Patent Publication Number: US-8538998-B2

Title: Caching and memory optimizations for multi-layer XML customization

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application incorporates by reference for all purposes the entire contents of: 
     (1) U.S. Non-Provisional application Ser. No. 12/029,600, titled MULTI-LAYER XML CUSTOMIZATION, filed Feb. 12, 2008; 
     (2) U.S. Non-Provisional application Ser. No. 12/029,605, titled CUSTOMIZATION SYNTAX FOR MULTI-LAYER XML CUSTOMIZATION, filed Feb. 12, 2008; 
     (3) U.S. Non-Provisional application Ser. No. 12/029,724, titled CUSTOMIZATION RESTRICTIONS FOR MULTI-LAYER XML CUSTOMIZATION, filed Feb. 12, 2008; and 
     (4) U.S. Non-Provisional application Ser. No. 12/029,615, titled CUSTOMIZATION CREATION AND UPDATE FOR MULTI-LAYER XML CUSTOMIZATION, filed Feb. 12, 2008. 
     BACKGROUND OF THE INVENTION 
     Embodiments of the present invention relate to computer software, and more particularly relate to techniques for customizing aspects of a metadata-driven software application. 
     Software applications are often customized to meet the needs of specific customers. This is particularly true in the field of enterprise software, where each organization may have a unique set of business requirements and data integration needs. Customizations generally fall into one of several categories, including site customizations (i.e., customizations that apply to all users of a deployment), group customizations (i.e., customizations that apply to specific groups or categories of users), and user customizations (i.e., customizations that apply to a single user). 
     In recent years, an increasing number of enterprise applications are being built using a metadata-driven approach. These applications (referred to herein as metadata-driven applications) are structured such that aspects of their content, behavior, and/or appearance are specified via metadata rather than program code. Generally speaking, the customization of metadata-driven applications amounts to customizing their corresponding metadata. 
     According to one known approach for customizing a metadata-driven application, a customer may make direct modifications to the application&#39;s base metadata and thereby tailor the application to its needs. However, this technique is problematic for several reasons. For example, if an upgrade or patch modifying the base metadata is released, the customer must expend considerable time and effort in re-implementing and retesting the modifications against the new version of the metadata. This process must be repeated for each new release. Further, this approach cannot be used to implement conditional customizations (i.e., customizations that apply to specific users or groups of users), which generally require modifications to the application code. 
     According to another known approach, the application developer may implement predefined customizations directly in the application metadata/code. However, this greatly increases the complexity of the application, resulting in higher development and maintenance costs. In addition, since the number of predefined customizations is necessarily limited, this approach cannot address the diverse customization requirements of all customers. Although the application developer may implement a mechanism for allowing customers to create their own customizations, this further increases application complexity and cost. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the present invention address the foregoing and other such problems by providing techniques for customizing aspects of a metadata-driven software application. In particular, embodiments of the present invention provide (1) a self-contained metadata engine for generating customized metadata documents from base metadata documents and customizations; (2) a customization syntax for defining customizations; (3) a customization creation/update component for creating and updating customizations; (4) a customization restriction mechanism for restricting the creation of new customizations by specific users or groups of users; and (5) memory and caching optimizations for optimizing the storage and lookup of customized metadata documents. 
     According to one embodiment of the present invention, a method for customizing a software application comprises receiving a base metadata document defining one or more characteristics of at least a portion of the software application, and identifying one or more customizations defining modifications to the base metadata document. A customized metadata document is then generated based on the one or more customizations and the base metadata document. 
     In one embodiment, the one or more customizations are stored separately from the base metadata document. For example, the one or more customizations may be stored in a predefined location relative to the base metadata document. Thus, the base metadata document may be upgraded without affecting the one or more customizations. Further, embodiments of the present invention may support an arbitrary number of customizations per base metadata document. 
     In another embodiment, the steps of receiving the base metadata document, identifying the one or more customizations, and generating the customized metadata document are performed by a metadata engine at runtime of the software application. The metadata engine may be a generic component configured to interoperate with any software application or system software component that makes use of metadata. 
     In another embodiment, the step of identifying the one or more customizations comprises determining one or more customizations levels, and for each customization level, identifying a customization based on one or more runtime attributes of the software application. The one or more customization levels are configurable by users of the software application. Further, the one or more customization levels may be sorted in a precedence order, where the customized metadata document is generated by applying the one or more customizations to the base metadata document in the precedence order. In one embodiment, the one or more customization levels are stored in an XML-based configuration file. 
     In another embodiment, the one or more customizations may be created by a supplier/developer of the software application, and packaged as part as of the software application. The one or more customizations may also be created by a user of the software application. In one embodiment, a customization creation interface is provided for creating a customization at any customization level. 
     In another embodiment, the customized metadata document generated from the one or more customizations and the base metadata document may be used to generate a user interface of the software application. 
     According to another embodiment of the present invention, a system for customizing software applications is provided. The system comprises a repository configured to store a base metadata document and one or more customizations, where the base metadata document defines one or more characteristics of at least a portion of the software application, and where the one or more customizations define modifications to the base metadata document. The one or more customizations are stored separately from the base metadata document. The system further comprises a metadata engine configured to receive the base metadata document, identify the one or more customizations, and generate a customized metadata document based on the one or more customizations and the base metadata document. 
     According to yet another embodiment of the present invention, a machine-readable medium is disclosed, the machine-readable medium having stored thereon a series of instructions which, when executed by a processing component, cause the processing component to customize a software application. In various embodiments, the series of instructions cause the processing component to receive a base metadata document defining one or more characteristics of at least a portion of the software application, and identify one or more customizations defining modifications to the base metadata document. The one or more customizations are stored separately from the base metadata document. The series of instructions further cause the processing component to generate a customized metadata document based on the one or more customizations and the base metadata document. 
     According to one embodiment of the present invention, a syntax for customizing a metadata document defining one or more characteristics of at least a portion of a software application is disclosed. The syntax comprises at least one element used to define a modification to a source element of the metadata document, where the at least one element includes a required attribute that must be set to a value that uniquely indentifies the source element. The value may be a unique identifier associated with the source element. Alternatively, if there is no unique identifier, the value may be an XPath expression that selects the source element. In various embodiments, the syntax is expressed as an XML schema. 
     In one embodiment, the at least one element may include a modify element used to modify an attribute of the source element, a move element used to reorder or re-parent the source element, and/or a replace element used to replace or remove the source element. The syntax may further include a second element used to insert a new element into the metadata document. 
     According to another embodiment of the present invention, a customization for a metadata document is provided. The customization includes at least one element that corresponds to a modification of a source element of the metadata document, where the at least one element includes an attribute that is set to a value that uniquely identifies the source element. In various embodiments, the customization is an XML document. 
     According to one embodiment of the present invention, a method for restricting the customizability of a base metadata document comprises receiving a type-level customization policy defined for an object type of an object included in the base metadata document, the type-level customization policy indicating whether the object type may be customized by users of a software application. The type-level customization policy is then enforced at runtime of the software application. This enforcement may include determining a policy value from a session context of the software application, the policy value indicating a username, group, or role of a current user of the software application, and determining, based on the policy value, the type-level customization policy, and the set of default behavior rules, whether an instance of the object included in the base metadata object may be customized by the current user. In various embodiments, the type-level customization policy is implemented as one or more type annotations to the object type. 
     In one embodiment, the method above further includes receiving an instance-level customization policy defined for the object included in the base metadata document, the instance-level customization policy indicating whether an instance of the object may be customized by users of the software application, and enforcing the instance-level customization policy at runtime of the software application. This enforcement may include determining, based on the previously determined policy value, the type-level customization policy, the instance-level customization policy, a set of precedence rules, and a set of default behavior rules, whether the instance of the object may be customized by the current user. In one embodiment, the instance-level customization policy is implemented as one or more object annotations in the base metadata document. 
     If there is a conflict between the type-level customization policy and the instance-level customization policy, the set of precedence rules indicates which policy overrides the other. Further, if no customization policy is defined, the set of default behavior rules indicate whether the instance of the object may be customized by users of the software application. 
     According to another embodiment of the present invention, a system for restricting the customizability of a base metadata document defining one or more characteristics of at least a portion of the software application is provided. The system comprises a declarative syntax for defining a customization policy, the customization policy indicating whether an instance of an object included in the base metadata object may be customized by users of the software application, and an evaluation engine configured to enforce the customization policy at runtime of the software application. In various embodiments, the system may further comprise a design tool for viewing/editing the customization policy, and an application programming interface (API) for programmatically determining whether the instance of the object is customizable for a given user of the software application. 
     According to yet another embodiment of the present invention, a machine-readable medium is disclosed, the machine-readable medium having stored thereon a series of instructions which, when executed by a processing component, cause the processing component to restrict the customizability of a base metadata document defining one or more characteristics of a software application. In various embodiments, the series of instructions cause the processing component to receive a type-level customization policy defined for an object type of an object included in the base metadata document, the type-level customization policy indicating whether instances of objects having the object type may be customized by users of the software application, and enforcing the type-level customization policy at runtime of the software application. 
     According to one embodiment of the present invention, a method for caching customized metadata documents for a metadata-driven software application comprises receiving a request for a customized version of a base metadata document, where the base metadata document defines one or more characteristics of at least a portion of the software application, and where the base metadata document is to be customized for a first user among a plurality of users of the software application. The method further comprises identifying a first customization for the base metadata document, where the first customization is determined to be applicable to the plurality of users; identifying a second customization for the base metadata document, where the second customization is determined to be applicable only to the first user; generating a first customized metadata document based on the first customization and the base metadata document; and generating a second customized metadata document based on the first customized metadata document and the second customization. The first customized metadata document is then stored in a shared cache applicable to application sessions of the plurality of users, and the second customized metadata document is stored in a session cache applicable only to an application session of the first user. In various embodiments, the second customized metadata document is returned to the requestor. 
     In one embodiment, the method above further comprises receiving another request for another customized version of the base metadata document, where the base metadata document is to be customized for a second user among the plurality of users; identifying a third customization for the base metadata document, where the third customization is determined to be applicable only to the second user; retrieving the first customized metadata document from the shared cache; and generating a third customized metadata document based on the retrieved first customized metadata document and the third customization. The third customized metadata document is then stored in another session cache applicable only to an application session of the second user. In various embodiments, the third customized metadata document is returned to the requestor. 
     In another embodiment, the first customized metadata document includes a first element and a second element, and the second customized metadata document includes the first element and a modified version of the second element. Further, storing the first customized metadata document in the shared cache comprises storing the first customized metadata document in a working memory as a first tree memory structure, the first tree memory structure including an allocated memory segment for the first element and an allocated memory segment for the second element, and storing the second customized metadata document in the session cache comprises storing the second customized metadata document in the working memory as a second tree memory structure, the second tree memory structure including a pointer to the allocated memory segment for the first element in the first tree memory structure and an allocated memory segment for the modified version of the second element. 
     According to another embodiment of the present invention, a layered cache for caching customized metadata documents for a metadata-driven software application is disclosed. The layered cache comprises a shared cache configured to store data for application sessions for a plurality of users of the software application, where the data includes a first set of customized metadata documents, each customized metadata document in the first set being based on one or more customizations applicable to the plurality of users. The layered cache further comprises a plurality of session caches, each session cache configured to store data for an application session of a single user, where the data stored in said each session cache includes a second set of customized metadata documents, each customized metadata document in the second set based on one or more customizations applicable only to the single user. In various embodiments, the layered cache is utilized by a metadata engine for efficiently providing customized application metadata to one or more components of the software application. 
     According to another embodiment of the present invention, a system for caching customized metadata documents for a metadata-driven software application is provided. The system comprises a working memory component including a shared cache configured to store data for application sessions of a plurality of users of the software application, and a session cache configured to store data only for an application session of a first user in the plurality of users. The system further includes a processing component configured to receive a request for a customized version of a base metadata document, where the base metadata document defines one or more characteristics of at least a portion of the software application, and where the base metadata document is to be customized for the first user; identify a first customization for the base metadata document, where the first customization is determined to be applicable to the plurality of users; identify a second customization for the base metadata document, where the second customization is determined to be applicable only to the first user; generate a first customized metadata document based on the first customization and the base metadata document; and generate a second customized metadata document based on the first customized metadata document and the second customization. The processing component is further configured to store the first customized metadata document in the shared cache, and store the second customized metadata document in the session cache. In various embodiments, the second customized metadata document is returned to the requester. 
     According to yet another embodiment of the present invention, a machine-readable medium is disclosed, the machine-readable medium having stored thereon a series of instructions which, when executed by a processing component, cause the processing component to cache customized metadata documents for a metadata-driven software application. In various embodiments, the series of instructions cause the processing component to receive a request for a customized version of a base metadata document, where the base metadata document defines one or more characteristics of at least a portion of the software application, and where the base metadata document is to be customized for a first user among a plurality of users of the software application; identify a first customization for the base metadata document, where the first customization is determined to be applicable to the plurality of users; identify a second customization for the base metadata document, where the second customization is determined to be applicable only to the first user; generate a first customized metadata document based on the first customization and the base metadata document; and generate a second customized metadata document based on the first customized metadata document and the second customization. The series of instructions further cause the processing component to store the first customized metadata document in a shared cache applicable to application sessions of the plurality of users, and store the second customized metadata document in a session cache applicable only to an application session of the first user. In various embodiments, the second customized metadata document is returned to the requestor. 
     A further understanding of the nature and advantages of the embodiments disclosed herein may be realized by reference to the remaining portions of the specification and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments in accordance with the present invention will be described with reference to the drawings, in which: 
         FIG. 1  is a flowchart illustrating the steps performed in customizing a base metadata document of a software application in accordance with an embodiment of the present invention. 
         FIG. 2  is flowchart illustrating the steps performed in identifying customizations for a base metadata document in accordance with an embodiment of the present invention. 
         FIG. 3  is a simplified block diagram illustrating a metadata engine in accordance with an embodiment of the present invention. 
         FIG. 4  is a code sample illustrating a customization configuration file in accordance with an embodiment of the present invention. 
         FIG. 5  is a second flowchart illustrating the steps performed in customizing a base metadata document of a software application in accordance with an embodiment of the present invention. 
         FIG. 6  is a code sample illustrating an exemplary customization in accordance with an embodiment of the present invention. 
         FIG. 7  is a flowchart illustrating the steps performed in restricting the customizability of a base metadata document using a type-level customization policy in accordance with an embodiment of the present invention. 
         FIG. 8  is a flowchart illustrating the steps performed in restricting the customizability of a base metadata document using a type-level customization policy and an instance-level customization policy in accordance with an embodiment of the present invention. 
         FIG. 9  is a simplified block diagram illustrating an object type hierarchy in accordance with an embodiment of the present invention. 
         FIG. 10  is a code sample illustrating a customization policy in accordance with an embodiment of the present invention. 
         FIGS. 11A and 11B  are flowcharts illustrating the steps performed in caching customized metadata documents in accordance with an embodiment of the present invention. 
         FIGS. 12A and 12B  are simplified block diagrams illustrating tree memory structures for representing metadata documents in working memory in accordance with an embodiment of the present invention. 
         FIG. 13  is a simplified block diagram illustrating a system environment that may be used in accordance with an embodiment of the present invention. 
         FIG. 14  is a simplified block diagram illustrating a computer system that may be used in accordance with an embodiment of the present invention. 
     
    
    
     In the drawings, the use of like reference numbers in different drawings indicates similar components. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form. 
     Embodiments of the present invention provide a framework for customizing metadata-driven software applications. According to one set of embodiments, the framework includes a metadata engine configured to receive a base metadata document and identify one or more sets of modifications (referred to herein as customizations) for the document based on one or more criteria. The metadata engine is further configured to generate a customized metadata document from the base metadata document and the one or more customizations, and present the customized metadata document to an application for processing. In one embodiment, the customizations are stored separately from the base metadata document. This allows the base metadata document to be modified (e.g., via an application patch/upgrade) without affecting the customizations. In another embodiment, the customization capability described above is completely contained within the metadata engine. Accordingly, any application or system software component driven by metadata may use the metadata engine to support customization with minimal changes. 
     In various embodiments, the framework further includes a customization syntax, a customization restriction mechanism, and a plurality of memory and caching optimizations. The customization syntax is a proprietary syntax that allows for efficient representation of modifications (e.g., modify, move, replace, insert, etc.) to a base metadata document. In one embodiment, the customization syntax is expressed as an Extensible Markup Language (XML) schema. 
     The customization restriction mechanism provides a technique for controlling whether certain elements of a base metadata documents may be customized, and by whom. As described in greater detail below, customization restrictions may be defined at a metadata object type level, a metadata object instance level, or both. 
     The memory and caching optimizations optimize storage and lookup of customized metadata documents by the metadata engine at application runtime. These optimizations include a layered cache for caching customized variants of metadata documents in a memory area configured to store data for application sessions of all users or groups of users (i.e., a shared cache), or in a memory area configured to store data for the application session of a single user (i.e., a session cache). These optimizations further include a specialized, hierarchical memory structure for efficiently storing representations of customized metadata documents in working memory. 
     As used herein, a “base metadata document” is a document that defines one or more base (i.e., non-customized) characteristics of at least a portion of a software application. In various embodiments, these characteristics may correspond to the content, behavior, and/or appearance of the application. For example, a base metadata document “X.xml” may define a page layout for a user interface of an application. 
     A “customization” is a document that defines one or more modifications to a base metadata document. Thus, a customization “custX.xml” may specify a rearrangement of the page layout defined in base metadata document “X.xml.” As described in greater detail below, embodiments of the present invention allow an arbitrary number of customizations to be defined for, and applied to, a given base metadata document. 
     Embodiments of the present invention may be used in a variety of different domains and contexts. Certain embodiments are particularly applicable to enterprise software applications, which are commonly customized to meet the business requirements of specific customers. However, the customization techniques described herein may be used to provide customization functionality for any system or application framework that relies on metadata. 
       FIG. 1  is a flowchart  100  illustrating the steps performed in customizing a base metadata document of a software application in accordance with an embodiment of the present invention. In various embodiments, the processing of flowchart  100  may be implemented in software, hardware, or combinations thereof. As software, flowchart  100  may be implemented as part of a metadata engine configured to provide customized metadata to one or more components of the application. An exemplary metadata engine is discussed in greater detail with respect to  FIG. 3  below. 
     At step  102 , a base metadata document for the application is received, where the base metadata document defines one or more characteristics of at least a portion of the application. As described previously, these characteristics may correspond to the content, behavior, and/or appearance of the application. In one set of embodiments, the base metadata document is formatted in Extensible Markup Language (XML). However, embodiments of the present invention may be adapted to customize metadata documents formatted in any type of markup language, such as HTML, SGML, and the like. 
     At step  104 , one or more customizations defining modifications to the base metadata document are identified. Like the base metadata document, the one or more customizations may be formatted in XML or any other type of markup language. In an exemplary embodiment, the one or more customizations are identified based on one or more criteria, such as runtime attributes of the application. This identification process is discussed in greater detail with respect to  FIG. 2  below. 
     In one set of embodiments, the one or more customizations are stored separately from the base metadata document. In other words, the one or more customizations and the base metadata document are stored as separate files (in the case of a file-based repository) or in separate database tables or rows (in the case of a DB-based repository). This allows the base metadata document to be modified or upgraded (for example, via an application patch or upgrade) without affecting the customizations. The one or more customizations and the base metadata document may be stored in the same repository (e.g., a centralized metadata repository), or in separate repositories. 
     In further embodiments, the one or more customizations are stored in a predetermined location relative to the location of the base metadata document. For example, the one or more customizations may be stored in file folder located at a predetermined hierarchical location relative to a file folder containing the base metadata document. In this manner, the location of a base metadata document may be used to easily determine the location of the customizations for the base metadata document. Alternatively, the one or more customizations may be stored in the same location as the base metadata document, or a well-known centralized location common to all base metadata documents. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     At step  106 , a customized metadata document is generated based on the one or more customizations and the base metadata document. If multiple customizations are merged with the base metadata document, the customizations are applied cumulatively. For example, if customization “A.xml” includes a modification to element A and customization “B.xml” includes a modification to element B, the customized metadata document will include the modifications to both elements A and B. In some cases, a modification defined in a first customization may conflict with a modification defined in a second customization. This situation may be resolved by defining a precedence order for customizations, which is discussed in greater detail with respect to  FIGS. 4 and 5  below. 
     In an exemplary embodiment, the processing of flowchart  100  is performed at runtime of the software application. For example, the processing of flowchart  100  may be initiated in response to a request for a user interface of the application. In this case, steps  102 ,  104 ,  106  may be executed in real-time (or near real-time). The customized metadata document generated at step  106  may then be used to present a customized version of the requested user interface to the requestor. 
     It should be appreciated that the specific steps illustrated in  FIG. 1  provide a particular method for customizing a base metadata document of a software application according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, while  FIG. 1  illustrates that all of the customizations for a base metadata document are identified (step  104 ) prior to generating a customized metadata document (step  106 ), other sequences of steps are possible. In one alternative embodiment, each customization may be applied incrementally to the base metadata document as it is identified. Further, the individual steps illustrated in  FIG. 1  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Yet further, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 2  is a flowchart  200  illustrating the steps performed in identifying customizations for a base metadata document in accordance with an embodiment of the present invention. Specifically, flowchart  200  corresponds to an exemplary implementation of step  104  of  FIG. 1 . At step  202 , one or more customization levels associated with the base metadata document are determined. In various embodiments, the one or more customization levels correspond to one or more “layers” of customization that may be applied to the base metadata document simultaneously. For example, the one or more customization levels may include user, group, site, vertical market, and the like. In one embodiment, the one or more customization levels are fully configurable by users of the software application. Thus, users may define an arbitrary number of customization levels, and provide a custom label/name for each (e.g., department, organization, jobt type, etc.). Further, an arbitrary number of distinct customization levels may be associated with a given base metadata document. 
     Once the customization levels for the base metadata document have been determined, a customization is identified for each customization level based on one or more corresponding runtime attributes of the application (step  204 ). For example, assume the base metadata document is associated with a user customization level and a group customization level. In this case, a runtime attribute corresponding to the user customization level (e.g., username) will be used to determine an appropriate user-level customization for the base metadata document. Similarly, a runtime attribute corresponding to the group customization level (e.g., group) will be used to determine an appropriate group-level customization for the base metadata document. In this manner, the customizations applied to the base metadata document may change dynamically based on the current runtime context of the application. 
     In one set of embodiments, the one or more customization levels associated with a base metadata document may be configured by users of the software application. In addition, a precedence order for applying the customizations corresponding to the customization levels may be defined. This functionality is discussed with respect to the customization configuration file of  FIG. 4  below. 
       FIG. 3  is a simplified block diagram  300  illustrating a metadata engine  302  in accordance with an embodiment of the present invention. In various embodiments, metadata engine  302  is configured to customize base metadata documents for a software application as described in flowcharts  100  and  200  of  FIGS. 1 and 2 . As shown, metadata engine  302  is communicatively coupled with a software application  314 , a file-based metadata repository  306 , and a database-based metadata repository  308 . One or more of repositories  306 ,  308  is configured to store a base metadata document (e.g.,  304 ) for application  314 . Further, one or more of repositories  306 ,  308  is configured to store customizations (e.g.,  310 ,  312 ) for the base metadata document. 
     According to one runtime flow, metadata engine  302  receives base metadata document  304  from repository  306 / 308 , where the base metadata document corresponds to an application page requested by user 1  of application  314 . Metadata engine  302  then identifies, as per the processing of  FIG. 2 , one or more customizations applicable to user 1 . In the example of  FIG. 3 , a site level customization  310  and a user-level customization  312  are identified. These customizations are applied to base metadata document  304  to generate a customized metadata document. The customized metadata document is passed to application  314 , where it used to generate a customized application page  316  for user 1 . 
     According to another runtime flow, metadata engine  302  receives base metadata document  304  from repository  306 / 308 , where the base metadata document corresponds to an application page requested by user 2  of application  314 . Metadata engine  302  then identifies, as per the processing of  FIG. 2 , one or more customizations applicable to user 2 . In the example of  FIG. 3 , only site level customization  310  is identified; in other words, user 2  does not have a user-level customization ( 318 ). Accordingly, only site-level customization  310  is applied to base metadata document  304  to generate a customized metadata document. The customized metadata document is passed to application  314 , where it used to generate a customized application page  320  for user 2 . 
     Thus, as shown, metadata engine  302  may dynamically determine the customizations to be applied to a given base metadata document based on runtime attributes (e.g., current user) of application  314 . For example, site customization  310  and user customization  312  are applied to base metadata document  304  for user 1 , while only site customization  310  is applied to base metadata document  304  for user 2 . 
     Further, metadata engine  302  may apply multiple customizations to a base metadata document. Although a maximum of two customizations are applied in the example of  FIG. 3  (site customization  310  and user 1  customization  312 ), any number of customizations are supported. 
     Yet further, the steps of customizing base metadata document  304  are completely contained within metadata engine  302 . As a result, any software application or software component driven by metadata may use metadata engine  302  to support customization with minimal code changes. For example, application  314  need only be modified to call metadata engine  302  to access base metadata document  304 , rather than retrieving the document directly from repository  306  or  308 . Metadata engine  302  transparently applies the appropriate customizations and passes the customized version of the base metadata document to the application. 
       FIG. 4  is a code sample illustrating a customization configuration file  400  for a software application in accordance with an embodiment of the present invention. As discussed above, each base metadata document of the application may be associated with one or more customizations levels that represent the applicable customization “layers” for the document. This information may be defined declaratively in configuration file  400 . In one set of embodiments, configuration file  400  is editable by users of the software application, thereby allowing the users to modify the customization behavior of metadata engine  302  to meet their particular business requirements. 
     As shown, configuration file  400  includes three “customization class” lists  402 ,  404 , and  406 . Generally speaking, a customization class is a function/class corresponding to a particular customization level. For example, customization class “UserCC” is a function/class corresponding to user-level customization, customization class “OrgCC” is a function/class corresponding to organization-level customization, etc. In one set of embodiments, each customization class is configured to evaluate the current runtime context of the application and return a value indicating the appropriate customization for its corresponding customization level based on said runtime context. Thus, if the current user is “user 1 ,” the “UserCC” customization class will return a value indicating the user-level customization for “user 1 .” Similarly, if the organization of the current user is “Sales,” the “OrgCC” customization class will return a value indicating the organization-level customization for “Sales.” 
     In an exemplary embodiment, a predefined set of customization classes may be implemented and provided by a supplier/developer of the software application. This predefined set may include classes that correspond to commonly used customization levels such as user, group, organization, site, vertical market, and the like. Users of the software application may also implement their own customization classes. 
     Customization class lists  402 ,  404 ,  406  represent three types of mappings between customization classes and base metadata documents. List  402  represents a mapping based on document path. Thus, base metadata documents with the document path of “/ORACLE/APPS” are associated with the enumerated customization classes of list  402 . List  404  represents a mapping based on the XML namespace of the top-level element in the metadata document. Thus, base metadata documents having a top-level element with the XML namespace “http://www.oracle.com/OA” are associated with the enumerated customization classes of list  404 . List  406  represents a default mapping for all other base metadata documents not mapped in  402  or  404 . Thus, all other base metadata documents are associated with the customization classes enumerated in list  406 . 
     In one set of embodiments, the order of customization classes in each list  402 ,  404 ,  406  defines a precedence order for applying customizations corresponding to those classes. For example, assume a base metadata document “X.xml” falls within the matching scope of list  404  (i.e., the XML namespace of its top-level element is “http://www.oracle.com/OA”). When X.xml is customized, a customization “SimpleSiteX.xml” corresponding to “SimpleSiteCC” will be applied to X.xml prior to a customization “SimpleUserX.xml” corresponding to “SimpleUserCC.” 
     In a further set of embodiments, the precedence order may be used to resolve conflicts between customizations. For example, in one embodiment, a customization that is higher in the precedence order may override a conflicting customization that is lower in the precedence order. Thus, customization “SimpleSiteX.xml” will override customization “SimpleUserX.xml.” In an alternative embodiment, a customization that is lower in the precedence order may override a customization that is higher in the precedence order. 
     Although only three customization class lists are illustrated in  FIG. 4 , any number of lists may be supported. In addition, each list may contain an arbitrary number of distinct customization classes. As a result, a given base metadata document may be associated with an arbitrary number of customization levels. 
     Further, although configuration file  400  is shown as using a specific XML-based syntax for representing lists  402 ,  404 ,  406 , any other appropriate syntax may be utilized. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 5  is a flowchart  500  illustrating the steps performed in customizing a base metadata document using a configuration file such as configuration file  400  of  FIG. 4  in accordance with an embodiment of the present invention. In various embodiments, the processing of flowchart  500  may be implemented in software, hardware, or combinations thereof. As software, flowchart  500  may be implemented as part of a metadata engine configured to provide customized metadata to one or more components of the application, such as metadata engine  302  of  FIG. 3 . 
     At step  502 , the configuration file is retrieved. In one embodiment, the configuration file may be cached in a working memory, such as a working memory of metadata engine  302  or application  314 , and retrieved from the cache. In other embodiments, the configuration file may be retrieved from a storage repository. 
     At step  504 , one or more customization classes associated with the base metadata document are determined. In one embodiment, this step comprises determining the appropriate customization class list (e.g., list  402 ,  404 ,  406  of  FIG. 4 ) for the base metadata document. 
     Once the customization classes have been determined, an instance of a first customization class in the one or more customization classes is executed to determine a customization (steps  506 ,  508 ). As discussed above, this determination is based one or more runtime attributes of the software application. If the customization returned by the first customization class exists, then the customization is retrieved and cumulatively applied to the base metadata document (step  510 ). Steps  508  and  510  are iterated until all of the customization classes for the base metadata document have been executed (step  512 ). In an exemplary embodiment, steps  508  and  510  are iterated over customization classes in the precedence order defined in the configuration file. This ensures that customizations are applied to the base metadata document according to the precedence order. 
     It should be appreciated that the specific steps illustrated in  FIG. 5  provide a particular method for customizing a base metadata document according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in  FIG. 5  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 6  is a code sample illustrating an exemplary customization  600  for a base metadata document of a software application in accordance with an embodiment of the present invention. In the sample shown, customization  600  is formatted in XML. However, customizations may be formatted in any type of markup language, such as HTML, SGML, and the like. In one set of embodiments, customization  600  may be a seeded customization that is provided by a supplier/developer of the application. In another set of embodiments, customization  600  may be a user customization that is created by a user of the application. 
     As shown, customization  600  uses a specific customization syntax. According to one embodiment, this syntax includes a top-level “CUSTOMIZATION” element  602  that is used to reference the base metadata document customized by the present customization. For example, element  602  includes a “CUSTOMIZES” attributes that indicates the base metadata document customized by customization  600  is “/ORACLE/APPS/HR/PAGE1.JSPX.” In other embodiments, element  602  may be omitted. In various embodiments, a customization may only reference, and therefore customize, a single base metadata document. In alternative embodiments, a customization may be configured to customize multiple base metadata documents. 
     The customization syntax illustrated in  FIG. 6  further includes one or more elements for defining modifications to a source element of the base metadata document. According to one aspect, if a modification applies to an attribute of top-level element in the base metadata document, the modification may be expressed as an attribute tag identifying the attribute name and the modified value. For example, attribute tag  604  illustrates a modification to the attribute “TITLE.” However, all other modifications to a source element are expressed as an element that includes a required “ELEMENT” attribute. The value of the ELEMENT attribute uniquely identifies the source element being modified. In most cases, the value of the ELEMENT attribute will be set to a unique identifier of the source element. If no unique identifier exists, the ELEMENT attribute may be set to an XPath expression configured to select the source element. 
     The one or more elements for defining modifications to a source element may include a “MODIFY” element for inserting, replacing, or removing attributes of the source element. For example, customization  600  includes a “MODIFY” element  606  defining a replacement for an attribute value of the source element “EMPLOYEELIST.” The one or elements may also include a “MOVE” element for reordering or re-parenting the source element, and a “REPLACE” element for replacing the source element with another element, or removing the source element altogether. 
     In various embodiments, the customization syntax may further include an “INSERT” element for inserting a new element into the base metadata document. In one embodiment, the INSERT element includes one or more position-related attributes such as “AFTER” or “BEFORE.” The position-related attributes identify where the new element will be inserted in relation to an existing source element. For example, the INSERT element  608  of  FIG. 6  indicates that a new element “COMMANDBUTTON” will be inserted after the existing element “APPLYBTN.” 
     Although not shown in  FIG. 6 , the customization syntax used therein may be expressed as an XML schema. In one embodiment, the schema may included in the same file as customization  600 . In other embodiments, the schema may be included in a separate XML Schema Definition (XSD) file. 
     In various embodiments, customization  600  may created/updated via a customization creation/update component included in metadata engine  302  of  FIG. 3 . The customization creation/update component may be used to create or update customizations at any level (e.g., user, group, site, etc.). Further, the customization creation/update component may be configured to interoperate with any pre-existing user interface designed to create/update metadata documents. For example, the customization creation/update component may be configured to interoperate with an XML editor designed to create/update XML documents using standardized DOM-based or JAXB-based method calls. In this manner, customizations may be created or updated transparently using pre-existing metadata authoring tools. 
     In one set of embodiments, a customization for a metadata document is created/updated by retrieving the metadata document, and receiving one or more changes to the metadata document using standardized method calls. For example, the metadata document may be retrieved as a DOM document object, and the changes may be in the form of standardized DOM method calls. Alternatively, the metadata document may be retrieved as a Java Architecture for XML Bindings (JAXB) based in-memory representation, and the changes may be in the form of standardized JAXB method calls. The changes are then automatically transformed into one or more customization instructions that are stored as a customization (such as customization  600 ). The customization is stored separately from the metadata document. 
     If there are multiple customization classes (i.e., customization levels) associated with the metadata document, the customization instructions are stored in a customization corresponding to the last customization class to be applied to the document (also known as the “tip” customization class). If this customization already exists, the existing customization is updated. If this customization does not exist, a new customization is created. 
     The feature of creating/updating customizations from standard XML updates is useful both at application runtime and design time. For example, it may be incorporated into a design time tool (such as Oracle JDeveloper) to enable the creation/update of seeded customizations. Further, the customization creation/update component may be incorporated into one or more runtime user interfaces (e.g., a page editor within a software application) to enable the creation/update of user customizations at application runtime. 
     In one set of embodiments, the customization creation/update component is configured to generate the best customization instruction to ensure that a patch/upgrade of a base metadata document can be done. For example, if a modified element in the base metadata document has a unique identifier, that unique identifier is used to reference the modified element in the corresponding customization instruction. This makes the customization instruction robust against possible structural changes made to the base metadata document (e.g., as part of a patch or upgrade). If the modified element does not have a unique identifier, but has a “locally-unique” identifier (i.e., unique within its siblings), the modified element is referenced using an XPath expression that includes the locally-unique identifier. Finally, if the modified element does not have a unique identifier or a locally-unique identifier, the modified element is referenced using an XPath expression. 
     In the case of an insert instruction, one of three existing elements in the base metadata document may be used to identify the location of the insert: an “after” sibling, a “before” sibling, or a parent element. Each of these three elements are checked to determined whether they have a unique identifier or a locally-unique identifier. Preference is given to the element that has a unique identifier. If more that one of the three have a unique identifier, the above order is used to select one. 
     In some cases, a subtree may be removed from one location in the metadata document and added either at a different location under the same parent, or to a different parent. In this situation, the customization creation/update component will detect such consecutive changes and will generate a move instruction. Generally speaking, using a move instruction rather than a series of replace and insert instructions is preferable for preserving customizations in the face of structural changes to the metadata document. Note that this can be achieved even if the metadata document is being updated using an API that does not support a “move” operation natively (e.g., DOM API). 
     In one set of embodiments, the customization instructions generated as part of a create or update operation may be optimized prior to being stored as a customization. These optimizations may include (1) combining multiple MODIFY elements for a single source element into a single MODIFY element; (2) combining an INSERT element and one or more MODIFY elements for portions of the inserted element&#39;s sub-tree into a single INSERT element; and (3) removing an INSERT element and a subsequent REPLACE element that deletes the inserted element. These optimizations provide the dual benefits of compacting the size of customization documents and improving the performance of metadata engine  302  when applying customizations to base metadata documents at runtime. 
     According to one embodiment, the optimizations described above may apply to changes performed across sessions (i.e., changes performed and saved at different points in time). For example, a new element may be inserted at one point in time and the customization may be saved. At a later point in time, a modification may be made to a subtree of the element inserted earlier. This modification may be combined with the prior insertion, even though the two operations were performed at different times. 
     According to another embodiment, the optimization process takes into account situations where a customization may apply to two or more other customizations. For example, consider a user-level customization that may be applied on top of both a “role-manager” level customization and a “role-employee” level customization. In this situation, the user-level customization may include certain customization instructions that apply to elements included in the role-manager customization, but are not included in the role-employee customization. Such customization instructions are automatically identified and preserved during the optimization process (even though they do not apply to the “role-employee” context). 
     As discussed above, embodiments of the present invention also provide a mechanism for restricting the customizability of a base metadata document (or portions thereof) by one or more users. This mechanism is enabled through a declarative syntax for defining customization restrictions, and an evaluation engine for enforcing the customization restrictions. As used herein, a customization restriction is a control that affects the customizability of a given metadata object instance. A first type of customization restriction controls whether customization is allowed. A second type of restriction identifies the users that can customize (if customization is allowed). And a third type of restriction controls whether specific types of customization (i.e., modify, move, replace, etc.) are allowed. One or more restrictions for a given object may be grouped into a customization policy. An exemplary customization policy is illustrated in  FIG. 10 . 
     In an exemplary embodiment, customization restrictions (and thus, customization policies) may be defined for a metadata object type or a metadata object instance. Customization restrictions defined for a given object type will apply to all object instances having that type. Customization restrictions defined for a given object instance will apply only to that instance. In further embodiments, customization restrictions may be defined at the element type or attribute type level (for object types), or at the element instance or attribute instance level (for object instances). 
       FIG. 7  is a flowchart  700  illustrating one technique for restricting the customizability of a base metadata document in accordance with an embodiment of the present invention. In various embodiments, the processing of flowchart  700  may be implemented in software, hardware, or combinations thereof. As software, the processing of flowchart  700  may be implemented as part of metadata engine  302  of  FIG. 3 . 
     At step  702 , a type-level customization policy defined for an object type of an object included in a base metadata document is received, where the type-level customization policy indicates whether instances of objects having the object type may be customized by users of a software application. In an exemplary embodiment, the customization restrictions included in the type-level customization policy are implemented as one or more type annotations to the object type. For example, if the object type is defined in an XSD file, the customization restrictions may be implemented as type annotations within the XSD file. Alternatively, the type annotations may be included in a standalone XML file or tag library. 
     At step  704 , a policy value is determined from a session context of the software application. The policy value is a value used to determine whether the customization restrictions included in the type-level customization policy should be enforced for the current application session. Typically, customization restrictions will be enforced based on the identity of the current user of the application. Accordingly, the policy value will generally correspond to a user-related attribute, such as username, group, role, or the like. Of course, other types of policy values may also be used based on the customization requirements for the application. 
     At step  706 , a determination is made whether an instance of the object included in the base metadata document may be customized by the current user of the application. This determination is based, at least in part, on the policy value determined at step  704  and the type-level customization policy. Thus, if the username of the current user is “User 1 ,” and the type-level customization policy includes a restriction disallowing customization by User 1 , the current user will be restricted from customizing the object instance. In various embodiments, the restrictions applied to the object instance may also depend on customization policies defined at the object instance level (rather than the object type level), and on customization policies defined for parent object types of the object&#39;s object type. This is discussed in greater detail with respect to  FIGS. 8 and 9  below. 
     In some cases, there may be no customization policies applicable to an object instance of a base metadata document. In those situations, the set of default behavior rules are used to determine whether the object instance may be customized. In one embodiment, the default behavior rules may indicate that the object cannot be customized by any user if there are no applicable customization policies. In alternative embodiments, the default behavior rules may allow customization by a specific group of users (e.g., administrators), or by all users under certain conditions. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
       FIG. 8  is a flowchart  800  illustrating another technique for restricting the customizability of a base metadata document in accordance with an embodiment of the present invention. Specifically,  FIG. 8  illustrates the steps performed in enforcing restrictions on an object included in the base metadata document when both type-level and instance-level customization restrictions apply. 
     At step  802 , a type-level customization policy defined for the object is received, where the type-level customization policy indicates whether instances of objects having the object type may be customized by users of the software application. At step  804 , an instance-level customization policy is received, where the instance-level customization policy indicates whether an instance of the object may be customized by the users of the application. In an exemplary embodiment, the customization restrictions in the instance-level customization policy are implemented as one or more object annotations to the object. The object annotations may be stored in the base metadata document, or in one or more extended metadata files associated with the base metadata document. 
     At step  806 , a policy value is determined from a session context of the application as in step  704  of  FIG. 7 . A determination is then made, based on the policy value, the type-level customization policy, the instance-level customization policy, a set of precedence rules, and a set of default behavior rules, whether an instance of the object may be customized by the current user (step  808 ). In this type of situation where multiple customization policies may apply to a given object instance, the set of precedence rules determine which customization policies actually apply. According to an exemplary set of precedence rules, restrictions in higher-precedence policies are automatically inherited by lower-precedence policies. Further, type-level policies have a higher precedence than instance-level policies. Accordingly, since the object instance in step  808  has both a type-level and object-level customization policy defined, the restrictions in the both the type-level and instance-level policies will apply. 
     In some cases, the type-level policy and instance-level policy may contain conflicting restrictions. In these situations, the set of precedence rules may also determine which restrictions override others. Generally speaking, lower-precedence restrictions cannot expand the scope of higher-precedence restrictions. For example, if a higher-precedence restriction disables customization for an object type/instance, a lower precedence restriction cannot enable customization of the object type/instance. However, lower-precedence restrictions can further limit the scope of higher-precedence restrictions. For example, if a higher-precedence restriction limits customization of an object type/instance to users in the group “Admin,” a lower-precedence restriction can further limit customization of the object type/instance to users in the group “Sales.” In this example, an instance of the object will be customizable only by users in both “Admin” and “Sales.” 
     It should be appreciated that the specific steps illustrated in  FIGS. 7 and 8  provide particular methods for restricting the customizability of a base metadata document according to embodiments of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in  FIGS. 7 and 8  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     In some embodiments, the customization restrictions that apply to a given object instance will depend on how the object instance is instantiated. For example, assume a set of precedence rules where restrictions in higher-precedence customization policies are automatically inherited by lower-precedence policies, where type-level policies have a higher precedence than object-level policies, and where super-type-level policies have a higher precedence than type-level policies. Further, assume a collection of object type definitions  900  as illustrated in  FIG. 9 . As shown, object type definitions  900  include three object types: BaseTypeA ( 902 ), ExtendedTypeB ( 906 ), and BaseTypeC ( 910 ). BaseTypeA includes ElementA ( 902 ). ExtendedTypeB extends from BaseTypeA and includes ElementB ( 908 ). BaseTypeC includes ElementC 1  ( 912 ), which is of type BaseTypeA, and Element C 2  ( 914 ), which is of type ExtendedTypeB. 
     Given the above, the customization policies that apply to an instance of ElementA will depend on how ElementA is instantiated. If ElementA is instantiated as part of an instance of BaseTypeA, the customization policies of the instance document and BaseTypeA will apply. If ElementA is instantiated as part of an instance of ExtendedTypeB, the customization policies of the instance document, BaseTypeA, and ExtendedTypeB will apply. If ElementA is instantiated as part of an instance of BaseTypeC/ElementC 2 , the customization policies of the instance document, BaseTypeA, ExtendedTypeB, and BaseTypeC will apply. 
       FIG. 10  is a code sample illustrating a exemplary type-level customization policy  1000  for a software application in accordance with an embodiment of the present invention. In the sample shown, customization policy  1000  is formatted in XML. However, customization policies may be formatted in any type of markup language, such as HTML, SGML, and the like. In one set of embodiments, customization policy  1000  may be provided by a supplier/developer of the application. In another set of embodiments, customization policy  1000  may be created by a user of the application. In the latter case, a design tool may be provided by the supplier/developer for creating, viewing, and/or editing customization policies. 
     As shown, customization policy  1000  is implemented as a plurality of type annotations  1002 ,  1004 ,  1006 . Annotation  1002  defines a customization restriction indicating that the attribute type “QUANTITY” may be customized by users in the groups of “ADMIN” and “SALES.” Annotation  1004  defines a customization restriction indicating that the attribute type “ITEMREF” cannot be customized. And annotation  1006  defines a customization restriction indicating that attribute type “DESCRIPTION” can be customized by any user. Although annotations  1002 ,  1004 ,  1006  utilize a specific syntax for defining customization restrictions, any other suitable syntax may be employed. One of ordinary skill in the art would recognized many variations, modifications, and alternatives. 
       FIGS. 11A and 11B  are flowcharts  1100 ,  1150  illustrating the steps performed in caching customized metadata documents in accordance with an embodiment of the present invention. In particular, flowcharts  1100 ,  1150  describe the use of a layered cache to optimize the lookup of customized metadata documents at application runtime. In various embodiments, the processing of flowcharts  1100 ,  1150  may be implemented in software, hardware, or combinations thereof. As software, the processing of flowcharts  1100 ,  1150  may be implemented as part of metadata engine  302  of  FIG. 3 . 
     At step  1102 , a request for a customized version of a base metadata document is received, where the base metadata document is to be customized for a first user among a plurality of users of a software application. Upon receiving the request, a first customization for the base metadata document is identified, where the first customization is determined to be applicable to the plurality of users (step  1104 ). For example, the first customization may be a site-level customization. Additionally, a second customization for the base metadata document is identified, where the second customization is determined to be applicable to only the first user (step  1106 ). For example, the second customization may be a user-level customization. In various embodiments, the first and second customizations are determined according to the steps of  FIG. 2  as described above. 
     At step  1108 , a first customized metadata document is generated based on the first customization and the base metadata document. The first customized metadata document can be considered an “intermediate” version of the final customized document because only the first customization has been applied. The first customized metadata document is then stored in a shared cache configured to store data for application sessions of the plurality of users ( 1110 ). Since the first customization applies to every user in the plurality of users, it is likely that the first customized metadata document will need to be generated again in response to other user requests (e.g., as a base for applying further customizations). Thus, by caching the first customized metadata document in the shared cache, the document can be retrieved quickly and efficiently. 
     At step  1112 , a second customized metadata document is generated based on the first customized metadata document and the second customization. The second customized metadata document can be considered the “final” customized version of the base metadata document. The second customized metadata document is then stored in a session cache configured to store data only for an application session of the first user (step  1114 ). Thus, the second customized metadata document can be retrieved quickly and efficiently from the session cache if requested again by an application session of the first user. The second customized metadata document may then be returned to the requestor. 
     At a later point in time, another request for another customized version of the base metadata document is received, where the base metadata document is to be customized for a second user among the plurality of users (step  1152 ). In response, a third customization for the base metadata document is identified, where third customization is determined to be applicable only to the second user (step  1154 ). For example, the third customization may be a user-level customization for the second user. In addition, since the third customization needs to be applied on top of the base metadata document and the first customization, the first customized metadata document is retrieved from the shared cache (step  1156 ). Note that the base metadata document and the first customization do not need to be retrieved and merged again, thereby substantially improving performance. A third customized metadata document is then generated from the first customized metadata document and the third customization, and the third customized metadata document is stored in another session cache configured to store data only for an application session of the second user (steps  1158 ,  1160 ). The third customized metadata document may then be returned to the requestor. 
     Using the techniques of  FIGS. 11A and 11B , metadata engine  302  of  FIG. 3  may intelligently cache customized variants of a base metadata document and thereby optimize the generation of customized metadata documents that share common customizations. In one set of embodiments, metadata engine  302  may automatically determine whether to store a customized metadata document in the shared cache or a session cache based on the customizations applied. In other embodiments, a cache hint may be included in each customization. The cache hint indicates to metadata engine  302  whether to cache an application of the customization, and which cache to use. 
     Although  FIGS. 11A and 11B  illustrate a specific case where only two customization levels (a group level customization and a user level customization) apply to a base metadata document, the layered cache of the present invention may also be used in situations where more than two customization levels apply. For example, assume a scenario where three customization levels (division, department, user) are associated with a base metadata document. Further, assume that user “user 1 ,” who is a member of division “Engineering” and department “cooling systems,” requests the document. In this scenario, a first customized metadata document based on the customization for division “Engineering” will be generated and stored in the shared cache. The first customized metadata document will be used as a starting point for customizing the base metadata document in cases where the base metadata document is requested by another user in the division “Engineering” (but in a different department). Further, a second customized metadata document based on the customizations for division “Engineering” and department “cooling systems” will be generated and stored in the shared cache. The second customized metadata document will be used as a starting point for customizing the base metadata document in cases where the base metadata document is requested by another user in division “Engineering” and department “cooling systems.” Finally, a third customized metadata document based on the customizations for division “Engineering,” department “cooling systems,” and user “user 1 ” will be generated and stored in a session cache specific to user 1 . The third customized metadata document will be used to service future requests by user 1  for the base metadata document. 
       FIGS. 12A and 12B  are simplified block diagrams illustrating tree memory structures  1200 ,  1250  for representing metadata documents in working memory in accordance with an embodiment of the present invention. In one set of embodiments, tree structures such as  1200 ,  1250  may be used to optimize the storage of customized metadata documents in the layered cache described above. 
     Tree structure  1200  is an exemplary data structure for a first metadata document stored in working memory. Assume that the first metadata document includes five elements A, B, C, D, E. As shown, tree structure  1200  includes five nodes  1202 ,  1204 ,  1206 ,  1208 ,  1210 , corresponding to elements A, B, C, D, E respectively. In addition, the hierarchical relationships between nodes  1202 ,  1204 ,  1206 ,  1208 ,  1210  correspond to the hierarchical relationships between elements A, B, C, D, E. Each node includes an allocated memory area for storing data pertaining to its corresponding element. 
     Tree structure  1250  is an exemplary data structure for a second metadata document stored in working memory. Assume that the second metadata document includes five elements A, B, C′, D, E, where elements A, B, D, E are identical to elements A, B, D, E in the first metadata document, and where C′ is a modified version of element C. In this case, since the data for elements A, B, D, E are already stored in working memory at nodes  1202 ,  1204 ,  1208 ,  1210  of tree structure  1200 , that data does not need to be duplicated in tree structure  1250 . Accordingly, nodes  1252 ,  1254 ,  1258 ,  1260  in tree structure  1250  can simply store pointers to corresponding nodes  1202 ,  1204 ,  1208 ,  1210 . Since element C′ has been changed from element C, a new memory area is allocated for element C′ at node  1256  in tree structure  1250 . 
     Using memory structures such as tree structures  1200 ,  1250 , customized metadata documents may be cached in the layered cache described above in a highly optimized manner. For example, customized variants of a base metadata document often share a large number of common elements. Since instances of common elements are not duplicated in memory as described with respect to structures  1200 ,  1250 , a large number of customized variants may be stored in a relatively small memory footprint. In one set of embodiments, only a modified element (e.g., node  1256 ) of a metadata document (e.g., tree  1250 ) is allocated a new memory area if a variant of the metadata document (e.g., tree  1200 ) is already stored in working memory. In other embodiments, the modified element and all of its parent elements (e.g., nodes  1256 ,  1252 ) may be allocated new memory areas, even if the parent elements are identical to the parent elements in the existing variant. 
       FIG. 13  is a simplified block diagram illustrating components of an exemplary system environment  1300  that may be used in accordance with an embodiment of the present invention. As shown, system environment  1300  includes one or more client computing devices  1302 ,  1304 ,  1306 ,  1308 , which are configured to operate a client application such as a web browser, proprietary client (e.g., Oracle Forms), or the like. In various embodiments, client computing devices  1302 ,  1304 ,  1306 ,  1308  may be used to interact with a metadata-driven software application such as application  314  of  FIG. 3 . 
     Client computing devices  1302 ,  1304 ,  1306 ,  1308  may be general purpose personal computers (including, by way of example, personal computers and/or laptop computers running various versions of Microsoft Windows and/or Apple Macintosh operating systems), cell phones or PDAs (running software such as Microsoft Windows Mobile and being Internet, e-mail, SMS, Blackberry, or other communication protocol enabled), and/or workstation computers running any of a variety of commercially-available UNIX or UNIX-like operating systems (including without limitation the variety of GNU/Linux operating systems). Alternatively, client computing devices  1302 ,  1304 ,  1306 ,  1308  may be any other electronic device, such as a thin-client computer, Internet-enabled gaming system, and/or personal messaging device, capable of communicating over a network (e.g., network  1310  described below). Although exemplary system environment  1300  is shown with four client computing devices, any number of client computing devices may be supported. 
     In most embodiments, system environment  1300  includes a network  1310 . Network  1310  may be any type of network familiar to those skilled in the art that can support data communications using any of a variety of commercially-available protocols, including without limitation TCP/IP, SNA, IPX, AppleTalk, and the like. Merely by way of example, network  1310  can be a local area network (LAN), such as an Ethernet network, a Token-Ring network and/or the like; a wide-area network; a virtual network, including without limitation a virtual private network (VPN); the Internet; an intranet; an extranet; a public switched telephone network (PSTN); an infra-red network; a wireless network (e.g., a network operating under any of the IEEE 802.11 suite of protocols, the Bluetooth protocol known in the art, and/or any other wireless protocol); and/or any combination of these and/or other networks. 
     System environment  1300  also includes one or more server computers  1312  which may be general purpose computers, specialized server computers (including, by way of example, PC servers, UNIX servers, mid-range servers, mainframe computers, rack-mounted servers, etc.), server farms, server clusters, or any other appropriate arrangement and/or combination. In various embodiments, server  1312  may be adapted to run one or more services or software applications described in the foregoing disclosure. For example, as shown in  FIG. 13 , server  1312  may correspond to an application server configured to execute metadata-driven application  314  of  FIG. 3 . Server  1312  may also be configured to run metadata engine  302  of  FIG. 3 . In alternative embodiments, metadata engine  302  may be executed on a server computer separate from application server  1312 . 
     Server  1312  may run an operating system including any of those discussed above, as well as any commercially available server operating system. Server  1312  may also run any of a variety of additional server applications and/or mid-tier applications, including HTTP servers, FTP servers, CGI servers, Java servers, database servers, and the like. Exemplary database servers include without limitation those commercially available from Oracle, Microsoft, Sybase, IBM and the like. 
     System environment  1300  may also include one or more databases  1314 ,  1316 . For instance, databases  1314 ,  1316  may include an application database  1314  configured to store transactional data for applications hosted on server  1312 , and a metadata database  1316  configured to store metadata for the applications as well as a metadata engine such as metadata engine  302  of  FIG. 3 . Databases  1314 ,  1316  may reside in a variety of locations. By way of example, one or more of databases  1314 ,  1316  may reside on a storage medium local to (and/or resident in) server  1312 . Alternatively, databases  1314 ,  1316  may be remote from server  1312 , and in communication with server  1312  via a network-based or dedicated connection. In one set of embodiments, databases  1314 ,  1316  may reside in a storage-area network (SAN) familiar to those skilled in the art. Similarly, any necessary files for performing the functions attributed to server  1312  may be stored locally on server  1312  and/or remotely, as appropriate. In one set of embodiments, databases  1314 ,  1316  may include relational databases, such as Oracle 10g, that are adapted to store, update, and retrieve data in response to SQL-formatted commands. 
       FIG. 14  illustrates an exemplary computer system  1400  that may be used in accordance with embodiments of the present invention. In various embodiments, system  1400  may be used to implement any of the computers  1302 ,  1304 ,  1306 ,  1308 ,  1312  described above. Computer system  1400  is shown comprising hardware elements that may be electrically coupled via a bus  1424 . The hardware elements may include one or more central processing units (CPUs)  1402 , one or more input devices  1404  (e.g., a mouse, a keyboard, etc.), and one or more output devices  1406  (e.g., a display device, a printer, etc.). Computer system  1400  may also include one or more storage devices  1408 . By way of example, the storage device(s)  1408  may include devices such as disk drives, optical storage devices, and solid-state storage devices such as a random access memory (RAM) and/or a read-only memory (ROM), which can be programmable, flash-updateable and/or the like. 
     Computer system  1400  may additionally include a computer-readable storage media reader  1412 , a communications subsystem  1414  (e.g., a modem, a network card (wireless or wired), an infra-red communication device, etc.), and working memory  1418 , which may include RAM and ROM devices as described above. In some embodiments, computer system  1400  may also include a processing acceleration unit  1416 , which can include a digital signal processor (DSP), a special-purpose processor, and/or the like. 
     Computer-readable storage media reader  1412  can further be connected to a computer-readable storage medium  1410 , together (and, optionally, in combination with storage device(s)  1408 ) comprehensively representing remote, local, fixed, and/or removable storage devices plus storage media for temporarily and/or more permanently containing computer-readable information. Communications system  1414  may permit data to be exchanged with network  1310  and/or any other computer described above with respect to system environment  1300 . 
     Computer system  1400  may also comprise software elements, shown as being currently located within working memory  1418 , including an operating system  1420  and/or other code  1422 , such as an application program (which may be a client application, Web browser, mid-tier application, RDBMS, etc.). In an exemplary embodiment, working memory  1418  may include executable code and associated data structures (such as caches) for metadata engine  302  of  FIG. 3 . It should be appreciated that alternative embodiments of computer system  1400  may have numerous variations from that described above. For example, customized hardware might also be used and/or particular elements might be implemented in hardware, software (including portable software, such as applets), or both. Further, connection to other computing devices such as network input/output devices may be employed. 
     Storage media and computer readable media for containing code, or portions of code, can include any appropriate media known or used in the art, including storage media and communication media, such as but not limited to volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage and/or transmission of information such as computer readable instructions, data structures, program modules, or other data, including RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, data signals, data transmissions, or any other medium which can be used to store or transmit the desired information and which can be accessed by a computer. 
     While the present invention has been described using a particular combination of hardware and software, it should be recognized that other combinations of hardware and software are also within the scope of the present invention. The present invention may be implemented only in hardware, or only in software, or using combinations thereof. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Many variations of the invention will become apparent to those skilled in the art upon review of the disclosure. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the pending claims along with their full scope or equivalents.