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Elastic Load Balancing
Application Load Balancers

Health Checks for Your Target Groups

Your Application Load Balancer periodically sends requests to its registered targets to test their status. These tests are called health checks.

Each load balancer node routes requests only to the healthy targets in the enabled Availability Zones for the load balancer. Each load balancer node checks the health of each target, using the health check settings for the target group with which the target is registered. After your target is registered, it must pass one health check to be considered healthy. After each health check is completed, the load balancer node closes the connection that was established for the health check.

If no Availability Zone contains a healthy target, the load balancer nodes route requests to all targets.

Note that health checks do not support WebSockets.

Health Check Settings

You configure health checks for the targets in a target group using the following settings. The load balancer sends a health check request to each registered target every HealthCheckIntervalSeconds seconds, using the specified port, protocol, and ping path. It waits for the target to respond within the response timeout period. If the health checks exceed the threshold for consecutive failed responses, the load balancer takes the target out of service. When the health checks exceed the threshold for consecutive successful responses, the load balancer puts the target back in service.

Setting Description

HealthCheckProtocol

The protocol the load balancer uses when performing health checks on targets. The possible protocols are HTTP and HTTPS. The default is the HTTP protocol.

HealthCheckPort

The port the load balancer uses when performing health checks on targets. The default is to use the port on which each target receives traffic from the load balancer.

HealthCheckPath

The ping path that is the destination on the targets for health checks. The default is /.

HealthCheckTimeoutSeconds

The amount of time, in seconds, during which no response from a target means a failed health check. The range is 2 to 60 seconds. The default is 5 seconds.

HealthCheckIntervalSeconds

The approximate amount of time, in seconds, between health checks of an individual target. The range is 5 to 300 seconds. The default is 30 seconds.

HealthyThresholdCount

The number of consecutive successful health checks required before considering an unhealthy target healthy. The range is 2 to 10. The default is 5.

UnhealthyThresholdCount

The number of consecutive failed health checks required before considering a target unhealthy. The range is 2 to 10. The default is 2.

Matcher

The HTTP codes to use when checking for a successful response from a target. You can specify values or ranges of values between 200 and 499. The default value is 200.

Target Health Status

Before the load balancer sends a health check request to a target, you must register it with a target group, specify its target group in a listener rule, and ensure that the Availability Zone of the target is enabled for the load balancer. Before a target can receive requests from the load balancer, it must pass the initial health checks. After a target passes the initial health checks, its status is Healthy.

The following table describes the possible values for the health status of a registered target.

Value Description

initial

The load balancer is in the process of registering the target or performing the initial health checks on the target.

healthy

The target is healthy.

unhealthy

The target did not respond to a health check or failed the health check.

unused

The target is not registered with a target group, the target group is not used in a listener rule for the load balancer, or the target is in an Availability Zone that is not enabled for the load balancer.

draining

The target is deregistering and connection draining is in process.

Health Check Reason Codes

If the status of a target is any value other than Healthy, the API returns a reason code and a description of the issue, and the console displays the same description in a tooltip. Note that reason codes that begin with Elb originate on the load balancer side and reason codes that begin with Target originate on the target side.

Reason code Description

Elb.InitialHealthChecking

Initial health checks in progress

Elb.InternalError

Health checks failed due to an internal error

Elb.RegistrationInProgress

Target registration is in progress

Target.DeregistrationInProgress

Target deregistration is in progress

Target.FailedHealthChecks

Health checks failed

Target.InvalidState

Target is in the stopped state

Target is in the terminated state

Target is in the terminated or stopped state

Target is in an invalid state

Target.IpUnusable

The IP address cannot be used as a target, as it is in use by a load balancer.

Target.NotInUse

Target group is not configured to receive traffic from the load balancer

Target is in an Availability Zone that is not enabled for the load balancer

Target.NotRegistered

Target is not registered to the target group

Target.ResponseCodeMismatch

Health checks failed with these codes: [code]

Target.Timeout

Request timed out

Check the Health of Your Targets

You can check the health status of the targets registered with your target groups.

To check the health of your targets using the console

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. On the navigation pane, under LOAD BALANCING, choose Target Groups.

  3. Select the target group.

  4. On the Targets tab, the Status column indicates the status of each target.

  5. If the status is any value other than Healthy, view the tooltip for more information.

To check the health of your targets using the AWS CLI

Use the describe-target-health command. The output of this command contains the target health state, and it includes a reason code if the status is any value other than Healthy.

Modify the Health Check Settings of a Target Group

You can modify the health check settings for your target group at any time.

To modify the health check settings of a target group using the console

  1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

  2. On the navigation pane, under LOAD BALANCING, choose Target Groups.

  3. Select the target group.

  4. On the Health checks tab, choose Edit.

  5. On the Edit target group page, modify the settings as needed, and then choose Save.

To modify the health check settings of a target group using the AWS CLI

Use the modify-target-group command.