This topic describes the features of Classic Load Balancer (CLB). CLB provides Layer 4 and Layer 7 load balancing, and supports features such as health checks, session persistence, and domain name-based forwarding to ensure the high availability and low latency of backend services.

In the following table, "✔" indicates that the feature is supported and "–" indicates that the feature is not supported.

Feature Layer 4 CLB Layer 7 CLB
Routing algorithms

CLB supports three routing algorithms: round-robin (RR), weighted round-robin (WRR), and consistent hashing (CH).

✔ ✔
Note CLB does not support the CH algorithm for Layer 7 traffic.
Health checks

CLB checks the health status of backend servers. When an unhealthy backend server is detected, CLB stops distributing inbound traffic to the backend server. Network traffic is distributed only to healthy backend servers.

✔ ✔
Session persistence

CLB supports session persistence. After session persistence is enabled, CLB distributes requests from the same session to the same backend server.

✔ ✔
Network access control lists (ACLs)

CLB uses allowlists and denylists to control access to your applications.

✔ ✔
High availability

CLB distributes inbound traffic to backend servers across zones. CLB also supports cross-zone deployment in most regions. If the primary zone fails, network traffic is automatically distributed to the secondary zone to avoid service interruptions.

✔ ✔
Security

You can integrate CLB with Apsara Stack Security to protect your applications from DDoS attacks of up to 5 Gbit/s.

✔ ✔
Network types

Alibaba Cloud provides Internet-facing and internal-facing CLB instances. To process network traffic in a virtual private cloud (VPC), you can create an internal-facing CLB instance. To process network traffic from the Internet, you can create an Internet-facing CLB instance.

✔ ✔
Monitoring

You can view the monitoring data of CLB instances through CloudMonitor. The data includes the number of connections and the volumes of inbound and outbound traffic.

✔ ✔
IPv6 support

CLB can forward requests from IPv6 clients.

✔ ✔
Health check logs

By default, CLB stores the health check logs that are generated in the last three days. Object Storage Service (OSS) can be used to store all CLB health check logs, allowing you to troubleshoot health check failures. OSS must be enabled before you can use this feature.

✔ ✔
Domain name-based forwarding and URL-based forwarding

You can configure domain name-based forwarding rules and URL-based forwarding rules for the Layer 7 listeners of CLB. The listeners route requests from different domain names and URLs to different backend servers based on the forwarding rules.

– ✔
Certificate management

CLB supports centralized management of certificates for HTTPS listeners. You do not need to upload certificates to backend servers. Requests are decrypted on CLB before the requests are sent to backend servers. This reduces the processing workload on backend servers.

– ✔
SNI

CLB supports Server Name Indication (SNI). SNI allows you to associate multiple certificates with an HTTPS listener. This way, the listener can route requests from different domain names to different backend servers.

– ✔
Redirection

CLB can redirect HTTP requests to HTTPS.

– ✔
WS and WSS

CLB supports using WebSocket (WS) and WebSocket Secure (WSS). WS is an HTML5 protocol that provides full-duplex communication channels between clients and servers. This implements real-time communication between clients and servers while saving server resources and bandwidth.

– ✔
HTTP/2

Hypertext Transfer Protocol Version 2 (HTTP/2) is the second version of Hypertext Transfer Protocol (HTTP). It provides significant performance improvements and is backward compatible with HTTP/1.X.

– ✔