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 loads 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. Compared with self-managed hardware load balancers, SLB can handle a great number of user requests in scenarios such as large-scale promotional events without incurring large upfront investments. Compared with self-managed, open source load balancers, SLB is supported by an experienced and professional technical 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).

  • ALB: provides ultra-high Layer 7 processing capabilities such as HTTPS offloading. Each ALB instance supports up to one million queries per second (QPS). An ALB instance serves as a cloud-native gateway on Alibaba Cloud and provides advanced content-based routing features. ALB instances can forward, redirect, and rewrite messages based on user-defined HTTP headers, cookies, and query strings. For more information, see What is ALB?.
  • CLB: supports TCP, UDP, HTTP, and HTTPS. CLB provides robust processing capabilities at Layer 4 and basic processing capabilities at Layer 7. For more information, see What is CLB?.
Table 1. Differences between ALB and CLB
Service scope
  • Robust processing capabilities at Layer 7 and advanced routing features
  • Supports protocols such as HTTP, HTTPS, and QUIC at the application layer
  • Application-oriented
  • Robust processing capabilities at Layer 4
  • Supports protocols such as TCP, UDP, HTTP, and HTTPS
  • Network-oriented
Architecture and performance
  • Based on the network function virtualization (NFV) platform and supports auto scaling
  • One million QPS per instance
  • Developed on top of a physical machine architecture
  • Five million QPS per instance
Forwarding capabilities
  • Supports content-based routing
  • Supports HTTP rewrites, redirects, overwrites, and throttling
  • Basic features at Layer 7
  • Supports only domain name-based and URL-based forwarding
Operations and maintenance
  • Supports auto scaling
  • Supports auto scaling to withstand traffic spikes
  • Provides different specifications
  • Requires an estimate of traffic spikes
  • Requires manual upgrades of specifications
Cloud-native support
  • Serves as a cloud-native gateway
  • Supports traffic splitting, mirroring, canary releases, and blue-green deployments
  • Must be used in combination with container services such as Container Service for Kubernetes (ACK) and Serverless Kubernetes (ASK)
  • Supports high-performance auto scaling for Internet applications at Layer 7
  • Maintains low network latency for audio and video streaming applications in high traffic scenarios
  • Supports canary releases and blue-green deployments for cloud-native applications
  • Ensures high reliability when CLB distributes requests to websites and systems at Layer 4
  • Supports high concurrency and ensures high performance in traffic distribution
  • Supports zone-disaster recovery and cross-region disaster recovery


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 API operations.

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.
  • Supports cluster deployment and session synchronization. Backend servers are grouped into a cluster and sessions to these backend servers are synchronized to avoid single points of failure (SPOFs). In addition, hot upgrades are also supported to avoid service disruptions.

Higher security and reliability

Provides built-in protection capabilities to reduce the costs of secure data transmission.
  • 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 is resilient against attacks at Layer 4 and also integrated with Web Application Firewall (WAF) to protect your service at the application layer.
  • Certificate management: SLB provides certificate management for HTTPS and QUIC to ensure the stability and security of data transmission.

High performance

Alibaba Cloud is the first cloud service provider of load balancers with guaranteed performance.
  • High-performance SLB instances: Alibaba Cloud provides high-performance SLB instances of different specifications to meet your business requirements. The processing capability of each SLB instance varies by specification.
  • Extra large-sized SLB instances: You can specify an extra large-sized SLB instance that provides ultra-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 management: You can create, configure, and manage SLB instances in the console, by calling API operations in OpenAPI Explorer, or by using Alibaba Cloud SDKs.
  • 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.