I've been running into a strange issue with my Multi-AZ Network Load Balancer (NLB) setup. Here's the situation: I have multiple target groups (TG) where all my EC2 instances are in just one availability zone (AZ). When I did an nslookup, I only got one IP back from the NLB, which was fine. However, things took a turn when I stopped all the EC2 instances in one target group, leaving it with no healthy targets. Meanwhile, the other target groups still had healthy targets. Suddenly, the NLB provisioned an extra IP in a different AZ where there weren't any targets available. This led to mixed results; my application using the WebSocket NLB endpoint sometimes worked and sometimes didn't. After some digging, I discovered that out of the two NLB DNS IPs, only one worked correctly, corresponding to the AZ with healthy targets. I'm quite puzzled by this behavior and would appreciate any insights or explanations regarding this.
1 Answer
The 'fail-open' behavior means that when the load balancer detects no healthy targets, it still forwards traffic as if all targets are healthy. It’s a bit odd since it does that even without healthy targets in any AZ, but it helps keep things available if your health checks aren't working correctly.
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