OAF的XML文件有:

• UIX Pages and Regions.
• OAF Personalizations
• BC4J Substitutions (EO, VO Substitutions)
• BC4J Components (EO.xml, VO.xml, AM.xml, AO.xml, VL.xml)

OAF的很多組件都是以XML的形式存在，但到最終的系統里，OAF的XML文件會存儲到數據庫表中(JDR_表)。MDS提供API(JDR_UTILS)從JDR表讀取，并構建XML的定義。

Table	Details
JDR_PATHS	Stores document paths, packages and there parent child relationship. Primary Key: PATH_DOCID
JDR_COMPONENTS	Stores components on documents and OA Framework pages. Primary Key: COMP_DOCID, COMP_SEQ
JDR_ATTRIBUTES	Stores attribute/properties of components on documents and OA Framework pages. Primary Key: ATT_COMP_DOCID, ATT_COMP_SEQ, ATT_SEQ
JDR_ATTRIBUTES_TRANS	Stores translated attribute values of document components or OA framework pages. Primary Key: ATL_COMP_DOCID, ATL_LANG, ATL_COMP_REF, ATL_NAME

JDR_UTILS API

How to get page or region contents from MDS repository
BEGIN
jdr_utils.printDocument('/oracle/apps/fnd/framework/toolbox/tutorial/webui/HelloWorldPG',1000);
END;

Lists all the MDS documents of specified directory currently imported into database.
BEGIN
jdr_utils.listdocuments ('/oracle/apps/per/selfservice.webui', TRUE ) ;
END

Delete a Page from MED repository
BEGIN
jdr_utils.deletedocument ('p_document => /oracle/apps/per/selfservice/webui/somepagePG') ;
END ;

In jDeveloper, when I build a page and its regions, it looks I am building an XML file. Is page definition stored as XML file in OA Framework?
The storage page definition happens in JDR tables, where page components are not stored as XML . But MDS provides API's to build XML definition from the data in JDR tables.
Hence there are two provisions
1. When you design a page, you store "page definiton" in XML format on your pc. When deploying to your system/server, you load this XML file into JDR tables by using command xmlimporter
2 . When a user runs the page, OA Framework does the following steps:-
Step a . OA Fwk Requests page definition/structure from (MDS) --note its cached too
Step b . MDS engine returns a xml file to OA Framework
Step c . Each node/component in XML(of Step b) is translated into a web bean object. Let's say your page has
Region-Main
field1
Regionchild
Button

In this case, four web beans objects will be instantiated by OA Framework. A bean object is nothing but an object representation components like fields, buttons, regions etc . A bean object also has methods like setRendered, setRequired, getRequired etc.
Step d . Not only we have beans created for that page, those beans are nested as well, in exactly the same sequence of components within Region-Main. Hence parent child relationship is retained.
Step e . After rendering the page,OA Framework then calls the controller class for that MDS page. The page is displayed to user after processRequest in Controller is completed.

refer: http://oracle.anilpassi.com/mds-in-oa-framework-what-exactly-it-is-2.html

MetaData Service (MDS) in OA Framework