This topic provides an overview of Server Load Balancer (SLB). SLB is a service that can distribute network traffic to a group of backend servers to increase the throughput of your applications. You can use SLB to prevent service interruption caused by single points of failure (SPOFs) and improve the availability of applications.


Alibaba Cloud provides an SLB service that is fully managed on the cloud. This service is out-of-box, high-throughput, stable, and reliable. You can dynamically scale SLB resources and pay for these resources on demand. Alibaba Cloud SLB is intended for balancing the heavy load of hyper-scale Internet applications. For example, you can use SLB to handle traffic spikes during large-scale online campaigns or improve the availability of Internet of Things (IoT) applications that run with a great number of concurrent connections. SLB saves the expenses on backend servers that are used to encrypt and decrypt data. It can redirect HTTP requests to HTTPS requests or redirect HTTPS requests back to HTTP requests while the backend servers only need to process HTTP requests.

Alibaba Cloud provides two types of SLB instances that run at the transport layer (Layer 4 in the OSI model) and application layer (Layer 7 in the OSI model). The Layer 4 SLB instances use Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) listeners to distribute network traffic. Each Layer 4 SLB instance supports up to tens of millions of concurrent connections and millions of new connections per second.

Layer 7 SLB instances run at the application layer (Layer 7 in the OSI model). Layer 7 SLB instances support various application protocols, such as HTTP, HTTPS, HTTP2, WebSocket Secure (WSS), Quick UDP Internet Connections (QUIC), and Google Remote Procedure Call (gRPC). Each Layer 7 SLB instance can handle up to one million queries per second (QPS). Layer 7 SLB instances support SSL, HTTPS, and TLS offloading.

Compared with hardware load balancing solutions that require large up-front investment, Alibaba Cloud SLB is highly scalable and provides pay-as-you-go services. It can balance the heavy load of applications during large-scale online campaigns. Compared with self-managed open source load balancing solutions, Alibaba Cloud SLB has an experienced and professional technical support team that offers 24/7 services. This ensures service stability and relieves you from operations and maintenance (O&M) work.

SLB instance types

Alibaba Cloud provides two types of SLB instances:

  • Classic Load Balancer (CLB): formerly known as SLB. CLB instances support TCP, UDP, HTTP, and HTTPS. CLB instances also provide powerful processing capabilities for Layer 4 load balancing and basic processing capabilities for Layer 7 load balancing.
  • Application Load Balancer (ALB): ALB instances provide ultra-high performance to process network traffic at Layer 7. For example, you can use HTTPS offloading with ALB instances. Each ALB instance supports up to one million QPS. An ALB instance serves as a cloud-native gateway of Alibaba Cloud and also provides advanced routing features. ALB instances can forward, redirect, and rewrite messages based on user-defined HTTP headers, cookies, and query strings.


If your applications run over HTTP, HTTPS, or QUIC, we recommend that you use ALB instances.

The following table describes the benefits of CLB (SLB) and ALB.

CLB (formerly known as SLB) ALB
Service scope
  • Provides powerful processing capabilities at Layer 4 and basic processing capabilities at Layer 7
  • Intended for network services
  • Supports HTTP, HTTPS, and QUIC, and is intended for processing network traffic at Layer 7
  • Intended for application services
  • Developed on top of an architecture of physical machines
  • Each instance supports up to 50,000 QPS
  • Insufficient elasticity and scalability at Layer 7
  • Developed on top of Cloud Network Management 2.0
  • Each instance supports up to 1,000,000 QPS
  • Higher elasticity and scalability
  • Provides basic processing capabilities at Layer 7
  • Supports only domain name-based and URL-based forwarding
  • Supports content-based routing
  • Forwards, redirects, and rewrites messages based on HTTP headers
Startup costs Fixed cost > CNY 80/month Fixed cost = CNY 36/month
Cloud-native support Weak Serves as a cloud-native gateway that can distribute and mirror traffic, and supports canary release and blue-green deployments.