JDeveloper, WebCenter and MDS: Write Directory

With PS3 of JDeveloper, WebCenter introduced a configured "Portal" application.  This out-of-the-box application runs immediately from JDeveloper.  It can also be further modified at runtime through the "administration" link after logging in as "weblogic/weblogic1". 

Looking under the covers, the JDeveloper application that gets created is using Oracle's MDS (Metadata Services) layer to store customizations and new base documents in a configurable "write" directory.  
To view this folder, look under the "Application Properties" and select "Run->MDS" from the dialog

Details of this dialog can be found in the doc but it's basically just telling you where all those changes you make at runtime are stored.  It also allows you to maintain those runtime changes between runs but beware that these values aren't included when the application is deployed.  It's like a little local sandbox you can play in while developing the app.

Base documents are under the resource directory:
>./oracle/webcenter/portalapp/pagehierarchy/Page1.jspx
>./pageDefs/oracle/webcenter/portalapp/pagehierarchy/Page1PageDef.xml

Customizations to base documents end up in an "mdssys" directory:
>./oracle/webcenter/portalapp/pagehierarchy/mdssys/cust/site/site/pages.xml.xml
>./oracle/webcenter/portalapp/pagehierarchy/mdssys/cust/site/site/Page1.jspx.xml

One word of warning;  if you turn on the "Preserve customizations across applications runs" option then the customization or base document change may hide any changes you make to the same document at design-time.