Abstract:
Techniques are described for replicating data from one to one or more heterogeneous data processing or communication devices for the purpose of remote backup, redundancy, content distribution, communications, observations or measurements. In a first phase, the attributes of the data that are modified or created on a device or that are passing through the device are tracked and journaled in volatile or non-volatile storage in real-time. In a second phase, the attributes that match patterns pre-specified in a configuration are used to determine which data to replicate and which modifications to make the devices. In a third phase, the data is replicated. In a preferred embodiment, the described techniques comprise an application that runs on a host device or is embedded in a logic or memory device. The described invention is designed to be transparent for system redundancy and error recovery processes such as error correction, re-transmission on links, and raid configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 10/980,875, now U.S. Pat. No. 7,836,014, filed on Nov. 3, 2004. U.S. patent application Ser. No. 10/980,875 claims priority from U.S. Provisional Patent Application No. 60/517,253, filed on Nov. 4, 2003. U.S. patent application Ser. No. 10/980,875 and U.S. Provisional Patent Application No. 60/517,253 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to replication of data and, more particularly, replication of data from one to one or more heterogeneous data processing and/or communication devices. 
     BACKGROUND 
     Data replication is used to protect data from loss, to ensure business continuity and to distribute data to all points of use while keeping the total cost of ownership down. Data replication requires making copies of data from a source device to one or more target devices. Target devices can reside on the same host or can be remotely located on multiple hosts. Data replication is performed for several reasons including device synchronization, disaster recovery planning and business continuance, content distribution, backup consolidation and server migration. 
     Safeguarding corporate data is of primary importance. Disaster can result from electrical outages, natural disasters such as floods, tornadoes, human caused disasters such as fires, and other such events that can cause physical loss of information technology (IT) infrastructure and the data it houses. Several steps have conventionally been taken to protect corporate data assets from such events. These often include utilization of offsite backups combined with mirroring technologies, fault tolerant hardware, and data replication. 
     Delivering data when needed to the points where it is used can be costly and challenging. The points of use may be multiple web servers, computational cluster nodes, spatially distributed points of ingestion by database engines, collaborative servers, data brokers, data resellers, distance learning end points, communication devices, display devices, archival or backup service points. Another user for data replication is to distribute content to use locations where it is needed. 
     SUMMARY 
     In general, the invention is directed to techniques that allow real-time data replication from one to one or more heterogeneous data processing devices. In particular, hybrid real-time data replication techniques are described that capture all data changes synchronously while performing replication asynchronously. The described hybrid real-time data replication techniques combine replication of modified and pass-through data. 
     Unlike conventional data replication techniques, which perform either synchronous or asynchronous data replication, the described hybrid real-time data replication techniques allow data integrity to be preserved while eliminating the limits due to latency and network fault sensitivity imposed by synchronous data replication over long haul networks. The described techniques extend to computer devices as well as intelligent devices, such as embedded storage devices, flash memories, cell phones, displays, cameras, medical imaging apparatuses or other such intelligent devices. Additionally, the described techniques are not limited to the source and destination devices being of the same type, architecture or configuration. 
     The described techniques can be used for both business continuance and content distribution. For example, the described techniques can be used to replicate data between two servers in a 1:1 uni-direction or bi-directional configuration or from one host to one or more hosts simultaneously in a 1:N configuration. Additionally, the described techniques provide a solution for business continuance, content distribution, and backup consolidation. In particular embodiments, the described techniques that replicates data to various versions of UNIX including Solaris, HP-UX, IBM AIX, and LINUX. 
     In one embodiment, the invention is directed to a data replication method comprising accepting a request from a client device to modify data, adding data attributes of the modified data to a message queue, saving the data attributes of modifications on a storage device, performing modifications and saving a status of the data modification operation, and communicating the status of the operation to the client device if the client device requests that the status be communicated. 
     In another embodiment, the invention is directed to a machine-readable medium containing instructions. The instructions cause a programmable processor to accept a request from a client device to modify data, add data attributes of the modified data to a message queue, save the data attributes of modifications on a storage device, perform modifications and saving a status of the data modification operation, and communicate the status of the operation to the client device if the client device requests that the status be communicated. 
     In yet another embodiment, the invention is directed to a system for replication of data across a distributed computing system, the system comprising a pass-through component and a data replication engine. The pass-through component intercepts data modification requests and the data replication engine receives the data modification requests from the pass-through component and replicates the modifications on one or more remote storage devices by accepting a request from a client device to modify data through the pass-through component, adding data attributes of modified data to a message queue, saving the data attributes on one or more of the storage devices, performing modifications and saves a status of the data alteration operation, and communicating the status of the operation to the device that requested the change if the device requests that the status be communicated. 
     The invention may be capable of providing one or more advantages. For example, the invention provides techniques for real-time data replication from one to one or more heterogeneous data processing devices. Unlike conventional data replication techniques that require that the source and destination devices be of the same type and architecture or at least have the same configuration, the described techniques allow data replication for devices such as computers, storage devices, communication devices, sensor devices, observation and measurement devices that are capable of sending and receiving data to and from other similar or dissimilar devices. Moreover, the described techniques capture all data changes synchronously while performing replication asynchronously. Furthermore, the described techniques combine replication of changed and pass-through data. 
     Additionally, the described techniques advantageously provide data replication for safeguarding customer data for business continuance and disaster recovery by consolidating backups and building backup appliances. The described techniques may also automate content distribution. Consequently, the described techniques may reduce the total cost of ownership of an organization&#39;s data while offering maximum protection and high availability without substantially impacting performance. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating hybrid real-time data replication in a single source device and a single destination device configuration according to an embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating hybrid real-time data replication in a single source device and multiple destination devices configuration according to an embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating hybrid real-time data replication in a multiple source device and a single destination device configuration according to an embodiment of the present invention. 
         FIG. 4  is a block diagram illustrating hybrid real-time data replication in a cascaded or fan-out single source and multiple destination device configuration according to an embodiment of the present invention. 
         FIG. 5  is a block diagram illustrating an example embodiment of hybrid real-time data replication system according to the present invention. 
         FIG. 6  is a flowchart illustrating an example process of the pass-through component in  FIG. 5 . 
         FIG. 7  is a diagram illustrating an example embodiment of the data replication engine in  FIG. 5 . 
         FIG. 8  is a flowchart illustrating an example process of the input thread in  FIG. 7 . 
         FIG. 9  is a flowchart illustrating an example process of the journal thread in  FIG. 7 . 
         FIG. 10  is a flowchart illustrating an example process of a remote thread in  FIG. 7 . 
         FIG. 11  is a flowchart illustrating an example process of a transport thread in  FIG. 7 . 
         FIG. 12  is a flowchart illustrating an example process of a complete thread in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an example operating environment  2  in which an example configuration of the present invention is implemented. In the illustrated embodiment, a source device  30  is connected to client devices  10 A- 10 N, hereafter collectively referred to as client devices  10 , via a network  20 . Source device  30  replicates data  40  modified, created by, or passing through source device  30  to a destination device  50 . 
     Destination device  50  is optionally connected to a set of client devices  11 A- 11 N, hereafter collectively referred to as client devices  11 , via network  21 . In general, one or more client devices  10  modifies or creates the content on source device  30  or, alternatively, sends data  40  to another one or more of client devices  10  by passing data  40  through source device  30 . The data modifications that occur on source device  30  or pass through source device  30  are replicated to destination device  50 . Consequently, data replication from one to one or more heterogeneous data processing devices is achieved by capturing all data changes synchronously while performing replication asynchronously. As such, the data replication techniques described herein enable data synchronization and/or distribution of data content from one to one or more similar or dissimilar devices. Alternatively, the data replication techniques described here enable data synchronization and/or distribution of data within the same device. 
     Client devices  10  and  11  may be any one or combination of data processing devices including storage devices, flash memories, cell phones, cameras, medical imaging apparatuses, and other such communication, observation and measurement devices capable of sending and receiving data to and from other data processing devices. Each of networks  20  and  21  may be any type of network including satellite, wireless, packet radio, leased lines, Ethernet, ATM, DSL, broadband, and any other network capable of transmitting data between client devices  10  and  11 . 
     The hybrid real-time data replication techniques are configured to run as an application on source device  30  or, alternatively, destination device  50 . In a preferred embodiment, source device  30  and destination device  50  are host computer devices running various versions of UNIX or other operating systems including LINUX, Solaris, HP-UX, IBM, and AIX. However, source device  30  and destination device  50  are not limited to devices being the of the same type and architecture or have the same configuration. Additionally, the hybrid real-time data replication system may also be embedded in a logic device and memory device such as EEPROM or gate arrays in addition to other hardware, firmware, and software based implementations. Those skilled in the art will realize that that example environment  2  is merely illustrative of one exemplary configuration of the use of the invention, and that alternative configurations may be used without departing from the scope of the present invention. 
     For example, in the illustrated 1:1 configuration, the described hybrid real-time data replication techniques can be used to replicate data between two servers, i.e., source device  30  and destination device  50 . Data modified by one or more clients  10  using NFS, direct connection, SAMBA, CIFS, and the like is replicated from one server to another independently of the underlying file system or operating system. Although not shown, the two servers may be connected using a local area network (LAN) or a long-haul network such as the Internet. If one of the two servers fails or is lost in a disaster event, data is safe on the replica server and recovery can be immediate. 
     In another example, one or more distributed heterogeneous production servers or devices residing on a computer network or network of devices may use the described techniques to replicate data to one or more remote devices or storage backup appliances. As the data on the distributed heterogeneous devices or servers is modified, the devices or servers rely on the described techniques to replicate some or all of the changing data to one or more remote devices, storage backups appliances or remote servers to create an online mirror of data for disaster recovery for high availability purposes or to synchronize device content. The data on the storage backup appliances or remote servers can then be archived to other permanent or temporary storage without impacting the data on the production servers. Embodiments of the invention executing on the remote devices or servers can be temporarily paused to produce a point-in-time snapshot copy of the data on the devices or storage backup appliance. 
     In another example, it may be desirable to track the data that is changing within a device or that is simply passing through the device and apply the same data changes to one or more other devices in a given configuration: For example, a user may want to keep many computers or devices synchronized in such a way that at the end of each given time period, the content of the computers or devices is the same. The user may use the invention to synchronize storage between many remote devices. 
     In yet another example, the described techniques may run as an application on an intelligent storage device within a computer. The device may integrate it&#39;s own operating system with the described invention or rely on the operating system and the described invention running on the host computer. This device synchronizes itself with other intelligent devices by distributing entire data objects or partial data objects among each other. 
     In another example, a camera or sensor is attached to a communication device. As the camera or sensor device captures the data, or the data passes through the device, some or all of the data is copied to one or many remote devices using the described techniques. Configuration filters are used to decide what data to distribute. 
     The hybrid real-time data replication techniques described herein allow data integrity to be preserved while eliminating the limits due to latency and network fault sensitivity imposed by typical synchronous data replication over long haul networks. Additionally, the hybrid real-time data replication techniques may provide particular advantage when employed as a solution for safeguarding data for business continuance and disaster recovery by consolidating backups and building backup appliances. Moreover, the techniques described herein may also advantageously automate content distribution. Consequently, the techniques described herein may reduce the total cost of ownership of an organization&#39;s data while offering maximum protection and high availability without substantially impacting performance. 
       FIG. 2  is a block diagram illustrating another example operating environment  62  in which an example configuration of the present invention is implemented. In the illustrated embodiment, a source device  90  replicates data  100  modified, created, or passed through source device  90  to multiple destination devices  110 A- 110 N, hereafter referred to as multiple destination devices  110 . Source device  90  is connected to client devices  70 A- 70 N, hereafter referred to as client devices  70 , via network  80 . 
     In general, one or more client devices  70  modifies or creates the content on source device  90  or, alternatively, sends data  100  to another one or more of client devices  70  by passing data  100  through source device  90 . The data modifications that occur on source device  90  or pass through source device  90  are replicated to destination devices  110 . Specifically, data replication is achieved by capturing all data changes synchronously while performing replication asynchronously. As such, the data replication techniques described herein enable data synchronization and/or distribution of data content from one to one or more similar or dissimilar devices. Alternatively, the data replication techniques described here enable data synchronization and/or distribution of data within the same device. 
     In the illustrated 1:N configuration, the described hybrid real-time data replication techniques can be used to replicate data from one host to many hosts simultaneously. For example, the techniques described herein may be used by a health care provider to distribute data in real-time from a single host running LINUX to several heterogeneous architectures running LINUX, AIX and Solaris separated by large distances. 
       FIG. 3  is a block diagram illustrating an alternative example operating environment  112  in which an example configuration of the present invention is implemented. In the illustrated embodiment, multiple source devices  140 A- 140 N, hereafter referred to as multiple source devices  140 , replicate data  150  modified, created, or passed through one or more of multiple source devices  140  to destination device  160 . Multiple source devices  140  are connected to client devices  120 A- 120 N, hereafter referred to as client devices  120 , via network  130 . Destination device  160  is optionally connected to a set of client devices  170 A- 170 N, hereafter collectively referred to as client devices  170 , via network  131 . 
     In general, one or more client devices  120  modifies or creates the content on one or more of multiple source devices  140  or, alternatively, sends data  150  to another one or more of client devices  120  by passing data  150  through one or more multiple source devices  140 . The data modifications that occur on one or more of multiple source devices  140  or pass through one or more of multiple source devices  140  are replicated to destination device  160 . 
     Consequently, data replication is achieved by capturing all data changes synchronously while performing replication asynchronously. As such, the data replication techniques described herein enable data synchronization and/or distribution of data content from one to one or more similar or dissimilar devices. Alternatively, the data replication techniques described here enable data synchronization and/or distribution of data within the same device. 
     In the illustrated N:1 configuration, the described hybrid real-time data replication techniques can be used to consolidate backups and build backup appliances. For example, a financial company may construct single and multiple backup appliances that consolidate all backups while keeping the data online for immediate recovery in case of failure of the primary site. In other words, backups from multiple source devices  140  may be consolidated using single destination source  160 . 
     Consequently, the described techniques may reduce the cost associated with backups while allowing the company to schedule backups in any time window while users are using the primary systems. In other words, the described techniques allow automatic online backup that takes place in real-time. Additionally, tape backup can be performed any time of the day. As a result, substantial payroll and good-will dollars may be saved by allowing users to access data in a 24/7 operational environment and by reducing staff overtime previously required to perform backups during off-peak hours. 
     For expanded security, the same financial company could build a flexible schedule that alternates between a first and a second appliance to create a complete history of the data changes and to give preference to other data traffic on the network. Using the “pause” and “resume” features of the hybrid real-time data replication techniques described herein, the user can suspend replication during designated periods of time. Once replication is resumed, all changes made during the suspended period are replicated to the destination appliances. The hybrid real-time data replication techniques described herein may be configured to provide both temporal and spatial business continuity. 
       FIG. 4  is a block diagram illustrating data replication in a cascaded or fan-out single source  180  and multiple destination source device  200  configuration  172  according to an embodiment of the present invention. 
     Importantly, the described invention is not limited to the previously described configurations. For example, a mesh of 1:1, 1:N, and cascaded configurations may be stored in a single repository (e.g., file) that is centrally managed and distributed to all participants. Local IT managers may retain authoritative administration if they choose. 
     For example, a consortium of international universities and national laboratories could use the described hybrid real-time data replication techniques to distribute content to each other and protect shared global climate change and biosciences data by replicating among the participating sites. Data collected by scientists at one site would be immediately available to all other sites. 
     Any analysis or transformations performed on the data by one scientist would be immediately and transparently available across all sites worldwide. For example, data created or modified by scientists in Sydney, Australia may be sent to Seattle and Chicago as well as Madrid, Spain. In a second phase, data may be sent from intermediary hosts to the remaining hosts. Consequently, the described hybrid real-time data replication techniques may be used to streamline and simplify the management of the replication matrix, distribute content in real-time, automate software installations, and ensure business continuity. 
       FIG. 5  is a block diagram illustrating one embodiment of hybrid real-time data replication device  202  according to the present invention. Device  202  includes software components configured to execute as an application on a source device or, alternatively, a destination device. In a preferred embodiment, the source device and the destination device are host computer devices running various versions of UNIX or other operating systems including but not limited to LINUX, Solaris, HP-UX, IBM, and AIX. In the illustrated embodiment, input/output (I/O) interface  212  exists between device  202  and components of a host device or devices in which the invention is embedded. Device  202  comprises pass-through component  214 , one or more modification queues  220  (hereafter referred to as modification queues  220 ), and data replication engine  230 . Pass-through component  214 , modification queues  220 , and data replication engine  230  may be divided into sub-components or combined into a single component without departing from the scope of the invention as described herein. 
     Pass-through component  214  is inserted between I/O interface  212  of the host device to other client devices and the physical transmission or storage abstraction layers  216  of the host device. As data modification requests  210  pass through pass-through component  214 , the data attributes are saved in modification queue  220  for later retrieval by data replication engine  230 . Data modification requests  210  are also passed through to storage abstraction layer  216  in order to modify data locally. 
       FIG. 6  is a flowchart illustrating an example process of pass-through component  214  ( FIG. 5 ). First, the requested data modification operation is performed ( 240 ) and, if the operation is successful ( 242 ), the attributes describing the data modification are added ( 244 ) to modification queue  220 . A return status indicating the successful modification operation is also provided ( 246 ). If the modification operation is not successful, a return status indicating the modification failure is provided. 
       FIG. 7  is a diagram illustrating an example embodiment of data replication engine  230  ( FIG. 5 ). Data replication engine  230  has one or more configuration files or configuration commands (not shown). The configuration files or commands include information as to which devices will receive data, what data to replicate, when to suspend replication, when to resume replication, and other such replication policies. Data replication engine  230  has one or more replication pathways  280 A- 280 N, hereafter referred to collectively and individually as pathways  280 . Data replication engine  230  includes input thread  250 , journal thread  270 , complete threads  330 A- 330 N, remote threads  300 A- 300 N, and transport threads  320 A- 320 N that execute concurrently to perform data replication functions. Each pathway  280  includes a corresponding one of complete threads  330 A- 330 N, remote threads  300 A- 300 N, and transport threads  320 A- 320 N. Additionally, each pathway  280  includes a corresponding one of pathway journals  290 A. 
     Input thread  250  retrieves modification attributes from modification queue  220  ( FIG. 5 ) of pass-through component  214  ( FIG. 5 ) and stores the attributes on journal queue  260 . Concurrently, journal thread  270  retrieves attributes from journal queue  260  and inserts each attribute, or item, into each pathway journal  290 A- 290 N of pathways  280 . Journal thread  270  also increments the reference count of the item if a given item is already present in journal queue  260 . 
     Within each pathway  280 , remote threads  300 A- 300 N concurrently retrieve items from the corresponding pathway journals  290 A- 290 N. If a given item is not already present in work journals  310 A- 310 N, the item is stored in work journals  310 A- 310 N and passed to transport threads  320 A- 320 N. If the item is already present in work journals  310 A- 310 N, a reference count for that item is incremented. When transport threads  320 A- 3320 N have completed replicating the data represented by the attribute item, it passes the item to complete threads  330 A- 330 N. The item is deleted from work journals  310 A- 310 N by complete threads  330 A- 330 N and if the reference count in pathway journals  290 A- 290 N is zero, the item is also removed from pathway journals  290 A- 290 N. Those skilled in the art of software design will realize that using another number of threads, concurrent, serial, or parallel components may be used without departing from the scope of the invention as described herein. 
       FIG. 8  is a flowchart illustrating an example process of input thread  250  ( FIG. 7 ). First data attributes are retrieved ( 340 ) from modification queue  220  ( FIG. 5 ) used by pass-through component  214  ( FIG. 5 ). If an item was successfully retrieved ( 342 ), the item is stored in journal queue  260  ( 344 ). If the item was not successfully retrieved, input thread  250  waits ( 346 ) for more items to become available and attempts to retrieve another item ( 340 ). 
       FIG. 9  is a flowchart illustrating an example process of journal thread  270  ( FIG. 7 ). If data is not present in journal queue  260  ( 352 ), journal thread  270  waits for data in journal queue  260  ( 350 ). If data is present in journal queue  260 , the first data attribute, or item, is retrieved ( 354 ). For each configured pathway  280  ( FIG. 7 ), the retrieved data item is compared against configuration data for that pathway  280  ( 356 ). If the data attribute matches the configuration data for the particular pathway  280  ( FIG. 7 ), the data attribute is stored in the corresponding one of pathway journals  290 A- 290 N ( 358 ). If each of pathways  280  ( FIG. 7 ) has been processed ( 360 ), journal thread  270  searches for data in journal queue  260 . If each of pathways  280  ( FIG. 7 ) has not been processed ( 360 ), the process is performed on the next pathway  280  ( 362 ). 
       FIG. 10  is a flowchart illustrating an example process of remote threads  300 A- 300 N ( FIG. 7 ) within each pathway  280  ( FIG. 7 ). When data replication is not suspended and data is present in the corresponding one of pathway journals  290 A- 290 N ( FIG. 7 ), the data is retrieved and stored within the corresponding one of work journals  310 A- 310 N ( 370 ). If the corresponding one of transport threads  320 A- 320 N ( FIG. 7 ) is not busy ( 372 ), the data is sent to the appropriate transport thread ( 374 ). If corresponding one of transport threads  320 A- 320 N ( FIG. 7 ) is busy ( 372 ), the data replication waits for notification ( 376 ) from the appropriate transport thread that it will accept further input. 
       FIG. 11  is a flowchart illustrating an example process of transport threads  320 A- 320 N ( FIG. 7 ) within each pathway  280  ( FIG. 7 ). If data is not available ( 380 ) from the corresponding one of remote threads  300 A- 300 N ( FIG. 7 ), the corresponding one of transport threads  320 A- 320 N ( FIG. 7 ) waits for notification that data is available ( 382 ). When data is available from the appropriate remote thread ( FIG. 7 ), that data is retrieved ( 384 ) and sent to a remote device ( 386 ). If the replication of data was successful ( 388 ), the data item is marked as successful ( 390 ) and sent ( 392 ) to the corresponding one of complete data threads  330 A- 330 N ( FIG. 7 ). If the replication of data was not successful, the data item is marked as failed ( 394 ). After a data attribute is marked, the data is sent to the appropriate one of complete threads  330 A- 330 N ( FIG. 7 ) and the corresponding transport thread  320 A- 320 N ( FIG. 7 ) check is more data is available ( 380 ). Transport threads  320 A- 320 N ( FIG. 7 ) negotiate with corresponding remote threads  310 A- 310 N ( FIG. 7 ) over what compression methods and level to use (not shown). Transport threads  320 A- 320 N ( FIG. 7 ) monitor the transfer rates and change the compression methods and level to obtain increased transfer rates or reduced rates according to the policy in the configuration (not shown). 
       FIG. 12  is a flowchart illustrating an example process of complete threads  330 A- 330 N ( FIG. 7 ) within each pathway  280  ( FIG. 7 ). If data is not available ( 400 ) from the corresponding one of transport threads  320 A- 320 N ( FIG. 7 ), then the corresponding one of complete threads  330 A- 330 N ( FIG. 7 ) waits for notification that data is available ( 402 ). When data is available, from the appropriate transport thread ( FIG. 7 ), the data is retrieved ( 404 ). If the appropriate one of transport threads  320 A- 320 N ( FIG. 7 ) marked the data as successful ( 406 ), the data is deleted ( 408 ) from the corresponding one of pathway journals  290 A- 290 N ( FIG. 7 ). The data is then deleted ( 410 ) from the corresponding one of work journals  310 A- 310 N ( FIG. 7 ). When the appropriate one of transport threads  320 A- 320 N ( FIG. 7 ) did not mark the data as successful, the data is deleted from the corresponding one of work journals  310 A- 310 N ( FIG. 7 ). 
     The described hybrid real-time data replication techniques may use a general-purpose computing system that is well known in the art for an operating environment in which the described invention may be implemented. The operating environment is only one example of a suitable operating environment, and should not be taken as limiting the use or functionality of the described invention. Other well-known computing systems, environments and/or configurations that may be suitable for use with the invention include, but are not limited to, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, programmable consumer electronics, network personal computers (PCs), minicomputers, mainframe computers, distributed computing environments the include any of the above systems or devices or other environments. 
     If implemented in software, a machine-readable or computer-readable medium may store computer readable instructions, i.e., program code, that can be executed by a processor to carry out one of more of the techniques described above. For example, the machine-readable or computer-readable medium may comprise random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), gate arrays, electrically erasable programmable read-only memory (EEPROM), flash memory, compact disk-ROM (CD-ROM), digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by processing devices. The machine-readable or computer-readable medium may comprise computer readable instructions that when executed, cause the device to carry out one or more of the techniques described herein. These and other embodiments are within the scope of the following claims.