A Classic Load Balancer (CLB) instance is a running entity of the CLB service. To use the CLB service, you must create a CLB instance and add listeners and backend servers to the instance.

Instance overview

Instance types

Alibaba Cloud provides Internet-facing and internal-facing Classic Load Balancer instances.

Internet-facing CLB instances

When you create an Internet-facing Classic Load Balancer instance, the system allocates a public IP address to the instance. You can associate your domain name with the public IP address. This way, the CLB instance can receive requests from the Internet and distribute requests to backend servers based on the rules that you configure for the listener.

An Internet-facing Classic Load Balancer instance has the following features when it provides services to the Internet:
  • A public IP address is allocated to the Classic Load Balancer instance and cannot be disassociated from the Classic Load Balancer instance.
  • Subscription Internet-facing Classic Load Balancer instances support only the pay-by-bandwidth metering method. Pay-as-you-go Internet-facing CLB instances support the pay-by-data-transfer and pay-by-bandwidth metering methods.

Internal-facing CLB instances

Internal-facing Classic Load Balancer provides external services by using private IP addresses. Client requests from the same virtual private cloud (VPC) are distributed to backend servers based on specified load balancing policies.

Internal-facing Classic Load Balancer instances that are associated with elastic IP addresses (EIPs) can process requests from the Internet. An internal-facing Classic Load Balancer instance has the following features when it provides services to the Internet:
  • A public IP address is allocated to the EIP. You can associate the EIP with the CLB instance and disassociate the EIP from the CLB instance based on your requirements.
  • An EIP that is associated with an EIP bandwidth plan supports the 95th percentile bandwidth billing method in addition to the subscription and pay-as-you-go billing methods.
The network types supported by internal-facing Classic Load Balancer instances vary based on billing methods.
  • Subscription CLB instances support classic network and VPC.
    • VPC

      If you choose VPC for an internal-facing Classic Load Balancer instance, the IP address of the Classic Load Balancer instance is allocated from the CIDR block of the vSwitch that is attached to the VPC. The CLB instance can be accessed only by Elastic Compute Service (ECS) instances in the VPC.

    • Classic network
      If you choose classic network for an internal-facing Classic Load Balancer instance, the IP address of the Classic Load Balancer instance is allocated and maintained by Alibaba Cloud. The instance can be accessed only by ECS instances in the classic network.
      Notice Internal-facing Classic Load Balancer instances of the classic network type are no longer available.
  • Pay-as-you-go internal-facing Classic Load Balancer instances support only VPC.

    In addition to standard features, these CLB instances also support PrivateLink. CLB instances that use PrivateLink can receive requests from other VPCs and distribute the requests to backend servers based on configured load balancing policies. For more information, see What is PrivateLink?.

Instance specifications

Classic Load Balancer provides high-performance instances and shared-resource instances.

  • 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 an SLB 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.

    Alibaba Cloud provides the following specifications for high-performance CLB instances.
    Instance type Specification Maximum number of connections CPS QPS Purchase method
    Type 1 Small I (slb.s1.small) 5,000 3,000 1,000 Available for purchase from the official website of Alibaba Cloud.
    Type 2 Standard I (slb.s2.small) 50,000 5,000 5,000 Available for purchase from the official website of Alibaba Cloud.
    Type 3 Standard II (slb.s2.medium) 100,000 10,000 10,000 Available for purchase from the official website of Alibaba Cloud.
    Type 4 Higher I (slb.s3.small) 200,000 20,000 20,000 Available for purchase from the official website of Alibaba Cloud.
    Type 5 Higher II (slb.s3.medium) 500,000 50,000 30,000 Available for purchase from the official website of Alibaba Cloud.
    Type 6 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 specification, you can purchase Application Load Balancer (ALB) instances. For more information, see What is Application Load Balancer (ALB)?.

    For information about how to select the specification of a high-performance instance, see FAQ about high-performance SLB instances.

  • Shared-resource CLB instances
    All shared-performance CLB instances share CLB resources. This indicates that the instance performance cannot be guaranteed.
    Notice Shared-performance CLB instances are no longer available.

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

Feature High-performance CLB instance Shared-resource CLB instance
Resource allocation Exclusive resources Shared resources
Service uptime guaranteed by terms of service level agreement 99.95% Not supported
IPv6 -
Server Name Indication (SNI) certificates -
Blacklists and whitelists -
Elastic network interface (ENI) mounting -
Adding 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.