Backend Sets for Network Load Balancers
Use backend sets to create logical entities consisting of a network load balancing policy, health check policy, and a list of backend servers for a network load balancer.
A backend set is a logical entity defined by a load balancing policy, a health check policy, and a list of backend servers. To create a backend set, you must specify a load balancing policy and health check script, and then add a list of backend servers (compute instances). A backend set must be associated with a single listener for the network load balancer to work.
You can perform the following backend set management tasks:
List the backend sets in a network load balancer.
Create a new backend set under a network load balancer.
Edit a backend set's settings.
Enable source preservation in a backend set.
Types of Backend Sets
You're prompted to specify the type of mode for a backend set is when you create it. You can configure a backend set as being either Default or Non-preemption mode.
Default Mode
A Default mode backend set contains an unlimited number of backend servers.
Non-Preemption Mode
A Non-preemption mode backend set only has a maximum of two backend servers. You can configure one of the two backend servers as a backup. If the active backend server is unavailable because of an issue or routine maintenance, the backup takes over.
When you view details of a non-preemptive mode backend set, both the Configured State and Operational State status are displayed. The Configured State reflects how the backend server is configured (default or non-preemptive). The Operational State reflects the actual operational functionality at that moment. If a non-preemption backup backend server is active because the usual active backend server is down, then that backup backend server has an Operational Statue as Active.
You can't edit a backend server that's part of a non-preemptive mode backend server.