<GlobalNamingResources>
  ...
  <Resource name="sharedDataSourceA"
            global="sharedDataSourceA"
            type="javax.sql.DataSource"
            factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
            alternateUsernameAllowed="true"
            username="bar"
            password="barpass"
            ...
<Resource name="sharedDataSourceB"
            global="sharedDataSourceB"
            type="javax.sql.DataSource"
            factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
            alternateUsernameAllowed="true"
            username="bar"
            password="barpass"
            ...
  ...
</GlobalNamingResources>

<Context path="/a"...>
  ...
  <ResourceLink
            name="appDataSource"
            global="sharedDataSourceA"
            type="javax.sql.DataSource"
            factory="org.apache.naming.factory.DataSourceLinkFactory"
            username="foo"
            password="foopass"
  ...
</Context>
<Context path="/b"...>
  ...
  <ResourceLink
            name="appDataSource"
            global="sharedDataSourceA"
            type="javax.sql.DataSource"
  ...
</Context>