Server Load Balancer (SLB) is a service that distributes network traffic across groups of backend servers to increase the throughput of your applications. You can use SLB to prevent service disruptions caused by single points of failure (SPOFs) and improve the availability of applications.


SLB is a fully managed load balancing service provided by Alibaba Cloud. This service is out-of-the-box, high-throughput, stable, and reliable. You can dynamically scale SLB resources and pay for these resources on demand. 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 promotions or improve the availability of Internet of Things (IoT) applications that receive a great number of concurrent connections. SLB reduces the expenses that are used to encrypt and decrypt data. It can redirect HTTP requests to HTTPS and redirect HTTPS requests back to HTTP. Requests are encrypted and decrypted by SLB. The backend servers need only to process HTTP requests.

Compared with hardware load balancing solutions that require large up-front investment, SLB is highly scalable and allows you to pay only for the resources that you use. It can balance the heavy load of applications during large-scale online promotions. 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 improves the efficiency of operations and maintenance.

SLB instance types

Alibaba Cloud provides two types of Server Load Balancer (SLB) instances: Classic Load Balancer (CLB) and Application Load Balancer (ALB).

  • Classic Load Balancer (CLB): supports TCP, UDP, HTTP, and HTTPS. CLB provides robust processing capabilities at Layer 4 and basic processing capabilities at Layer 7.
  • Application Load Balancer (ALB): provides ultra-high Layer 7 processing capabilities such as HTTPS offloading. Each ALB instance supports up to one million QPS. An ALB instance serves as a cloud-native gateway on Alibaba Cloud and provides advanced routing features. ALB instances can forward, redirect, and rewrite messages based on user-defined HTTP headers, cookies, and query strings.
Table 1. Differences between CLB and ALB
Type Classic Load Balancer (CLB) Application Load Balancer (ALB)
Service scope
  • Robust processing capabilities at Layer 4
  • Supports protocols such as TCP, UDP, HTTP, and HTTPS
  • Network-oriented
  • Robust processing capabilities at Layer 7 and advanced routing features
  • Supports protocols such as HTTP, HTTPS, and QUIC at the application layer
  • Application-oriented
  • Developed on top of a physical machine architecture.
  • 50,000 QPS per instance
  • Based on the NFV platform
  • One million QPS per instance
  • Higher elasticity and scalability
  • Basic features at Layer 7
  • Supports only domain name-based and URL-based forwarding
  • Supports content-based routing
  • Supports HTTP rewrites, redirects, overwrites, and throttling
Operations and maintenance
  • Provides different specifications for purchase
  • Requires an estimate of traffic spikes
  • Requires manual upgrades of specifications
  • Supports auto scaling
  • Supports auto scaling based on traffic spikes
  • Limited support
  • Serves as a cloud-native gateway
  • Supports traffic splitting, mirroring, canary releases, and blue-green deployments
  • Ensures high reliability when CLB distributes requests to websites and systems at Layer 4
  • Supports high concurrent connections and ensures high performance in traffic distribution
  • Provides zone-disaster recovery and cross-region disaster recovery
  • Supports high-performance auto scaling for Internet applications at Layer 7
  • Maintains low network latency for audio and video applications that handle a large amount of network traffic
  • Supports canary releases and blue-green deployments for cloud-native applications


Multiple protocols

Supports multiple protocols to meet your business requirements in various scenarios.
  • Basic protocols: SLB supports Layer 4 load balancing over TCP or UDP, and Layer 7 loading balancing over HTTP or HTTPS.
  • Advanced protocols: SLB supports the QUIC protocol. QUIC is an advanced protocol that ensures faster and safer transmission for audio, video, and mobile applications. In addition, SLB also supports Google Remote Procedure Call (gRPC) to facilitate communication among microservices by calling APIs.

Multi-level disaster recovery

Provides disaster recovery at multiple levels to ensure high availability.
  • Monitors the health status of backend servers on a regular basis. SLB does not distribute network traffic to unhealthy backend servers. This ensures the availability of your service.
  • Supports multi-zone deployment in specific regions to provide zone-disaster recovery.
  • SLB supports cluster deployment and session synchronization. You can perform hot upgrades, monitor the health status and performance of machines in real time.

Higher security and reliability

Provides built-in protection capabilities free of charge, which can secure transmission and reduce costs.
  • Layer 4 security: Layer 4 SLB is resilient against DDoS, SYN flood, UDP flood, ACK flood, ICMP flood, and DNS flood attacks.
  • Layer 7 security: Layer 7 SLB provides Layer 4 security capabilities and supports the configuration read-only mode. You can integrate SLB with Web Application Firewall (WAF) to secure your applications.
  • Certificate management: SLB provides certificate management for HTTPS and QUIC to ensure the stability and security of transmission.

High performance

Alibaba Cloud is the first cloud service provider of high-performance SLB instances.
  • High-performance SLB instances: Alibaba Cloud provides high-performance SLB instances of different specifications. The processing capabilities of high-performance SLB instances vary by specification.
  • Extra large-sized SLB instances: You can specify an extra large-sized SLB instance that has high processing capabilities to meet your business requirements.

Flexible scheduling

Supports multiple scheduling algorithms and forwarding methods to improve the flexibility of application deployment.
  • Scheduling algorithms: SLB supports weighted round-robin (WRR) and consistent hash (CH). You can specify a scheduling algorithm based on your business requirements.
  • Domain name-based forwarding and URL-based forwarding: SLB can distribute network traffic across backend servers based on domain names and URLs. This improves the flexibility of application deployment.

Flexible management and billing

You can specify the instance type and billing method based on your business requirements. Alibaba Cloud allows you to use multiple methods to manage your SLB service.
  • Flexible billing: SLB supports the subscription and pay-as-you-go billing methods, and the pay-by-data-transfer and pay-by-bandwidth metering methods.
  • Flexible management: You can create, configure, and manage SLB instances in the console, by calling APIs in OpenAPI Explorer, or by using Alibaba Cloud SDKs.