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.
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 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.
- 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.
- 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 by only ECS instances in classic networks.Notice Internal-facing CLB instances of the classic network type are no longer available for purchase.
- VPC
- 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
- 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 connections | CPS | QPS | How to purchase |
|---|---|---|---|---|
| Small I (slb.s1.small) | 5,000 | 3,000 | 1,000 | Available for purchase from the official website of Alibaba Cloud. |
| Medium I (slb.s2.small) | 50,000 | 5,000 | 5,000 | Available for purchase from the official website of Alibaba Cloud. |
| Medium II (slb.s2.medium) | 100,000 | 10,000 | 10,000 | Available for purchase from the official website of Alibaba Cloud. |
| Large I (slb.s3.small) | 200,000 | 20,000 | 20,000 | Available for purchase from the official website of Alibaba Cloud. |
| Large II (slb.s3.medium) | 500,000 | 50,000 | 30,000 | Available for purchase from the official website of Alibaba Cloud. |
| Super Large I (slb.s3.large) | 1,000,000 | 100,000 | 50,000 | Available for purchase from the official website of Alibaba Cloud. |
For more information, see FAQ about high-performance CLB instances.
Shared-resource CLB instance
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) for multiple certificates | √ | - |
| Blacklists and whitelists | √ | - |
| Associating 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 | √ | - |