An idempotent request is one that does not cause any change or inconsistency in an application when retried. In HTTP, some methods (such as GET) are idempotent, while other methods (such as POST) are not. Retrying an idempotent URL must not cause values to change on the server or in the database. The only difference is a change in the response received by the user.
Examples of idempotent requests include search engine queries and database queries. The underlying principle is that the retry does not cause an update or modification of data.
To enhance the availability of deployed applications, configure the environment to retry failed idempotent HTTP requests on all the instances serviced by a load balancer. This option is used for read-only requests, for example, to retry a search request.
Configure idempotent URLs in the sun-web.xml file. When you export the load balancer configuration, idempotent URL information is automatically added to the loadbalancer.xml file.
For more information on configuring idempotent URLs, see Configuring Idempotent URL Requests in Sun GlassFish Enterprise Server 2.1 Developer’s Guide.