A Classic Load Balancer (CLB) instance receives requests from clients and distributes the requests to backend servers. To use CLB, you must create a CLB instance and add listeners and backend servers to the CLB instance.
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 assigns a public IP address to the instance. You can associate a domain name with the public IP address. The CLB instance receives requests from clients over the Internet and distributes the requests to backend servers based on the rules that you configure for listeners.
Internet-facing CLB instances have the following features:
An Internet-facing CLB instance is assigned a public IP address. You cannot disassociate the public IP address from the CLB instance.
The pay-by-data-transfer and pay-by-bandwidth metering methods are supported for Internet data transfer.
Internal-facing CLB instances
Internal-facing CLB instances provide external services by using private IP addresses. Requests from the internal network of Alibaba Cloud are distributed to backend servers based on the rules that you configure for listeners.
You can associate elastic IP addresses (EIPs) with internal-facing CLB instances to provide Internet-facing services. Internal-facing CLB instances provide the following capabilities to support Internet access:
You can associate EIPs with internal-facing CLB instances and disassociate EIPs from internal-facing CLB instances based on your business requirements. For more information, see Associate and manage EIPs.
You can associate EIPs with pay-as-you-go Internet Shared Bandwidth instances.
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?
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.
| 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 |
The maximum capacity of a pay-as-you-go CLB instance is the same as the capacity of 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 about NLB, see What is NLB?
For more information about ALB, see What is ALB?
For more information about how to select the specification of a high-performance CLB instance, see FAQ about CLB.
Shared-resource CLB instances
Resources are shared among all shared-resource CLB instances. Therefore, the performance of shared-resource CLB instances is not guaranteed.
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 CLB instances |
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 Elastic Compute Service (ECS) instances | √ | - |
HTTP-to-HTTPS redirection | √ | - |
Consistent hashing | √ | - |
TLS security policies | √ | - |
HTTP2 | √ | - |
WebSocket or WebSocket Secure | √ | - |
In the preceding table, a check mark (√) indicates that the feature is supported, and a hyphen (-) indicates that the feature is not supported.