Use a single instance of a resource adapter

We can restrict certain resource adapters to a single runtime instance inside the Java Virtual Machine (JVM).

Enabling this setting imposes a highly restrictive environment on the system and should be used with caution.

Using the single-instance resource adapter configuration option on some resource adapters can allow us to set up an environment that optimally behaves. Some resource adapters that support inbound communications from the enterprise information system (EIS) might require single-instance behavior. By enabling this setting, server startup time can be optimized. Other resource adapters might not require this setting. We need to determine if we should configure the resource adapter for single-instance behavior.

Consider using the single-instance resource adapter configuration for testing and troubleshooting problems. Placing the single-instance restriction on some resource adapters might work as a corrective action for problems; enabling single-instance behavior on one or more resource adapters thought to be involved in a problem can help isolate the specific issue.

This design does not allow two resource adapter JavaBeans instances that would return true from the equals method to coexist in the same JVM, if any one of them is configured as single-instance. For example, if two applications that have embedded the same resource adapter, or one application that embeds a resource adapter and the same resource adapter is installed in the server as a stand-alone resource adapter, are configured on the same server such that even though some of their config attributes are different, the ones that the equals() method evaluates are equal, this will no longer be allowed, and will return a ResourceException.

The vendor of a resource adapter which cannot tolerate multiple instances does not have a JCA-defined method of communicating this. Therefore, it is up to the deployer to recognize the need, and configure resource adapter(s) for single-instance behavior.