Keep-Alive, or keepalive, is a signal sent from one device to another in order to maintain the connection between the two devices.
You can have a keep-alive signal for more than two devices at a time.
By default, DigitalOcean Load Balancers ignore the Connection: keep-alive header of HTTP responses from Droplets to load balancers and close the connection upon completion. When you enable backend keepalive, the load balancer honors the Connection: keep-alive header and keeps the connection open for reuse. This allows the load balancer to use fewer active TCP connections to send and to receive HTTP requests between the load balancer and your target Droplets.
Enabling this option generally improves performance (requests per second and latency) and is more resource efficient. For many use cases, such as serving web sites and APIs, this can improve the performance the client experiences. However, it is not guaranteed to improve performance in all situations, and can increase latency in certain scenarios.
The option applies to all forwarding rules where the target protocol is HTTP or HTTPS. It does not apply to forwarding rules that use TCP, HTTPS, or HTTP/2 passthrough.
There are no hard limits to the number of connections between the load balancer and each server. However, if the target servers are undersized, they may not be able to handle incoming traffic and may lose packets. See Best Practices for Performance on DigitalOcean Load Balancers.
You can use WebSockets with or without backend keepalive enabled.
By default, load balancers ignores the Connection: keep-alive headers returned by target Droplets. You can configure the load balancer to use fewer active TCP connections by enabling the backend keepalive setting.