Ensure that your AWS Elastic Load Balancers have at least two healthy EC2 backend instances assigned, in order to provide a better fault-tolerant load balancing configuration.
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Having just one EC2 instance behind your Elastic Load Balancer (ELB), even if the ELB is associated with an Auto Scaling Group (ASG) that can add instances automatically, increases the risk of downtime. To achieve fault tolerance with zero downtime, always register at least two EC2 instances with your ELB.
A common scenario where an ELB utilizes a single EC2 instance is when the instance is a part of an Auto Scaling Group with min, max and desired capacity set to 1. To determine if each ELB available in your account distributes the traffic to at least two healthy backend instances, perform the following:
Remediation / Resolution
To register additional healthy EC2 backend instances with your Elastic Load Balancers, perform the following:
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ELB Minimum Number Of EC2 Instances
Risk level: High