24 Managing Resource Versions

Overview of Oracle XML DB Versioning

Versioning lets you create and manage different versions of a resource in Oracle XML DB Repository. A record, or history, is kept of all changes to an Oracle XML DB resource that is under version control. When you update a version-controlled resource, Oracle XML DB stores the pre-update contents as a separate resource version – a snapshot for the historical record.

Versioning features include the following:

• Version control for a resource.

  You can turn version control on or off for an Oracle XML DB Repository resource.

• Updating a version-controlled resource.

  When Oracle XML DB updates a version-controlled resourc, it creates a new version of the resource. This new version is not deleted from the database when you delete the version-controlled resource.

• Accessing a version-controlled resource.

  You can access a version-controlled resource the same way you access any other resource.

• Accessing a resource version.

  To access a particular version of a resource, you use the resource ID of that version. The resource ID can be obtained from the resource version history or from the version-controlled resource itself. See "Versioning and Resource IDs".

Table 24-1 lists some terms used in this chapter.

Versioning and Resource IDs

A resource object ID, or resource ID, is a unique, constant, system-generated identifier for an Oracle XML DB resource. Each resource has a resource ID. This includes version resources, which are system-generated resources that do not have any path names. A resource ID is sometimes called a RESID.

You use PL/SQL package DBMS_XDB_VERSION to put a resource under version-control and manage different versions of it. Some of the DBMS_XDB_VERSION routines accept the path name of a version-controlled resource as argument and return the resource ID of the relevant version resource.

For example, you use function DBMS_XDB_VERSION.makeVersioned to put a resource under version control, that is, to turn it into a version-controlled resource. It accepts as argument a repository path to the resource.

You need not use the same path name for a given version-controlled resource when you perform various versioning operations on it, but the path names you use must all refer to the same resource.

Whenever a path name is passed as an argument representing a version-controlled resource, it is the latest (that is, the current) version of the resource that is used. A path name always stands for the latest version. The only way you can refer to a version other than the current version is to use its resource ID.

The resource ID of a given version is constant. Remember that a version is itself a resource, and the resource ID of a resource never changes.

Each time you check in a version-controlled resource, Oracle XML DB creates a new version resource. A version resource is a snapshot of a resource (its content and metadata) together with a resource ID. The collection of version resources for a given version-controlled resource constitutes a historical sequence of previous versions, the version series or history of the resource.

When you check in a version-controlled resource that has resource ID R, Oracle XML DB creates a new resource ID, P, which refers to a snapshot of the resource (both content and metadata), as it was before it was last checked out. The snapshot was made before check-out, but the associated version resource (and its resource ID P) are created at check-in time. Together, the new resource ID P and the snapshot it refers to thus represent the previous, not the current, version of the resource. Resource ID R continues to refer to the current version.

Put another way, when you check in a version-controlled resource, a version resource is created that represents the previous state of the version-controlled resource. Like any new resource, this new version resource is allocated a new resource ID (P).

You can think about making a version resource (check-in) the way you think about making a backup copy of a file: Just as you give a new name to the backup file, so the previous-version snapshot of a resource is given a new resource ID. The current resource retains the original resource ID, just as your working file keeps its original name.

What this means is that when you check in a resource, in order to "create a new version", what's really new is the version resource (resource ID P and the snapshot it references) that represents the old (previous) version. The newest, or latest, version of the resource (R) is really just the current version. Remember: new version resource = old (previous) version of the resource content and metadata.

Resource ID R refers to the current version of the version-controlled resource throughout its lifetime, from the moment it was put under version control until it is deleted. You can always access the latest version of a resource using its original resource ID.

When you need to refer to a previous version of a resource, you must use its resource ID to reference it; you cannot use a path name. You can use function DBMS_XDB_VERSION.getPredsByRESID to obtain the resource ID of the previous version of a given resource.

Versioning and ACLs

A version resource is immutable; it is a snapshot of resource content and metadata, plus a resource ID, and both snapshot and ID are static. Likewise, the ACL of a version resource cannot be changed.

You can modify the ACL of a version-controlled resource that you have checked out. When you check it in, the modified ACL continues to be associated with the current (latest) version of the resource, and the previous version, that is, the newly created version resource, is associated with the ACL before it was modified. That is, the previous version is associated with the previous ACL, and the current version is associated with the updated ACL.

What is important to keep in mind is this:

• Different versions of a resource can have different ACLs associated with them.

• You can modify the ACL associated with the current version after you check out the resource.

• Check-in associates the ACL as it was before check-out with the newly created version resource, that is, with the previous version of the resource.

• The ACL associated with a given version remains the same.

Resource Versioning Examples

This section presents examples that do the following:

• Put a resource under version control, that is, create a version-controlled resource – Example 24-2

• Retrieve the content of the resource by referring to the resource ID – Example 24-3

• Check out the version-controlled resource (for all users) – Example 24-4

• Update the resource content – Example 24-5

• Check in the resource – Example 24-6

• Retrieve the content and metadata of both the new and old versions of the resource – Example 24-7, Example 24-8, Example 24-9

• Cancel a resource check-out – Example 24-10

Example 24-3 creates an Oracle XML DB Repository resource at repository path /public/t1.txt. The resource has as content the text Mary had a little lamb. The example uses SQL*Plus command VARIABLE to declare bind variables targetPath, current_RESID, and previous_RESID, which are used in other examples in this section.

VARIABLE targetPath      VARCHAR2(700)
VARIABLE current_RESID   VARCHAR2(32)
VARIABLE previous_RESID  VARCHAR2(32)

DECLARE
  res BOOLEAN;
BEGIN
  :targetPath  := '/public/t1.txt';
  IF (DBMS_XDB.existsResource(:targetPath))
     THEN DBMS_XDB.deleteResource(:targetPath);
  END IF;
  res := DBMS_XDB.createResource(:targetPath, 'Mary had a little lamb');
END;
/

The new resource is not version-controlled. Example 24-2 uses PL/SQL function makeVersioned to put it under version control. This function returns the resource ID of the first version resource for the version-controlled resource. The function does not auto-commit; you must use COMMIT.

DECLARE
  resid DBMS_XDB_VERSION.RESID_TYPE;
BEGIN
  resid := DBMS_XDB_VERSION.makeVersioned(:targetPath);
  :current_RESID := resid;
  COMMIT;
END;
/

Example 24-2 also copies the resource ID of the new version resource to bind variable current_RESID. Example 24-3 shows how to use type constructor XDBUritype together with PL/SQL function createOIDPath to retrieve the resource content by referencing the resource ID.

SELECT XDBURIType(DBMS_XDB.createOIDPath(:current_RESID)).getClob() FROM DUAL;

XDBURITYPE(DBMS_XDB.CREATEOIDPATH(:CURRENT_RESID)).GETCLOB()
------------------------------------------------------------
Mary had a little lamb
 
1 row selected.

Example 24-4 checks out the verson-controlled resource (and commits), so that it can be modified. Note that any user can modify a resource that has been checked out.

BEGIN
  DBMS_XDB_VERSION.checkOut(:targetPath);
  COMMIT;
END;
/

Example 24-5 updates the content of the checked-out resource. Before the (LOB) content can be updated, you must lock the resource. The example uses a dummy update of the resource display name (a scalar attribute) to do this.

DECLARE
  content        BLOB;
  newContentBlob BLOB;
  newContentClob CLOB;
  source_offset  INTEGER := 1;
  target_offset  INTEGER := 1;
  warning        INTEGER;
  lang_context   INTEGER := 0;
BEGIN
  -- Lock the resource using a dummy update.
  UPDATE RESOURCE_VIEW
    SET RES =
      updateXML(RES, '/Resource/DisplayName/text()',
                XMLCast(XMLQuery(
                          'declare namespace ns =
                           "http://xmlns.oracle.com/xdb/XDBResource.xsd"; (: :)
                           $r/ns:Resource/ns:DisplayName/text()'
                          PASSING RES AS "r" RETURNING CONTENT)
                        AS VARCHAR2(128)))
    WHERE equals_path(res, :targetPath) = 1;
  -- Get the LOB locator.
  SELECT XMLCast(XMLQuery('declare namespace ns =
                           "http://xmlns.oracle.com/xdb/XDBResource.xsd"; (: :)
                           $r/ns:Resource/ns:XMLLob'
                          PASSING RES AS "r" RETURNING CONTENT)
                 AS BLOB)
    INTO content FROM RESOURCE_VIEW
    WHERE equals_path(RES, :targetPath) = 1;
  -- Update the LOB.
  newContentClob := 'Hickory dickory dock, the mouse ran up the clock';
  DBMS_LOB.createTemporary(newContentBlob, false, DBMS_LOB.CALL);
  DBMS_LOB.convertToBlob(newContentBlob, newContentClob,
                         DBMS_LOB.getLength(newContentClob),
                         source_offset, target_offset,
                         nls_charset_id('AL32UTF8'), lang_context, warning);
  DBMS_LOB.open(content, DBMS_LOB.lob_readwrite);
  DBMS_LOB.trim(content, 0);
  DBMS_LOB.append(content, newContentBlob);
  DBMS_LOB.close(content);
  DBMS_LOB.freeTemporary(newContentBlob);
  DBMS_LOB.freeTemporary(newContentClob);
  COMMIT;
END;
/

Example 24-5 retrieves the LOB content using the LOB locator, which is element /ns:Resource/ns:XMLLob. It empties the existing content and adds new content using PL/SQL procedures trim and append in package DBMS_LOB. It commits the content change.

At this point, the content has been modified, but this change has not been recorded in the version series. Example 24-6 checks in the resource and commits the check-in.

DECLARE
  resid DBMS_XDB_VERSION.RESID_TYPE;
BEGIN
  resid := DBMS_XDB_VERSION.checkIn(:targetPath);
  :previous_RESID := DBMS_XDB_VERSION.getPredsByRESID(resid)(1);
  COMMIT;
END;
/

PL/SQL function checkIn returns the resource ID of the current version, which is the same as current_RESID. Example 24-6 passes this value to PL/SQL function getPredsByRESID. This function returns the list of resource IDs for the (immediate) predecessors of its argument resource.^Foot 1  Example 24-6 assigns the first (and only) element of this list to bind variable previous_RESID.

At this point, the value of current_RESID is the resource ID of the current version, and the value of previous_RESID is the resource ID of the previous version.

You can retrieve the content or metadata of a resource using any of the following methods:

• XDBURIType together with PL/SQL function DBMS_XDB.createOIDPath – Retrieve content. See Example 24-3 and Example 24-7.

• PL/SQL function DBMS_XDB_VERSION.getContentsCLOBByRESID – Retrieve content. See Example 24-8.

• PL/SQL function DBMS_XDB_VERSION.getResourceByRESID – Retrieve metadata. See Example 24-9.

You can use XDBURIType with createOIDPath to access resource content using protocols. For example, you could have Oracle XML DB serve up various versions of a graphic image file resource for a Web page, setting the HREF for the HTML IMAGE tag to a value returned by createOIDPath.

Example 24-7 through Example 24-9 use these different methods to retrieve the two versions of the resource addressed by bind variables current_RESID and previous_RESID after check-in.

SELECT XDBURIType(DBMS_XDB.createOIDPath(:current_RESID)).getClob() FROM DUAL;
 
XDBURITYPE(DBMS_XDB.CREATEOIDPATH(:CURRENT_RESID)).GETCLOB()
------------------------------------------------------------
Mary had a little lamb
 
1 row selected.
 
SELECT XDBURIType(DBMS_XDB.createOIDPath(:previous_RESID)).getClob() FROM DUAL;
 
XDBURITYPE(DBMS_XDB.CREATEOIDPATH(:PREVIOUS_RESID)).GETCLOB()
-------------------------------------------------------------
Hickory dickory dock, the mouse ran up the clock
 
1 row selected.

SELECT DBMS_XDB_VERSION.getContentsCLOBByRESID(:current_RESID) FROM DUAL;

DBMS_XDB_VERSION.GETCONTENTSCLOBBYRESID(:CURRENT_RESID)
-------------------------------------------------------
Mary had a little lamb
 
1 row selected.
 
SELECT DBMS_XDB_VERSION.getContentsCLOBByRESID(:previous_RESID) FROM DUAL;
 
DBMS_XDB_VERSION.GETCONTENTSCLOBBYRESID(:PREVIOUS_RESID)
--------------------------------------------------------
Hickory dickory dock, the mouse ran up the clock
 
1 row selected.

SELECT XMLSerialize(DOCUMENT DBMS_XDB_VERSION.getResourceByRESID(:current_RESID)
                    AS CLOB INDENT SIZE = 2)
  FROM DUAL;
<Resource xmlns="http://xmlns.oracle.com/xdb/XDBResource.xsd" Hidden="false"
          Invalid="false" VersionID="2" ActivityID="0" Container="false"
          CustomRslv="false" VersionHistory="false" StickyRef="true">
  <CreationDate>2009-05-06T12:33:34.012133</CreationDate>
  <ModificationDate>2009-05-06T12:33:34.280199</ModificationDate>
  <DisplayName>t1.txt</DisplayName>
  <Language>en-US</Language>
  <CharacterSet>UTF-8</CharacterSet>
  <ContentType>text/plain</ContentType>
  <RefCount>1</RefCount>
  <ACL>
    <acl description="Public:All privileges to PUBLIC"
         xmlns="http://xmlns.oracle.com/xdb/acl.xsd" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd 
                             http://xmlns.oracle.com/xdb/acl.xsd"
         shared="true">
      <ace>
        <grant>true</grant>
        <principal>PUBLIC</principal>
        <privilege>
          <all/>
        </privilege>
      </ace>
    </acl>
  </ACL>
  <Owner>HR</Owner>
  <Creator>HR</Creator>
  <LastModifier>HR</LastModifier>
  <SchemaElement>http://xmlns.oracle.com/xdb/XDBSchema.xsd#text</SchemaElement>
  <Contents>
    <text>Mary had a little lamb</text>
  </Contents>
  <VCRUID>69454F2EF12E3375E040578C8A1764B5</VCRUID>
  <Parents>69454F2EF12F3375E040578C8A1764B5</Parents>
</Resource>
 
1 row selected.
 
SELECT XMLSerialize(DOCUMENT DBMS_XDB_VERSION.getResourceByRESID(:previous_RESID)
                    AS CLOB INDENT SIZE = 2)
  FROM DUAL;
<Resource xmlns="http://xmlns.oracle.com/xdb/XDBResource.xsd" Hidden="false"
          Invalid="false" VersionID="1" Container="false" CustomRslv="false" 
          VersionHistory="false" StickyRef="true">
  <CreationDate>2009-05-06T12:33:34.012133</CreationDate>
  <ModificationDate>2009-05-06T12:33:34.012133</ModificationDate>
  <DisplayName>t1.txt</DisplayName>
  <Language>en-US</Language>
  <CharacterSet>UTF-8</CharacterSet>
  <ContentType>text/plain</ContentType>
  <RefCount>0</RefCount>
  <ACL>
    <acl description="Public:All privileges to PUBLIC"
         xmlns="http://xmlns.oracle.com/xdb/acl.xsd" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.oracle.com/xdb/acl.xsd
                             http://xmlns.oracle.com/xdb/acl.xsd"
         shared="true">
      <ace>
        <grant>true</grant>
        <principal>PUBLIC</principal>
        <privilege>
          <all/>
        </privilege>
      </ace>
    </acl>
  </ACL>
  <Owner>HR</Owner>
  <Creator>HR</Creator>
  <LastModifier>HR</LastModifier>
  <SchemaElement>http://xmlns.oracle.com/xdb/XDBSchema.xsd#text</SchemaElement>
  <Contents>
    <text>Hickory dickory dock, the mouse ran up the clock</text>
  </Contents>
  <VCRUID>69454F2EF12E3375E040578C8A1764B5</VCRUID>
</Resource>
 
1 row selected.

You can cancel a check-out using PL/SQL function DBMS_XDB_VERSION.unCheckOut. Example 24-10 illustrates this.

DECLARE
  resid DBMS_XDB_VERSION.RESID_TYPE;
BEGIN
  resid := DBMS_XDB_VERSION.unCheckOut(:targetPath);
END;
/

Table 24-2 summarizes the subprograms in PL/SQL package DBMS_XDB_VERSION that are mentioned in this chapter.