What does AWS do with unhealthy Instance under Load Balancer?

Does it send the traffic to it or not? Is there a chance of request loss?

If it is the only instance left in the target group, then yes it will attempt to keep sending. If not it will route requests to other hosts. I would Set up a custom health check if necessary. If the instance is in an Asg it will get terminated if unhealthy and respawned.

It depends on if you are using CLB/ALB/NLB. If you are using CLB and none of the targets are healthy, CLB will send 503 straight away to client.
In case of ALB (Application Load Balancer) / NLB (Network Load Balancer) we need to see If ALL targets in Target group are unhealthy. If all targets in Target group are unhealthy, ALB/NLB will fail-open and send the request to target anyhow and whatever the response target returns will be proxied back to client. If there are any healthy instance in the Target group then ALB/NLB will route the request to healthy target only.

Unhealthy instances receive traffic till load balancer realize that instance is unhealthy. Once it marks instance as unhealthy it doesn’t send till it becomes healthy again. Understand how healthcheck works in load balance.

Load Balancer sends health check requests to make sure that instances are healthy. In case they fail those health checks specified number of times it removes the instance from balancing and does not forward requests to unhealthy instance anymore. In addition if the Load Balancer is associated with Auto Scaling Group it requests a new instance in place of the unhealthy one.
However though there’s still possibility of losing request in the time between instance became unhealthy and configured treshold of failed health checks is reached so that it is removed from the pool. Usually that would be around a minute or two.

load-balancer

with clb you can register vpc instances as well. Clb is old but support both layer4 and layer7 load balancing. If you would like layer4 load balancing go for nlb , if you want layer7 load balancing go for alb. There is no need to use CLB anymore unless you have specific usecase which doesn’t support in nlb or alb.

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