Using AZ Affinity to Minimize Access Latency

AZ Affinity Overview

ELB can distribute traffic across AZs. By default, a load balancer forwards traffic to all backend servers in a backend server group.

If AZ affinity is enabled for a backend server group, traffic will be distributed across the backend servers in the same AZ as the load balancer, minimizing the access latency.

Forwarding Even Unhealthy will not work if AZ affinity is enabled for a backend server group.

Figure 1 Traffic forwarding with AZ affinity enabled

Resource AZs

An AZ contains one or more physical data centers. Each AZ has independent cooling, fire extinguishing, moisture-proof, and electricity facilities. Within an AZ, compute, networking, storage, and other resources are logically divided into multiple clusters.

• Load balancer AZ

  Dedicated load balancers can be deployed across AZs. If you select multiple AZs, a load balancer is created in each selected AZ.

  You can change the AZ of a load balancer by referring to Changing an AZ.

• Backend server AZ

  Table 1 Backend server AZ

  Backend Server Type	AZ
  Cloud server	The AZ where the cloud servers are running.
  Supplementary network interface	The AZ where their primary network interfaces are running.
  IP as backend server	The AZ you select when adding IP as backend servers.

AZ Affinity Failover Threshold

The health of each AZ is independent from each other and is determined based on the number or percentage of healthy backend servers in that AZ.

• If the percentage is 0% or the quantity is 0, the AZ is always considered as healthy and traffic will be distributed across the servers in this AZ.
• If health check is not enabled for a backend server group, all backend servers in the group are considered as healthy by default. So the AZ is also healthy.
• If there is no backend server in the AZ of the load balancer, the AZ is considered unhealthy, and the AZ affinity failover function takes effect.
• The health check result of a backend server with the weight of 0 is also used to calculate the AZ affinity failover threshold.

If you enable AZ affinity, traffic forwarding may be interrupted if the failover threshold and forwarding actions are not properly configured. So deploy your backend servers in different AZs.

How Traffic Is Forwarded When AZ Affinity Fails

If the failover threshold is met, AZ affinity fails. In this case, traffic will be distributed based on how you set Forward Traffic To.

If your service requires high availability, you are advised to select Healthy backend servers in all AZs.

Ensure that traffic can be forwarded in the way as you wish regardless of whether AZ affinity fails.

Configuring AZ Affinity

1. Go to the backend server group list page.
2. On the Backend Server Groups page, locate the backend server group and click Edit in the Operation column.
3. In the Modify Backend Server Group dialog box, enable or disable AZ Affinity.

   Configure Failover Threshold and Forward Traffic To.

4. Click OK.