What Is ELB?

Elastic Load Balance (ELB) automatically distributes incoming traffic across multiple backend servers based on the listening rules you configure. ELB expands the service capabilities of your applications and improves their availability by eliminating single points of failure (SPOFs).

As shown in the example in the following figure, ELB distributes incoming traffic to three application servers, and each server processes one third of the requests. ELB also provides health checks, which can detect unhealthy servers. Traffic is distributed only to servers that are running normally, improving the availability of applications.

Figure 1 Using a load balancer

ELB Components

ELB consists of the following components:

Load balancer: distributes incoming traffic across backend servers in one or more availability zones (AZs).
Listener: uses the protocol and port you specify to check for requests from clients and route the requests to associated backend servers based on the listening rules and forwarding policies you configure. You can add one or more listeners to a load balancer.
Backend server group: contains one or more backend servers to receive requests routed by the listener. You need to add at least one backend server to a backend server group.
You can set a weight for each backend server based on their performance.

You can also configure health checks for a backend server group to check the health of each backend server. When a backend server is unhealthy, the load balancer stops routing new requests to this server.

Figure 2 ELB components