Classic Load Balancer (CLB) provides load balancing services by distributing requests from clients across groups of backend servers. To use the CLB service, you must create a CLB instance and add listeners and backend servers to the CLB instance.

Components

Instance types

Alibaba Cloud provides Internet-facing and internal-facing CLB instances.

Internet-facing CLB instances

When you create an Internet-facing CLB instance, the system automatically assigns a public address to the CLB instance. You can associate your domain name with the public IP address. CLB instances receive requests from clients over the Internet and forward requests to backend servers based on the forwarding rules that you configure for listeners.

An Internet-facing CLB instance provides the following features:
  • An Internet-facing CLB instance is assigned a public IP address to provide services over the Internet. You cannot disassociate the public IP address from the CLB instance.
  • Subscription CLB instances support only the pay-by-bandwidth metering method. Pay-as-you-go CLB instances support the pay-by-bandwidth and pay-by-data-transfer metering methods.

Internal-facing CLB instances

An internal-facing CLB instance is assigned a private IP address. An internal-facing CLB instance provides services by distributing requests from the internal networks of Alibaba Cloud to backend servers based on the forwarding rules that you configure for listeners.

If you want an internal-facing CLB instance to provide services over the Internet, you can associate an elastic IP address (EIP) with the CLB instance. After you associate an EIP with the CLB instance:
  • The CLB instance can use the EIP to provide services over the Internet. You can disassociate the EIP from the CLB instance if you no longer need the EIP.
  • You can associate the EIP with an EIP bandwidth plan. An EIP that is associated with an EIP bandwidth plan supports the subscription, pay-as-you-go, and pay-by-95th-percentile-bandwidth billing methods.
The network types supported by internal-facing CLB instances vary based on the billing method.
  • Subscription internal-facing CLB instances support the following network types: classic network and virtual private cloud (VPC).
    • VPC

      If an internal-facing CLB instance is deployed in a VPC, the IP address of the CLB instance is allocated from the CIDR block of the vSwitch in the VPC. In this case, the internal-facing CLB instance can be accessed by only Elastic Compute Service (ECS) instances in the same VPC.

    • Classic network
      If the internal-facing CLB instance is deployed in a classic network, the private IP address of the CLB instance is allocated and managed by Alibaba Cloud. In this case, the internal-facing CLB instance can be accessed only by ECS instances in classic networks.
      Important Internal-facing CLB instances of the classic network type are no longer available for purchase.
  • Pay-as-you-go internal-facing CLB instances support only the VPC network type.

    Internal-facing CLB instances support PrivateLink. Internal-facing CLB instances can receive requests from other VPCs through PrivateLink connections and distribute the requests to backend servers based on the forwarding rules that you configure for listeners. For more information, see What is PrivateLink?

Instance specifications

High-performance CLB instances

The following section describes three key metrics of high-performance CLB instances:
  • Maximum number of connections

    The maximum number of concurrent connections that a CLB instance supports. When the number of existing concurrent connections reaches the upper limit, new connection requests are dropped.

  • Connections per second (CPS)

    The number of new connections that are established per second. When the CPS value reaches the upper limit, new connection requests are dropped.

  • Queries per second (QPS)

    The number of HTTP or HTTPS queries (requests) that can be processed per second. This metric is specific to Layer 7 listeners. When the QPS value reaches the upper limit, new connection requests are dropped.

Table 1. Specifications of high-performance CLB instances
Specification Maximum number of connections CPS QPS
Small I (slb.s1.small) 5,000 3,000 1,000
Medium I (slb.s2.small) 50,000 5,000 5,000
Medium II (slb.s2.medium) 100,000 10,000 10,000
Large I (slb.s3.small) 200,000 20,000 20,000
Large II (slb.s3.medium) 500,000 50,000 30,000
Super Large I (slb.s3.large) 1,000,000 100,000 50,000
Note
  • The maximum capacity of a pay-as-you-go CLB instance is the same as a Super Large I (slb.s3.large) CLB instance.
  • If you require more concurrent connections at Layer 4, use Network Load Balancer (NLB). If you require a higher queries per second (QPS) value at Layer 7, use Application Load Balancer (ALB).

For more information how to select a specification, see FAQ about high-performance CLB instances.

Shared-resource SLB instance

Resources are shared among all shared-resource CLB instances. Therefore, the performance of shared-resource CLB instances is not guaranteed.
Important Shared-resource CLB instances are no longer available for purchase.

Differences between shared-resource and high-performance CLB instances

Item High-performance CLB instances Shared-resource SLB instance
Resource allocation Exclusive resources Shared resources
Service uptime guaranteed by the service-level agreement (SLA) 99.95% Not supported
IPv6 ✓ -
Server Name Indication (SNI) for multiple certificates ✓ -
Blacklists and whitelists ✓ -
Association with elastic network interfaces (ENIs) ✓ -
Adding backend servers by specifying the secondary IP addresses of ENIs that are attached to ECS instances ✓ -
HTTP-to-HTTPS redirection ✓ -
Consistent hashing ✓ -
TLS security policies ✓ -
HTTP2 ✓ -
WebSocket or WebSocket Secure ✓ -
Note In the preceding table, a check mark (√) indicates that the feature is supported, and a hyphen (-) indicates that the feature is not supported.