Patent Publication Number: US-7912945-B2

Title: Exchange of management information via application messages

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to distributed systems, and more specifically to exchanging management information between machines in a service oriented architecture. 
     BACKGROUND 
     Each machine within a distributed computing system typically includes a management application that monitors the activities and operating conditions of other applications, services and machines in the distributed computing system. To effectively monitor such activities and operating conditions, the management applications periodically exchange management information with other management applications via management messages. A management message is generated by a management application at an application level (e.g., of the internet control message protocol (ICMP) or the open systems interconnection basic reference model (OSI model)), and includes operating statistics of a machine on which the management application operates and/or applications and services that operate on the machine. Examples of information included in management messages include how long a service has been active, how many users a service has had, the number of services and clients active on a machine, computing power of the machine, the present and past workload of a service, software versions of the service, etc. The exchange of management messages consumes bandwidth of the distributed computing system. 
    
    
     
       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: 
         FIG. 1  illustrates an exemplary distributed computing system, in which embodiments of the present invention may operate; 
         FIG. 2  illustrates a data flow diagram showing a message flowing through a distributed computing system, in accordance with one embodiment of the present invention; 
         FIG. 3  illustrates a flow diagram of one embodiment for a method of exchanging management information between machines in a distributed computing system; 
         FIG. 4  illustrates a flow diagram of another embodiment for a method of exchanging management information between machines in a distributed computing system; and 
         FIG. 5  illustrates a block diagram of an exemplary computer system, in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a method and apparatus for appending management information to messages that flow through a distributed computing system. In one embodiment, a message addressed to an application operating on a destination machine is received at an intermediate machine. Management information is attached to the message, the management information including operating statistics of the intermediate machine. The message is then forwarded to the destination machine. The message may be received at multiple different intermediate machines as it flows through a distributed computing system. Each of the intermediate machines may add management information to the message. In one embodiment, an amount of management information attached to the message is proportional to a number of machines through which the message has flowed. Upon processing the message, the destination machine may generate a response message, and attach the management information to the response message. The response message may then be transmitted back to a source machine that generated the message. 
     In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
     Some portions of the detailed descriptions which follow are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “sending”, “receiving”, “attaching”, “forwarding”, “caching”, or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     The present invention also relates to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions, each coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will appear as set forth in the description below. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. 
     The present invention may be provided as a computer program product, or software, that may include a machine-readable medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to the present invention. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable (e.g., computer-readable) medium includes a machine (e.g., a computer) readable storage medium (e.g., read only memory (“ROM”), random access memory (“RAM”), magnetic disk storage media, optical storage media, flash memory devices, etc.), a machine (e.g., computer) readable transmission medium (electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.)), etc. 
       FIG. 1  illustrates an exemplary distributed computing system  100 , in which embodiments of the present invention may operate. In one embodiment, the distributed computing system  100  includes a service oriented architecture (SOA). A service oriented architecture (SOA) is an information system architecture that organizes and uses distributed capabilities (services) for one or more applications. SOA provides a uniform means to offer, discover, interact with and use capabilities (services) distributed over a network. Through the SOA, applications may be designed that combine loosely coupled and interoperable services. 
     The distributed computing system  100  may include multiple machines (e.g., first machine  118 , second machine  122  and third machine  132 ) connected via a network  135 . Each of the machines may be a notebook computer, desktop computer, server, cellular phone, or other computing device. Each machine may include one or more services (e.g., first service  110  and additional services  130 ) and/or clients (e.g., first client  105  and additional clients  125 ) that operate on the machine. Each machine may also include a management application (e.g., first management application, second management application, and third management application  148 ) that operates on the machine, and is connected with clients and/or services that operate on the same machine. 
     In one embodiment, first client  105  operates on a first machine  118 , first service  110  operates on a second machine  122 , and content based router  120  operates on a third machine  132 . In a further embodiment, additional clients  125  and/or additional services  130  operate on the first machine  118 , second machine  122  and/or third machine  132 . In another embodiment, first client  105 , first service  110  and/or content based router  120  may operate on the same machine (e.g., they may all operate on first machine  118 ). 
     The network  135  may be a public network (e.g., Internet), a private network (e.g., Ethernet or a local area Network (LAN)), or a combination thereof. In one embodiment, the network  135  includes an enterprise service bus (ESB). An ESB is an event-driven and standards-based messaging engine that provides services for more complex architectures. The ESB provides an infrastructure that links together services  110 ,  130  and clients  105 ,  125  to enable distributed applications and processes. The ESB may be implemented to facilitate an SOA. In one embodiment, the ESB is a single bus that logically interconnects all available services and clients. Alternatively, the ESB may include multiple busses, each of which may logically interconnect different services and/or clients. 
     Clients  105  and  125  are applications that run on machines. In the SOA, clients  105  and  125  include applications that access services  110  and  130 . Clients  105  and  125  may be fat clients (clients that perform local processing and data storage), thin clients (clients that perform minimal or no local processing and minimal to no data storage), and/or hybrid clients (clients that perform local processing but little to no data storage). 
     Services  110  and  130  may be discretely defined sets of contiguous and autonomous functionality (e.g., business functionality, technical functionality, etc.). Each service  110  and  130  may represent a process, activity or other resource that can be accessed and used by other services or clients on network  135 . Each service  110  and  130  may be independent of other services  110  and  130 , and may be accessed without knowledge of its underlying platform implementation. 
     In an example for a business function of “managing orders,” services may include, for example, create order, fulfill order, ship order, invoice order, cancel/update order, etc. Each such service may be autonomous from the other services that are used to manage orders, and may be remote from one another and have different platform implementations. However, the services may be combined and used by one or more applications to manage orders. 
     Each of the clients  105  and  125  and services  110  and  130  may be both message producers and message consumers. In the illustrated embodiment, the first client  105  is a message producer, and the first service  110  is a message consumer. Alternatively, additional clients  125 , first service  110  or additional services  130  may be message producers, and/or first client  105 , additional clients  125 , or additional services  130  may be message consumers. 
     A message producer is a client or service that generates a message. Messages include data that may convey information to, or initiate an event on, a message consumer (e.g., a service or client). A message may be generated for any of a myriad of purposes. For example, the message may be generated to report a purchase of a good, to request contact information, to begin a remote process (e.g., initiate a service), etc. 
     A message consumer receives a message generated by a message producer, and sent to an address (e.g., service address or client address) of the message consumer. Based on the content of the message, the message consumer may store information contained in the message, generate a response message to send to a service or client, undergo a state change, and/or initiate some other event. A state change initiated by a message may be dependent on contents of the message (e.g., the message body, message context, etc.), rules governing responses to the message, etc. 
     Each service may have a service address to which messages directed to the service are sent. The service address is included in a header of a message. A format of the service address may depend on a transport mechanism used by the service. For example, if the transport mechanism is HTTP or FTP, then the service address may include a host number and a port number. If, on the other hand, the transport mechanism is JMS, the service address may be a JMS queue number. If a message is sent to an incorrect service address, a message cannot be processed by the service. 
     In one embodiment, the distributed computing system  100  includes a content based router  120 . The content based router  120  is a service of the ESB that monitors the network  135  for messages, and transparently routes the messages between clients and services. 
     In one embodiment, the content based router  120  acts as a forwarder for one or more services (e.g., first service  110 ) on an ESB. When the content based router  120  intercepts a message from a message producer (e.g., first client  105 ), the content based router  120  may examine a header and/or other contents of the message to determine whether it is directed to an active service address or addresses of the service. Those messages directed to a specific service address that is no longer active may be forwarded to a specified different service address. A service may therefore receive messages that are sent to old service addresses that are no longer active. For example, content based router  120  may be set to forward all messages addressed to a first (old) service address of first service  110  to a second (new) service address of first service  110 . First service  110  may therefore be moved as necessary without inconvenience to clients  105 ,  125 . 
     In another embodiment, content based router  120  acts as a switchboard, directing some or all message traffic between clients  105 ,  125  and services  110 ,  130 . In such an embodiment, some or all messages may be directed to the content based router  120 . A client  105 ,  125  may not need to know a service address of an intended service to send a message to it. Instead, a client may only need to know a name or other identifying information of the service  110 ,  130 . Content based router  120  may then direct incoming messages to appropriate services based on service names or other identifying information. 
     In one embodiment, first machine  118 , second machine  122  and third machine  132  include first management application  128 , second management application  138  and third management application  148 , respectively. Management applications may gather operating statistics of applications and/or services to which the management applications are connected (e.g., those applications and services that operate on a machine on which a management application operates). For example, first management application may collect operating statistics of first client  105 , third management application may collect operating statistics of content based router  120 , etc. Management applications may collect data on, for example, how long a service has been active, how many clients have used a service, the current and past workload of a service, software versions being used by an application or service, etc. Management applications may also gather operating statistics of the machine on which the management application operates (e.g., how long the machine has been active, number of services operating on the machine, machine capabilities, etc.). 
     In one embodiment, a management application attaches management information to outgoing messages (whether those messages were generated by a local application or service, or whether the messages were generated by an application/service on a remote machine. When those messages are received at another machine, a management application that operates on the other machine may copy the received management information, and/or remove the received management information form the message. That other machine may then attach additional management information to the message before forwarding it on to yet another machine (e.g., to a machine on which a destination application operates). Management applications therefore may use existing messages as a medium for exchanging management information. The amount of management information that is added to a message may depend on a number of machines that receive a message, and/or a number of services, clients, etc. that operate on the machines. 
     In one embodiment, management applications also generate management messages that query other management applications about services and/or clients operating on other machines. Such queries may be sent, for example, if a management application lacks recent data on specified services or applications running on a remote machine. Management messages may also include management information that has been attached by the sending management application. 
       FIG. 2  illustrates a data flow diagram  200  showing a message flowing through a distributed computing system, in accordance with one embodiment of the present invention. In one embodiment, the distributed computing system corresponds to distributed computing system  100  of  FIG. 1 . 
     Returning to  FIG. 2 , a source machine  205  includes a source application  235 , a first management application  240  and a first dispatcher  245 . A dispatcher is a service that receives a message as an input, and outputs the message. Dispatchers may be configured to perform operations on all messages that they receive. Alternatively, dispatchers may examine each received message and determine whether or not to operate on it based on, for example, a rules engine and accompanying rules. Dispatchers may determine where the messages are addressed to, and operate on the message or perform an action based on the addressee. Dispatchers may also examine the contents of the message, and determine whether to operate on the message based on identified contents. For example, a dispatcher may be configured only to operate on messages having a specific time stamp, or including specific information. 
     The source application  235  generates a message  225  directed to a destination application  255 , and sends it to a first intermediate machine  210  in a first transmission  270 . In one embodiment, the management application  240  attaches management information to the message  225 . The management information may be unrelated with a purpose and/or body of the message. For example, the message  225  may be to process a credit card transaction, and the management information may include operating statistics on a source machine  205 . In another embodiment, first management application  240  directs dispatcher  245  to attach management information to the message  225 . For example, first management application  240  may periodically or continuously forward management information to first dispatcher  245 . When a new message is ready to be transmitted, first dispatcher  245  may attach latest management information that it has received from first management application  240 . In yet another embodiment, source application  235  attaches management information to message  225 . For example, source application  235  may periodically be provided with management information from first management application  240 , which source application  235  may attach to messages that it generates. Alternatively, no management information may be attached to the message at the source machine  205 . 
     In one embodiment, first intermediate machine  210  is a machine on which destination application  255  once operated. Alternatively, first intermediate machine  210  may be a router, gateway, or other processing device configured to receive and transmit messages. First intermediate machine  210  includes a first forwarder  250 , a second management application  248  and a second dispatcher  262 . In one embodiment, second dispatcher  262  receives message  225  and forwards it to first forwarder  250 . If any management information were attached to message  225 , second dispatcher  262  could copy the management information and forward it to second management application  242 . 
     First forwarder  250  forwards the message  225  to a second intermediate machine  215  in a second transmission  275 . First forwarder  250  may have been installed at the first intermediate machine  210  to forward messages that are directed to a destination application  255  that once operated on the first intermediate machine  210 . Such messages may be forwarded to a second intermediate machine  215  to which the destination application  220  was migrated. The first forwarder  250  may be a simple forwarder that is configured to forward all received messages to second intermediate machine  215 . First forwarder  250  may forward messages to the second intermediate machine  215  regardless of message context, message header, or other message content. Moreover, first forwarder  250  may forward messages to second intermediate machine  215  without examining the messages. Alternatively, first forwarder  250  may examine messages to determine where they should be forwarded. For example, first forwarder  250  may include a list of forwarding addresses for multiple different services that once operated on first intermediate machine  210 . Upon receipt of message  225 , first forwarder  250  may examine message  225  to determine that message  225  is directed to destination application, and find an entry in the list of forwarding addresses that directs first forwarder  150  to forward message  225  to second intermediate machine  215 . 
     In one embodiment, second management application  242  attaches first management information  260  to the message  225 . The first management information  260  may include operating statistics on one or more services and/or applications that operate on first intermediate machine  210 . The first management information  260  may also include operating statistics of the first intermediate machine  210 . Alternatively, second management application  215  may direct second dispatcher  262  to attach first management information  260  to the message  225 . 
     Second intermediate machine  215  receives message  225  in second transmission  275 . In one embodiment, third dispatcher  258  receives messages. Third dispatcher  268  can then copy first management information  260 , and forward it to third management application  248 . Third dispatcher can also forward message  225  to second forwarder  252 . 
     Third management application  248  stores first management information  260  (e.g., in a local or remote data store). Third management application  248  may attach second management information  265  pertaining to second intermediate machine  215  and/or services/applications running on second intermediate machine  215  to the message  225 . Alternatively, third dispatcher  268  or second forwarder  252  may attach the second management information  265  to message (e.g., if second forwarder  252  or third dispatcher  268  periodically receives updated management information from third management application  248 ). Second forwarder  252  then forwards the message  225  to a destination machine  220  in a third transmission  280 . 
     Destination machine  220  receives message  225 . Destination machine  220  includes a fourth dispatcher  272 , a destination application  255 , and a fourth management application  258 . In one embodiment, fourth dispatcher  272  receives the message  225 , and removes the first management information  260  and second management information  265  from the message  225 . Fourth dispatcher  272  then forwards message  225  to destination application  255 , and forwards first management information  260  and second management information  265  to fourth management application  258 . If the destination application  255  were to receive the message  225  with the management information attached, the destination application  255  may not be able to process the message  225 . Alternatively, destination application  255  may discard the attached management information, and process the message  225 . By intercepting the message  225  prior to receipt by destination application  255 , fourth dispatcher  272  can ensure that the first management information  260  and second management information  265  will not be lost. 
     Fourth management application  258  stores first management information  260  and second management information  265  (e.g., in a data store). Destination application  255  processes message  225 , or places it on a message queue for future processing, upon receiving the message  225 . Once destination application  255  has processed message  225 , destination application  255  generates a response message  230  and sends the response message  230  to source application  235  in a fourth transmission  285 . In one embodiment, fourth management application  258  attaches first management information  260  and second management information  265  to the response message  230 . Alternatively, fourth dispatcher  272  may attach first management information  260  and second management information  265  to response message  230 . 
     Upon source machine  205  receiving response message  230 , first dispatcher  245  removes the first management information  260  and second management information  265  (and any additional management information) from the response message  230 . First dispatcher  245  then forwards the first management information  260  and second management information  265  to first management application  240 , and forwards response message  230  to source application  235 . Source application  235  can then process response message  230 . 
     By adding management information to messages that are otherwise flowing through a distributed computing system, management applications can exchange management information without requiring the generation or transmission of additional messages. This can reduce network traffic without reducing an amount of management information that is exchanged. An amount of management information that is attached to a message as it flows through machines of the distributed computing system may be proportional to the number of machines through which it flows. The amount of management information attached to a message may also be proportional to a number of services that operate on the machines through which the message flows. 
       FIG. 3  illustrates a flow diagram of one embodiment for a method  300  of exchanging management information between machines in a distributed computing system. In one embodiment, the distributed computing system is a service oriented architecture (SOA) that includes an enterprise service bus (ESB). The method may be performed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), or a combination thereof. In one embodiment, method  300  is performed by a machine of distributed computing system  100  of  FIG. 1 . 
     Referring to  FIG. 3 , method  300  includes receiving a message at in intermediate machine (block  305 ). In one embodiment, the intermediate machine includes a content based router that receives the message. In another embodiment, the intermediate machine may include a forwarder that receives the message. Alternatively, the intermediate machine may include a router, gateway, or other computing device through which messages may flow in a distributed computing system. The received message is addressed to an application or service operating at a destination machine. 
     At block  310 , processing logic determines whether existing management information is attached to the message. Such a determination may be made, for example, by a dispatcher or other application. If existing management information is attached to the message, the method continues to block  315 . If no existing management information is attached to the message, the method proceeds to block  325 . 
     At block  315 , the existing management information is cached at the intermediate machine. Caching the existing management information may include storing the management information in a data store. At block  320 , the existing management information is forwarded to a management application that runs on the intermediate machine. 
     At block  325 , processing logic attaches management information of the intermediate machine to the message. The management information may include operating statistics of the machine and/or of applications and services running on the machine. The management information may be attached by a dispatcher, the management application, a forwarder, a content based router, or some other application. At block  330 , the message is forwarded to a destination machine. 
       FIG. 4  illustrates a flow diagram of another embodiment for a method  400  of exchanging management information between machines in a distributed computing system. In one embodiment, the distributed computing system is a service oriented architecture (SOA) that includes an enterprise service bus (ESB). The method may be performed by processing logic that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), or a combination thereof. In one embodiment, method  400  is performed by a machine of distributed computing system  100  of  FIG. 1 . 
     Referring to  FIG. 4 , method  400  includes receiving a message at a destination machine (block  405 ). The message may be addressed to an application or service that operates on the destination machine. At block  410 , processing logic determines whether management information from intermediate machines is attached to the message. If no management information is attached to the message, the method proceeds to block  425 . If management information is attached to the message, the method continues to block  415 . 
     At block  415 , the management information is removed from the message. The management information may be removed by a dispatcher (e.g., dispatcher  245  of  FIG. 2 ). At block  420 , the management information is forwarded to a management application that operates on the destination machine. At block  425 , the message is forwarded to the application to which it is addressed. 
     At block  430 , a response message is generated. At block  435 , management information is attached to the message. The management information may be attached to the message by the management application, or by a dispatcher. Alternatively, the management information may be attached to the response message by the application. Additional management information that pertains to the destination machine and/or to applications and services that run on the destination machine may also be attached to the response message. At block  440 , the response message is transmitted to a source application that generated the original message. The method then ends. 
       FIG. 5  illustrates a diagrammatic representation of a machine in the exemplary form of a computer system  500  within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In alternative embodiments, the machine may be connected (e.g., networked) to other machines in a Local Area Network (LAN), an intranet, an extranet, or the Internet. The machine may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital Assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines (e.g., computers) that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. 
     The exemplary computer system  500  includes a processor  502 , a main memory  504  (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) such as synchronous DRAM (SDRAM) or Rambus DRAM (RDRAM), etc.), a static memory  506  (e.g., flash memory, static random access memory (SRAM), etc.), and a secondary memory  518  (e.g., a data storage device), which communicate with each other via a bus  530 . 
     Processor  502  represents one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processor  502  may be a complex instruction set computing (CISC) microprocessor, reduced instruction set computing (RISC) microprocessor, very long instruction word (VLIW) microprocessor, processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processor  502  may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processor  502  is configured to execute the processing logic  526  for performing the operations and steps discussed herein. 
     The computer system  500  may further include a network interface device  508 . The computer system  500  also may include a video display unit  510  (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device  512  (e.g., a keyboard), a cursor control device  514  (e.g., a mouse), and a signal generation device  516  (e.g., a speaker). 
     The secondary memory  518  may include a machine-readable storage medium (or more specifically a computer-readable storage medium)  531  on which is stored one or more sets of instructions (e.g., software  522 ) embodying any one or more of the methodologies or functions described herein. The software  522  may also reside, completely or at least partially, within the main memory  504  and/or within the processing device  502  during execution thereof by the computer system  500 , the main memory  504  and the processing device  502  also constituting machine-readable storage media. The software  522  may further be transmitted or received over a network  520  via the network interface device  508 . 
     The machine-readable storage medium  531  may also be used to store a dispatcher and/or application manager (e.g., the first dispatcher  245  and first management application  240  of  FIG. 2 ), and/or a software library containing methods that call management applications and/or dispatchers. While the machine-readable storage medium  531  is shown in an exemplary embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instructions for execution by the machine and that cause the machine to perform any one or more of the methodologies of the present invention. The term “machine-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, and optical and magnetic media. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.