Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to a system of processing updated software programs and, more particularly, is related to a system and method for transferring state data between different versions of software programs. 
     2. Description of Related Art 
     As known in the computer and software arts, software programs undergo many changes during their product life. The changes either enhance the system through updates or remove problems (i.e., bugs) from the program steps. In either circumstance, the software program is usually replaced with a new version of the program. 
     Typically, when a software program is replaced with a new version, the old version of the program is terminated and the new version is started. This, however, creates a problem where the program operation is interrupted for some time period. 
     While there are some solutions for on-line software version changes, these techniques suffer from the following problems. One problem with computer systems in the prior art that are designed to allow real-time software updates is that they require a special operating system or support changes within the operating system. These changes may include modifications to the compilers and/or linkers. 
     Another problem is that operating systems designed to allow software program updates on-line or on-the-fly, use indirect addressing tables to link different modules together utilizing complicated mechanisms. The use of indirect addressing tables impacts the performance of the overall system software. 
     The prior art also suffers the problem of transferring the state from the old process to the new process, along with transferring control using the stack monitoring system. However, this transferring of the state to the new version program in the new process suffers the following performance problems. 
     First, the main function of the program in either the old version or the new version can never change. 
     While the stack can be used and transferred from the old program in the old process to the new program in the new process, no new procedure area layers can be added. New procedure area layers cannot be added because they would cause errors in the processing since they would be returning to procedures at different addresses. 
     Furthermore, when transferring control from an old version of a program in an old process to a new version of a program in a new process, the assumption that the program counter can be converted implies that the offset to a function or procedure may never change due to the offset addressing within the version of the program. 
     Transferring from an old program/process to a new version program/process also assumes that no extra global or static variables can be added or deleted. The reason is that this would cause errors in the execution of the new program/process. 
     Next, the transfer from an old program/process to a new version program/process implies that the addresses of the data remain constant across the two version, i.e., there is no reordering or optimization changes, etc. 
     For the return value or parameter format changes, an intraprocedure is written that adds extra overhead on each function call and make it more difficult to maintain the software. 
     Additionally, stack monitoring techniques cannot guarantee that the old program/process is not performing a time critical task when the state transfer routine is initiated. 
     Also, the new program/process loses attributes associated with the old process such as the process identification (ID), all the network connections, file connections, and the like, when the old process is terminated to restart within the new process. 
     In addition, to initiate state transfer, one inserts an illegal instruction and assumes there can be no other causes for this illegal instruction or trap, which would thereby make it possible to initiate a state transfer at the wrong time. 
     Hence, software users have lacked the ability to allow a program running in a process to be replaced with a new version of a program within the same process without loss of service or state. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to an apparatus and method system and method for transferring state data between software programs within the same process. In accordance with one aspect of the invention, the apparatus and method repeatedly determine if a newer online program exists for an executing online program, save online program data if a newer online program exists, terminate the online program, execute the newer online program to provide the requested service, and utilize the preserved current program data during the newer online program&#39;s execution. The apparatus and method for on-line replacement of a program running in a process do not require a special operating system, but instead use a few standard operating system calls (i.e, fork and exec, etc.). Therefore, the apparatus is highly portable between different operating system machine types. 
     In accordance with one embodiment of the apparatus and method of the present invention, a checkpoint methodology allows the programmer maximum flexibility in addressing upgrade/replacement program issues and determines where, when and how the transition to a new version of a program should occur with least impact to the services. 
     In accordance with another embodiment of the present invention, procedures have complete flexibility to change arguments or return values and change calling order, function names, function code and the like within the new version of the program since stack monitoring techniques are not being used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description, serve to explain the principles of the invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. In the drawings: 
     FIG. 1 is a block diagram of a computer system having user system processes in the operating system. 
     FIG. 2 is a block diagram showing the prior arts of state transition from a first program running within process A and transferring state to a second program within process B. 
     FIG. 3 is a block diagram of the present invention showing the flow between the old program within the parent process to the new program running within the parent process utilizing a child process to save data variables, including the state information of the present invention. 
     FIG. 4 is a block diagram showing the memory layout of a process within the operating system as shown in FIG.  1 . 
     FIG. 5 is a flow chart of the prior art method for transitioning from program  1  in process A to an updated program B in process  2  as shown in FIG.  2 . 
     FIG. 6 is a flow chart of the method to update/replace the program within process A by utilizing check points and saving state information by forking a child process as shown in FIG.  3 . 
     FIG. 7A is a flow chart of the child process transfer routine of the present invention, as shown in FIG.  6 . 
     FIG. 7B is a flow chart of the parent state transfer routine of the present invention, as shown in FIG.  6 . 
     FIG. 8A is a flowchart of the send next data item subroutine of the present invention, as shown in FIG.  7 A. 
     FIG. 8B is a flowchart of the parent process retrieve data subroutine, as shown in FIG.  7 B. 
     FIG. 9A is a block diagram of the variable data packet used to transfer a data item from the old child process to the new parent process. 
     FIG. 9B is a block diagram for the hash table variable data translation process. 
     FIG. 10A is a flowchart of an alternative embodiment for the sending the next data item routine of the present invention as referenced in FIG.  7 A. 
     FIG. 10B is a flowchart of an alternative embodiment for receiving the data item as referenced in FIG.  7 B. 
     FIG. 11 is a block diagram of an alternative embodiment for the heap allocation variable data translation process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the description of the invention as illustrated in the drawings. While the invention will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed therein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the invention as defined by the appended claims. 
     As illustrated in FIG. 1 is a computer system  12  which generally comprises a processor  21 , a storage device(s)  22 , and system memory  31  with an operating system  32 . Both the storage device  22  and memory  31  include instructions that are executed by the processor  21 . Storage device(s)  22  can be, for example, in any one or combination of the following: compact disc read only memory (CD-ROM), disk, diskette, cartridge, cassette, ROM or the like. The memory  31  can be either one or a combination of the common types of memory such as for example, but not limited to, erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, programmable read only memory (PROM), random access memory (RAM), read only memory (ROM), flash memory, Dynamic random access memory (DRAM), Static random access memory (SRAM), system memory, or the like. The processor  21  accepts data from memory  31  or storage device  22  over the local interface or bus  23 . Direction from the user can be signaled by using an input device(s) for example, a mouse  24 , keyboard  25 , or the like. The action input and result output are displayed on the display terminal  26 . Parent process  61  executes an old version of the program  62  and child process  65  is used to save and transfer the state data. 
     Parent process  61 , the old program  62 , the new program  63 , a checkpoint and swap management library  64 , a child process  65 , and a registry  68 , all of which can be implemented in hardware, software, firmware, or a combination thereof. In the preferred embodiment(s), the parent process  61 , old program  62 , new program  63 , checkpoint and swap management library  64 , child process  65  and the registry  68  are implemented in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. 
     Illustrated in FIG. 2 is a well known method of updating a running process. Normally, when a process  41  is to be updated on-line, the program calls a state saving routine  42  that saves the state information of the program executing in process A  41 . Once the state data is saved, process A  41  terminates and process B  51  is executed. Process B  51  retrieves the state data utilizing the retrieve state data routine  52 . Then, while process B  51  is not done, it performs the program task, and if an update is requested, then it is done and it continues the loop processing and repeats the steps in program running in process B at  51 . This method is time consuming and requires an extra data storage area for saving the state information. 
     Illustrated in FIG. 3 is online replacement of a program running in a process using the checkpoint system of the present invention. As parent process  61  executes the old program  62 , the old program  62  executes code which periodically checks to see if old program  62  is done. If the old program  62  is not done, parent process  61  continues to perform the program  62  tasks checks if program update is requested  71  and continues loop processing. In the preferred embodiment, the check if a program update is requested is accessing the Checkpoint and Swap Management Library  64  via  71 , which further checks the registry file  68  via link  72 . 
     Checkpoint and swap management library  64 , when accessed, checks the registry file or program version service number  68 , via the check registry communication  72 . If the registry file or program version service  68  indicates that no new program version has been put in service, then the checkpoint in swap management library  64  returns to the old program  62  via return of check  71 . 
     In the event that a new program version is to be placed in service, the checkpoint and swap management library  64  forks a child process  65  via link  74 . The child process  65  is created with a complete copy of the parent process  61 , including variables and state data information. The checkpoint and swap management library  64  terminates the old program  62  and executes the new program  63 , via the exec command  75 . 
     The new program  63  may contain new or deleted variables and/or routines. The new program  63  initializes its data and retrieves the state data from the child via communication link  76 . The new program  63  performs any needed data transformations on the retrieved state data. The new program  63  continues processing from the checkpoint utilizing the state information acquired from the child process  65 , and, while not done, performs the registry check  71 , and the program task, and loops until done as described above with regard to program  62 . The apparatus and method for online replacement of a program running in a process using checkpoints will be explained further with regard to FIG. 6,  7 A and  7 B. 
     Illustrated in FIG. 4 is the memory map of each user process for the system illustrated in FIG.  1 . The kernel  33  provides interprocess communication  78  facilitator interprocess communications via link  76 . Process A memory  81  includes a stack area  82 , a heap area  83 , an uninitialized data area  84 , an initialized data area  85 , and a text or program code area  86 . 
     Process B memory  91  and any other processes also include stack area  92 , heap area  93 , the uninitialized data area  94 , initialized data area  95 , and the text or program code area  96  in their process. The Process B (i.e. the child process) is an exact copy of the parent process A memory  81  when the fork command is executed, and explained above with reference to FIG.  3 . 
     Illustrated in FIG. 5 is the prior methodology of performing an online update of a program version. The current process  41  is initialized at step  111  and executed at step  112 . A check for updates or changes to an application in the current process  41  is performed at step  113 . If the update or change to current process check is negative at step  114 , then the current process  41  continues executing at step  112 . If the update/change to the current process  41  is affirmative at step  114 , then the state information is saved to a file at step  115 . Next, the current process  41  is terminated at step  116 . 
     Execution of the new version of a program in the new process  51  is performed at step  121 . The new version of a program in the new process  51  with a new process ID first retrieves the state information from a file at step  122 . The new version of the program in the new process  51  then initializes the new process  51  with the saved state information retrieved from the stack or file at step  123 , and thereafter the new process  51  attempts to restart from the termination point of the old process  41  at step  124 . Finally, the new process  51  is set as a current process in step  129  and continues execution at step  112 . As illustrated in FIG. 5, the prior art requires execution of the new version of the program in a new process. 
     The flow charts of FIGS. 6-10B show the architecture, functionality, and operation of a possible implementation of the replacing a running program code and data within the same process software. In this regard, each block represents a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that in some alternative implementations, the functions noted in the blocks may occur out of the order. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved, as will be further clarified hereinbelow. 
     Illustrated in FIG. 6 is the apparatus and method of the present invention. The apparatus and method provides a distinct advantage in that the termination of the old program and process is not required in order to execute a new version of a program. 
     First, the parent process  61  (FIG. 3) is initialized at step  131  and, in step  132 , the parent process  61  executes the old program  62  while not done at step  132 . At step  133 , the old program  62  checks the registry  68  (FIG.  3 ), via link  72  (FIG.  3 ), for updates or changes to the old program  62 . The check for updates or changes to the old program  62 , also includes accessing a program version service as described with regard to FIG.  3 . If, in step  134 , there is not an update or change to the old program  62 , the parent process  61  returns to step  132  to continue execution of old program  62 . 
     If at step  134 , the update or change to a old program  62  (FIG. 3) within the parent process  61  is indicated, then the old program  62  runs a checkpoint  73  and saves the state information by forking a child process  65  (FIG. 3) at step  135 , herein further defined with regard to FIG.  7 A. Next, the parent process  61  replaces the entire memory area  81  (FIG. 4) with the code and data of the new version of the program  63  (FIG. 3) at step  141 . 
     Next, the new program version  63 , in the parent process  61 , is executed at step  142 . In the preferred embodiment, this is performed by utilizing the exec system call within the Unix operating system. It is well known that there are equivalent system calls in other operating systems that would perform the same function. 
     The new program  63  in the parent process  61  then progresses to block  143  in which the state information is retrieved from the child  65 , using a state retrieval routine  160 , herein further defined with regard to FIG. 7B at step  143 . Once the state information has been retrieved and the new program  63  is initialized within parent process  61 , the new program  63  then jumps to the checkpoint code, at step  149 , for subsequent execution and then returns to step  132  to execute within the parent process  61  and repeats the foregoing method. 
     The transfer of the process state from the old process (parent) to the new process (child) can be accomplished using several means. These include Sun&#39;s XDR/RPC, The Open Group&#39;s DCE RPC and ASN.1 compilers. Since the present invention doesn&#39;t require the remote capabilities of the RPC methods, so the present invention is a simplification of those processes. In fact, the use of the SUN XDR methodology used in conjunction with a simple, local interprocess communication mechanism is sufficient. Other similar methods can be constructed that take advantage of the local nature of the transfer. 
     The greater area of difficulty is automating the transfer of the process state so that an application programmer does not have to be made aware of the details of the transfer. 
     Illustrated in FIG. 7A is the initialization of the child state transfer routine  150  of the preferred embodiment. The child state transfer routine  150  is initialized at step  151 . Next, the child process  65  (FIG. 3) tries to detect the parent at step  152 . If no parent  61  (FIG. 3) is detected, the child process  65  then continues to repeat the check until a parent process  61  is detected. Once a parent process  61  is detected at step  152 , the child process  65  checks if all data has been sent at step  153 . If all the data is not sent, the child process  65  sends the next data identifier, type, size and value at step  154 , herein further defined with regard to FIGS. 8A,  9 A or  10 A. The child process  65  then returns to step  153  to again check if all data has been sent. Once all the data has been sent, the child process  65  terminates at step  159 . 
     Illustrated in FIG. 7B is the state transfer routine  160  of the parent process  61  (FIG.  3 ), according to the preferred embodiment of the present invention. First, the parent process  61  state transfer routine  160  tests if there is a child process  65  (FIG. 3) present at step  161 . If no child process  65  is detected at step  161 , the state transfer routine  160  progresses to continue step  169  where the new program  63  (FIG. 3) is executed. 
     If a child process  65  is detected, the parent process  61  state transfer routine  160  moves to step  162  and establishes contact with the child process  65  by setting up interprocess communication  76  and  78  between the parent process  61  and the child process  65 . In the preferred embodiment, the interprocess communication  76  (FIG. 3) and  78  (FIG. 4) is facilitated using sockets. Thereafter, the parent state transfer routine  160  establishes an identifier hash table  211  (FIG.  9 B), a mapping address table  256  (FIG.  11 ), and a reference list  261  (FIG.  11 ). Then, at step  164 , parent process  61  state transfer routine  160  tests to determine if all data has been retrieved. If there is more data to be retrieved, then at step  165 , parent process  61  state transfer routine  160  retrieves the data identifier  204  (FIG.  9 A), type  205  (FIG.  9 A), size  206  (FIG. 9A) and value  207  (FIG. 9A) from the child process  65 . The parent process  61  receives data routine  165  is herein further defined with regard to FIGS. 8B or  10 B. 
     The parent state transfer routine  160  then returns to step  164  to again check if all data items have been retrieved, and loops between steps  164  and  165  until all data items have been retrieved. Once all the data items have been retrieved, then the parent process  61  state transfer routine  160  progresses to step  166  and checks if there is reference list data  261  (FIG. 11) to be processed. If there is reference list data  261  to be processed at step  166 , then the parent state transfer routine  160  takes the old address  262 / 265  (FIG. 11) and finds a corresponding new address  258  (FIG. 11) and updates that reference since it has the reference address  263 / 266  (FIG. 11) at step  167 . The updating of the reference list  261  is further defined with regard to FIG.  11 . 
     After the next reference new address is determined and the reference updated at step  167 , the routine then returns to check if there is more reference data to be processed at step  166 , and repeats the steps  167  and  166  loop if there is more reference data to be processed. If there is no more reference data to be processed, parent process  61  state transfer routine  160  then continues to step  169 , which returns for continued execution of the new program  63 . 
     Illustrated in FIG. 8A is the flowchart of the send next data item subroutine  154 A, in which the data variable is sent from the child process  65  (FIG. 3) to the parent process  61  (as referenced in FIG. 7A, step  154 ). First, the child process  65  retrieves the ID of the next variable in step  181 . The child process  65  converts the variable data into a data packet  201  (FIG. 9A) using the address and size to retrieve the data value at step  182 . The data packet  201  is further defined in detail with regard to FIG.  9 A. 
     The child process  65  then sends the data packet to the parent process  61  at step  183  and then exits the send next data subroutine  154 A at step  189 . The send next data item subroutine called from FIG. 7A, step  154 . 
     Illustrated in FIG. 8B is the flowchart of the parent process  61  retrieve data subroutine  165 A. First, in step  191 , the parent process  61  reads a data packet  201  (FIG. 9A) into a memory. The parent process  61  performs a hash table lookup using the ID string at step  192 . Thereafter, the parent process  61  next checks to see if the data item has a data type or size change at step  193 . If the data item has changed its data type or its size, the parent process  61  executes a conversion or mapping routine  254  (FIG. 11) at step  194 . After the conversion or mapping routine  254  is executed at step  194  or if the data item or size has not changed in step  194 , then the parent process  61  replaces the variable value with a new value at step  195 . The parent process retrieve data subroutine  165 A is then exited at step  191  and returns to step  165  (FIG.  7 B). 
     Illustrated in FIG. 9A is the block diagram of the variable data packet  201  (FIG. 9A) used by the child process  65  (FIG. 3) to send the parent process  61  (FIG. 3) the data items to be transferred from the child process  65  to the new parent process  61 . The first data segment within the variable data packet  201  is the packet type field  202 , used to indicate the type of packet and format (e.g., control data). The next data field is packet size field  203 , which indicates the length of the variable data packet  201 . This data packet size field  203  allows the parent process  61  to identify the actual length of the variable data packet  201 . 
     The next item in the variable data packet  201  is the identifier field  204 . The identifier field  204  indicates the data item reference or identification name. The data type field  205  indicates the type of data for the variable. As known in the art, there are numerous different data types, including, but not limited to, integer, real number, text character, floating point numbers, arrays, linked lists, and the like. The size field  206  indicates the size of the variable. The value of the variable is identified in the value field  207 . 
     Illustrated in FIG. 9B is the block diagram for the hash table static data translation process  210 . The hash table  211  contains numerous addresses of verification data structures  213 . The parent process  61  (FIG. 3) uses the variable packet  201  identifier  204  to find the corresponding data variable utilizing the hash table  211 . The hash table determines the address of  213  based on the hash of ID  204 . The parent process  61  next verifies that the data type  205 , data size  206 , and data type from  213  match and the size from  213  match before using the value address in  213  to update the actual variable residing in the parent process  61  data segment  212 . 
     One alternative embodiment method for automating the transfer uses variable description information from a compiler (not shown) usually made available to a debugger. Using this information, the state transfer mechanism can transfer all variables allocated in the global data portion of the process. The transfer of heap allocated data values needs to be handled as part of the processing of the global values that reference the heap. The transfer mechanism needs to traverse all data structures allocated in the heap and transfer them as a hierarchy of components. This transfer might require programmer supplied descriptions of the data structures involved or explicit management of the data transfer depending on the data types used. 
     Using the system described in the commonly-assigned and co-pending U.S. Patent Application entitled “MEMORY MANAGEMENT TECHNIQUES FOR ON-LINE REPLACEABLE SOFTWARE”, Ser. No. 09/120,261 filed on jul. 21, 1998, herein incorporated by reference, to show how the heap allocated data can be transferred when all data to be preserved in the transfer is allocated using the system described therein. 
     In summary, an allocation of enduring memory, that is memory to be preserved, includes an application specific ID that the application can use for future reference to indicate the allocated data&#39;s type. The system described therein also includes routines for iterating through all of the memory allocated using the system. 
     Illustrated in FIG. 10A is a flowchart of an alternate heap data send subroutine  154 B for sending heap data as performed within step  154  in FIG.  7 A. First, the child process  65  (FIG. 3) reads the heap allocation data at step  221 . If there is more heap allocation data to be processed, the child process then transfers the heap allocation data using the appropriate packet conversion function  274  at step  222 . Thereafter, the child process  65  encodes each reference with the old address at step  223 . The child process  65  then packages and sends the data to the parent process  61  (FIG. 3) at step  224 . The process then transfers to step  229  which exits the subroutine and returns to step  154  in FIG.  7 A. 
     Illustrated in FIG. 10B is a flowchart of an alternate heap data receive subroutine  165 B for receiving heap data as referenced in FIG. 7B, at step  165 . First, the parent process  61  (FIG. 3) reads the data packet  201  (FIG. 9A) at step  231 . The parent process  61  then decodes the received heap object at step  232 , using the conversion function  254  (FIG.  11 ). Next, the parent process  61  allocates new storage utilizing the e-malloc function (tags and tracks malloc&#39;d data) at step  233 . The parent process  61  then stores the received heap object at step  234 . 
     The parent process  61  saves the new address created in the e-malloc function at step  235  in a mapping address table  256 , herein described in further detail with reference to FIG.  11 . The parent process  61  saves the old references and reference addresses in a reference list  261  herein defined in further detail with regard to FIG. 11, at step  236 . It then exits the subroutine at step  239 . 
     Illustrated in FIG. 11 is the block diagram for the heap allocation data translation process of the parent process  61  (FIG. 3) and child process  65  (FIG. 3) utilizing parent process translation apparatus  251  and child process translation apparatus  271 . For every data type that is to be allocated, the application program  62  (FIG. 3) creates a heap type dispatch table  252  between the ID  253  used when allocating that type and the conversion routine  254  used to convert that data type to and from a data packet  201  (FIG.  9 A). The transfer mechanism, utilizing the iteration method, will find all heap allocated objects as shown in FIG. 4, convert them to data packets  201  (FIG.  9 A), and transfer them using the appropriate conversion routine  254  applying the reverse conversion in the parent process variable translation  251 . 
     Since it is likely that data will not be placed at the same address in the new program  63 , pointer linkages need to be updated in the parent process translation apparatus  251 . This is handled by sending a tag, consisting of the old location, along with every pointed to object as shown in FIG.  9 A. The old address  257  is placed in a table  256  in the new process  251  as part of the transfer. The data contents of the table are the new addresses  258  in the new process  251 . All pointers references are transferred as a special type that is placed into a linked list  261  in the new process  251  with the original address  262  and a pointer to the new pointer  263 . When the transfer is complete, the linked list  261  is processed an element at a time. The old address  262  is used as the key to the table  256  where the new address  258  is retrieved and placed into the new referencing pointer. 
     The on-line replacement of a running program comprises an ordered listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). 
     Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. 
     The embodiment or embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly and legally entitled.

Technology Category: 3