Preparations for Creating a Load Balancer

Region

AZ

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

Load balancers in these AZs work in active-active or multi-active mode and requests are distributed by the nearest load balancer in the same AZ.

To reduce network latency and improve access speed, you are suggested to deploy your load balancer in the AZ where backend servers are running.

If disaster recovery is required, create load balancers based on the scenario:

• One load balancer in multiple AZs (disaster recovery at the AZ level)

  If the number of requests does not exceed what the largest specifications (large II) can handle, you can create a load balancer and select multiple AZs. In this way, if the load balancer in a single AZ is abnormal, the load balancer in other AZs can route the traffic, and disaster recovery can be implemented among multiple AZs.

• Multiple load balancers and each load balancer in multiple AZs (disaster recovery at both the load balancer and AZ level)

  If the number of requests exceeds what the largest specifications (large II) can handle, you can create multiple load balancers and select multiple AZs for each load balancer. In this way, if a single load balancer is abnormal, other load balancers can distribute the traffic, and disaster recovery can be implemented among multiple load balancers and AZs.

NOTE:

• If requests are from the Internet, the load balancer in each AZ you select routes the requests based on source IP addresses. If you deploy a load balancer in two AZs, the requests the load balancers can handle will be doubled.
• For requests from a private network:
  • If clients are in the AZ you selected when you created the load balancer, requests are distributed by the load balancer in this AZ. If the load balancer is unavailable, requests are distributed by the load balancer in another AZ you select.

    If the load balancer is available but the connections that the load balancer needs to handle exceed the amount defined in the specifications, service may be interrupted. To address this issue, you need upgrade specifications. You can monitor traffic usage on private network by AZ.

  • If clients are in an AZ that is not selected when you create the load balancer, requests are distributed by the load balancer in each AZ you select based on source IP addresses.
• If clients are in a VPC that is different from where the load balancer works, the load balancer in the AZ where the original VPC subnet resides routes the requests. If the load balancer in this AZ is unavailable, requests are distributed by the load balancer in another AZ.

Network Type

Specifications

Dedicated load balancers provide a broad range of specifications to meet your requirements in different scenarios. Specifications for network load balancing are suitable for TCP or UDP requests, while specifications for application load balancing are broadly used to handle HTTP or HTTPS requests. Select appropriate specifications based on your traffic volume and service requirements.

Protocol

ELB provides load balancing at both Layer 4 and Layer 7.

• If you choose TCP or UDP, the load balancer routes requests directly to backend servers. In this process, the destination IP address in the packets is changed to the IP address of the backend server, and the source IP address to the private IP address of the load balancer. A connection is established after a three-way handshake between the client and the backend server, and the load balancer only forwards the data.
  Figure 1 Layer-4 load balancing
  
• Load balancing at Layer 7 is also called "content exchange". After the load balancer receives a request, it works as a proxy of backend servers to establish a connection (three-way handshake) with the client and then determines to which backend server the request is to be routed based on the fields in the HTTP/HTTPS request header and the load balancing algorithm you selected when you add the listener.
  Figure 2 Layer-7 load balancing
  

Backend Servers

Before you use ELB, you need to create cloud servers, deploy required applications on them, and add the cloud servers to one or more backend server groups. When you create ECSs or BMSs, note the following:

• Cloud servers must be in the same region as the load balancer.
• Cloud servers that run the same OS are recommended so that you can manage them more easily.