Jboss Datasource configuration for MySQL - MysqlXA

I have been looking at several examples for configuring a datasource for MySQL in Jboss 7. All references i have seen for the element looks like this:

<driver name="com.mysql" module="com.mysql">
    <driver-class>com.mysql.jdbc.Driver</driver-class>
    <xa-datasource-class>com.mysql.jdbc.jdbc2.optional.MysqlXADataSource</xa-datasource-class>
</driver>

I know what the <driver-class> is but what exactly is the <xa-datasource-class> what is its purpose?

When i configured a datasource on Tomcat before i did not need to specify the xa-datasource for any database. Why is it different here?

Thanks

According to jdbc 4.0 specification (12.2): XA datasources produces XA connections capable to be used in global/distributed transactions. You might need such a connection if you need a transaction to span more than one database or a JMS calls. You can find a clear explanation of the concept here: http://www.theserverside.com/discussions/thread.tss?thread_id=21385#95346

If you don't have such a distributed transactions scenario you don't need to specify a xa-datasource, a simple datasource configuration is enough. So, if you use a simple datasource there is no need to specify a xa-datasource-class when you declare your driver.

<datasources>
    <datasource jndi-name="java:/myDatasource" pool-name="MyDS" enabled="true" jta="true" use-java-context="true" use-ccm="true">
                <connection-url>
                    jdbc:mysql://localhost:3306/mydb
                </connection-url>
                <driver>
                    mysql
                </driver>
                <transaction-isolation>
                    TRANSACTION_READ_COMMITTED
                </transaction-isolation>
                <pool>
                    <min-pool-size>
                        5
                    </min-pool-size>
                    <max-pool-size>
                        10
                    </max-pool-size>
                    <prefill>
                        true
                    </prefill>
                    <use-strict-min>
                        false
                    </use-strict-min>
                    <flush-strategy>
                        FailingConnectionOnly
                    </flush-strategy>
                </pool>
                <security>
                    <user-name>
                        username
                    </user-name>
                    <password>
                        password
                    </password>
                </security>
            </datasource>
            <drivers>
                <driver name="mysql" module="com.mysql"/>
            </drivers>
        </datasources>