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Following resources can be useful while migrating from Tomcat to Geronimo
1) http://geronimo.apache.org/schemas-2.1/docs/geronimo-module-1.2.xsd.html - Look for inverse-classloading. You may also want to know more on dependencies, hidden-classes, non-overridable-classes. These all elements will be helpful in modifying the geronimo-web.xml.
2) Learn how to configure JMS with apache geronimo - http://cwiki.apache.org/GMOxDOC21/web-application-for-jms-access.html
3) Learn how to configure JDBC with apache geronimo- http://cwiki.apache.org/GMOxDOC21/web-application-for-jdbc-access.html
4) For security related configurations you can refer- http://cwiki.apache.org/GMOxDOC21/configuring-security.html
5) More on configuring various services- JMS, JDBC http://cwiki.apache.org/GMOxDOC21/configuring-services.html