Backend Sets for Network Load Balancers
A backend set combines a network load balancing policy, a 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 a health check script. You must 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
Specify the mode when you create a backend set. You can configure a backend set in either Default or Non-preemption mode.
Default Mode
A backend set in Default mode supports an unlimited number of backend servers.
Non-Preemption Mode
To use non-preemption mode for backend sets, you must create the network load balancer in an availability domain-local subnet.
A backend set in non-preemption mode supports a maximum of two backend servers. You can configure one of the two backend servers as a backup. If the active backend server becomes unhealthy, the backup takes over as the operational active backend. When the configured active backend server becomes healthy again, it assumes the operational backup role and doesn't reclaim the operational active role.
When you view the details of a backend set in non-preemption mode, both Configured State and Operational State are displayed. Configured State reflects the configured role of each backend server. Operational State reflects each backend server's role based on its current operating state. These states are independent: a configured active backend can serve as the operational backup, and a configured backup backend can serve as the operational active backend.
You can't edit a backend server that's part of a non-preemptive mode backend server.