Abstract:
Provided are a method, system, and program for using a heartbeat signal to maintain data consistency for writes to source storage copied to target storage. A copy relationship associates a source storage and target storage pair, wherein writes received at the source storage are transferred to the target storage. A determination is made whether a signal has been received from a system within a receive signal interval. A freeze operation is initiated to cease receiving writes at the source storage from an application in response to determining that the signal has not been received within the receive signal interval. A thaw operation is initiated to continue receiving write operations at the source storage from applications after a lapse of a freeze timeout in response to the freeze operation, wherein after the thaw operation, received writes completed at the source storage are not transferred to the target storage.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method, system, and program for using a heartbeat signal to maintain data consistency for writes to source storage copied to target storage. 
   2. Description of the Related Art 
   Disaster recovery systems typically address two types of failures, a sudden catastrophic failure at a single point in time or data loss over a period of time. In the second type of gradual disaster, updates to volumes may be lost. To assist in recovery of data updates, a copy of data may be provided at a remote location. Such dual or shadow copies are typically made as the application system is writing new data to a primary storage device. Different copy technologies may be used for maintaining remote copies of data at a secondary site, such as International Business Machine Corporation&#39;s (“IBM”) Extended Remote Copy (XRC), Coupled XRC (CXRC), Global Copy, and Global Mirror Copy. These different copy technologies are described in the IBM publications “The IBM TotalStorage DS6000 Series: Copy Services in Open Environments”, IBM document no. SG24-6783-00 (September 2005) and “IBM TotalStorage Enterprise Storage Server: Implementing ESS Copy Services with IBM eServer zSeries”, IBM document no. SG24-5680-04 (July 2004). 
   In data mirroring systems, data is maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Primary and secondary control units, also known as storage controllers or enterprise storage servers, may be used to control access to the primary and secondary storage devices. In certain backup system, a sysplex timer is used to provide a uniform time across systems so that updates written by different applications to different primary storage devices use consistent time-of-day (TOD) value as a time stamp. Application systems time stamp data sets when writing such data sets to volumes in the primary storage. The integrity of data updates is related to ensuring that updates are done at the secondary volumes in the volume pair in the same order as they were done on the primary volume. The time stamp provided by the application program determines the logical sequence of data updates. 
   In peer-to-peer remote copy operations (PPRC), multiple primary control units may have source/target pairs, i.e., volume pairs, included in consistency groups so that data copied to target volumes by the different primary control units maintains data consistency. A host system includes a program, referred to as a consistency manager, to maintain data consistency across the different primary control units having source/target pairs in a consistency group. In the current art, if a primary control unit detects an error, such as a failure with the connection to secondary control unit managing access to the target storage in the source/target pair, then the primary control unit may initiate a freeze operation to block any further writes to the source volumes. In response to the freeze operation, application programs blocked from writing data would not write any more data to any primary control unit. After initiating the freeze operation, the primary control unit would send an interrupt to the consistency manager identifying the freeze and set a freeze timeout timer. At the expiration of the freeze timeout timer, the primary control unit would initiate a thaw operation to start accepting writes from the application to the source storage in the source/target pair, but not copy the writes to the target storage. 
   In the current art, if the primary control unit cannot communicate the interrupt to the consistency manager to allow the consistency manager to send freeze commands to all primary control units, then applications writing to primary control units other than the primary control unit where the freeze occurred may have their data writes transferred to the target storage even though data at the primary control unit where the freeze occurred would not copy writes to the target storage. This may result in data inconsistency at the target storage. 
   For these reasons, there is a need in the art to provide techniques for maintaining data consistency. 
   SUMMARY 
   Provided are a method, system, and program for using a heartbeat signal to maintain data consistency for writes to source storage copied to target storage. A copy relationship associates a source storage and target storage pair, wherein writes received at the source storage are transferred to the target storage. A determination is made whether a signal has been received from a system within a receive signal interval. A freeze operation is initiated to cease receiving writes at the source storage from an application in response to determining that the signal has not been received within the receive signal interval. A thaw operation is initiated to continue receiving write operations at the source storage from applications after a lapse of a freeze timeout in response to the freeze operation, wherein after the thaw operation, received writes completed at the source storage are not transferred to the target storage. 
   In an additional embodiment, there is information on multiple source storage and target storage pairs maintained by control units, wherein the control unit maintaining the pair copies writes to the source storage to the target storage. A determination is made of freeze timeouts used by control units maintaining the source and target pairs. In response to a freeze operation with respect to one source and target pair managed by one control unit, the control unit blocks writes to the source storage. The control unit initiates a thaw operation to continue receiving write operations at the source storage after a lapse of the freeze timeout for the source and target pair in response to the freeze operation. After the thaw operation, received writes completed at the source storage are not transferred to the target storage. A determination is made of a send signal interval based on the determined freeze timeouts. A signal is communicated at the send signal interval to the control units maintaining the source and target pairs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an embodiment of a network computing environment. 
       FIG. 2  illustrates an embodiment of information maintained for a copy relationship. 
       FIG. 3  illustrates an embodiment of copy relationship and other information maintained at the primary control units. 
       FIG. 4  illustrates an embodiment of session information. 
       FIG. 5  illustrates an embodiment of consistency group information. 
       FIG. 6  illustrates an embodiment of primary control unit information. 
       FIG. 7  illustrates an embodiment of operations to register source/target pairs maintained by primary control units to include in a consistency group. 
       FIG. 8  illustrates an embodiment of operations performed by a consistency manager to send heartbeat signals to primary control units. 
       FIG. 9  illustrates an embodiment of operations for a primary control unit to monitor for heartbeat signals from a consistency manager to maintain data consistency. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an embodiment of a network computing environment. A network  2  includes a plurality of primary control units  4   a  . . .  4   n ; primary storages  6   a  . . .  6   n ; secondary storages  12   a  . . .  12   n ; a host  14  writing updates to the primary storages  6   a  . . .  6   n ; and a consistency manager  16  maintaining data consistency among source and target storage pairs managed by the primary  4   a  . . .  4   n  and secondary  10   a  . . .  10   n  control units. The components  4   a  . . .  4   n ,  6   a  . . .  6   n ,  12   a  . . .  12   n ,  14 ,  16 , and  18 , are connected to the network  2  and the network  2  enables communication among these components. The network  2  may include one or more switches to provide one or more paths of communication between the different network  2  elements. 
   The primary  4   a  . . .  4   n  and secondary  10   a  . . .  10   n  control units include copy manager software  20   a  . . .  20   n  and  22   a  . . .  22   n , respectively, that manages the copying of writes to locations in the primary storages  6   a  . . .  6   n  in a source/target copy pair to target storage  10   a  . . .  10   n  indicated in the source/target copy pair information. The primary copy manager  20   a  . . .  20   n  may read updates from the primary storages  6   a  . . .  6   n  and send the writes to the primary control unit  4   a  . . .  4   n  that manages the copying of the writes in the order in which they were written to the primary storages  6   a  . . .  6   n  to the corresponding secondary storage  12   a  . . .  12   n  (target). The dependent order of the writes may be maintained by writing the data synchronously, so that the data will be on the target and source storage before the application  24  is allowed to proceed with a next write. Therefore, the data will be consistent on the targets as a result of the application  24  using ordered dependent writes for data that needs to be consistent with itself. Thus, when data is recovered from the target storage, i.e., secondary storage  12   a  . . .  12   n , the recovered data will be consistent. 
   The copy managers  20   a  . . .  20   n ,  22   a  . . .  22   n  may copy data by sending the writes to the primary control units  4   a  . . .  4   n , which then manage and initiate the synchronously copying from the source to the storage using a technique such as peer-to-peer remote copy (PPRC). Complete may be returned to the application  24  providing the writes upon completing the write at the primary control unit  4   a  . . .  4   n  or the secondary control unit  10   a  . . .  10   n . Alternatively, the primary control units  4   a  . . .  4   n  may copy data asynchronously using remote copy technology. 
   The consistency manager  16  maintains consistency across storage/target pairs managed by primary control units  4   a  . . .  4   n . Each primary control unit  4   a  . . .  4   n  includes information on one or more copy relationship, each copy relationship specifying source locations in the primary storage  6   a  . . .  6   n , e.g., LSSs, volumes, etc., copied to corresponding target locations in the secondary storage  12   a  . . .  12   n.    
   The network  2  may comprise a Storage Area Network (SAN), Local Area Network (LAN), Intranet, the Internet, Wide Area Network (WAN), peer-to-peer network, arbitrated loop network, etc. The storages  6   a  . . .  6   n ,  12   a  . . .  12   n  may comprise an array of storage devices, such as a Just a Bunch of Disks (JBOD), Direct Access Storage Device (DASD), Redundant Array of Independent Disks (RAID) array, virtualization device, tape storage, flash memory, etc. 
   The consistency manager  16  may be implemented within one of the primary or secondary control units or in a separate system, such as shown in  FIG. 1 . 
     FIG. 2  illustrates an embodiment of copy relationship information maintained by the copy managers  20   a  . . .  20   n  and, in certain embodiments,  22   a  . . .  22   n . Each copy relationship  50  instance includes: a copy relationship identifier (ID)  52 ; the source storage  54  locations, e.g., LSS, in the primary storages  6   a  . . .  6   n  involved in the copy relationship; the corresponding target storage  56  locations in the secondary storages  12   a  . . .  12   n  to which writes to the source storage  54  locations are copied; and a freeze timeout  58  for the copy relationship  52 , e.g., LSS pair. For instance, if a freeze operation is performed at the primary control unit  4   a  . . .  4   n  due to some error, then after the freeze timeout time  58  has elapsed for the particular copy relationship  52 , the primary control unit  4   a  . . .  4   n  automatically initiates a thaw operation to start accepting writes to the source storage  54  locations from the application  24  without copying the writes to the corresponding target storage  56  location. In one embodiment, the copy manager  20   a  . . .  20   n  may issue the thaw to the primary control unit  4   a  . . .  4   n  before the timeout time if the copy manager  20   a  . . .  20   n  determines that all the source LSS pairs have been frozen to ensure data consistency. In this way, the copy manager  20   a  . . .  20   n  may maintain different freeze timeouts for different source storage locations  54  involved in copy relationships to allow writes to resume at different times for different source storage locations  54 , depending on the freeze timeout times  58  defined in the copy relationship information  50  for that storage location  54 . 
     FIG. 3  illustrates further information maintained in a primary control unit  4   a  . . .  4   bn  for use by the copy manager  20   a  . . .  20   n , including one or more copy relationships  50 , a minimum freeze timeout time  60  indicating a consistency group minimum freeze timeout time used across all the copy relationships in the primary control units  4   a ,  4   b  . . .  4   n  that are managed by the consistency manager  16  in a single consistency group. The consistency manager  16  may provide the copy manager  20   a  . . .  20   n  in the primary control units  4   a  . . .  4   n  with this value. The receive heartbeat interval  62  is an interval in which the copy manager  20   a  . . .  20   n  expects to receive a heartbeat signal from the consistency manager  16 . 
   If the copy manager  20   a  . . .  20   n  does not receive the heartbeat signal within the receive heartbeat interval  62 , then the copy manager  20   a  . . .  20   n  will initiate a freeze operation to quiesce further writes. The freeze operation may be issued to those source-target locations, e.g., LSS pairs, registered in the sessions managed by the copy manager  20   a  . . .  20   n . In one embodiment, the copy manager  20   a  . . .  20   n  calculates the receive heartbeat interval  62  as a function of the consistency group minimum freeze timeout time  60 , such that the receive heartbeat interval  62  is less than the consistency group minimum freeze timeout time  60 . Using the consistency group minimum freeze timeout time to determine the receive heartbeat interval ensures that any one primary control unit  4   a  . . .  4   n  would perform a freeze operation before another primary control unit  4   a  . . .  4   n  would thaw as a result of the expiration of that primary control unit&#39;s  4   a  . . .  4   n  freeze timeout times. For instance, if a primary control unit  4   a  . . .  4   n  loses connection with the consistency manager  16 , then there is a concern that another primary control unit  4   a  . . .  4   n  may initiate a freeze operation as a result of some failure to copy writes to the target storage. If one primary control unit lost its connection with the consistency manager  16 , then it may continue to copy writes to the target storage after the primary control unit that performed the freeze operation thaws. If this occurs, then target storage may include inconsistent data because one primary control unit is writing dependent data to the target side, while other primary control units that performed the freeze operation do not copy dependent data, resulting in data inconsistency at the target side. With the described embodiments, if the consistency manager  16  is assumed to send the heartbeat signal more frequently than the receive heartbeat interval  62  and the receive heartbeat interval  62  is less than the consistency group minimum freeze timeout time  60  across all primary control units  4   a  . . .  4   n , than all primary control units will freeze before any one of them thaws and permits the application  14  writes to continue. This ensures that all primary control units  4   a  . . .  4   n  will not send any further data to the target after any other primary control unit thaws because all primary control units involved in the consistency group will have initiated a freeze operation before any of them would thaw and permit writes after a freeze. 
   In one embodiment, the receive heartbeat interval  62  may be calculated by subtracting from the minimum freeze timeout time  60  the time it would take the copy manager  20   a  . . .  20   n  to issue a freeze operation to all copy relationships  50  maintained at the primary control unit  4   a  . . .  4   n , also known as a command runtime. This takes into account the command runtime for the freeze to be implemented at all copy relationships  50 , i.e., all LSSs, so that a primary control unit will issue a freeze operation in enough time to allow the freeze to be implemented at all of its copy relationships  50  before any other primary control unit can thaw and allow the application  14  to continue writes to all primary control units  4   a  . . .  4   n.    
   In one embodiment, the consistency manager  16  may maintain a consistency group comprised of one or more sessions. A session includes source/target pairs on one or more primary control units  4   a  . . .  4   n  and multiple sessions may include source/target pairs on the same or different primary control units  4   a  . . .  4   n .  FIG. 4  illustrates an embodiment of session information  70  having: a session identifier (ID)  72  and then one or more source/target pair instances for each source/target pair included in the session. For each source/target pair included in the session  72 , the session information  70  includes the primary control unit  74   a  . . .  74   n  and the source/target pair  76   a  . . .  76   n  in the primary control unit  74   a  . . .  74   n  included in the session  72 . The source/target pair  76   a  . . .  76   n  information may identify an LSS pair or other storage unit pairs in the primary  6   a  . . .  6   n  and secondary  12   a  . . .  12   n  storages. 
     FIG. 5  illustrates an embodiment of consistency group information  80  the consistency manager  16  maintains for each consistency group being managed. The consistency group information  80  includes a consistency group identifier (ID)  82 ; the one or more sessions  84  included in the consistency group  82 , where each session includes one or more source/target pairs in one or more of the connected primary control units  4   a  . . .  4   n ; a consistency group minimum freeze timeout time  86  indicating the minimum freeze timeout time across all primary control units  4   a ,  4   b  . . .  4   n  including source/target pairs in the consistency group  82 ; and a send heartbeat interval  88  calculated from the consistency group minimum freeze timeout time  86  at which the consistency manager  16  sends heartbeat signals to the primary control units  4   a  . . .  4   n  managing source/target pairs in the consistency group  82 . 
     FIG. 6  illustrates an embodiment of primary control unit information  90  the consistency manager  16  maintains for each primary control unit including source/target pairs in a one consistency group  80 . The primary control unit information  90  indicates the control unit  92  and the minimum freeze timeout time  94  of the source/target pairs at that control unit  92 . 
     FIG. 7  illustrates an embodiment of operations performed by the consistency manager  16  and the copy manager  20   a  . . .  20   n  in the primary control units  4   a  . . .  4   n  to exchange information to maintain data consistency with respect to the freeze operation. The consistency manager  16  performs the operations at blocks  100 - 110  and the copy manager  20   a  . . .  20   n  performs the operations at blocks  150 - 156 . Upon the consistency manager  16  initiating (at block  100 ) operations to register source/target pairs from the primary control units  14   a  . . .  14   n  in a consistency group  82  ( FIG. 5 ), the consistency manager  16  sends (at block  102 ) a registration to each connected primary control unit  4   a  . . .  4   n . Upon receiving this registration request, the copy managers  20   a  . . .  20   n  at the primary control units  4   a  . . .  4   n  determine (at block  150 ) a minimum freeze timeout across all source/target pairs (e.g., LSS pairs) to be added to the consistency group being registered. The minimum freeze timeout may be determined across all source/target pairs registered in the sessions managed by the copy managers  20   a  . . .  20   n . The copy manager  20   a  . . .  20   n  sends (at block  152 ) the consistency manager  16  the minimum freeze timeout time at the primary control unit  4   a  . . .  4   n  and the source/target pairs to register in the consistency group. The consistency manager  16  saves (at block  104 ) the received information for source/target pairs (e.g., LSS pairs) for the primary control unit  4   a  . . .  4   n  and the minimum freeze timeout time  94  ( FIG. 6 ) for the primary control unit  92  with the primary control unit information  90 . 
   Upon receiving registrations from all the primary control units  4   a  . . .  4   n , the consistency manager  16  determines and saves (at block  106 ) the consistency group minimum freeze timeout time  86  ( FIG. 5 ) as the determined minimum of the received control unit minimum freeze timeout times  94  ( FIG. 6 ). The consistency manager  16  determines (at block  108 ) the send signal interval as a function of the consistency group minimum freeze timeout time  86 . In one embodiment, the send signal interval comprises a fraction of the receive heartbeat interval  62  ( FIG. 3 ) used by the copy managers  20   a  . . .  20   n . In this way, the consistency manager  16  sends the heartbeats to the primary control units  4   a  . . .  4   n  at a higher frequency then the receive heartbeat interval to ensure that the primary control units  4   a  . . .  4   bn  initiate freeze operations before other primary control units  4   a  . . .  4   n  thaw and begin allowing application  24  writes. The consistency manager  16  sends (at block  110 ) the determined consistency group minimum freeze timeout time  86  to each primary control unit  4   a  . . .  4   n  including source/target pairs in the consistency group  82  ( FIG. 5 ) at issue. 
   Upon the copy manager  20   a  . . .  20   n  at the primary control unit  4   a  . . .  4   n  receiving (at block  154 ) the consistency group minimum freeze timeout  94 , the copy manager  20   a  . . .  20   n  calculates (at block  156 ) the receive signal interval as a function of the consistency group minimum freeze timeout time. As discussed, the calculated receive heartbeat interval  62  may comprise the consistency group minimum freeze timeout time  86  less then the freeze command runtime. In an alternative embodiment, the consistency manager  16  may calculate the receive heartbeat interval  62  and then transmit that calculated value to the copy managers  20   a  . . .  20   n  to use. 
     FIG. 8  illustrates an embodiment of consistency related operations performed by the consistency manager  16 . The consistency manager  16  initiates consistency operations (at block  200 ) and communicates (at block  202 ) a heartbeat signal at the send heartbeat interval to the primary control units  4   a  . . .  4   n  maintaining the source and target pairs in the consistency group  82  ( FIG. 5 ) being managed. The consistency manager  16  may send the heartbeat signals at the send heartbeat interval  88  ( FIG. 5 ) rate to all primary control units  74   a  . . .  74   n  ( FIG. 4 ) in all sessions  84  ( FIG. 5 ) identified in the consistency group information  80  for the consistency group  82  being managed. The consistency manager  16  may perform such operations for multiple consistency groups. 
     FIG. 9  illustrates an embodiment of operations performed by the copy managers  20   a  . . .  20   n  to perform heartbeat signal management related operations. Upon initiating (at block  220 ) heartbeat signal monitoring from the consistency manager  16 , the copy manager  20   a  . . .  20   n  sets (at block  222 ) a timer for the receive heartbeat interval  62  ( FIG. 3 ). If (at block  224 ) a signal (heartbeat) is received from the consistency manager  16  before the timer expires, then control proceeds back to block  222  to reset the timer and wait for the next heartbeat. Otherwise, if a heartbeat signal is not received from the copy manager  20   a  . . .  20   n  within the timer period (receive heartbeat interval  62 ), then the copy manager  20   a  . . .  20   n  initiates (at block  226 ) a freeze operation to block further writes from applications  24  ( FIG. 1 ) for all source/target pairs managed by the primary control unit  4   a  . . .  4   n . The freeze operation may be sent to source/target pairs in the sessions registered with the copy manager  20   a  . . .  20   n . In response to being blocked, the applications  24  would stop sending writes to any primary control unit  4   a  . . .  4   n  until the application  24  is notified that writes are allowed as part of the thaw operation. The copy manager  20   a  . . .  20   n  further sends (at block  228 ) an interrupt to the consistency manager  16  indicating a freeze. If the connection is available and the consistency manager  16  receives this interrupt, then the consistency manager  16  sends freeze commands to all the primary control units  4   a  . . .  4   n  in the consistency group including the primary control unit from which the interrupt was received. After commencing the freeze operation, the copy manager  20   a  . . .  20   n  starts (at block  230 ) the freeze timeout timer for each source/target pair, where after a freeze timeout timer expires, the source (primary control unit) may initiate the thaw procedure and accept writes for that source storage, e.g., LSS. After the freeze thaws, the copy manager  20   a  . . .  20   n  would not copy writes over to the target storage (secondary storage  12   a  . . .  12   n ), so that data consistency is maintained at the secondary (target) storages  12   a  . . .  12   n.    
   In a further embodiment, if a source/target pair is added or removed to a consistency group  82  ( FIG. 5 ), then the consistency manager  16  may perform the operations of  FIG. 7  to recalculate the consistency group minimum freeze timeout time  86  to allow adjustment of the send  88  ( FIG. 5 ) and receive  62  ( FIG. 3 ) heartbeat intervals. 
   Described embodiments provide a technique to ensure that all primary control units having source/target pairs in a consistency group will all initiate freeze operations if one primary control unit initiates a freeze operation before any primary control unit thaws, or begins accepting writes after a freeze. With described embodiments, a primary control unit maintaining communication with a consistency manager initiates a freeze operation if the consistency manager sends a freeze command in response to being notified of a freeze command by another control unit. Alternatively, if a primary control unit loses its connection with the consistency manager, then that primary control unit would automatically begin a freeze operation if it did not receive a heartbeat signal from the consistency manager before any other primary control unit could thaw after its freeze timeout time. 
   Additional Embodiment Details 
   The described operations may be implemented as a method, apparatus or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. The described operations may be implemented as code maintained in a “computer readable medium”, where a processor may read and execute the code from the computer readable medium. A computer readable medium may comprise media such as magnetic storage medium (e.g., hard disk drives, floppy disks, tape, etc.), optical storage (CD-ROMs, DVDs, optical disks, etc.), volatile and non-volatile memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs, DRAMs, SRAMs, Flash Memory, firmware, programmable logic, etc.), etc. The code implementing the described operations may further be implemented in hardware logic (e.g., an integrated circuit chip, Programmable Gate Array (PGA), Application Specific Integrated Circuit (ASIC), etc.). Still further, the code implementing the described operations may be implemented in “transmission signals”, where transmission signals may propagate through space or through a transmission media, such as an optical fiber, copper wire, etc. The transmission signals in which the code or logic is encoded may further comprise a wireless signal, satellite transmission, radio waves, infrared signals, Bluetooth, etc. The transmission signals in which the code or logic is encoded is capable of being transmitted by a transmitting station and received by a receiving station, where the code or logic encoded in the transmission signal may be decoded and stored in hardware or a computer readable medium at the receiving and transmitting stations or devices. An “article of manufacture” comprises computer readable medium, hardware logic, and/or transmission signals in which code may be implemented. A device in which the code implementing the described embodiments of operations is encoded may comprise a computer readable medium or hardware logic. Of course, those skilled in the art will recognize that many modifications may be made to this configuration without departing from the scope of the present invention, and that the article of manufacture may comprise suitable information bearing medium known in the art. 
   The terms “an embodiment”, “embodiment”, “embodiments”, “the embodiment”, “the embodiments”, “one or more embodiments”, “some embodiments”, and “one embodiment” mean “one or more (but not all) embodiments of the present invention(s)” unless expressly specified otherwise. 
   The terms “including”, “comprising”, “having” and variations thereof mean “including but not limited to”, unless expressly specified otherwise. 
   The enumerated listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise. 
   The terms “a”, “an” and “the” mean “one or more”, unless expressly specified otherwise. 
   Devices that are in communication with each other need not be in continuous communication with each other, unless expressly specified otherwise. In addition, devices that are in communication with each other may communicate directly or indirectly through one or more intermediaries. 
   A description of an embodiment with several components in communication with each other does not imply that all such components are required. On the contrary a variety of optional components are described to illustrate the wide variety of possible embodiments of the present invention. 
   Further, although process steps, method steps, algorithms or the like may be described in a sequential order, such processes, methods and algorithms may be configured to work in alternate orders. In other words, any sequence or order of steps that may be described does not necessarily indicate a requirement that the steps be performed in that order. The steps of processes described herein may be performed in any order practical. Further, some steps may be performed simultaneously. 
   When a single device or article is described herein, it will be readily apparent that more than one device/article (whether or not they cooperate) may be used in place of a single device/article. Similarly, where more than one device or article is described herein (whether or not they cooperate), it will be readily apparent that a single device/article may be used in place of the more than one device or article or a different number of devices/articles may be used instead of the shown number of devices or programs. The functionality and/or the features of a device may be alternatively embodied by one or more other devices which are not explicitly described as having such functionality/features. Thus, other embodiments of the present invention need not include the device itself. 
   The illustrated operations of  FIGS. 7 ,  8 , and  9  show certain events occurring in a certain order. In alternative embodiments, certain operations may be performed in a different order, modified or removed. Moreover, steps may be added to the above described logic and still conform to the described embodiments. Further, operations described herein may occur sequentially or certain operations may be processed in parallel. Yet further, operations may be performed by a single processing unit or by distributed processing units. 
   The foregoing description of various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.