Abstract:
Techniques and systems are provided for efficiently managing hierarchical relational data stored in object-relational database system by extending the syntax of the database language supported by the database system (e.g. SQL) to incorporate several new “hierarchical” operators. These operators are evaluated based on the hierarchical relationship defined for hierarchical relational data. The operators maybe incorporated in SQL queries along with relational operators.

Description:
RELATED APPLICATIONS 
   The present application claims priority from the following U.S. Provisional Patent Applications, the entire contents of which are incorporated herein by reference for all purposes: 
   U.S. Provisional Patent Application No. 60/326,052, filed on Sep. 28, 2001, entitled FILE BASED ACCESS PROVIDED WITH A DATABASE SYSTEM, by ERIC SEDLAR and VISWANATHAN KRISHNAMURTHY; 
   U.S. Provisional Patent Application No. 60/378,800, filed on May 7, 2002, entitled SQL ACCESS TO DATA THAT PROVIDES A FILE SYSTEM ABSTRACTION, by NIPUN AGARWAL, RAVI MURTHY, ERIC SEDLAR, SIVASANKARAN CHANDRASEKAR, FEI GE, SYAM PANNALA, NEEMA JALALI and MURALIDHAR KRISHNAPRASAD. 
   The present application is also related to the following U.S. patent applications, the entire contents of which are incorporated herein by reference for all purposes: 
   U.S. patent application Ser. No. 10/260,384, filed on Sep. 27, 2002, entitled PROVIDING A CONSISTENT HIERARCHICAL ABSTRACTION OF RELATIONAL DATA, by NIPUN AGARWAL, ERIC SEDLAR, RAVI MURTHY and NAMIT JAIN; 
   U.S. patent application Ser. No. 10/259,278, filed on Sep. 27, 2002, entitled MECHANISM FOR MAPPING XML SCHEMAS TO OBJECT-RELATIONAL DATABASE SYSTEMS, by RAVI MURTHY, MURALIDHAR KRISHNAPRASAD, SIVASANKARAN CHANDRASEKAR, ERIC SEDLAR, VISWANATHAN KRISHNAMURTHY and NIPUN AGARWAL; 
   U.S. patent application Ser. No. 10/260,161, filed on Sep. 27, 2002, entitled INDEXING TO EFFICIENTLY MANAGE VERSIONED DATA IN A DATABASE SYSTEM, by NIPUN AGARWAL, ERIC SEDLAR and RAVI MURTHY; 
   U.S. patent application Ser. No. 10/256,524, filed on Sep. 27, 2002, entitled MECHANISMS FOR STORING CONTENT AND PROPERTIES OF HIERARCHICALLY ORGANIZED RESOURCES, by RAVI MURTHY, ERIC SEDLAR, NIPUN AGARWAL, and NEEMA JALALI; 
   U.S. patent application Ser. No. 10/259,176, filed on Sep. 27, 2002, entitled MECHANISM FOR UNIFORM ACCESS CONTROL IN A DATABASE SYSTEM, by RAVI MURTHY, ERIC SEDLAR, NIPUN AGARWAL, SAM IDICULA and NICK MONTOYA; 
   U.S. patent application Ser. No. 10/256,777, filed on Sep. 27, 2002, entitled LOADABLE UNITS FOR LAZY MANIFESTATION OF XML DOCUMENTS by SYAM PANNALA, ERIC SEDLAR, BHUSHAN KHALADKAR, RAVI MURTHY, SIVASANKARAN CHANDRASEKAR, and NIPUN AGARWAL; 
   U.S. patent application Ser. No. 10/260,381, filed on Sep. 27, 2002, entitled MECHANISM TO EFFICIENTLY INDEX STRUCTURED DATA THAT PROVIDES HIERARCHICAL ACCESS IN A RELATIONAL DATABASE SYSTEM, by NEEMA JALALI, ERIC SEDLAR, NIPUN AGARWAL, and RAVI MURTHY; 

   FIELD OF THE INVENTION 
   The present invention relates to relational database systems and, more specifically, to techniques for managing hierarchical data within relational database systems. 
   BACKGROUND OF THE INVENTION 
   Humans tend to organize information in categories. The categories in which information is organized are themselves typically organized relative to each other in some form of hierarchy. For example, an individual animal belongs to a species, the species belongs to a genus, the genus belongs to a family, the family belongs to an order, and the order belongs to a class. 
   With the advent of computer systems, techniques for storing electronic information have been developed that largely reflected this human desire for hierarchical organization. Conventional computer file systems, for example, are typically implemented using hierarchy-based organization principles. Specifically, a typical file system has directories arranged in a hierarchy, and documents stored in the directories. Ideally, the hierarchical relationships between the directories reflect some intuitive relationship between the meanings that have been assigned to the directories. Similarly, it is ideal for each document to be stored in a directory based on some intuitive relationship between the contents of the document and the meaning assigned to the directory in which the document is stored. 
     FIG. 1  shows an example of a typical file system. The illustrated file system includes numerous directories arranged in a hierarchy. Two documents  118  and  122  are stored in the directories. Specifically, documents  118  and  122 , both of which are entitled “Example.doc”, are respectively stored in directories  116  and  124 , which are respectively entitled “Word” and “App4”. 
   In the directory hierarchy, directory  116  is a child of directory  114  entitled “Windows”, and directory  114  is a child of directory  110 . Similarly, directory  124  is a child of directory  126  entitled “VMS”, and directory  126  is a child of directory  110 . Directory  110  is referred to as the “root” directory because it is the directory from which all other directories descend. In many systems, the symbol “/” is used to refer to the root directory. 
   When electronic information is organized in a hierarchy, each item of information may be located by following a “path” through the hierarchy to the entity that contains the item. Within a hierarchical file system, the path to an item begins at the root directory and proceeds down the hierarchy of directories to eventually arrive at the directory that contains the item of interest. For example, the path to file  118  consists of directories  110 ,  114  and  116 , in that order. 
   Hierarchical storage systems often allow different items to have the same name. For example, in the file system shown in  FIG. 1 , both of the documents  118  and  122  are entitled “Example.doc”. Consequently, to unambiguously identify a given document, more than just the name of the document is required. 
   A convenient way to identify and locate a specific item of information stored in a hierarchical storage system is through the use of a “pathname”. A pathname is a concise way of uniquely identifying an item based on the path through the hierarchy to the item. A pathname is composed of a sequence of names. In the context of a file system, each name in the sequence of names is a “filename”. The term “filename” refers to both the names of directories and the names of documents, since both directories and documents are considered to be “files”. 
   Within a file system, the sequence of filenames in a given pathname begins with the name of the root directory, includes the names of all directories along the path from the root directory to the item of interest, and terminates in the name of the item of interest. Typically, the list of directories to traverse is concatenated together, with some kind of separator punctuation (e.g., ‘/’, ‘\’, or ‘;’) to make a pathname. Thus, the pathname for document  118  is /Windows/Word/Example.doc, while the pathname for document  122  is /VMS/App4/Example.doc. 
   The relationship between directories (files) and their contained content varies significantly between different types of hierarchically organized systems. One model, employed by various implementations, such as Windows and DOS file systems, requires each file to have exactly one parent, forming a tree. In a more complicated model, the hierarchy takes the form of a directed graph, where files can have multiple parents, as in the UNIX file system in which hard links are used. 
   In contrast to hierarchical approaches to organizing electronic information, a relational database stores information in tables comprised of rows and columns. Each row is identified by a unique RowID. Each column represents an attribute of a record, and each row represents a particular record. Data is retrieved from the database by submitting queries to a database management system (DBMS) that manages the database. The queries must conform to the database language supported by the database management system. Structured Query Language (SQL) is an example of a database language supported by many existing database management systems. 
   Each type of storage system has advantages and limitations. A hierarchically organized storage system is simple, intuitive, and easy to implement, and is a standard model used by most application programs. Unfortunately, the simplicity of the hierarchical organization does not provide the support required for complex data retrieval operations. For example, the contents of every directory may have to be inspected to retrieve all documents created on a particular day that have a particular filename. Since all directories must be searched, the hierarchical organization does nothing to facilitate the retrieval process. 
   A relational database system is well suited for storing large amounts of information and for accessing data in a very flexible manner. Relative to hierarchically organized systems, data that matches even complex search criteria may be easily and efficiently retrieved from a relational database system. However, the process of formulating and submitting queries to a database server is less intuitive than merely traversing a hierarchy of directories, and is beyond the technical comfort level of many computer users. 
   In the past, hierarchically organized systems and relationally organized systems have been implemented in different ways that were not compatible. With some additional processing, however, a relationally organized system can emulate a hierarchically organized system. This type of emulation is especially desirable when the storage capability and flexibility of a relational system is needed, but the intuitiveness and ubiquity of the hierarchical system is desired. 
   When hierarchically organized information is stored in a relational database system, the power of the relational system may be tapped by accessing the information using the access mechanism that is native to the relational database system. Specifically, using the database language supported by the database, complex queries can be formulated to extract the desired information from the database. However, the information that is of interest to a user may hinge on the position of the information within the applicable information hierarchy. Unfortunately, because database languages were designed based on the relational database model, the concept of hierarchical relationships is largely foreign to them. Consequently, it is desirable to provide a mechanism for querying information, stored within a relational database, based on the position of the information within the applicable hierarchy. It is further desirable to provide a mechanism for using queries to easily extract information about the position of information within the applicable information hierarchy. 
   SUMMARY OF THE INVENTION 
   Techniques and systems are provided for efficiently managing hierarchical relational data stored in object-relational database system by extending the syntax of the database language supported by the database system (e.g. SQL) to incorporate several new “hierarchical” operators. These operators are evaluated based on the hierarchical relationship defined for hierarchical relational data. The operators may be incorporated in SQL queries along with relational operators. 
   For example, assume rows of a table represent nodes in an XML tree. Each row is associated with a path for the node that corresponds to the row. An “UNDER_PATH” operator takes an argument that specifies a node in the hierarchy (the “input” node) and returns true for a row if the node associated with the row falls on or below the input node. The operator “PATH” returns the path to the node associated with a row. In one embodiment, if a query evaluates multiple hierarchical operators, such as “UNDER_PATH” and “PATH”, for the same rows, then the path only needs to be computed once for each row. 
   Another variation is the EQUALS_PATH operator that returns TRUE for a row if the node associated with the row is the same as the input node. Also, the operator RELATIVE_PATH returns the path to the node associated with a row, relative to the input node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is a block diagram illustrating a hierarchical file system; 
       FIG. 2  is a block diagram illustrating an information hierarchy; 
       FIG. 3  is a block diagram illustrating tables that may be used to capture the information hierarchy illustrated in  FIG. 2  within a relational system according to an embodiment of the invention; and 
       FIG. 4  is a block diagram of a system on which embodiments of the invention may be implemented. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A method and apparatus for accessing hierarchical information stored in a relational database system is described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
   Functional Overview 
   Techniques are described herein for managing, within a relational database system, data items that belong to an information hierarchy. According to embodiments of the invention, the hierarchy to which the data items belong is not transparent to the database system. Rather, the database system explicitly recognizes and manages the hierarchy, and provides support for queries that access the data items based on the position of data items within the hierarchy. In one embodiment, the database language supported by the relational database system is extended to include operators that produce output based on the position of data items within that hierarchy. Such hierarchical operators may be used in conjunction with the many relational operators supported by relational database languages to formulate powerful queries to access the information in which users are interested. In one embodiment, the operators include a “PATH” operator and an “UNDER_PATH” operator, both of which shall be described in greater detail hereafter. 
   System Overview 
     FIG. 2  is a block diagram that illustrates a hierarchy  200  that is used in examples that are provided herein to facilitate an understanding of embodiments of the invention. Hierarchy  200  includes eight nodes. The highest node in the hierarchy is referred to as the “root” node. The nodes at the end of each branch in the hierarchy are “leaf” nodes. The nodes between the root node and the leaf nodes are “intermediate” nodes. In the illustrated hierarchy, nodes  1 ,  2 , and  3  are intermediate nodes, and nodes  4 ,  5 ,  6 , and  7  are leaf nodes. 
   In an information hierarchy, the nodes correspond to information. Typically, the piece of information associated with each node will have some form of name, and some type of content. For example, in a hierarchy that corresponds to a hierarchical file system, the nodes will typically correspond to files (where a “folder” or “directory” is one type of file). Each such file will have a name, and some form of contents. 
   Frequently, the names that are associated with the nodes in a hierarchy need not be unique. In the example shown in  FIG. 2 , both node  1  and node  7  have the name “a”. Though they have the same name, they are separate and distinct nodes, which occupy different positions in the hierarchy and may be associated with completely different content. 
     FIG. 3  is a block diagram of two tables  302  and  350  that may be used to represent hierarchy  200  in a relational database system. Table  302  includes one row for each node in the hierarchy. The row for a given node includes the name associated with the given node and the data associated with the given node. For example, row  304  corresponds to node  1 , and contains the name “a” and data  306  associated with node  1 . 
   Table  350  includes one row for each of the parent-child relationships in hierarchy  200 . A parent column  378  holds the value that specifies the parent in a parent-child relationship, and a child column  380  holds the value that specifies the child of the parent-child relationship. For example, row  352  indicates that the root node is the parent of node  1 . Similarly, rows  354  and  356  respectively indicate that node  1  is the parent of nodes  2  and  3 . 
   Tables  302  and  250 , capture all the information of the hierarchy  200  in relational format. While embodiments of the invention shall be described hereafter through the use of examples that refer to hierarchy  200  and tables  302  and  350 , such embodiments are merely exemplary. The way that a relational database system stores information about hierarchies may vary from implementation to implementation, and the present techniques are not limited to any particular implementation. 
   The UNDER_PATH Operator 
   According to one aspect of the invention, an “UNDER_PATH” hierarchical operator is provided to allow database statements to formulate explicit conditions related to whether nodes reside at or below a given node in a hierarchy. Specifically, in one embodiment, the UNDER_PATH operator has input parameters used to identify a node in the hierarchy. The node specified in the input parameters to the UNDER_PATH operator shall be referred to herein as the “input node”. The UNDER_PATH operator returns TRUE for all nodes that include the input node in their path up to the root node. Thus, the set of nodes that cause the UNDER_PATH operator to return TRUE are the “descendants” of the input node. 
   According to one embodiment, the UNDER_PATH operator can be invoked in any one of the following ways:
     INTEGER UNDER_PATH(resource_column, pathname)   INTEGER UNDER_PATH(resource_column, depth, pathname)   INTEGER UNDER_PATH(resource_column, pathname, correlation)   INTEGER UNDER_PATH(resource_column, depth, pathname, correlation)
 
where the arguments are defined as follows:
   resource_column: The column name or column alias of the ‘resource’ column   pathname: The path name of the input node   depth: The maximum depth to search.   correlation: An integer that can be used to correlate the UNDER_PATH operator (a primary operator) with ancillary operators such as PATH and DEPTH.   

   For example, referring to hierarchy  200 , the statement UNDER_PATH(node, ‘/a’), specifies node  1  as the input node for the UNDER_PATH operator. Similarly, the statement UNDER_PATH(node, ‘/a/c/a’) specifies node  7  as the input node for the UNDER_PATH operator. 
   When node  1  is specified as the input node to the UNDER_PATH operator, the UNDER_PATH operator returns “TRUE” for the rows that correspond to nodes  2 ,  3 ,  4 ,  5 ,  6  and  7 . When node  3  is specified as the input node to the UNDER_PATH operator, the UNDER_PATH operator returns “TRUE” for the rows that correspond to nodes  6  and  7 . When node  1  is specified as the input node to the UNDER_PATH operator and a maximum depth of 1 is specified, the UNDER_PATH operator returns “TRUE” for only the rows that correspond to nodes  2  and  3 . 
   UNDER_PATH Processing 
   A database server that represents hierarchy  200  through the use of tables  302  and  350  may execute the UNDER_PATH operation by scanning table  350  X+1 times, where X equals the length of the longest chain, within hierarchy  200 , between the input node and any leaf node. If a maximum depth is specified, then the number of scans performed is the lesser of X+1 and the specified depth. 
   For example, assume that the predicate of a query includes the expression UNDER_PATH(node, ‘/a’). Initially, the database server initializes a “matching set” that is to include the nodes for which UNDER_PATH(node, ‘/a’) evaluates to TRUE, and sets the matching set to NULL. 
   In the first pass, the database server scans table  350  to find the rows that have “ 1 ”, which corresponds to ‘/a’, in the parent column  378 . In the present example, rows  354  and  356  include “ 1 ” in the parent column  378 . From these rows, the database server adds the child nodes ( 2  and  3 , respectively) to the matching set. 
   During each subsequent pass, the database server searches for rows that contain, in the parent column  378 , the nodes that were added to the matching set in the previous pass. Thus, during the second pass, the database server searches for rows that have either  2  or  3  in the parent column  378 . Rows  358  and  360  have  2  in the parent column  378 , and rows  362  and  364  have  3  in the parent column  378 . From rows  358 ,  360 ,  362  and  364  the database server respectively adds  4 ,  5 ,  6  and  7  to the matching set. 
   During the third pass, the database server searches for rows that contain nodes  4 ,  5 ,  6  or  7  in the parent column  378  (the nodes that were added to the matching set in the previous pass). None of the rows of table  350  have  4 ,  5 ,  6 , or  7  in the parent column  378 , so nothing is added to the matching set during the third pass. When a pass fails to add anything to the matching set, the matching set is complete and the UNDER_PATH expression is evaluated based on the members of the matching set. Specifically, the UNDER_PATH expression returns TRUE when applied to a row that corresponds to any member of the matching set, and FALSE when applied to rows that correspond to all other nodes. 
   According to alternative embodiment, the evaluation of the UNDER_PATH operator is performed more efficiently through the use of one or more indexes. For example, a hierarchical index, such as the index described in may be used to return the paths under a particular path. Such hierarchical indexes speed up access walking down a pathname. If the UNDER_PATH operator is used in the predicate of a query, and other parts of the query predicate are very selective, a functional implementation of UNDER_PATH may be chosen that will walk back up the hierarchy. Under these circumstances, walking up the hierarchy can be more efficient, since a much smaller number of links may need to be traversed. 
   The EQUALS_PATH Operator 
   According to one embodiment, an EQUALS_PATH operator is also provided. The EQUALS_PATH operator is defined as follows:
         EQUALS_PATH: INTEGER EQUALS_PATH(resource_column, pathname)       

   The EQUALS_PATH operator is functionally equivalent to UNDER_PATH with a depth restriction of zero. Thus, it returns TRUE when evaluated against the row associated with the input node, and returns FALSE when evaluated against all other rows. 
   The DEPTH Operator 
   According to one embodiment, a DEPTH operator is provided. The DEPTH operator is defined as follows:
         DEPTH: INTEGER DEPTH(correlation)       

   The DEPTH operator returns a number that indicates the depth, within the hierarchy, of the node associated with the row to which the operator is being applied. For example, when applied to the rows for nodes  1 ,  2 ,  3 ,  4 ,  5 ,  6  and  7  of hierarchy  200 , the DEPTH operator returns the values 0, 1, 1, 2, 2, 2 and 2, respectively. 
   The PATH Operator 
   According to one aspect of the invention, a “PATH” hierarchical operator is provided. When applied to the row associated with a given node, the PATH operator produces a string made up of the names of the nodes that make up the chain that leads from the given node of the row up to the node specified by UNDER_PATH. The set of nodes identified in the string produced by applying the PATH operator to the row for a given node forms a relative path under the specified pathname in UNDER_PATH. 
   According to one embodiment, the PATH operator is defined as follows:
         PATH: VARCHAR2 PATH(correlation)
 
where VARCHAR2 represents a string type.
       

   A database server that represents hierarchy  200  through the use of tables  302  and  350  may evaluate the PATH expression for the row associated with a node by scanning table  350  X times, where X equals the level, within hierarchy  200 , of the node. According to one embodiment, the evaluation of PATH proceeds as follows: 
   Initially, to construct the PATH of a node, the database server creates a string that includes the name of the node associated with the row to which the PATH operator is being applied. For the purpose of explanation, it shall be assumed that the PATH operator is being applied to the row associated with node  5  of hierarchy  200 . Thus, the database server initially creates the string “/e”. 
   After the initial string is created, the database server scans table  350  to identify the row in which 5 is the value of the child column  380 . In the present example, the database server would locate row  360 . From row  360 , the database server would read that node  2  is the parent of node  5 . From table  302 , the database server would determine that the name of node  2  is “b”. Based on this information, the string “/b” is added to the string “/e” to create the string “/b/e”. 
   Once again the database server scans table  350 , this time to identify the row that contains 2 in child column  380 . In the present example, the database server would locate row  354 . From row  354 , the database server would read that node  1  is the parent of node  2 . Table  302  indicates that “a” is the name of node  1 . Node “a” is the root node, so the path construction operation is complete. Thus, the final result string is “/b/e”, which is relative to “/a”. 
   The process of scanning table  350  to find the parent to each new node that is added to the path is repeated until the input node is reached. Because the root node has no parent, the PATH string is complete when the root node is reached. In the present example, the string produced by applying the PATH operator to node  5  is “/b/e”. 
   Views 
   There are a variety of contexts in which hierarchical operators may be useful. Once such context is the definition of views. For example, a user may create a view (RV) of the hierarchical data based on the following view definition:
         RV: Select PATH from R$ where UNDER_PATH(root)=True;       

   In this example, R$ is table  302 . 
   When executed, view RV produces one row for each node in hierarchy  200 , where the row associated with a given node contains the relative path to the node. For example, the row produced in the view for node  2  would contain the path “/a/b”. Similarly, the row produced in the view for node  6  would contain the path “/a/c/f”. In this example, the condition “UNDER_PATH(root)=True” would filter out any rows in table  302  that do not correspond to nodes of hierarchy  200 . Such a filter is useful, for example, if table  302  is used to store information for numerous hierarchies. 
   Evaluation Optimizations 
   As mentioned above, the evaluation of hierarchical operators may involve performing numerous scans of the table  350  that stores the parent-child relationship information. One way to reduce the cost of the evaluations is to use an index, as mentioned above. Another way to reduce the cost of the evaluations involves using information obtained during the evaluation of one hierarchical operator for the evaluation of another hierarchical operator. For example, a single database statement may contain both the PATH and UNDER_PATH operators. The information obtained in the same scans of table  350  can be used to evaluate both operators. 
   Hardware Overview 
     FIG. 4  is a block diagram that illustrates a computer system  400  upon which an embodiment of the invention may be implemented. Computer system  400  includes a bus  402  or other communication mechanism for communicating information, and a processor  404  coupled with bus  402  for processing information. Computer system  400  also includes a main memory  406 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  402  for storing information and instructions to be executed by processor  404 . Main memory  406  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  404 . Computer system  400  further includes a read only memory (ROM)  408  or other static storage device coupled to bus  402  for storing static information and instructions for processor  404 . A storage device  410 , such as a magnetic disk or optical disk, is provided and coupled to bus  402  for storing information and instructions. 
   Computer system  400  may be coupled via bus  402  to a display  412 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  414 , including alphanumeric and other keys, is coupled to bus  402  for communicating information and command selections to processor  404 . Another type of user input device is cursor control  416 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  404  and for controlling cursor movement on display  412 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
   The invention is related to the use of computer system  400  for implementing the techniques described herein. According to one embodiment of the invention, those techniques are performed by computer system  400  in response to processor  404  executing one or more sequences of one or more instructions contained in main memory  406 . Such instructions may be read into main memory  406  from another computer-readable medium, such as storage device  410 . Execution of the sequences of instructions contained in main memory  406  causes processor  404  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and software. 
   The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor  404  for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  410 . Volatile media includes dynamic memory, such as main memory  406 . Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  402 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
   Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, or any other magnetic medium, a CD-ROM, any other optical medium, punchcards, papertape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read. 
   Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor  404  for execution. For example, the instructions may initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  400  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  402 . Bus  402  carries the data to main memory  406 , from which processor  404  retrieves and executes the instructions. The instructions received by main memory  406  may optionally be stored on storage device  410  either before or after execution by processor  404 . 
   Computer system  400  also includes a communication interface  418  coupled to bus  402 . Communication interface  418  provides a two-way data communication coupling to a network link  420  that is connected to a local network  422 . For example, communication interface  418  may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  418  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  418  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
   Network link  420  typically provides data communication through one or more networks to other data devices. For example, network link  420  may provide a connection through local network  422  to a host computer  424  or to data equipment operated by an Internet Service Provider (ISP)  426 . ISP  426  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  428 . Local network  422  and Internet  428  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  420  and through communication interface  418 , which carry the digital data to and from computer system  400 , are exemplary forms of carrier waves transporting the information. 
   Computer system  400  can send messages and receive data, including program code, through the network(s), network link  420  and communication interface  418 . In the Internet example, a server  430  might transmit a requested code for an application program through Internet  428 , ISP  426 , local network  422  and communication interface  418 . 
   The received code may be executed by processor  404  as it is received, and/or stored in storage device  410 , or other non-volatile storage for later execution. In this manner, computer system  400  may obtain application code in the form of a carrier wave. 
   In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.