[<WAR_HOME>/WEB-INF/web.xml]
<web-app>
    <resource-ref>
        <description>My DataSource</description>
        <res-ref-name>jdbc/myds</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
        <res-sharing-scope>Shareable</res-sharing-scope>
    </resource-ref>
</web-app>

[<WAR_HOME>/META-INF/context.xml]
<Context path="/mywebapp">
    <ResourceLink name="jdbc/msDS" global="jdbc/myDS" type="javax.sql.DataSource" />
</Context>

[<TOMCAT_HOME>/server.xml]
<Server>
    <GlobalNamingResources>
        <Resource name="jdbc/myDS"
                  auth="Container"
                  url="…"
                  user="…"
                  password="…"
                  type="javax.sql.DataSource"
                  driverClassName="…"
                  maxActive="10"
                  maxIdle="5"/>
    </GlobalNamingResources>
</Server>