number of HTTP response codes generated by registered instances. This count does not include any response codes generated by the load balancer. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Sum. Note that Minimum, Maximum, and Average are all 1. Example: Suppose that your load balancer has 2 instances in us-west-2a and 2 instances in us-west-2b, and that requests sent to 1 instance in us-west-2a result in HTTP 500 responses. The sum for us-west-2a includes these error responses, while the sum for us-west-2b does not include them. Therefore, the sum for the load balancer equals the sum for us-west-2a. HTTPCode_ELB_4XX [HTTP listener] The number of HTTP 4XX client error codes generated by the load balancer. Client errors are generated when a request is malformed or incomplete. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Sum. Note that Minimum, Maximum, and Average are all 1. Example: Suppose that your load balancer has us-west-2a and us-west-2b enabled, and that client requests include a malformed request URL. As a result, client errors would likely increase in all Availability Zones. The sum for the load balancer is the sum of the values for the Availability Zones. HTTPCode_ELB_5XX [HTTP listener] The number of HTTP 5XX server error codes generated by the load balancer. This count does not include any response codes generated by the registered instances. The metric is reported if there are no healthy instances registered to the load balancer, or if the request rate exceeds the capacity of the instances (spillover) or the load balancer. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Sum. Note that Minimum, Maximum, and Average are all 1. Example: Suppose that your load balancer has us-west-2a and us-west-2b enabled, and that instances in us-west-2a are experiencing high latency and are slow to respond to requests. As a result, the surge queue for the load balancer nodes in us-west-2a fills and clients receive a 503 error. If us-west-2b continues to respond normally, the sum for the load balancer equals the sum for us-west-2a.