While deploying some services which make use of a datasource (e.g. EJBTimerService) the following ERROR is thrown in the log file, why?

2007-08-29 10:41:29,857 ERROR [org.jboss.ejb.txtimer.GeneralPurposeDatabasePersistencePlugin] Cannot create timer table
java.lang.IllegalStateException: Cannot obtain type mapping from: jboss.jdbc:datasource=DefaultDS,service=metadata
     at org.jboss.ejb.txtimer.GeneralPurposeDatabasePersistencePlugin.createTableIfNotExists(GeneralPurposeDatabasePersistencePlugin.java:137)
     at org.jboss.ejb.txtimer.DatabasePersistencePolicy.startService(DatabasePersistencePolicy.java:104)

This exception occurs, when your have an incorrect

When you configure a datasource, please use the example templates given for your database server at : $JBOSS_HOME/docs/examples/jca

You can find more information on datasource configuration at : http://wiki.jboss.org/wiki/Wiki.jsp?page=ConfigDataSources