Abstract:
The present invention provides a method for managing a database connection, comprising the steps of: in responsive to a connection request of a first application, allocating an available connection for the first application; if there is no available connection, selecting a held connection being used by a second application; backing up connection context of the held connection; releasing the held connection; and allocating the held connection for the first application. Since connection context of the application has been restorably backed up, a connection of the application can be cut off and the connection re-scheduled when necessary. As a result, the connection utilization rate is enhanced. The present invention further provides a device and system that can manage a database connection based on the technology of connection pools.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims priority from prior Chinese Patent Application No. 200710126874.1, filed on Jun. 29, 2007, the entire disclosure of which is herein incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the computer field, and in particular, to a method and system for managing a database connection. 
       BACKGROUND OF THE INVENTION 
       [0003]    Nowadays a large amount of applications need to access databases in order to obtain necessary data. For example, a database-related application written in the Java language typically uses JDBC (Java Database Connection) to interact with a database. Here, a connection represents a channel, and various applications can read/write data to the database via such channels. 
         [0004]    To access data from application, a database connection needs to be set up for each application request. Due to frequent access to the database, the operation of setting up database connections consumes a large amount of memory. To reduce memory usage, a new mechanism named “pool” is introduced into management of the connections. That is, a certain number of connections are placed in a connection pool in advance, and when a connection needs to set up, one connection is taken out of the pool and then placed back after completion. However, the management of connections via connection pools in the prior art is restricted to the basic allocation of connections. Moreover, the usage of a connection by an application, whether the usage is reasonable or not, is not under control by application server. In fact, the improper usage of a connection by an application will cause resource conflicts, thread hangs and even system crashes. There is a variety of improper usages of connections. One example of an improper usage is a connection leak, i.e. some applications fail to close a connection after obtaining it and the connection is hence held for a long time. Another example of an improper usage is an inefficient connection resource usage, i.e. a certain connection is not released to the connection pool after having been used or the connection is held for a long time without being used. Still, another example of an improper usage is deadlock, i.e. connections are invoked in a recursive way and applications thus wait for each other to get resources that are being used by others. 
         [0005]    In the prior art there have been proposed several connection management schemes in order to solve the problem of the improper usage of connections. One technology is to manually detect connection leak on an application server (hereinafter referred to “AS” for short). In this method, a special diagnosis program is employed to detect connection leak on an AS in a poling way. The deficiency in this detection method lies in the incapability of automatically detecting connection leak, thereby causing the omission of detection. Additionally, frequent detection will lead to extra system overheads and utilization. 
         [0006]    A second technology is to automatically detect connection problems based on timeout. Compared with the diagnosis program described previously, this technology is automatic. An AS administrator specifies a timeout for a database connection (e.g. 75 seconds as a default value) and any application that still holds connection upon expiration of the timeout will get the timeout exception and its connection will thus be reclaimed by the system. This technology is especially suitable for the deadlock. Once the deadlock occurs, connection resources in a deadlock thread will be deprived by the system due to the timeout, and the deadlock is thus unlocked. However, the deficiency in this technology is that it is difficult to determine timeouts suitable for all transactions. For example, if a thread is a transaction that consumes a long time to be completed, then the timeout that is merely suitable for an ordinary transaction will deprive this time-consuming transaction of the connection, so the time-consuming transaction cannot proceed normally. If the AS administrator specifies a too long timeout, then the system cannot respond to a problem in time and will only discover a problem long after the problem occurred. Therefore, the overall connection utilization rate will be decreased. 
         [0007]    Another commonly used technology is to debug a program in advance, wherein the program is analyzed using program debugging tools. If an analysis result is that the program may be subject to deadlock or leak when it is run, then the programmer is notified to modify the program. It goes without saying that to debug a distributed application in a large scale enterprise environment is a labor-intensive task. Therefore, this method has relatively poor cost efficiency. 
         [0008]    According what is needed is a method and system to over come the problems encountered in the prior art and to improved technology for managing a database connection. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a database connection with better utilization rate and enhances overall system performance. The present invention can save and restore connection context of a database connection and re-allocate the connection resource from one application to another application, thereby reusing the connection. The term “connection context” mentioned here means state information of a connection. When an application accesses a database using a connection, such state information will be transferred over the connection.  FIG. 1  shows an example of such state information, which will be described in detail below. 
         [0010]    In a first aspect of the present invention, there is provided a method for managing a database connection, comprising the steps of: in responsive to a connection request of a first application, allocating an available connection for the first application; if there is no available connection, selecting an allocated connection being held by a second application; backing up connection context of the held connection; releasing the held connection; and allocating the held connection for the first application. 
         [0011]    According to another embodiment of the present invention, if the second application makes a request for restoring a previous connection state, then a new connection is allocated for the second application based on the backup of the connection context and the previous connection state is restored. 
         [0012]    According to another embodiment of the present invention, the application may be a Java application, and the connection may be a JDBC connection. The connection context that needs to be backed up comprises Connection Objects, Statement Objects, Resultset Objects of the connection. 
         [0013]    According to another embodiment of the present invention, during selection of a held connection, an application that has held a connection for more than a predetermined time can be selected. Alternatively, the selection can be made based on the Least Recently Used (LRU) algorithm. 
         [0014]    According to another embodiment of the present invention, a correspondence is established between connection context of the held connection and a key. For example, when connection context is being backed up, the application&#39;s connection context is placed to an index table along with the key, the connection context is taken out of the index table according to the key if necessary, and a new connection is allocated for the application based on the connection context and a previous connection state is restored. The key may be any character or an identifier capable of uniquely identifying a statement of the application, such as specific information in the connection context. 
         [0015]    In another embodiment of the present invention, there is provided a database connection manager, comprising: connection context backup means for backing up context of a connection; and connection scheduling means for scheduling the connection. The connection scheduling means comprises: means that allocates an available connection for a first application in responsive to a connection request of the first application; means that selects a held connection being used by a second application if there is no available connection; means that controls the connection context backup means to back up connection context of the held connection; means that releases the held connection; and means that allocates the held connection for the first application. 
         [0016]    In a third aspect of the present invention, there is provided a system for managing a database connection, comprising applications, a connection manager and a connection pool, wherein the connection manager can back up and restore the application&#39;s connection context and re-schedule a connection using the method described above. 
         [0017]    In a connection pool management solution of the prior art, an application gets a connection from a connection pool directly and releases this connection upon completion of an access to a database, so that the released connection can be used by other application requesting a connection. However, if all connections in the connection pool have been held and not released by applications upon completion of an access to the database in time, then other applications requesting a connection will not get a connection to access the database. According to the method of the present invention, when an application makes a request to the connection manager for a connection, then the connection manager allocates a connection according to a current connection usage state. Specifically, when all connections in the connection pool have been allocated, the connection manager will restorably back up connection context of at least one application with held connection, reclaim at least one connection and allocate the reclaimed connection for the new application requesting a connection if a new application requests a connection, thereby re-scheduling the connection. If the application with the released connection is an application that does not need the connection any more, this improper connection allocation state is naturally ended by the aforesaid processing. If the application with the released connection has not completed the operation, it can request another connection when necessary, and the manager will restore a connection according to connection context previously backed up during the reclamation operation, so that the application&#39;s transaction can proceed with the processing before the connection interruption without re-starting the processing. Therefore, the method of the present invention can achieve dynamic and resilient management of a database connection and thereby enhancing the utilization rate of connection resources and improve the system performance. 
         [0018]    These and other advantages of the present invention will become more apparent from the following description of other embodiments, taken in conjunction with the accompanying drawings. These embodiments are illustrative merely and not restrictive. 
     
    
     
       BRIEF DESCRIPTION ON THE DRAWINGS 
         [0019]      FIG. 1  schematically shows an example of connection context of an application, in which a JDBC connection according to an embodiment of the present invention; 
           [0020]      FIG. 2  schematically shows a flowchart of a method for managing a database connection according to an embodiment of the present invention; 
           [0021]      FIG. 3  schematically shows a flowchart of an operation for backing up connection context of a database connection according to an embodiment of the present invention; 
           [0022]      FIG. 4  schematically shows a flowchart of an operation for restoring the database connection based on the saved connection context according to an embodiment of the present invention; 
           [0023]      FIG. 5  schematically shows a block diagram of a system for managing a database connection which includes a database connection manager according to an embodiment of the present invention; 
           [0024]      FIG. 6  schematically shows a procedure in which the connection manager according to an embodiment of the present invention; and 
           [0025]      FIG. 7  schematically shows a computer device in which embodiments according to the present invention can be implemented. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  shows an example of connection text of an application, in which a JDBC connection according to an embodiment of the present invention. Information transferred over a JDBC connection comprises: configuration information, Statement and ResultSet information, server information, DataSource information, etc. The configuration information comprises: Autocommit, Isolationlevel, Readonly, Catalog, Timezone, etc. The DataSource information comprises host/port, user/password, database, and other information. Specifically, when a database needs to be accessed, a Java application requests a connection setup, generates statements over the connection set up, executes the SQL statement on the statements, and thereby acquires a result set. 
         [0027]    According to the present invention, connection context of the JDBC connection that needs to be backed up, i.e. state information of the JDBC connection comprises: Connection State, DataSource State, Transaction State, Statement State, and ResultSet State. Hereinafter, conceptual explanation or exemplary illustration will be given to these kinds of state information. 
         [0028]    The Connection State comprises Server URL, User/Password, Database Name, etc. Generally speaking, when a JDBC connection needs to be set up, a driver must be downloaded first, and a driver manager then invokes a connection method of the driver and passes a URL to the driver. The syntactic format of a JDBC URL recommended by the JDBC specification is generally as follows: 
         [0029]    jdbc: &lt;subprotocol&gt;:&lt;subname&gt;. 
         [0030]    wherein Subprotocol defines the kind of a database connectivity mechanism that may be supported by one or more drivers. The contents and syntax of subname depend on subprotocol. Subname always includes Server Address, User Name, Password, and Database Name. For example, for the Oracle SQL *net information on the particular database called ejbdemo on the machine dbmachine, the character string url is 
         [0000]      “jdbc:oracle:thin:user/password@(description=(address_list=(address=(protocol=tcp)(host=d amachine)(port=1521)))(source_route=yes)(connect_data=(sid=ejbdemo))).” 
         [0031]    For another example, for requesting to connect to the ejbdemo database on the local machine, the character string url is 
         [0000]      “jdbc:mysql://localhost/ejbdemo?user=user;password=pass.” 
         [0032]    An application interface of JDBC (JDBC API) defines a group of properties to identify and describe a Datasource implementation. Standard Datasource properties are as enumerated and listed in the following table. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Property Name 
                 Type 
                 Description 
               
               
                   
               
             
             
               
                 databaseName 
                 String 
                 Name of a particular database on a server 
               
               
                 dataSourceName 
                 String 
                 A data source name, used to name an 
               
               
                   
                   
                 underlying XADataSource object or 
               
               
                   
                   
                 ConectionPoolDataSource object when 
               
               
                   
                   
                 pooling of connections is done 
               
               
                 description 
                 String 
                 Description of this data source 
               
               
                 networkProtocol 
                 String 
                 Network protocol used to communicate 
               
               
                   
                   
                 with the server 
               
               
                 password 
                 String 
                 A database password 
               
               
                 portNumber 
                 int 
                 Port number where a server is listening 
               
               
                   
                   
                 for requests 
               
               
                 roleName 
                 String 
                 The initial SQL rolename 
               
               
                 ServerName 
                 String 
                 Database server name 
               
               
                 User 
                 String 
                 User&#39;s account name 
               
               
                   
               
             
          
         
       
     
         [0033]    Samples of Transaction State include, for example, Disable Auto-Commit mode: con.setAutoCommit(false) (assume con is a Connection object). 
         [0034]    Additionally, transaction Isolation Levels are
   TRANSACTION_NONE   TRANSACTION_READ_UNCOMMITTED   TRANSACTION_READ_COMMITTED   TRANSACTION_REPEATABLE_READ   TRANSACTION_SERIALIZABLE
 
which range from the least restrictive to the most restrictive.
   
 
         [0040]    Statements may be update statements such as SQL INSERT, UPDATE and DELETE, or they may be query statements using SELECT statement. Additionally, stored procedures may be invoked through a statement. Update statements such as INSERT, UPDATE and DELETE return a count that indicates how many rows were affected in the database. These statements do not return any other information. One statement is sent to each database server each time. 
         [0041]    Statement objects are created by Connection objects. For example, 
         [0042]    Connection conn=dataSource.getConnection(user, passwd); 
         [0043]    Statement stmt=conn.createStatement( ) 
         [0044]    The statement may be cached and then the execution path is pre-determined on the database server allowing it to be executed multiple times in an efficient manner. Such a statement is called PreparedStatement. For example,
   Connection conn=ds.getConnection(user, passwd);   PreparedStatement ps=conn.prepareStatement(   “INSERT INTO BOOKLIST” +   “(AUTHOR, TITLE, ISBN) VALUES (?, ?, ?)”);   ps.setString(1, “Zamiatin, Evgenii”);   ps.setString(2, “We”);   ps.setLong(3, 0140185852);   
 
         [0052]    Furthermore, CallableStatement is used for executing stored procedures on the database. For example,
   CallableStatement cstmt=con.prepareCall(“{CALL PROC(?, “Literal_Value”, ?)}”);   cstmt.setString(1, “First”);   cstmt.setString(2, “Third”).   
 
         [0056]    Query statements return a JDBC row result set. Rowset is one kind of result set. Individual columns in a row are retrieved either by name or by column number. There may be any number of rows in a result set. The row result set has metadata that describes the names of the columns and their types. Main concepts associated with the result set in the JDBC specification comprise rowset type, rowset concurrency, resultset holdability, rowset object. 
         [0057]    Rowset types comprise:
   TYPE_FORWARD_ONLY   TYPE_SCROLL_INSENSITIVE   TYPE_SCROLL_SENSITIVE;   Rowset concurrency comprises:   CONCUR_READ_ONLY   CONCUR_UPDATABLE;   Resultset holdability comprises:   HOLD_CURSORS_OVER_COMMIT   CLOSE_CURSORS_AT_COMMIT.   
 
         [0067]      FIG. 2  shows a flowchart of a connection management method according to a another embodiment of the present invention. The connection management method according to the present invention is now described with reference to  FIG. 2 . 
         [0068]    First, an application makes a request for a JDBC connection either with or without a key. If this request contains a key, it means that the application making the request will continue an interrupted but uncompleted statement, and the key will be used as an index for restoring a statement (this situation will be described in detail hereinafter). If this request contains no key, it means that the application will start a new statement, other than continue an interrupted but uncompleted statement. The request for a JDBC connection made in step  201  is a request without a key, whereas the request for a JDBC connection made in step  210  is a request with a key. 
         [0069]    In step  202 , whether or not there is an available connection in the connection pool is decided. If there is an available connection, then the flow goes to step  205  in which a connection is allocated for the application. If there is no idle connection, then the flow goes to step  203  in which a held connection is reclaimed. If all held connections are executing statements, then the request will have to wait. If there is an idle held connection, then, preferably, a held connection is selected based on timeout or using a prior art such as LRU (Least Recently Used). A simplest selection algorithm is that there is a time stamp, which records the time when the last statement is completed, on a JDBC connection; and the connection having the largest value by subtracting the time stamp on each connection from a current time is selected for subsequent reclamation and scheduling processing. 
         [0070]    In step  204 , all state information of the selected connection and the key of a statement to be interrupted are saved, the statement is then suspended, and the selected connection is released to the connection pool finally. The key must be associated with relevant state information of the statement to be backed up. If necessary, the key can be used as an index to retrieve relevant state information of the statement so as to restore the statement. Relevant state information of the statement is called “connection context” in the present invention. The key may be saved in the same database as state information of a corresponding statement. 
         [0071]    Afterwards, the flow goes to step  205 . Since there is currently a released idle connection in the connection pool, a JDBC connection may be allocated for the application. 
         [0072]    Step  202  to step  205  mentioned above form the main technical solution of the present invention, and the procedure in which a JDBC connection is allocated for an application is completed till step  205 . Hereinafter, description will be given depending on whether there is key information in a request. If the request contains a key, it means that the request needs to restore an existing interrupted statement. If the request contains no key, it means that the request needs to create a new statement. In this case, there are comprised the following steps: 
         [0073]    In step  206 , whether or not the request contains a key is detected. If the request contains no key, then the flow goes to step  207  in which a key is created for and bound to the statement. If the request contains a key, then the flow goes to step  208  in which the previously saved connection context is retrieved using the key, state information of the connection context is restored to the connection and the statement is activated. 
         [0074]    The performance of either step  207  or step  208  results in a connection having a key already. Therefore, in step  209 , a SQL statement is executed on the activated statement. 
         [0075]    In step  211 , whether the statement is alive or not is detected according to the key. If the result is positive, then the SQL statement is executed via the JDBC connection in step  212 . Then, whether the statement needs to be closed or not is decided in step  213 . If the statement does not need to be closed, then the flow goes to step  217  in which whether the connection has been compulsorily interrupted or not is decided. If not, then the flow returns to step  209  in which the SQL statement is executed on the statement. 
         [0076]    If the statement is closed in step  213 , then whether the connection needs to be released or not is decided in step  214 . If not, then a statement is created in step  216 , and the flow subsequently goes to step  217  in which whether the connection has been compulsorily interrupted or not is decided. If not, then the flow returns to step  209  in which the SQL statement is executed on the statement. 
         [0077]    If the connection is released in step  214 , then in step  215  the connection is reclaimed, i.e. the connection is cut off, and the connection is placed in the connection pool. 
         [0078]    If it is decided in step  217  that the connection has been compulsorily interrupted, then the flow goes to step  210  in which a request for a JDBC connection is made. Since the request contains a key associated with the statement, the flow goes to step  202  in order to acquire a new connection once again and restore the previously interrupted but uncompleted statement according to the key in the request. 
         [0079]    In the aforesaid steps, “key” means a unique identifier for indexing connection context of a statement the application wants to restore. In concrete implementations, a key can be any character. Preferably, it can be a specific item of information or a combination of multiple items of information in connection context. The embodiments disclosed here should not be construed as limiting the present invention. 
         [0080]      FIG. 3  shows a flowchart of an operation for backing up connection context of a database connection according to a another embodiment of the present invention. Steps shown in  FIG. 3  are elaborate steps of the first part of step  204  in  FIG. 2 . In step  301 , a context object is created for packaging and storing the above mentioned state information of the connection. In step  302 , all statement objects are extracted from the JDBC connection object. In step  303 , SQL, rowset configuration information and rowsets are extracted from the statement objects and then placed into the context object created in step  301 . In step  304 , a key associated with the statement and the context object obtained from step  303  are placed in an index table so that the context object can be retrieved according to the key and state information of the connection can be restored by the context object later. 
         [0081]      FIG. 4  shows a flowchart of an operation for restoring the database connection based on the saved connection context according to a another embodiment of the present invention. Steps shown in  FIG. 4  are elaborate steps of the first part of step  208  in  FIG. 2 . In step  401 , the context object is extracted from the index table according to the key. In step  402 , all statement objects are extracted from the context object. In step  403 , all state information, including information in the statement, is restored on the allocated JDBC connection. In step  404 , context containing the key is deleted from the index table. Since the connection context has been restored and will never be used, the context backup had better be deleted immediately after restoration, in order to avoid a conflict with context associated with the key of the statement, which might be backed up next time. Also, during backing up context associated with the same key, the previous context backup can be replaced by the new context backup. 
         [0082]      FIG. 5  shows a block diagram of a system  50  for managing a database connection which includes a database connection manager according to a another embodiment of the present invention. System  50  comprises an application  51  such as a Java application and the like, a connection pool  53 , and a connection manager  52  according to the present invention. Connection manager  52  can comprise connection scheduling means  521 , connection context backup means  522 , and connection context restoration means  522 . Further, manager  52  can comprise key management means  523  and connection context repository  524 . 
         [0083]    Upon receipt of a connection request from an application, connection manager  52  detects whether or not there is an idle connection in connection pool  53 . If yes, a connection is allocated for the application via connection scheduling means  521 ; if not, then an application that has held a connection is selected via the same. Subsequently, connection context backup means  522  backs up connection context of the selected application to connection context repository  524 , suspends a statement of the connection, and releases the connection of the application. Then, connection scheduling means  521  allocates the released connection for the application that previously made a connection request. At this point, key management means  523  detects whether or not the request contains a key. If not, then a key is allocated for the application and bound to the statement. If yes, then connection context thereof is restored from connection context repository  524  via connection context restoration means  524  according to the key, and a connection is set up for the application based on the restored connection context. 
         [0084]      FIG. 6  shows a procedure in which the connection manager according to a another embodiment of the present invention interacts with a Java application. Connection manager  52  comprises connection scheduling means  521 , connection context backup means  522 , connection context restoration means  524 , and key management means  523  (not shown in  FIG. 6  for the purpose of simplicity). In step  601 , the Java applications makes a connection request GetConnection( ) to the connection scheduling means. If there is an idle connection, then the connection scheduling means allocates a connection for the Java application in step  62 . Otherwise, the connection scheduling means selects a held connection, backs up connection context of the held connection to the context repository, and releases the connection in step  63  shown by a dotted line in  FIG. 6 . In step  64 , the released connection is then allocated for the Java application. In step  65 , the Java application creates a statement Stmt=conn.createStatement( ) on the connection set up. In step  66 , the key management means in the connection manager generates a key for the station so that all statements on this connection then contain this key. The Java application executes on the created statement a SQL statement containing the key in step  67  and gets a returned result set in step  68 . Step  67  to step  68  can be repeated until the Java application executes a new SQL statement, during which the connection manager can reclaim the connection according to needs at any time. Such resilient management is denoted using step  69 . In step  69 , when the Java application executes a new SQL statement, there are probably three cases. In the first case, a result set is returned in step  70 . In the second case, the connection manager backs up context of the connection to the context repository in step  71  shown by a dotted line, and releases the connection in step  72 . In the third case, the connection manager retrieves a key from a statement of the Java application, and extracted the context backup from the context repository according to the key in steps  73  and  74  each shown by a dotted line, and then sets up a connection for the Java application according to the restored context in step  75 . 
         [0085]      FIG. 7  schematically shows a computer device in which the embodiments according to the present invention may be implemented. 
         [0086]    The computer system shown in  FIG. 7  comprises a CPU (Central Processing Unit)  701 , a RAM (Random Access Memory)  702 , a ROM (Read Only Memory)  703 , a system bus  704 , a Hard Disk controller  705 , a keyboard controller  706 , a serial interface controller  707 , a parallel interface controller  708 , a display controller  709 , a hard disk  710 , a keyboard  711 , a serial external device  712 , a parallel external device  713  and a display  714 . Among these components, connected to system bus  704  are CPU  701 , RAM  702 , ROM  703 , HD controller  705 , keyboard controller  706 , serial interface controller  707 , parallel interface controller  708  and display controller  709 . Hard disk  710  is connected to HD controller  705 , and keyboard  711  to keyboard controller  706 , serial external device  712  to serial interface controller  707 , parallel external device  713  to parallel interface controller  708 , and display  714  to display controller  709 . 
         [0087]    The functions of each component in  FIG. 7  are well known in the art, and the architecture shown in  FIG. 7  is conventional. Such an architecture applies to not only personal computers but also hand held devices such as Palm PCs, PDAs (personal data assistants), mobile telephones, etc. In different applications, for example, for implementing a user terminal containing the client module according to the present invention or a server host containing the web application server according to the present invention, some components may be added to the architecture shown in  FIG. 7 , or some of the components shown in  FIG. 7  may be omitted. The whole system shown in  FIG. 7  is controlled by computer readable instructions, which are usually stored as software in hard disk  710 , EPROM or other non-volatile memory. The software can also be downloaded from the network (not shown in the figure). The software, either saved in hard disk  710  or downloaded from the network, can be loaded into RAM  702 , and executed by CPU  701  for implementing the functions defined by the software. 
         [0088]    Although the computer system shown in  FIG. 7  is able to support the solution of managing a database connection according to the present invention, the computer system merely serves as an example of computer systems. Those skilled in the art may understand that many other computer system designs are also able to carry out the embodiments of the present invention. 
         [0089]    The present invention may further be implemented as a computer program product used by, for example the computer system shown in  FIG. 7 , which contains code for implementing the database connection management according to the present invention. The code may be stored in a memory of other computer system prior to the usage. For instance, the code may be stored in a hard disk or a removable memory like an optical disk or a floppy disk, or may be downloaded via the Internet or other computer network. 
         [0090]    As the preferred embodiments of the present invention have been described, those of ordinary skill in the art will appreciate that various modifications or alterations may be made without departing from the essence and scope of the present invention. Moreover, it is to be understood that the present invention is not limited to the specific embodiments disclosed here, and the scope thereof is defined by the appended claims.