Abstract:
A system, method, computer program and article of manufacture for sharing metadata among cursors is provided. A metadata object marked as a versioned object can have multiple versions. Program units can share these versions of object metadata as opposed to copying the non-versioned object metadata into their memory. These program units may contain a pointer to an object&#39;s metadata but are otherwise void of the metadata information for the object. A locking protocol for the metadata ensures concurrency control and triggers dependency tracking. A locking protocol for the cursors controls cursor dependency status.

Description:
CROSS REFERENCED AND RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/571,362 filed on May 14, 2004 and is related to co-pending U.S. application Ser. No. ______ filed on even date herewith entitled “System for Managing Versions of Cached Metadata,” which claims the benefit of U.S. Provisional Application Ser. No. 60/571,361, filed on May 14, 2004. These applications are hereby incorporated by reference in their entireties as if fully set forth herein.  
     
    
     BACKGROUND AND SUMMARY  
       [0002]     This invention related to computer systems, and more particularly to sharing metadata between program units.  
         [0003]     A system global cache can serve as a global repository of cached metadata for objects. As requests from clients are received, each request is converted through compilation into a program unit, or cursor. The program unit stores the information needed to execute, including the metadata for the object being acted upon. If the object being accessed is partitioned, the metadata contains information about each partition. The memory needs of the metadata for a partitioned object is proportional in size to the number of object partitions. Therefore, as the number of partitions increases, the metadata storage requirements for the cursor program unit increases. Program unit memory usage can become significant in a system with partitioned objects having a large number of partitions or is a system that receives a large number of requests which access partitioned objects. Significant program unit memory usage can impact scalability.  
         [0004]     A system, method, computer program, and article of manufacture for sharing metadata among program units is provided. A metadata object marked as a versioned object can have multiple versions. Program units can share these versions of object metadata as opposed to copying the non-versioned object metadata into their memory. These program units may contain a pointer to an object&#39;s metadata but are otherwise void of the metadata information for the object. A locking protocol for the metadata ensures concurrency control and triggers dependency tracking. A locking protocol for the program unit controls cursor dependency status.  
         [0005]     A method of sharing metadata can include: receiving a plurality of requests to access a resource, creating a program unit for each of the plurality of requests, creating an execution representation of the metadata object of the resource, and sharing the execution representation of the metadata among the plurality of program units.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a representation of cursor use of non-versioned metadata for a system with few partitions and a system with many partitions.  
         [0007]      FIG. 2  is a representation of the process of sharing metadata.  
         [0008]      FIG. 3A  is a representation of cursor sharing.  
         [0009]      FIG. 3B  is a representation of cursor sharing of versioned metadata.  
         [0010]      FIG. 4  is a representation of cursor operations.  
         [0011]      FIG. 5  is a representation of the versioned metadata locking protocol.  
         [0012]      FIG. 6  is a representation of the search and create process.  
         [0013]      FIG. 7  is a representation of versioned metadata shared by cursors over time.  
         [0014]      FIG. 8  is a representation of dependency structures over time.  
         [0015]      FIG. 9  is a representation of the aging-out process.  
         [0016]      FIG. 10  is a representation of a system in which metadata sharing takes place.  
     
    
     DETAILED DESCRIPTION  
       [0017]     The present invention is directed to a method and system for allowing object metadata to be shared between program units for concurrent read write access. In the following description, for the purposes of explanation, one or more embodiments having specific combinations of elements are set forth in order to provide a thorough description. It will be apparent, however, to one skilled in the art that the embodiments may be practiced without these specific combination of elements.  
         [0018]     Versioned data allows a new version of data to be created as needed for execution of an intended operation. One type of data where versions can be beneficial is metadata. For, example, when a request to access a resource is issued, the database system converts the request through compilation into a program unit, or cursor. The program unit stores the information needed to execute, including the metadata for the object being acted upon. If the object being accessed is partitioned, the metadata contains information about each partition. The memory needs of the metadata for a partitioned object is proportional in size to the number of object partitions. As the number of partitions increases, the metadata storage requirement for a program unit increases. Consequently, program unit memory usage can become significant, inhibiting scalability. Program unit use of non-versioned metadata is illustrated in  FIG. 1 .  
         [0019]      FIG. 1  is an example representation of a non-versioned metadata object. The base metadata TB 1   102  is stored as a non-versioned object. As a program unit is built, the base metadata is converted to a more readily executable form and stored in memory as TB 1  metadata instance  104 . TB 1  is an object with a small number of partitions. Consequently, TB 1  metadata  102  and TB 1  metadata instance  104  occupy a small memory footprint. Each program unit that needs access to TB 1  copies the metadata instance into the program unit memory. Cursor C 1   106  contains a copy of metadata TB 1  instance  107  and cursor C 2   108  contains a copy of TB 1  metadata instance  109 . The disadvantage of this environment is evident in the remaining elements of  FIG. 1 .  
         [0020]     Object TB 2  in  FIG. 1  is an object with a large number of partitions. As before, the base metadata is converted to a more readily executable form and stored in memory as TB 2  metadata instance  114 . Cursor C 3   116  copies one metadata TB 2  instance  117  and cursor C 4   118  copies another metadata TB 2  instance  119 . Notice that TB 2  metadata instance is much larger that TB 1  metadata instance. This variation in size is due to the metadata required for each partition in object TB 2 . Each cursor that needs access to TB 2  copies the metadata instance into the cursor memory. However, the metadata for an object with many partitions consumes more memory than the metadata for an object with few or no partitions. As a result the cursor memory requirement must continue to increase as the number of partitions increase. This presents a situation which is not conducive to scaling.  
         [0021]     To solve the scaling issue with metadata memory usage, in one embodiment, a metadata object is marked as a versioned object, i.e., capable of having many versions. As each request is compiled, the resulting program units share the metadata by linking to the most recent metadata execution representation version instead of copying the metadata into the program unit memory. Metadata sharing process  200  is illustrated in  FIG. 2 . At process action ( 202 ) a resource is requested. Instead of copying the metadata for the resource into the program unit memory, process action ( 204 ) searches for the metadata, and process action ( 206 ) determines if an execution representation version exists. If a metadata execution representation version exists for the requested object, the program unit shares this metadata execution representation with those program units already linked in process action ( 208 ). If the requested metadata does not have an execution representation, then an execution representation version is created in process action ( 210 ). This version would then be available to share with future program units or other objects. The search ( 204 ) and create ( 210 ) process actions are executed by the versioning infrastructure described below in the VERSIONING INFRASTRUCTURE section of the specification. A depiction of program unit sharing of versioned metadata is illustrated in  FIGS. 3A and 3B .  
         [0022]      FIG. 3A  is an example representation of metadata sharing. Here, when a program unit requests access to base metadata TB 1   302 , TB 1  execution representation is created. Both program units C 1   306  and C 2   308  do not copy the metadata version into their execution memory, but share the single execution representation version of TB 1  metadata  304  by linking to it.  
         [0023]     This program unit now exists which contains no metadata information regarding the objects on which it will act. The cursor may contain information regarding the intended operation and the location of, or pointers to, the metadata, but no actual metadata information for the object.  
         [0024]     Using the memory environment depicted in  FIG. 3A , an object could not be modified until all the cursors accessing the metadata of the object were finished executing. For example, when a program unit accesses an object, a lock can be placed on the metadata for that object. This lock prevents any changes to the object, and consequently, prevents any changes to the metadata because mid-execution changes may cause the program unit to fail or may invalidate the result. Changes to the metadata are blocked so that the program unit can execute the desired operation without mid-execution changes from other program units. This means that long running program units may lock the metadata for a long period of time, potentially blocking object changes that cause metadata updates even if those updates are known to have no affect on the executing program unit. To provide a mechanism to allow resource and metadata changes, the metadata object can be a versioned object.  
         [0025]      FIG. 3B  is a representation of versioned sharable metadata. The base metadata TB 1  is stored as a versioned object. A plurality of execution representation versions of TB 1  metadata can be created as illustrated by elements  320 ,  322 , and  324 . Each of the plurality of metadata execution representation versions can have linked program units, such as cursors. In another embodiment, nested linking is available (i.e., cursors may have other objects, metadata or cursors, link to them while they are linked to a metadata object). Note that although cursors are shown as ovals, and metadata as rectangles, a cursor is an example of metadata, and therefore, may be shown as the same entity type.  
         [0026]     Using versioned metadata as depicted in  FIG. 3B , a client may modify TB 1  because the existing program units will remain linked to the execution representation version of metadata TB 1  that they were sharing, while any new requests will share a newly built execution representation version of TB 1  metadata. When a new execution representation version of metadata is created the previous execution representation version of metadata is marked obsolete. Obsolete execution representation versions of metadata will remain in the system while any cursor that depends on it, or expressed interest in it, still exists.  
         [0027]     As explained above, metadata can be locked. Since cursors are examples of metadata, cursors can also be locked.  FIG. 4  illustrates the cursor locking process  400 . When a resource is accessed ( 402 ) the cursor shares the resource and becomes locked ( 404 ). It remains locked while it is executing its intended operation during process action ( 406 ). Upon finishing execution, the cursor is unlocked and becomes a freeable dependent at process action ( 408 ).  
         [0000]     Metadata Locking Protocol  
         [0028]     In another embodiment, a metadata locking protocol ensures concurrency control.  FIG. 5  depicts the metadata locking protocol  500 . The locking protocol  500  treats TB 1  and any execution representation versions as one object where locking is concerned. This helps maintain concurrency control by preventing edits to the base metadata while another cursor is parsing. When a cursor accesses TB 1  metadata ( 502 ), a lock is placed ( 504 ) on the base object, or persistent TB 1  metadata. An execution representation version of TB 1  metadata is created in process action ( 506 ). A lock is placed on the execution representation version metadata in process action ( 508 ). At process action ( 510 ) a request to add dependencies is issued. The relationship of the cursor to the execution representation metadata versions is expressed via dependencies.  
         [0029]     Dependencies are realized via Dependency Structures and Dependency References. Each child has a Dependency Structure and each parent has a Dependency Reference. During process action ( 510 ) the pointer to the parent Dependency Reference is added to the child Dependency Structure and the pointer to the child Dependency Structure is added to the parent Dependency Reference. The lock is released on the execution representation metadata version in process action ( 512 ). The parse completes, and the lock is released on base metadata TB 1  ( 514 ).  
         [0000]     Versioning Infrastructure  
         [0030]     In one embodiment, the versioning infrastructure includes a search and create process, an aging-out process, and dependency structures. In another embodiment the dependency structures are not included. The search and create process manages new and obsolete versions of metadata. The dependency structures provide a mechanism for a client or another object to declare interest in, or dependency on, a metadata object. The aging-out process deletes the obsolete metadata versions in the system when no longer needed (i.e., when dependencies no longer exist). In another embodiment, a viewing process allows the viewing of any and/or all active versions of a metadata.  
         [0031]      FIG. 6  depicts a representation of the search and create process  600  according to one embodiment. For the purposes of this example, assume that two types of requests may be issued to access the data. A first type of access is to create a new version of the metadata (e.g., a write or update operation request). A second type of request will not create a new version of metadata (e.g., a read operation request). The request may identify a resource, or object, of the system to be operated upon. A request to access a resource occurs at process action ( 602 ). It is determined at process action ( 604 ) if the request is a request for a new version of the metadata, i.e., a write request. If the request is for a new version, the current version of the metadata is marked obsolete in process action ( 606 ), and a new execution representation version of metadata is created in process action ( 608 ). If it is determined at process action ( 604 ) that the request is not a request for a new version of the metadata, i.e., a read request, then a search ( 610 ) is conducted for the most recent execution representation version of the metadata. It is determined at process action ( 612 ) if the retrieved most recent metadata execution representation has been marked obsolete. If it is determined at process action ( 612 ) that the retrieved most recent metadata execution representation has been marked obsolete, then a new execution representation version of metadata is created and the program unit is linked to the new execution representation version at process action ( 614 ). If it is determined at ( 612 ) that the retrieved most recent execution representation metadata has not been marked obsolete, then it can be assumed that the most recent execution representation version of the metadata has been found, and the process then links the program unit to the current execution representation metadata version ( 616 ).  
         [0032]     For example, suppose a request to create a new partition in a table was issued. Process action ( 604 ) would determine that the request was a request for a new version of metadata. The existing execution representation version of the table metadata would be marked obsolete in process action ( 606 ) and the new execution representation version of a table metadata based on the modified table is created in process action ( 608 ).  
         [0033]     In another example, suppose a request to select data in a table was issued. Process action ( 604 ) would determine that the request was not a request for a new version of metadata. Process action ( 610 ) would search for and return the most recent execution representation version of the table metadata. If the metadata execution representation returned is not obsolete ( 612 ), then the curser is linked to the metadata execution representation, thereby sharing the metadata execution representation with any presently linked cursors.  
         [0034]     In another scenario, the metadata execution representation returned in process action ( 610 ) is obsolete. This situation would occur if a previous create new execution representation version operation, such as that of process action ( 608 ), failed. Consequently, a new metadata execution representation version was never created but the previous version was already marked obsolete. In this case, process action ( 614 ) would then create a new metadata execution representation version and link the client cursor to it.  
         [0035]     To further explain the versioning infrastructure  FIG. 7  illustrates an embodiment of versioned metadata over time. The metadata is represented by rectangular structures labeled MDa such as metadata MDa at t 0   702  and metadata MDa′ at t 1   722 . Each Metadata structure has a “0” or “1” in an inset rectangle in the lower left corner. The inset rectangle represents the current version flag. A “1” in the current version flag indicates the metadata has not been marked obsolete. A “0” in the current version flag indicates that the metadata has been marked obsolete. The oval structures connected to the metadata by double arrow lines represent program units, such as cursors, linked to, or sharing, the metadata. Cursors shown with dashed lines indicate the cursor has completed its operation and become freeable. Program units or cursors removed from the drawing indicate the object has been freed. Program units or cursors added to the drawing over time indicate a new client request was issued and the resulting program unit was linked to the metadata. As mentioned previously, although cursors are shown as ovals, and metadata as rectangles, a cursor is an example of metadata, and therefore, may be shown as the same entity type. The double arrow lines indicate a parent/child relationship.  
         [0036]      FIG. 8  is a representation of an embodiment of the dependency structures of versioned metadata over time. The dependency structures are data storage devices containing information regarding the relationship between parent objects and child objects and are used to determine when an obsolete metadata execution version can be deleted. The structures are represented in  FIG. 8  by rectangular structures containing data fields. Each parent object stores dependency information in a Dependency Reference and each child object stores dependency information in a Dependency Structure. For example, each parent Dependency Reference contains a pointer to the child Dependency Structure of each child object with which it is linked. Each child Dependency Structure contains a pointer to the Dependency Reference of the parent with which it is linked. An object can be both a parent and a child, and can therefore have both a Dependency Reference and a Dependency Structure. This will become more clear as the figures are explained further.  
         [0037]      FIG. 7  and  FIG. 8  will be discussed in the following paragraphs in relation to the creation and linking process shown in  FIG. 6 .  
         [0038]     At time t 0 , three clients have requested to access object “a”. For each request the search and create process  600 : searches ( 610 ) and finds metadata execution representation version MDa  702 , determines that metadata MDa  702  is not obsolete ( 612 ) but is the current most recent metadata execution representation version indicated by the “1” in current version flag  710 , and links the cursor to the metadata execution representation MDa  702 . Child C 1   704 , C 2   706 , and C 3   708  are shown linked to the current version of metadata execution representation, MDa  702 , in representation  FIG. 7 . At this point there are three child objects, i.e., cursors C 1   704 , C 2   706 , and C 3   708  currently accessing object “a”, and sharing metadata MDa  702 . If another read request came in at this time, its cursor would also share metadata MDa  702  as a result of the search and create process  600 . The three cursors will have access to metadata MDa as long as they exist.  
         [0039]     Also at time t 0 , the Dependency Reference for parent MDa  802  contains pointers  881 ,  882 , and  883  to each of C 1  Dependency Structure  804 , C 2  Dependency Structure  806 , and the C 3  Dependency Structure  808 , respectively. Each of child dependency structures C 1  Dependency Structure  804 , C 2  Dependency Structure  806 , and C 3  Dependency Structure  808  contain a pointer,  884 ,  885 , and  886 , respectively, to the MDa Dependency Reference  802  for parent object MDa.  
         [0040]     At time t 1 , all three children C 1   714 , C 2   716 , and C 3   718  remain linked to MDa  712 . The MDa Dependency Reference at t 1   812  and cursor Dependency Structures  814 ,  816 , and  818  remain unchanged. However, a request for a new version of metadata, such as C 4   724 , was received in process action ( 604 ), i.e., a request to modify object “a”. As a result, metadata execution representation MDa at time t 1   712  is marked obsolete in process action ( 606 ) and a new metadata execution representation version MDa′  722  is created in process action ( 608 ). Current version flag  720  contains a “0” showing that metadata execution representation MDa  712  is obsolete and is no longer the current, i.e., most recent, metadata execution representation version. Metadata execution representation version MDa′  722  is shown at time t 1  in  FIG. 7  with a “1” in current version flag  726 , indicating that metadata execution representation MDa′  722  is not obsolete but is the current, most recent, metadata execution representation version. Since metadata execution representation MDa′  722  is the most recent not obsolete metadata execution representation version, cursor C 4   724  is linked to (i.e., shares) it. Along with a new metadata and cursor, also created are parent MDa′ Dependency Reference  822  containing the pointer to the child C 4  Dependency Structure  824  and C 4  Dependency Structure  824  containing the pointer to the MDa′ Dependency Reference  822 .  
         [0041]     At this point there is a current metadata execution representation version MDa′  722  and an obsolete metadata execution representation version MDa  712 . Cursors C 1   714 , C 2   714  and C 3   718  will maintain access to the obsolete metadata execution representation version  712  as long as they exist. Cursor C 4   724  will maintain access to the current metadata  722  as long as it exists. No new requests will obtain access to the obsolete metadata  712 . All new read requests received will be linked to metadata execution representation MDa′  722  by the search and create process  600 . Multiple versions of metadata can be created in this environment. Each one would be maintained while needed. In another embodiment, cursors C 1 , C 2 , and C 3  can have other objects linked to them while they are linked to metadata execution representation MDa.  
         [0042]     At time t 2 , cursors C 1   734 , C 2   736 , and C 3   738  are finished executing and can be removed from the system. New cursors will not be linked to metadata execution representation MDa  732  because MDa  732  is not the current metadata execution representation version, as it is marked obsolete. When an object having no dependents is no longer in use, it is marked freeable. As such, each cursor is marked freeable as indicated in  FIG. 7  by the dashed structures of C 1   734 , C 2   736  and C 3   738 , and as indicated in  FIG. 8  by bold pointers in Dependency Structures  834 ,  836 , and  838  and in metadata MDa Dependency Reference  832 .  
         [0043]      FIG. 7  at time t 2  also depicts that a request for a resource occurred in search and create process  600  process action ( 602 ) that was not a request for a new version ( 604 ). The resulting child cursor, C 5   746  was linked to and shared metadata execution representation version MDa′  742  in process action ( 616 ) because execution representation MDa′  742  was found to be the most recent metadata execution representation version ( 610 ) that was not obsolete ( 612 ). Execution representation MDa′  742  shows cursor C 4   744  remains linked and new cursor C 5   746  is linked. As such, parent Mda′ Dependency Reference at t 2   842  contains pointers for the child cursor C 4  Dependency Structure  844 , and the new child cursor C 5  Dependency Structure  846 . Each of child C 4  Dependency Structure  844  and child C 5  Dependency Structure  846  contain a pointer for the parent object MDa′ Dependency Reference  842 .  
         [0044]     At this point metadata execution representation version Mda′  742 , the most recent metadata version, is not obsolete and has two active dependents, C 4   744  and C 5   746 . Metadata execution representation version Mda  732  is obsolete and has no active dependents. There are three freeable dependents, cursors C 1   734 , C 2   736 , and C 3   738 , which will trigger the aging-out process  900  described further below.  
         [0045]     At time t 3 , execution representation version MDa′  762  is shown as the current metadata version with cursor C 5   764  linked. The child CS Dependency Structure  866  contains the pointer for the MDa′ Dependency Reference  862 . The parent MDa′ Dependency Reference  862  contains the pointer for the child C 5  Dependency Structure  866 . Cursor C 4  is no longer executing and is marked freeable, shown by the dashed oval in  FIG. 7  and the bold pointer to MDa Dependency Reference in the C 4  Dependency Structure  864  and the bold pointer to the C 4  Dependency Structure in the MDa Dependency Reference  862 . Children C 1 , C 2 , and C 3  have been freed as indicated by the removal of them from the drawing. As such, the C 1 , C 2 , and C 3 , memory can be reallocated as needed. Methods for handling freed metadata resources are performed by the resource allocation subsystem and are known in the art. Also at time t 3 , MDa  752  also becomes freeable as it has no dependents since C 1 , C 2  and C 3  have been freed. This is indicated by the dashed structure.  
         [0046]     As mentioned previously dependency structures are used to track the relationship between parent and child objects. A Dependency Reference is used by each parent to track each child dependent, and a child Dependency Structure is used by each child to track each parent on which it depends. The pointers in the dependent structures indicate interest or dependency. Each version of the metadata remains in the system while any client is interested in it or dependent on it. That is, dependency structures are used like a locking mechanism. An object is aged out only when it has no dependents and the memory it occupies is needed.  
         [0047]     At time t 2 , cursors C 1 , C 2  and C 3  become freeable dependents. In this example, assume they become freeable dependents in numerical order. The first one to become a freeable dependent, C 1 , will trigger the metadata aging-out process  900  at process action ( 902 ). The process determines if the freeable dependent is the last dependent of the metadata in process action ( 904 ). Since there are three cursors with a dependency on metadata execution representation version MDa, the first two times through process action ( 904 ), (for cursors C 1  and C 2 ) it is determined that they are not the last dependents, and the system will resume other activities ( 914 ). When cursor C 3 , the third and final cursor, is marked freeable and goes though the metadata aging-out process  900 , process action ( 904 ) determines that it is the last dependent. Process action ( 906 ) determines if the metadata execution representation version is locked by any other object. If the metadata execution representation version is locked, the system will resume other processing activities ( 914 ). In this example the metadata execution representation version is not locked by another object. Process action ( 908 ) determines if the metadata execution representation is obsolete. If it is not obsolete the metadata execution representation is marked “can be freed” in process action ( 912 ). Another client may request a resource and still be linked to a metadata execution representation that is marked “can be freed” because the metadata execution representation is still the most recent metadata version that has not been marked obsolete. In this instance, metadata execution representation version MDa is obsolete, and is freed in process action ( 910 ). Methods for handling freed metadata resources are performed by the resource allocation subsystem and are known in the art.  
         [0048]     In another embodiment, a viewing mechanism provides a client a way to view of all metadata execution representation versions, obsolete or not.  
         [0049]     Further details about versioned metadata is available in related U.S. patent application 60/571,361 filed May 14, 2004 entitled “System for Managing Transient Versions of Cached Metadata” which is fully incorporated as if set forth herein.  
         [0000]     System Architecture Overview  
         [0050]     The execution of the sequences of instructions required to practice the embodiments may be performed by a computer system  1400  as shown in  FIG. 10 . In an embodim to practice the invention is performed by a single computer system  1400 . According to other embodiments of the invention, two or more computer systems  1400  coupled by a communication link  1415  may perform the sequence of instructions required in coordination with one another. In order to avoid needlessly obscuring the embodiments, a description of only one computer system  1400  will be presented below; however, it should be understood that any number of computer systems  1400  may be employed.  
         [0051]     A computer system  1400  according to an embodiment will now be described with reference to  FIG. 10 , which is a block diagram of the functional components of a computer system  1400 . As used herein, the term computer system  1400  is broadly used to describe any computing device that can store and independently run one or more programs.  
         [0052]     Each computer system  1400  may include a communication interface  1414  coupled to the bus  1406 . The communication interface  1414  provides two-way communication between computer systems  1400 . The communication interface  1414  of a respective computer system  1400  transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. A communication link  1415  links one computer system  1400  with another computer system  1400 . For example, the communication link  1415  may be the internet in which case the communication interface  1414  may be a telephone line, a cable or a wireless modem, or the communication link  1415  may be a LAN, in which case the communication interface  1414  may be a LAN card, or the communication link  1415  may be a PSTN, in which case the communication interface  1414  may be an integrated services digital network (ISDN) card or a modem.  
         [0053]     A computer system  1400  may transmit and receive messages, data, and instructions, including program, i.e., application, code, through its respective communication link  1415  and communication interface  1414 . Received program code may be executed by the respective processor(s)  1407  as it is received, and/or stored in the storage device  1410 , or other associated non-volatile media, for later execution.  
         [0054]     In an embodiment, the computer system  1400  operates in conjunction with a data storage system  1431 , e.g., a data storage system  1431  that contains a database  1432  that is readily accessible by the computer system  1400 . The computer system  1400  communicates with the data storage system  1431  through a data interface  1433 . A data interface  1433 , which is coupled to the bus  1406 , transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. In some embodiments, the functions of the data interface  1433  may be performed by the communication interface  1414 .  
         [0055]     Computer system  1400  includes a bus  1406  or other communication mechanism for communicating instructions, messages and data, collectively, information, and one or more processors  1407  coupled with the bus  1406  for processing information. Computer system  1400  also includes a main memory  1408 , such as a random access memory (RAM) or other dynamic storage device, coupled to the bus  1406  for storing dynamic data and instructions to be executed by the processor(s)  1407 . The main memory  1408  also may be used for storing temporary data, i.e., variables, or other intermediate information during execution of instructions by the processor(s)  1407 .  
         [0056]     The computer system  1400  may further include a read only memory (ROM)  1409  or other static storage device coupled to the bus  1406  for storing static data and instructions for the processor(s)  1407 . A storage device  1410 , such as a magnetic disk or optical disk, may also be provided and coupled to the bus  1406  for storing data and instructions for the processor(s)  1407 .  
         [0057]     A computer system  1400  may be coupled via the bus  1406  to a display device  1411 , such as, but not limited to, a cathode ray tube (CRT), for displaying information to a user. An input device  1412 , e.g., alphanumeric and other keys, is coupled to the bus  1406  for communicating information and command selections to the processor(s)  1407 .  
         [0058]     According to one embodiment of the invention, an individual computer system  1400  performs specific operations by their respective processor(s)  1407  executing one or more sequences of one or more instructions contained in the main memory  1408 . Such instructions may be read into the main memory  1408  from another computer-usable medium, such as the ROM  1409  or the storage device  1410 . Execution of the sequences of instructions contained in the main memory  1408  causes the processor(s)  1407  to perform the processes described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and/or software. Logic refers to hardware, software or any combination of hardware and software.  
         [0059]     The term “computer-usable medium,” as used herein, refers to any medium that provides information or is usable by the processor(s)  1407 . Such a medium may take many forms, including, but not limited to, non-volatile, volatile and transmission media. Non-volatile media, i.e., media that can retain information in the absence of power, includes the ROM  1409 , CD ROM, magnetic tape, and magnetic discs. Volatile media, i.e., media that can not retain information in the absence of power, includes the main memory  1408 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise the bus  1406 . Transmission media can also take the form of carrier waves; i.e., electromagnetic waves that can be modulated, as in frequency, amplitude or phase, to transmit information signals. Additionally, transmission media can take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.  
         [0060]     The foregoing specification has been described with reference to specific embodiments. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope. For example, the reader is to understand that the specific ordering and combination of process actions shown in the process flow diagrams described herein is merely illustrative, and the invention can be performed using different or additional process actions, or a different combination or ordering of process actions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.