Classic Load Balancer (CLB) instances receive requests from clients and forward requests to backend servers. To use the load balancing service, you must create a CLB instance and add listeners and backend servers to the 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 specify for listeners.

An Internet-facing CLB instance has the following features:
  • The system assigns a public IP address to the CLB instance. You cannot disassociate the public IP address from the CLB instance.
  • When the subscription billing method is used, only the pay-by-bandwidth metering method is supported. When the pay-as-you-go billing method is used, the pay-by-bandwidth and pay-by-data-transfer metering methods are supported.

Internal-facing CLB instances

Internal-facing CLB instances provide services over private networks. Internal-facing CLB instances receive requests from internal networks and forward requests to backend servers based on listener rules.

Internal-facing CLB instances that are associated with elastic IP addresses (EIPs) can process requests from the Internet. An internal-facing CLB instance has the following features when it provides services over the Internet:
  • An internal-facing CLB instance associated with an EIP can provide services over the Internet. You can disassociate the EIP from the CLB instance as needed.
  • An EIP that is associated with an EIP bandwidth plan supports the pay-by-95th-percentile-bandwidth billing method in addition to the subscription and pay-as-you-go billing methods.
The network types supported by internal-facing CLB instances vary based on billing methods.
  • 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 that is attached to the VPC. 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. The internal-facing CLB instance can be accessed by only ECS instances in classic networks.
      Notice Internal-facing CLB instances of the classic network type are no longer available.
  • 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 listener rules. For more information, see What is PrivateLink?.

Instance types and specifications

High-performance CLB instances

The following section describes three key metrics of high-performance CLB instances:
  • Max Connection

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

Table 1. Specifications of high-performance CLB instances
Specification Maximum number of connections CPS QPS Purchase method
Small I (slb.s1.small) 5,000 3,000 1,000 Available for purchase from the official website of Alibaba Cloud.
Standard I (slb.s2.small) 50,000 5,000 5,000 Available for purchase from the official website of Alibaba Cloud.
Standard II (slb.s2.medium) 100,000 10,000 10,000 Available for purchase from the official website of Alibaba Cloud.
Higher I (slb.s3.small) 200,000 20,000 20,000 Available for purchase from the official website of Alibaba Cloud.
Higher II (slb.s3.medium) 500,000 50,000 30,000 Available for purchase from the official website of Alibaba Cloud.
Super I (slb.s3.large) 1,000,000 100,000 50,000 Available for purchase from the official website of Alibaba Cloud.
Note If you require a higher QPS, you can purchase Application Load Balancer (ALB) instances. For more information, see What is Application Load Balancer (ALB)?.

For more information, see FAQ about high-performance CLB instances.

Shared-resource CLB instance

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

The following table describes the differences between shared-resource and high-performance CLB instances.

Feature High-performance CLB instance Shared-resource CLB 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) -
Blacklists and whitelists -
Elastic network interface (ENI) mounting -
Assigning secondary IP addresses to ENIs that are associated with ECS instances -
HTTP-to-HTTPS redirection -
Consistent hashing -
TLS security policies -
HTTP2 -
WebSocket or WebSocket Secure -
Note In the preceding table, "√" indicates that a feature is supported, and "-" indicates that a feature is not supported.