SLB consists of three components:

  • SLB instances

    An SLB instance is a key load-balancing component in SLB. It receives traffic and distributes traffic to backend servers. To get started with SLB, you must create an SLB instance and add at least one listener and two ECS instances to the SLB instance.

  • Listeners

    A listener checks for connection requests from clients, forwards requests to backend servers, and performs health checks on backend servers.

  • Backend servers

    ECS instances are used as backend servers in SLB to receive and process distributed requests. ECS instances can be added to the default server group of an SLB instance. You can also add multiple ECS servers to VServer groups or primary/secondary server groups after the corresponding groups are created.


  • High availability

    SLB features full redundancy that avoids SPOFs and supports zone-disaster recovery. You can use SLB with Apsara Stack DNS to achieve geo-disaster recovery with an availability of up to 99.95%.

    SLB can be scaled based on network traffic to protect your services from outages caused by fluctuating traffic flows.

  • Strong scalability

    You can increase or decrease the number of backend servers to adjust the load balancing capacity for your applications.

  • Low costs

    SLB can save 60% of load balancing costs compared with using traditional hardware solutions.

  • Outstanding security

    You can use SLB with Alibaba Cloud Security to defend your applications against 5 Gbit/s distributed denial of service (DDoS) attacks.

  • High concurrency

    An SLB cluster supports hundreds of millions of concurrent connections, and a single SLB instance supports tens of millions of concurrent connections.