Elastic Load Balancing (ELB) is an AWS service used to dispatch incoming web traffic from your applications across your Amazon EC2 backend instances, which may be in different availability zones.

ELB is widely used by web and mobile applications to help ensure a smooth user experience and provide increased fault tolerance, handling traffic peaks and failed EC2 instances without interruption.

ELB continuously checks for unhealthy EC2 instances. If any are detected, ELB immediately reroutes their traffic until they recover. If an entire availability zone goes offline, Elastic Load Balancing can even route traffic to instances in other availability zones. With Auto Scaling, AWS can ensure your infrastructure includes the right number of EC2 hosts to support your changing application load patterns.

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https://www.datadoghq.com/blog/top-elb-health-and-performance-metrics/