Self-healing and dynamic optimization of VM server cluster management in multi-cloud platform

Virtual machine server clusters are managed using self-healing and dynamic optimization to achieve closed-loop automation. The technique uses adaptive thresholding to develop actionable quality metrics for benchmarking and anomaly detection. Real-time analytics are used to determine the root cause of KPI violations and to locate impact areas. Self-healing and dynamic optimization rules are able to automatically correct common issues via no-touch automation in which finger-pointing between operations staff is prevalent, resulting in consolidation, flexibility and reduced deployment time.

TECHNICAL FIELD

Embodiments of the present disclosure relate to the performance monitoring of network functions in a virtual machine server cluster. Specifically, the disclosure relates to using self-healing and dynamic optimization (SHDO) of virtual machine (VM) server cluster management to support closed loop automation.

BACKGROUND

A virtual network combines hardware and software network resources and network functionality into a single, software-based administrative entity. Virtual networking uses shared infrastructure that supports multiple services for multiple offerings.

Network virtualization requires moving from dedicated hardware to virtualized software instances to implement network functions. Those functions include control plane functions like domain name servers (DNS), Remote Authentication Dial-In User Service (RADIUS), Dynamic Host Configuration Protocol (DHCP) and router reflectors. The functions also include data plane functions like secure gateways (GW), virtual private networks (VPN) and firewalls.

The rapid growth of network function virtualization (NFV), combined with the shift to the cloud computing paradigm, has led to the establishment of large-scale software-defined networks (SDN) in the IT industry. Due to the increasing size and complexity of multi-cloud SDN infrastructure, a technique is needed for self-healing & dynamic optimization of VM server cluster management in the SDN ecosystem to achieve high and stable performance of cloud services.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As more organizations adopt virtual machines into cloud data centers, the importance of an error-free solution for installing, configuring, and deploying all software (operating system, database, middleware, applications, etc.) for virtual machines in cloud physical environments dramatically increases. However, performance monitoring of virtual network functions (VNF) and VM components in the network virtualization world is not always an easy task.

Presently disclosed is a solution for designing and deploying a self-healing virtual machine management tool that will support dynamic performance optimization. The solution provides flexibility, consolidation, increased customer satisfaction and reduced costs via no-touch automation. The new methodology uses historical statistics to determine a threshold of a given value and to autonomously trigger optimization within a self-healing environment. The disclosed techniques provide an innovative automated approach to design and deploy self-healing of VM server cluster management to support performance optimization management of VNF and VM.

In sum, the present disclosure develops a new methodology to derive adaptive quality metrics to autonomously trigger optimization of NFV and management structure of self-healing and dynamic optimization (SHDO) of VM server cluster management to support closed loop automation. In particular, an adaptive thresholding methodology is described for developing actionable quality metrics for benchmarking and anomaly detection. Further, real time analytics are described for root cause determination of key performance indicator (KPI) violation and impact areas. Additionally, self-healing policy management and dynamic optimizing performance tuning are implemented, including reducing system load, disk tuning, SCSI tuning, virtual memory system tuning, kernel tuning, network interface card tuning, TCP tuning, NFS tuning, JAVA tuning, etc.

In certain embodiments of the present disclosure, a method is provided for managing a virtual machine server cluster in a multi-cloud platform by monitoring a plurality of quality metrics. For each of the quality metrics, the quality metric is classified as one of a plurality of predetermined quality metric types, accumulated measurement values are recorded for the quality metric, a statistical value is calculated from the accumulated measurement values, and an adaptive threshold range is determined for the quality metric based on the statistical value and based on the predetermined quality metric type.

It is then determined that a statistical value for a particular quality metric is outside the adaptive threshold range for the particular quality metric. A self-healing and dynamic optimization task performed based on the determining that the statistical value is outside the adaptive threshold range.

In additional embodiments, a computer-readable storage device is provided having stored thereon computer readable instructions for managing a virtual machine server cluster in a multi-cloud platform by monitoring a plurality of quality metrics. Execution of the computer readable instructions by a processor causes the processor to perform operations comprising the following. For each of the quality metrics, the quality metric is classified as one of a plurality of predetermined quality metric types, the types including a load metric, a utilization metric, a process efficiency metric and a response time metric, accumulated measurement values are recorded for the quality metric, a statistical value is calculated from the accumulated measurement values, and an adaptive threshold range is determined for the quality metric based on the statistical value and based on the predetermined quality metric type.

It is then determined that a statistical value for a particular quality metric is outside the adaptive threshold range for the particular quality metric, and a self-healing and dynamic optimization task is performed based on the determining that the statistical value is outside the adaptive threshold range.

In another embodiment, a system is provided for managing a virtual machine server cluster in a multi-cloud platform by monitoring a plurality of quality metrics. The system comprises a processor resource, a performance measurement interface connecting the processor resource to the virtual machine server cluster, and a computer-readable storage device having stored thereon computer readable instructions.

Execution of the computer readable instructions by the processor resource causes the processor resource to perform operations comprising, for each of the quality metrics, classifying the quality metric as one of a plurality of predetermined quality metric types, the one of a plurality of predetermined quality metric types being a utilization metric; receiving, by the performance measurement interface, accumulated measurement values for the quality metric; calculating, by the processor, a statistical value from the accumulated measurement values; and determining, by the processor, an adaptive threshold range for the quality metric based on the statistical value and based on the predetermined quality metric type.

The operations also include determining, by the processor, that a statistical value for a particular quality metric is outside the adaptive threshold range for the particular quality metric; and performing, by the processor, a self-healing and dynamic optimization task based on the determining that the statistical value is outside the adaptive threshold range, the self-healing and dynamic optimization task comprising adding a resource if the statistical value is above an upper threshold and removing a resource if the statistical value is below a lower threshold.

A tool100for self-healing and dynamic optimization, shown inFIG. 1, is used in managing a virtual machine server cluster180. The server cluster180includes hosts181,182,183that underlie instances184,185,186of a hypervisor. Each hypervisor instance creates and runs a pool of virtual machines187,188,189. Additional virtual machines190may be created or moved according to orchestration from the network virtual performance orchestrator140.

The tool100for self-healing and dynamic optimization includes an adaptive performance monitoring management module110. The management module110performs a threshold configuration function112in which thresholds are initially set using predefined settings according to the target type, as described in more detail below. Using KPI trending, an anomaly detection function114is used to monitor data via a performance measurement interface171from a performance monitoring data collection module170such as a Data Collection, Analysis, Event (DCAE) component of AT&T's eCOMP™ Framework. KPI violations are identified using signature matching116or another event detection technique.

A threshold modification module118dynamically and adaptively adjusts the thresholds in real time according to changing conditions as determined in the adaptive performance monitoring management module110. In addition to the performance monitoring data170, the management module110utilizes various stored data including virtual machine quality metrics120, historical performance monitoring data121and topology awareness122in modifying thresholds.

A self-healing policy130is established based on output from the adaptive performance monitoring management module110. The self-healing policy130is applied in dynamic optimizing and performance tuning132, which is used in adjusting the virtual machine quality metrics120.

The self-healing policy130is implemented through virtual life cycle management134and in virtual machine consolidation or movement136. For example, CPUs may be consolidated if a utilization metric falls below 50%. The dynamic optimizing and performance tuning132, the virtual life cycle management134and the virtual machine consolidation or movement136are orchestrated by a network virtual performance orchestrator140, which oversees virtual machine clusters141if a user defined network cloud142.

The presently described application monitoring solution is capable of managing the application layer up/down and degraded. The monitoring infrastructure is also self-configuring. The monitoring components must be able to glean information from inventory sources and from an auto-discovery of the important items to monitor on the devices and servers themselves. The monitoring infrastructure additionally incorporates a self-healing policy that is able to automatically correct common VM server issues so that only issues that really require human attention are routed to operations.

The monitoring infrastructure is topology-aware at all layers to allow correlation of an application event to a root cause event lower in the stack. Specifically, the monitoring infrastructure is topology-aware at the device or equipment levels as shown by the VM cluster diagram200ofFIG. 2, including the virtual machine level 210, the vSwitch level 220 and at the virtual NIC level 230. The monitoring infrastructure is also topology-aware at the connection or link level, including the pNIC links250from VMNIC ports256to vSwitch uplink port255, and links from vSwitch host ports260to vNIC (VM) ports265.

Traditional server fault management tools used static thresholds for alarming. That approach leads to a huge management issue as each server had to be individually tweaked over time and alarms had to be suppressed if they were over the threshold but still “normal” for that individual server. As part of self-configuration, monitoring must be able to set its own threshold values. Using adaptive thresholding, it is possible to set a threshold on a “not normal” value defined as X standard deviations from the mean, where X is selected based on characteristics of the metric and of the overall system design.

“Adaptive thresholding” refers to the ability to utilize historical statistics to make a determination as to what the exact threshold value should be for a given threshold. In a methodology400to set a threshold in accordance with one aspect of the disclosure, shown inFIG. 4, upon instructions410to get a threshold, historical data is initially loaded at operation420from monitor history files in a performance monitoring database. The monitor history files may be data-interchange format files such as JavaScript Object Notation (.json) files that contain the accumulated local statistics. Since the statistics are based on accumulated values, no large database is needed. For example, each monitor with statistics enabled will generate its own history file named, for example, <monitor>.history.json. Statistics are accumulated in these files by hour and by weekday to allow trending hour by hour and from one week to the next.

Monitor value names in an example history file of the methodology are the names of the statistics. In most cases that is the same as the monitor name, but for some complex monitors, the <monitor value name> may match some internal key. The day of the week is a zero-indexed value representing the day of the week (Sunday through Saturday), with zero representing Sunday. The hour and minute of the day are in a time format and represent the sample time. As discussed in more detail below, the sample time is a timestamp tied to the configured SHDO interval and not necessarily the exact timestamp of the corresponding monitor values in monitor.json. That allows normalization of the statistics such that there are a predictable and consistent number of samples per day.

The actual statistics stored in the file do not equate exactly to the available statistics. The file contains the rolling values used to calculate the available statistics. For example, the mean is the sum divided by the count. The SHDO core functions related to statistics will take these values and calculate the available statistics at runtime.

If no historical data exists (decision430), then a configured static threshold is returned at operation435. Otherwise, a determination is made as to the type of threshold at decision440. Because there are a large number of metrics to configure for many different target types, using the standard method can be cumbersome. An implementation of actionable VM quality metrics, shown in the table600ofFIG. 6, is proposed to specify predefined settings for specific usage patterns to trigger the presently disclosed self-healing and dynamic optimization methodology. Not all metrics have adaptive thresholds. In order to apply an adaptive threshold, the adaptive threshold metrics must fall into one of the following four categories or types610, as shown inFIG. 6: (1) X1:Load, (2) X2:Utilization, (3) X3:Process Efficiency and (4) X4:Response Time. While those categories were found by the inventors to be particularly effective in the presently disclosed methodology, more, fewer or different categories may be used without departing from the scope of the disclosure. The presently disclosed methodology treats adaptive threshold metrics falling into one category differently than adaptive threshold metrics falling into another category. For example, different statistical values may be calculated for data relating to a load metric than for data relating to a utilization metric. Further, threshold ranges may be calculated differently for different metric categories.

Returning toFIG. 4, if the adaptive threshold metric type does not fall into one of those categories (decision440), then a configured static threshold is returned at operation435.

For an adaptive threshold metric type falling into one of the defined categories, an aligned time stamp is then established at operation450. History update functions are called if the disclosed SHDO package is called for execution with a defined interval. For purposes of the statistics functions in the presently disclosed SHDO methodology, sample time refers to the normalized timestamp to associate with a given sample value. That timestamp is derived from the expected run time of the interval. That is, this is the theoretical time at which the system would timestamp all values if SHDO methodology ran instantaneously. That timestamp is used in place of the true timestamp to ensure that samples are allocated to the correct time bucket in the history file. Sample time is determined by SHDO at the start of its run and that sample time is used for all history file updates for that run. Samples in the history file are stored by the zero-based day of week (0=Sunday, 7=Saturday) and normalized timestamp.

For example, SHDO is scheduled for execution on Tuesday at 10:05:00 with an interval of 5. Upon execution, SHDO calls the current time function which returns 10:05:02 (i.e., not precisely the same as the expected execution time). The sample time function is called on the current time. The sample time function normalizes the actual current time to a sample time value of “10:05” and a sample day value of 2 (Tuesday).

Partial sums are then retrieved at operation460from historical data for the aligned timestamp. Since those accumulated values will increase indefinitely, there is a possibility of hidden arithmetic overflow occurring. For example, in the Perl programming language, all numbers are stored in float (double), even on 32-bit platforms. Worst case, the max value of a positive Perl scalar is 2^53 for integers, or 16 places before or after the decimal without losing significance for floats. Thus, when storing new values, loss of significance must be considered. If loss of significance occurs, the sum, sum of the squares, and count values should all be divided by 2. That allows the values in the history file to remain within bounds and does not greatly alter the resulting statistics values. For example, the mean will be unchanged and standard deviation will change only very slightly due to the fact that this is sample and not population data. For most datasets, those rollover events should be very rare but they still must be accounted for.

Statistical values are then calculated at operation470for the historical data. A particular function call of the SHDO system is responsible for reading the history files for all statistics-enabled monitors and providing a hash of the statistics values for threshold use. That function is called prior to the execution of the monitors. The function indexes through all enabled monitors, checking for the existence of a corresponding .json history file, and then loads the available statistics. The applicable portion of the resulting hash is passed to threshold functions in a manner similar to how monitor data is passed. The value loaded from the history file is the value corresponding to the current sample time. Thus, the statistics are defined such as “the average value for the monitor for a Tuesday at 10:05”, not the average of all values at all times/days.

The statistical values may include the following: average, maximum value, minimum value, last value, standard, sum of historical values, sum of squares of historical values, and count of values. As noted, the particular statistical values that are loaded depends at least in part on the threshold type. The values are stored in the hash for monitor history.

The adaptive threshold range is then calculated at operation480based on the statistical values and on the type and configuration of the threshold. The adaptive threshold is then returned to the calling program at operation490. One example of an adaptive threshold for “alarm if not normal” is to alarm if the monitor value being thresholded is greater than two standard deviations from the historical mean.

A methodology500for updating statistics in accordance with one aspect of the disclosure is shown inFIG. 5. Monitor functions within the SHDO module are responsible for calling the function for updating statistics if they wish their data to be saved in history files. That function accepts the monitor name, statistic name, and statistic value as inputs. The sample time and path to history files is already known to the SHDO module so that information need not be supplied. The function opens the history file for the monitor, decodes the .json file into a hash, and writes the new values to the correct day-of-week and time for the statistic.

Upon instructions510to update statistics, it is initially determined whether historical data exist, as illustrated by decision block520. If historical data already exist, then that data is loaded, as shown by operation530. As noted above, the statistics are based on accumulated values, so no large database is needed. If no historical data exists, then a new data file is created at operation540. An aligned timestamp is then created at operation550for the statistics, as described above.

Partial sums are then calculated for the data at operation560. The computed partial sums may include a sum of all values, a sum of the squares of the values, a new count value, a new minimum value, a new maximum value and a last value. The particular partial sums computed at operation560depends on the type or category of the threshold.

The computed partial sums are then saved to a data file at operation570and the function is ended at operation580.

An example will now be illustrated of anomaly detection and handling based on a threshold range rule (CPU from 50% to 85%). The example uses actual performance monitoring data for CPU utilization620(FIG. 6), which is a Type X2 (utilization) threshold. The example uses a simple CPU Rule: add 1 VM if >85% CPU/VM and remove 1 VM if <50% CPU/VM. The rule therefore applies, in addition to the 85% of maximum KPI violation shown inFIG. 6, a minimum CPU utilization of 50%, below which CPU consolidation begins.

The graph700ofFIG. 7Ashows a simulated average CPU usage710, an actual CPU usage712over 10 virtual machines, and a number of virtual machines714instantiated over a time period720of approximately one week.

The graph750ofFIG. 7Bshows an actual total CPU usage without the rule (line760) and an actual total CPU usage with the rule (line762) over the same time period770of approximately one week. The graph750clearly shows a savings benefit in total CPU usage, with more savings at higher CPU usage.

The graphs700,750demonstrate the better utilization of server hardware resulting from implementation of the presently described virtualization. While most applications only use 5-15% of system resources, the virtualization increases utilization up to 85%. Additionally, a higher cache ratio results in more efficient use of CPU. Specifically, average relative CPU savings of 11% is expected. In one demonstration, total average CPU usage was 499%, versus 556% without implementing the rule.

Additional savings are possible by optimizing VM packing and turning off servers. Savings may also be realized from elasticity to serve other services during “non-peak” periods.

A flow chart800, shown inFIG. 8, illustrates an example logical flow of a dynamic optimization methodology according to the present disclosure. The process may be initiated at block842by an automatic detection of a performance monitoring alert or a customer-reported alert indicating a virtual machine performance issue. The alert may be forwarded to a network operating work center840and/or to an automatic anomaly detection process820. The automatic anomaly detection820or KPI trending is based on an abnormal event detected using signatures indicating virtual machine performance degradation.

Signature matching810is initially performed to detect KPI violations. The methodology performs a series of comparisons811-816according to metric types, to detect instances where thresholds are exceeded. If a utilization metric811or a response time metric812exceeds the KPI for that metric, then the methodology attempts to optimize the performance metric tuning at operation822. For example, the system may attempt to perform disk tuning, SCSI tuning, VM tuning, kernel tuning, NIC tuning, TCP tuning, NFS tuning, JAVA tuning, etc. If the optimization822is successful, then the problem is considered resolved within the closed loop at operation930.

If the optimization822fails, then the systems attempts virtual machine movement at operation832. If that fails, then the system performs an auto notification to a network operating work center at operation840, resulting in human/manual intervention.

If the virtual machine movement832is successful, then an automatic virtual machine orchestration management function832is called, and the problem is considered resolved at operation830.

If a virtual machine load metric813exceeds the KPI for that metric, the system attempts to reduce the system load at operation824. If that task fails, then optimizing PM tuning is performed at operation822as described above. A successful reduction of system load results in the problem being considered resolved at operation830.

If a virtual machine process metric814exceeds the KPI for that metric, the system attempts to perform virtual machine life cycle management at operation826. If that task fails, the system performs an auto notification to a network operating work center at operation840as described above. If the virtual machine life cycle management is successful, then the automatic virtual machine orchestration management function832is called, and the problem is considered resolved at operation830.

The example system also checks if any relevant KPIs, CPU, memory, HTTPD connections, etc. are below 50% at decision815. If so, the system attempts to perform virtual machine consolidation at operation828. If successful, then the automatic virtual machine orchestration management function832is called, and the problem is considered resolved at operation830. If that task fails, the system performs an auto notification to a network operating work center at operation840as described above.

Additional or miscellaneous metrics may also be checked (operation816) and additional actions taken if those metrics are found to be outside threshold limits.

Several use cases demonstrating the presently disclosed self-healing policy will now be discussed. A first case900, shown inFIG. 9, relates to preventing site/application overload using adaptive thresholding of the ProcRunTime metric. The methodology initially checks for processes exceeding normal runtime at operation910. A check_procruntime function retrieves the current process runtimes for all target processes at operation912, and calculates a threshold value using the adaptive threshold algorithm at operation914. If the runtimes are found not to exceed the threshold at decision916, then the workflow is ended at918.

If the threshold is exceeded, then the check_procruntime function reports an alarm to the SHDO system at operation920. The SHDO system matches the check_procruntime alarm to the ProcRunTime metric and executes the closed loop workflow at operation922. The workflow logs into the target server and kills the identified processes at operation924.

The SHDO system then checks at decision926whether the identified processes were successfully killed. If so, then the problem is considered resolved in the closed loop and the process ends at block928. If one or more of the identified processes were not successfully killed, then the workflow makes another attempt to kill those processes at operation930. Another check is made that the processes were killed at decision932. If so, then the problem is considered resolved. If not, then the process is transferred to manual triage at operation934.

In another use case1000, shown inFIG. 10, site/application overload is prevented using adaptive thresholding of memory. The methodology initially checks for physical memory utilization at operation1010. A check_mem function retrieves the current physical memory utilization at operation1012, and calculates a threshold value using the adaptive threshold algorithm at operation1014. If the retrieved utilization is found not to exceed the threshold at decision1016, then the workflow is ended at1018.

In the case where the threshold is exceeded, the check_mem function reports an alarm to the SHDO system at operation1020, and the SHDO system matches the check_mem alarm to a MemUtil closed loop workflow and executes at operation1022. The workflow logs into the target server and identifies any processes that are using significant memory at operation1024.

The workflow then consults the network configuration at operation1026to determine whether any of those identified processes are kill-eligible or belong to services that can be restarted. If the workflow determines at decision1028that no processes exists that can be killed or restarted, then the closed loop workflow is stopped and problem is assigned to manual triage at block1036. Otherwise, the workflow attempts to kill or restart the eligible processes and success is evaluated at decision block1030. If the attempt is unsuccessful, then the workflow resorts to manual triage1036. If the processes were successfully killed to restarted, then the workflow determines at decision1032whether memory utilization is back within bounds. If so, the workflow ends at block1034. If not, the workflow loops back to operation1024to identify additional processes using significant memory.

In another use case, virtual machine spawning is prevented from exhausting resources. This is required to protect against the case where a VM configuration error in a SIP client causes all spawning of new VM's to fail. The SIP client may keep spawning new VMs until there is no additional disk/memory/IP available. The lack of disk/memory/IP causes the performance of the operating VMs to deteriorate.

Additionally, the continued failure of VM spawning fills the redirection table of the SIP Proxy, preventing the SIP proxy from pointing to the correct VM (fail-over mechanism dysfunction). No functioning virtual machines therefore survive.

The problem of exhausting resources by VM spawning is addressed by several features of the presently described system wherein adaptive thresholding is used for VM memory to reduce incoming load. First, a sliding window is used for provisioning multiple VMs, allowing only a maximum of K ongoing tasks, where K is a predetermined constant established based on network characteristics. The sliding window prevents the system from continued provisioning when failure occurs. By fixing the size of the processing queue to K tasks, a new provisioning task is performed only when the queue has a free slot. Failed tasks and completed tasks are removed from the queue.

In another feature of the presently described system, failed tasks are moved from the processing queue to a fail queue. VM provisioning fails, as described above, are put in the fail queue and are removed from the fail queue after a timeout. When the size of the fail queue exceeds a threshold, the system stops admitting provisioning tasks. In that way, the system does not continue provisioning when a provisioning failure is recurring.

The system additionally enforces dual or multiple constraints on provisioning tasks. Specifically, each provisioning task is examined both (1) by the queues in the physical machine and (2) by the queues in the service group. The system detects instances where VM provisioning often fails in a certain physical machine, and instances where VM provisioning often fails in a certain service group (e.g., a region in USP, or an SBC cluster). The system addresses the detected problem by, for example, taking the problem physical machine off line, or diagnosing a problem in a service group.

The hardware and the various network elements discussed above comprise one or more processors, together with input/output capability and computer readable storage devices having computer readable instructions stored thereon that, when executed by the processors, cause the processors to perform various operations. The processors may be dedicated processors, or may be mainframe computers, desktop or laptop computers or any other device or group of devices capable of processing data. The processors are configured using software according to the present disclosure.

Each of the hardware elements also includes memory that functions as a data memory that stores data used during execution of programs in the processors, and is also used as a program work area. The memory may also function as a program memory for storing a program executed in the processors. The program may reside on any tangible, non-volatile computer-readable storage device as computer readable instructions stored thereon for execution by the processor to perform the operations.

Generally, the processors are configured with program modules that include routines, objects, components, data structures and the like that perform particular tasks or implement particular abstract data types. The term “program” as used herein may connote a single program module or multiple program modules acting in concert. The disclosure may be implemented on a variety of types of computers, including personal computers (PCs), hand-held devices, multi-processor systems, microprocessor-based programmable consumer electronics, network PCs, mini-computers, mainframe computers and the like, and may employ a distributed computing environment, where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, modules may be located in both local and remote memory storage devices.

An exemplary processing module for implementing the methodology above may be stored in a separate memory that is read into a main memory of a processor or a plurality of processors from a computer readable storage device such as a ROM or other type of hard magnetic drive, optical storage, tape or flash memory. In the case of a program stored in a memory media, execution of sequences of instructions in the module causes the processor to perform the process operations described herein. The embodiments of the present disclosure are not limited to any specific combination of hardware and software.

The term “computer-readable medium” as employed herein refers to a tangible, non-transitory machine-encoded medium that provides or participates in providing instructions to one or more processors. For example, a computer-readable medium may be one or more optical or magnetic memory disks, flash drives and cards, a read-only memory or a random access memory such as a DRAM, which typically constitutes the main memory. The terms “tangible media” and “non-transitory media” each exclude transitory signals such as propagated signals, which are not tangible and are not non-transitory. Cached information is considered to be stored on a computer-readable medium. Common expedients of computer-readable media are well-known in the art and need not be described in detail here.

The forgoing detailed description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the disclosure herein is not to be determined from the description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. It is to be understood that various modifications will be implemented by those skilled in the art, without departing from the scope and spirit of the disclosure.