Abstract:
Approaches are described herein to represent a delta using the extensible style language transformation language (XSLT) to describe the delta between versions of a data entity. XSLT is a language that defines operations for transforming a body of data (“source”) that conforms to the extended mark-up language (XML) into a different body of data (“target”) typically in another format, such as HTML. A set of XML instructions represents a delta by specifying operations needed to transform or change a source version of an XML entity into a target version of an XML entity.

Description:
RELATED APPLICATION 
     This patent application claims priority from U.S. Provisional Patent Application No. 60/298,437, entitled “MANAGING XML IN A DATABASE”, filed by Mark J. Barrenechea, on Jun. 15, 2001, the contents of which are herein incorporated by reference in its entirety; the patent also claims priority from U.S. Provisional Patent Application No. 60/384,693, entitled “REPRESENTING DELTAS BETWEEN VERSIONS USING XSLT”, filed by Tim Yu, et al. on May 31, 2002, the contents of which are herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to representing differences between versions of a body of data, and in particular, between versions of an XML entity. 
     BACKGROUND OF THE INVENTION 
     Successful software products persist, evolving into multiple versions, each of which may incorporate improvements over previous versions. Managing the development of software is an enormously complex undertaking, especially for evolving software. The practice of managing the evolution of software is referred to as source configuration management. 
     An important function of source configuration management is versioning. Versioning refers to the creating and tracking of versions of software components (e.g. source code files) in a particular software product. Versioning tools are software tools that provide or facilitate versioning. Versioning tools track versions of a software component, and provide information about differences between versions. Differences between versions of a data entity, such as software components, files, and text documents, or a portion thereof, are referred to herein as a delta. 
     Initial versioning tools stored a complete copy of each version of a software component. Information about the delta between particular versions was generated by comparing the complete copy of the versions. 
     Later versioning tools stored information describing only deltas between versions. Data representing the delta between particular versions is referred to herein as a delta representation. Storing delta representations requires less storage than storing complete copies of versions because, between consecutive versions of a software component, the complete delta comprises only a small percentage of the software component. 
     Most delta based versioning tools are developed by proprietary software vendors. A proprietary software vendor may use proprietary formats to format data generated by the vendor&#39;s versioning tools, including a proprietary format for representing deltas. Typically, a proprietary format is protected and/or not made public. This prevents other software vendors from developing tools that can process data generated by another vendor&#39;s versioning tool. Thus, versioning tools developed by one software vendor cannot be used to process data generated by another vendor&#39;s versioning tools. The user of a versioning tool thus is limited to one vendor. 
     Furthermore, a user (e.g. software development company) of a “legacy” versioning tool may wish to switch to another versioning tool. To do this, the proprietary formatted data (“legacy data”) of the legacy versioning tool must be converted to the format of the new versioning tool. The process of converting the format of data is inherently expensive, not only in terms of the resources expended to carry out the conversion, but also in terms of the interruption to the business operations of the user. Because the vendor of the legacy tool is the only one that may know about or is entitled to use the proprietary format of the legacy data, only that vendor can provide the capability to convert the legacy data. This gives a virtual monopoly to the vendor, with a bargaining power that is often exploited by the vendor. 
     Based on the foregoing, it is clearly desirable to provide an approach that uses non-proprietary formats for representing data generated by versioning tools, and in particular, for delta representations. 
     SUMMARY OF THE INVENTION 
     Approaches are described herein to represent a delta using the extensible style language transformation language (XSLT) to describe the delta between versions of a data entity. XSLT is a language that defines operations for transforming a body of data (“source”) that conforms to the extended mark-up language (XML) into a different body of data (“target”) typically in another format, such as HTML. According to an approach, a set of XML instructions represents a delta by specifying operations needed to transform or change a source version of an XML entity into a target version of an XML entity. 
     An advantage of representing deltas using XSLT is that the deltas are represented using an open, non-proprietary, and established standard. In general, information about the standard is openly available to all software developers, and provides a common, popular, and extensively supported format to represent the deltas. In fact, many widely available XSLT processors may be used to transform a source version of an XML entity into a target version of the XML entity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a flow diagram providing an architectural overview of an embodiment of the present invention. 
         FIG. 2  depicts in detail XML code of a version of an XML entity according to an embodiment of the present invention. 
         FIG. 3  depicts in detail XML code of another version of an XML entity according to an embodiment of the present invention. 
         FIG. 4  depicts in detail XSLT instructions of an XSLT delta representation according to an embodiment of the present invention. 
         FIG. 5  is a flowchart depicting a process for generating XSLT templates for an XSLT delta representation according to an embodiment of the present invention. 
         FIG. 6  is a diagram depicting multiple delta representations used to generate multiple versions of a data entity according to an embodiment of the present invention. 
         FIG. 7  is computer system that may be used to implement an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method and apparatus for representing the deltas between versions of a data entity are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     XML Technology 
     Because the present invention is based on XML related technology, a description of XML is useful. XML is rapidly becoming the common standard for representing data. XML describes and provides structure to a body of data, such as a file or data packet, referred to herein as an XML entity. The XML standard provides for tags that delimit the sections of an XML entity referred to as XML elements. Each XML element may contain one or more name-value pairs referred to as attributes. The following XML Segment A is provided to illustrate XML. 
     Segment A 
     &lt;book&gt;
         &lt;title&gt;XML Programming&lt;/title&gt;   &lt;publication publisher=“Doubleday”
           date=“January”&gt;&lt;/publication&gt;   
           &lt;Authors&gt;
           &lt;Author&gt;Mark Berry&lt;/Author&gt;   &lt;Author&gt;Mary Bay&lt;/Author&gt;   
           &lt;/Authors&gt;       

     &lt;/book&gt; 
     XML elements are delimited by a start tag and a corresponding end tag. For example, segment A contains the start tag &lt;title&gt; and the end tag &lt;/title&gt; to delimit an element. The data between the elements is referred to as the elements content. In the case of this element, the content of the element is the text value ‘XML Programming’. 
     Element content may contain various other types of data, which include attributes and other elements. Attributes of an element are represented by attribute name-value pairs. An attribute name-value pair specifies the attribute&#39;s name and value. For example, the element delimited by start and end tags &lt;publication&gt; and &lt;/publication&gt; contains the attribute name-value pair publisher=“Doubleday”, specifying an attribute name of publisher and an attribute value of the literal string “Doubleday”. An element is herein referred to by its start tag. For example, the element delimited by the start and end tags &lt;publication&gt; and &lt;/publication&gt; is referred to as the publication element. 
     The book element is an example of an element that contains one or more other elements. Book contains the element publication and authors. An element that is contained by another element is referred to as a descendant of that element. Thus, the publication and authors elements are descendents of the element book. The book element is an ascendant of the elements publication and author. Elements that are direct descendents of the same element are sibling elements. The siblings of title shown in segment A are publication and authors, exclusively. 
     Authors contains a pair of “iterative elements” defined by the multiple tags named author. They are referred to as iterative elements because they are sibling elements defined by tags of the same name. To facilitate reference to elements, a particular element may be referred to by the following notation element-name[n], where n refers to the order of the element among its sibling elements that are defined by the same tag of the same name, if any. For example, Author[1] refers to the first iterated element author within authors. 
     A path is a sequence of hierarchically related elements, where each element in the sequence is preceded by its ascendants, if any. A path identifies a particular element. For example, the path string “/book/Authors/Author[ ] ” identifies the first iterated element author. 
     By defining an element that contains attributes and descendant elements, the XML entity defines a hierarchical tree relationship between the element, its descendant elements, and its attribute. A set of elements that have such a hierarchical tree relationship is referred to herein as an XML tree. 
     Industry standards define structures for XML trees. One such standard is the Document Object Model (DOM), promulgated by the W3C. An XML tree that conforms to the DOM standard is herein referred to as a DOM tree. 
     In order for a computer to operate on an XML tree, an in-memory representation of the XML tree is generated. In general, an XML tree is read from a storage device (a disk that stores files that contain XML entities) to create in-memory data structures used to represent an XML tree. The in-memory data structures are manipulated by applications running on the computer. Typically, the applications access and manipulate the data structures through a set of routines or functions designed for this purpose. 
     Typically, an XML tree is represented in memory as a node tree, which is a set of hierarchically related linked nodes. A node in the node tree represents, for example, an element, an element&#39;s value, or an attribute of the element. Links between a node and another node represent a hierarchal tree relationship between the nodes, their corresponding elements, attributes, and value. For example, a node corresponding to a parent element node may be linked to nodes representing child elements of the parent element, and linked to nodes representing attributes of the parent element. 
     Overview 
       FIG. 1  is a flow diagram providing an architectural overview of an embodiment of the present invention. Referring to  FIG. 1 , its shows XSLT delta representation  140 . An XSLT delta representation, such as XSLT delta representation  140 , is a body of instructions that represents the delta between a base XML entity and a target XML entity. Instructions in an XSLT delta representation are applied to a source XML entity to produce a copy of the target XML entity. XSLT delta representation  140  represents the delta between target XML entity version  102  and source XML entity version  101 . An XSLT delta representation  140  may be one or more XSLT instructions stored in one or more files or other types of data containers. 
     Process  120  is representative of operations that are performed to determine the delta between target XML entity version  102  and source XML entity version  101 . The delta may be determined by generating a node tree representation of XML entity versions  102  and  101 , and comparing the node tree representations to determine the delta. 
     Process  130  is representative of operations performed to generate, based on the differences determined by process  120 , XSLT instructions that represent differences between XML entity versions  102  and  101 . The instructions generated by process  130  are stored as XSLT delta representation  140 . 
     Process  150  represents operations performed by an XSLT processor to interpret XSLT delta representation  140 . An XSLT processor is a software component configured to execute XSLT instructions. Interpretation of XSLT delta representation  140  causes the XSLT processor to transform source XML entity version  101  to generate a copy of a target XML entity version  102 , XML entity version  102 ′. 
     XML entity version  102  and  102 ′ may not be exact replica&#39;s of each other in that white space in XML entity version  102  may not be present in XML entity version  102 ′. White space refers to punctuation and formatting characters in an XML entity that are not preserved by an XSLT processor during parsing of the XML entity. White space includes, for example, carriage returns, tabs and space characters between attribute name-value pairs. In an embodiment, comments are not treated as white space (i.e. text contained in an element having the format &lt;!-- comment text --&gt;). 
     Source XML entity version  101 , target XML entity version  102 , and XSLT delta representation  140  are represented in greater detail in  FIGS. 2 ,  3 , and  4 , respectively. 
     Referring to  FIG. 2 , it shows node booklist  210  within source XML entity version  101 . Booklist  210  contains iterated book elements  220 ,  230 ,  240 ,  250 , and  260 . Each of the elements contain the elements title, author, publisher, and price. 
     Referring to  FIG. 3 , it shows element booklist  310 , a version of element booklist  210 . Element booklist  310  contains iterated book elements  320 ,  340 ,  350 ,  360 , and  370 . Most of these elements are copies or modified versions of the iterated book elements in booklist  210 . Elements  340  and  360  are copies of elements  240  and  260 . Element  320  is a modified version of element  220  having the following differences: the relative positions of the publisher and author elements within element  320  have been switched. Element  350  is a modified version of element  250 . The text value of the element price in element  350  reflects a change in value from “15.9” to “15.99”. 
     In addition, element  230  has been deleted from booklist  310 . Element  370  has been inserted into booklist  310 . 
       FIG. 4  shows XSLT instructions within XSLT delta representation  140 . XSLT is a declarative language, meaning it specifies how results should look like. XSLT processors operate on in-memory node tree representations of XML entities. XSLT instructions specify how those nodes should be reflected in the result, referred to herein as a target node tree. 
     In the following description of  FIGS. 2 and 3 , elements of target XML entity version  102  and source XML entity version  101  may be referred to as nodes. The reason these elements are referred to as nodes is that target XML entity version  102  and source XML entity version  101  are used to illustrate how an XSLT processor applies the XSLT instructions to an in-memory node tree representation of these XML entities. Referring to the elements as nodes allows operations of the XSLT processor to be more conveniently and concisely expressed. 
     XSLT delta representation  140  contains multiple XSLT templates, each XSLT template specifying how one or more nodes should appear in a target “node tree”. An XSLT template may include a match attribute, which identifies which one or more nodes in a “source” node tree the XSLT template is to be applied to. As mentioned before, a node tree represents elements and their element attributes as nodes. Thus, when an XSLT template identifies a node, it may be identifying a node that corresponds to an element or an element attribute. Nodes in a node tree are herein referred by their corresponding element. 
     XSLT constructs conform to XML. Each XSLT template is an element; the instructions in the XSLT template are also elements. For example, XSLT template  420  is an element having a beginning tag &lt;xsl:template match=“node ( )|@”&gt; and ending tag &lt;/xsl:template&gt;. The matching attribute identifies the node title within element  220 . 
     The following is a description of how an XSLT processor executes the XSLT templates in XSLT delta representation  140 . Further details about the syntax of XSLT and the operations they specify, and how XSLT processors execute them may be found in  XSL Transformations  ( XSLT )  Version  1.0, W3C Recommendation, 16 Nov. 1999, and  XSL Transformations  ( XSLT )  Version  1.1, W3C Working Draft, 24 Aug. 2001. The present invention is not limited to any particular version of XSLT, version of XML, or any other version of any other computer language. 
     In the following description, XSLT instructions are referred to as performing actions, such as copying a node. However, this is just a convenient way of expressing that an XSLT processor or some other executing entity is performing these actions in response to executing the instructions. For example, the statement an “instruction copies a node” is just a convenient way of expressing that execution by an XSLT processor of the instruction causes the XSLT processor to copy the node. 
     Furthermore, operations performed by the XSLT processor are described as being performed on a node in a source tree or as being performed on a corresponding element or attribute. However, this is just a convenient way of expressing how those operations affect the target source tree and the XML entity represented by the target source tree. For example, the statement that “a node&#39;s value is modified” is just a convenient way of expressing that the value of a corresponding version of the node in the target node tree is made to be different. The statement that an “element is deleted” is just a convenient way of expressing that no corresponding node is created in the target node tree. 
     Referring to  FIG. 4 , in XSLT template  410 , the match attribute identifies all nodes in the source node tree. XSLT instructions  412  copies all nodes that satisfy the matching attribute, except for nodes that match another XSLT template in XSLT delta representation  140 . 
     XSLT templates  420  and  430  operate in tandem to “move” publisher from below author within element  220  to above the author in element  320 . XSLT template  420  inserts publisher into element  320  above author, and XSLT template  430  deletes the element from below author. 
     XSLT template  420  specifies an insertion operation by (1) specifying a “placeholder” node, which is a node which fixes a place within a node tree to insert one or more other nodes, (2) specifying instructions for copying the placeholder node, and (3) specifying instructions for inserting nodes. Here, the placeholder node is title within element  220 . 
     The matching criteria of XSLT template  420  identifies element  220  by specifying the path “/booklist[1]/book[ ]/title[1]”. Thus, XSLT template  420  is applied to the node for the element  220 . Instructions  422  in XSLT template  420  copy title into target XML entity version  102 . Instructions  423  insert the publisher after element title just copied into element  320 . 
     With respect to XSLT template  430 , its matching criteria specifies the path “/booklist[1]/book[1]/publisher[1]”, which identifies publisher in element  230 . XSLT template  430  specifies no action, deleting the element in effect by not specifying that anything is to be changed in the target node tree. 
     XSLT template  440  deletes element  230  from XSLT delta representation  140 . The matching criteria of XSLT template  440  specifies the path “/booklist[1]/book[1]/publisher[1]”, which identifies element  230 . XSLT template  440  specifies no action, deleting the element  220  by not specifying that anything is to be changed in the node tree. 
     XSLT template  450  modifies the value of the price in element  250 . The element is identified by the match attribute of XSLT template  450 , which specifies path “/booklist[1]/book[4]/price[1]”. Element  452  sets the value of the price element in element  350  to “15.99”. 
     XSLT template  460  inserts element  370  into target XML entity version  102 . The matching attribute of element  360  identifies element  250 , a place holder node. Instructions  466  add nodes after the place holder node. 
     Creating Instructions 
       FIG. 5  shows steps that may be used to create XSLT templates that represent one or more differences of a delta. The process is executed for one or more differences identified by process  150 . Such differences include deletion of nodes, insertion of nodes, modification of nodes, and movement of nodes (which may be represented by a deletion and an insertion). 
     Referring to  FIG. 5 , at step  510 , the value of the matching attribute of the XSLT template is generated, the value typically being a path string. The path generated depends on the type of difference being processed at this step.  FIG. 4  provides examples of the type of path generated to reflect a particular type of difference in the target node tree. For example, if a node in the source tree is being deleted, then the path identifies the node to delete. 
     If the difference involves an insertion of nodes, then the path identifies a placeholder node. A placeholder node may be an adjacent sibling node or a placeholder node. 
     A set of multiple differences may be based on the same placeholder node. In such cases, one XSLT template may be generated for the complete set of differences. 
     For some differences, more than one XSLT template is generated. In this case, a match attribute value is generated for each XSLT template. An example of when multiple XSLT templates are generated is the XSLT templates generated to move a node. As was illustrated by XSLT templates  420  and  430 , two XSLT templates are generated to move a node, one to insert the node at a new location, another to delete the node from its former location. 
     At step  530 , XSLT instructions are generated for the XSLT template that cause the changes in target node tree that are needed to reflect the difference. The particular instructions generated depend on the type of difference being processed, as illustrated by  FIG. 4 . 
     Generating Multiple Representations 
     For purposes of illustration,  FIG. 1  depicts only one XSLT delta representation for one version of a data entity. However, the present invention is of course not so limited. As mentioned earlier, it is advantageous for a versioning system to represent and track multiple versions of an XML entity by maintaining any XSLT delta representations that represent deltas between versions. 
     In such a versioning system, a data entity and its various versions would be represented by a base version, which is a complete copy of the data entity, and one or more deltas for unmaterialized versions. An unmaterialized version is a version for which there is no copy that is necessarily persistently stored. Each unmaterialized version is associated with a delta representation representing the delta between the unmaterialized version and another version. 
       FIG. 6  depicts an example of a base version and multiple delta representations for unmaterialized versions of the base version that are stored in a versioning system. Referring to  FIG. 6 , it shows base version  610 , and subsequent unmaterialized versions  612 ,  614 , and  616 . Because unmaterialized versions  612 ,  614 , and  616  are unmaterialized, complete copies of them are not actually maintained in the versioning system. Instead, a delta representation is stored for each of them. XSLT delta representation  622  represents the delta between base version  610  and unmaterialized version  612 , XSLT delta representation  624  represents the delta between unmaterialized version  612  and unmaterialized version  614 , XSLT delta representation  626  represents the delta between unmaterialized version  614  and unmaterialized version  616 . 
     An unmaterialized version that is not a base version may have zero or more predecessor versions between it and a base version. Each of the predecessor versions are associated with an XSLT delta representation. To generate a complete copy of the version, the XSLT delta representation of the predecessors are cumulatively applied to the base version. For example, to generate a complete copy of unmaterialized version  616 , XSLT delta representation  622  is applied to base version  610  to generate a copy of unmaterialized version  612 . XSLT delta representation  624  is then applied to the copy of unmaterialized version  612  to generate a copy of unmaterialized version  614 . XSLT delta representation  626  is applied to the copy of unmaterialized version  614  to generate a copy of unmaterialized version  616 . 
     Hardware Overview 
       FIG. 7  is a block diagram that illustrates a computer system  700  upon which an embodiment of the invention may be implemented. Computer system  700  includes a bus  702  or other communication mechanism for communicating information, and a processor  704  coupled with bus  702  for processing information. Computer system  700  also includes a main memory  706 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  702  for storing information and instructions to be executed by processor  704 . Main memory  706  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  704 . Computer system  700  further includes a read only memory (ROM)  708  or other static storage device coupled to bus  702  for storing static information and instructions for processor  704 . A storage device  710 , such as a magnetic disk or optical disk, is provided and coupled to bus  702  for storing information and instructions. 
     Computer system  700  may be coupled via bus  702  to a display  712 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  714 , including alphanumeric and other keys, is coupled to bus  702  for communicating information and command selections to processor  704 . Another type of user input device is cursor control  716 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  704  and for controlling cursor movement on display  712 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     The invention is related to the use of computer system  700  for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system  700  in response to processor  704  executing one or more sequences of one or more instructions contained in main memory  706 . Such instructions may be read into main memory  706  from another computer-readable medium, such as storage device  710 . Execution of the sequences of instructions contained in main memory  706  causes processor  704  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
     The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  704  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  710 . Volatile media includes dynamic memory, such as main memory  706 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  702 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
     Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  704  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  700  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  702 . Bus  702  carries the data to main memory  706 , from which processor  704  retrieves and executes the instructions. The instructions received by main memory  706  may optionally be stored on storage device  710  either before or after execution by processor  704 . 
     Computer system  700  also includes a communication interface  718  coupled to bus  702 . Communication interface  718  provides a two-way data communication coupling to a network link  720  that is connected to a local network  722 . For example, communication interface  718  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  718  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  718  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  720  typically provides data communication through one or more networks to other data devices. For example, network link  720  may provide a connection through local network  722  to a host computer  724  or to data equipment operated by an Internet Service Provider (ISP)  726 . ISP  726  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  728 . Local network  722  and Internet  728  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  720  and through communication interface  718 , which carry the digital data to and from computer system  700 , are exemplary forms of carrier waves transporting the information. 
     Computer system  700  can send messages and receive data, including program code, through the network(s), network link  720  and communication interface  718 . In the Internet example, a server  730  might transmit a requested code for an application program through Internet  728 , ISP  726 , local network  722  and communication interface  718 . 
     The received code may be executed by processor  704  as it is received, and/or stored in storage device  710 , or other non-volatile storage for later execution. In this manner, computer system  700  may obtain application code in the form of a carrier wave. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.