Sun Java System Application Server Enterprise Edition 8.1 2005Q2 High Availability Administration Guide

- Previous: Enabling Applications for Load Balancing
- Next: Exporting the Load Balancer Configuration File

## Creating the HTTP Health Checker

The load balancer’s health checker periodically checks all the configured Application Server instances that are marked as unhealthy. A health checker is not required, but if no health checker exists, or if the health checker is disabled, the periodic health check of unhealthy instances is not performed.

The load balancer’s health check mechanism communicates with the application server instance using HTTP. The health checker sends an HTTP request to the URL specified and waits for a response. A status code in the HTTP response header between 100 and 500 means the instance is healthy.

### Creating a Health Checker

To create the health checker, use the `asadmin` `create-http-health-checke` r command. Specify the following parameters:

Table 4–2 Health Checker Parameters

Parameter | Description | Default
--- | --- | ---
url | Specifies the listener’s URL that the load balancer checks to determine its state of health. | “/”
interval | Specifies the interval in seconds at which health checks of instances occur. Specifying 0 disables the health checker. | 30 seconds
timeout | Specifies the timeout interval in seconds within which a response must be obtained for a listener to be considered healthy. | 10 seconds

If an application server instance is marked as unhealthy, the health checker polls the unhealthy instances to determine if the instance has become healthy. The health checker uses the specified URL to check all unhealthy application server instances to determine if they have returned to the healthy state.

If the health checker finds that an unhealthy instance has become healthy, that instance is added to the list of healthy instances.

For more information see the documentation for `create-http-health-checker` and `delete-http-health-checker`.

### Additional Health Check Properties for Healthy Instances

The health checker created by `create-http-health-checker` only checks unhealthy instances. To periodically check healthy instances set some additional properties in your exported `loadbalancer.xml` file.

Note –

These properties can only be set by manually editing `loadbalancer.xml` after you’ve exported it. There is no equivalent asadmin command to use.

To check healthy instances, set the following properties:

Table 4–3 Health-checker Manual Properties

Property | Definition
--- | ---
active-healthcheck-enabled | True/false flag indicating whether to ping healthy server instances to determine whether they are healthy. To ping server instances, set the flag to true.
number-healthcheck-retries | Specifies how many times the load balancer’s health checker pings an unresponsive server instance before marking it unhealthy. Valid range is between 1 and 1000. A default value to set is 3.

Set the properties by editing the `loadbalancer.xml` file. For example:

    <property name="active-healthcheck-enabled" value="true"/>
    <property name="number-healthcheck-retries" value="3"/>

If you add these properties, then subsequently edit and export the `loadbalancer.xml` file again, you must add these properties to the file again, since the newly exported configuration won’t contain them.

- Previous: Enabling Applications for Load Balancing
- Next: Exporting the Load Balancer Configuration File

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