Elastic Load Balancing Metrics and Dimensions
Elastic Load Balancing supports two types of load balancers: Classic Load Balancers and Application Load Balancers. Elastic Load Balancing sends metrics to CloudWatch for both types of load balancers.
Contents
Application Load Balancer Metrics
The AWS/ApplicationELB
namespace includes the following metrics.
Metric  Description 

ActiveConnectionCount 
The total number of concurrent TCP connections active from clients to the load balancer and from the load balancer to targets. Statistics: The most useful statistic is 
ClientTLSNegotiationErrorCount 
The number of TLS connections initiated by the client that did not establish a session with the load balancer. Possible causes include a mismatch of ciphers or protocols. Statistics: The most useful statistic is 
HealthyHostCount 
The number of targets that are considered healthy. Statistics: The most useful statistics are 
HTTPCode_ELB_4XX_Count 
The number of HTTP 4XX client error codes that originate from the load balancer. Client errors are generated when requests are malformed or incomplete. These requests have not been received by the target. This count does not include any response codes generated by the targets. Statistics: The most useful statistic is 
HTTPCode_ELB_5XX_Count 
The number of HTTP 5XX server error codes that originate from the load balancer. This count does not include any response codes generated by the targets. Statistics: The most useful statistic is 
HTTPCode_Target_2XX_Count ,
HTTPCode_Target_3XX_Count ,
HTTPCode_Target_4XX_Count ,
HTTPCode_Target_5XX_Count 
The number of HTTP response codes generated by the targets. This does not include any response codes generated by the load balancer. Statistics: The most useful statistic is 
NewConnectionCount 
The total number of new TCP connections established from clients to the load balancer and from the load balancer to targets. Statistics: The most useful statistic is 
ProcessedBytes  The total number of bytes processed by the load balancer. 
RejectedConnectionCount 
The number of connections that were rejected because the load balancer had reached its maximum number of connections. Statistics: The most useful statistic is 
RequestCount 
The number of requests received by the load balancer. Statistics: The most useful statistic is 
TargetConnectionErrorCount 
The number of connections that were not successfully established between the load balancer and target. Statistics: The most useful statistic is 
TargetResponseTime 
The time elapsed, in seconds, after the request leaves the load balancer
until a response from the target is received. This is equivalent to the
Statistics: The most useful statistics are 
TargetTLSNegotiationErrorCount 
The number of TLS connections initiated by the load balancer that did not establish a session with the target. Possible causes include a mismatch of ciphers or protocols. Statistics: The most useful statistic is 
UnHealthyHostCount 
The number of targets that are considered unhealthy. Statistics: The most useful statistics are 
Metric Dimensions for Application Load Balancers
To filter the metrics for your Application Load Balancer, use the following dimensions.
Dimension  Description 

AvailabilityZone 
Filter the metric data by Availability Zone. 
LoadBalancer 
Filter the metric data by load balancer. Specify the load balancer as follows: app/loadbalancername/1234567890123456 (the final portion of the load balancer ARN). 
TargetGroup 
Filter the metric data by target group. Specify the target group as follows: targetgroup/targetgroupname/1234567890123456 (the final portion of the target group ARN). 
Classic Load Balancer Metrics
The AWS/ELB
namespace includes the following metrics.
Metric  Description 

BackendConnectionErrors 
The number of connections that were not successfully established between the load balancer and the registered instances. Because the load balancer retries the connection when there are errors, this count can exceed the request rate. Note that this count also includes any connection errors related to health checks. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Example: Suppose that your load balancer has 2 instances in uswest2a and 2 instances in uswest2b, and that attempts to connect to 1 instance in uswest2a result in backend connection errors. The sum for uswest2a includes these connection errors, while the sum for uswest2b does not include them. Therefore, the sum for the load balancer equals the sum for uswest2a. 
HealthyHostCount 
The number of healthy instances registered with your load balancer.
A newly registered instance is considered healthy after it passes the first health check.
If crosszone load balancing is enabled, the number of healthy instances for the Reporting criteria: There are registered instances Statistics: The most useful statistics are Example: Suppose that your load balancer has 2 instances in uswest2a
and 2 instances in uswest2b, uswest2a has 1 unhealthy instance, and uswest2b has no unhealthy instances.
With the 

[HTTP listener] The 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 Example: Suppose that your load balancer has 2 instances in uswest2a and 2 instances in uswest2b, and that requests sent to 1 instance in uswest2a result in HTTP 500 responses. The sum for uswest2a includes these error responses, while the sum for uswest2b does not include them. Therefore, the sum for the load balancer equals the sum for uswest2a. 
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 Example: Suppose that your load balancer has uswest2a and uswest2b 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 Example: Suppose that your load balancer has uswest2a and uswest2b enabled, and that instances in uswest2a are experiencing high latency and are slow to respond to requests. As a result, the surge queue for the load balancer nodes in uswest2a fills and clients receive a 503 error. If uswest2b continues to respond normally, the sum for the load balancer equals the sum for uswest2a. 
Latency 
[HTTP listener] The time elapsed, in seconds, after the request leaves the load balancer until the headers of the response are received. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Example: Suppose that your load balancer has 2 instances in uswest2a and 2 instances in uswest2b, and that requests sent to 1 instance in uswest2a have a higher latency. The average for uswest2a has a higher value than the average for uswest2b. 
RequestCount 
The number of requests completed or connections made during the specified interval (1 or 5 minutes). [HTTP listener] The number of requests received and routed, including HTTP error responses from the registered instances. [TCP listener] The number of connections made to the registered instances. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Example: Suppose that your load balancer has 2 instances in uswest2a
and 2 instances in uswest2b, and that 100 requests are sent to the load balancer. There are 60 requests sent to uswest2a,
with each instance receiving 30 requests, and 40 requests sent to uswest2b, with each instance receiving 20 requests.
With the 
SpilloverCount 
The total number of requests that were rejected because the surge queue is full. [HTTP listener] The load balancer returns an HTTP 503 error code. [TCP listener] The load balancer closes the connection. Reporting criteria: There is a nonzero value Statistics: The most useful statistic is Example: Suppose that your load balancer has uswest2a and uswest2b enabled, and that instances in uswest2a are experiencing high latency and are slow to respond to requests. As a result, the surge queue for the load balancer node in uswest2a fills, resulting in spillover. If uswest2b continues to respond normally, the sum for the load balancer will be the same as the sum for uswest2a. 
SurgeQueueLength 
The total number of requests that are pending routing. The load balancer queues
a request if it is unable to establish a connection with a healthy instance in order to route the request.
The maximum size of the queue is 1,024. Additional requests are rejected when the queue is full.
For more information, see Reporting criteria: There is a nonzero value. Statistics: The most useful statistic is Example: Suppose that your load balancer has uswest2a and uswest2b enabled, and
that instances in uswest2a are experiencing high latency and are slow to respond to requests. As a result, the
surge queue for the load balancer nodes in uswest2a fills, with clients likely experiencing increased response times.
If this continues, the load balancer will likely have spillovers (see the 
UnHealthyHostCount 
The number of unhealthy instances registered with your load balancer. An instance is considered unhealthy after it exceeds the unhealthy threshold configured for health checks. An unhealthy instance is considered healthy again after it meets the healthy threshold configured for health checks. Reporting criteria: There are registered instances Statistics: The most useful statistics are Example: See 
Metric Dimensions for Classic Load Balancers
To filter the metrics for your Classic Load Balancer, use the following dimensions.
Dimension 
Description 

AvailabilityZone

Filter the metric data by the specified Availability Zone. 
LoadBalancerName

Filter the metric data by the specified load balancer. 