Abstract:
The present invention describes a method and system for creating a cluster availability model that takes into account availabilities of software components in the cluster. Aspects of the disclosure include defining a repair model and failure parameters for a repair mode in the repair model, and modeling availabilities of software components based on the repair mode and failure parameters. Additional aspects include combining availability models of software components in a computationally feasible manner and determining repair rates of node and cluster reboots. A cluster availability model is determined using the combined availability models of software components and the repair rates of node and cluster reboots.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 09/850,183 filed May 7, 2001 and entitled “A MEANS FOR INCORPORATING SOFTWARE INTO AVAILABILITY MODELS,” which in turn claims benefit U.S. Provisional Patent Application No. 60/202,154 filed May 5, 2000, and entitled “MEANS FOR INCORPORATING SOFTWARE INTO AVAILABILITY MODELS,” both of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cluster availability model that takes into account availability of software components in a cluster. More particularly, the present invention relates to a method and system for modeling the availability of a cluster by aggregating availability information of individual software components in the cluster in a computationally efficient manner. 
     2. Discussion of the Related Art 
     Availability modeling of a cluster is becoming increasingly important. Such modeling reduces costs of implementing a cluster because errors and problems can be identified early in the design process. In addition, different components within the cluster may be changed, added, or deleted during testing and evaluation to reflect advances in technology or network requirements. Components may be hardware devices, software applications and/or a combination of both. An availability model preferably incorporates information about each of the components in a cluster, their reliability, and the behavior of the system in cases of component failure, to yield an overall availability prediction for the entire system. 
     A hardware repair may be relatively simple. Typically, repairing involves manual operations by human technicians. For example, a service technician may replace a defective component. As such, the repair rates of hardware may be determined by response time, travel time, spare parts availability, and the time to perform specific service operations. With hardware, the interdependencies between components tend to be tree structured, and failure modes and repair actions associated with different modules tend to exhibit a high degree of independence. Because of the modularity, it is often possible to model complex hardware systems based on the outputs of models of the individual components. It may not be necessary for system models to include all of the details from the individual component models. There may be a large number of models to deal with, but they are rarely prohibitively complex. 
     Software repairs, however, differ from hardware repairs in many respects. First, there may be a multiplicity of ways to repair a particular problem, such as restarting the program, rebooting the node, or rebooting the entire cluster. Second, each of the possible repair techniques can take a different amount of time. Third, as initial repair efforts often fail, it is necessary to associate an efficacy (likelihood of success) with each repair technique. Fourth, software repairs may involve a hierarchical escalation of repair measures. For example, if a particular problem is not fixed by restarting the program, the next step may be to reboot the entire node. The above differences make it difficult to arrive at an availability model of software components in the cluster. 
     Further, with software, there tends to be many more cross-level interactions, and many repair actions (e.g., node rebooting) which affect a large number of components. Because of this, an availability model for a complex software environment may have to incorporate detailed models for each software component, thus making the whole-system model very complex (perhaps exponential in the number of components). Because of this complexity, system architects often try to avoid incorporating detailed software failure and recovery behavior into their system availability models. 
     The functionality of newer systems is becoming increasingly dominated by software, and many key elements of the repair/recovery process are now being performed by software. As such, it is no longer practical to ignore software behavior when attempting to model system availability. A realistic system availability model must model the failure and recovery behavior of the software in that system. There is a great need for availability modeling techniques that include the failure and recovery behavior of all of the system&#39;s software components, while still yielding models of manageable complexity with reasonably determinable parameters. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a cluster availability model. In particular, the present invention provides a method and system for modeling availability of a cluster with software components with at least one node in a computationally feasible manner. 
     To achieve these and other advantages and in accordance with the purposes of the present invention as embodied and broadly described herein, the present invention provides a method for modeling the availability of a cluster, the cluster having a plurality of software components and at least one node, the method including determining a plurality of component availability models using a repair model and a plurality of failure parameters, each of the plurality of component availability models corresponding to one of the plurality of software components; combining the plurality of component availability models; determining repair rates for node and cluster reboots; and constructing an availability model based on the repair rates and the combined plurality of component availability models. 
     In one embodiment, the invention provides a system for modeling the availability of a cluster, the cluster having a plurality of software components and at least one node, the system including means for determining a plurality of component availability models using a repair model and a plurality of failure parameters, each of the plurality of component availability models corresponding to one the plurality of software components; means for combining the plurality of component availability models; means for determining repair rates for node and cluster reboots; and means for constructing an availability model based on the repair rates and the combined plurality of component availability models. 
     In an alternate embodiment, the invention provides a method for modeling the availability of a cluster, the cluster having a plurality of software components and at least one node, the method including specifying a repair model, the repair model having one or more repair modes; specifying a plurality of failure parameters, for each software component in the plurality of software components, assigning values to the plurality of failure parameters for each appropriate repair mode for the software component; combining values of the plurality of failure parameters of the plurality of software components for each repair mode in the repair model; determining repair rates for node and cluster reboots; and constructing an availability model based on the repair rates and the combined plurality of failure parameters. 
     In an alternate embodiment, the invention provides a computer program product comprising a computer useable medium having computer readable code embodied therein for modeling the availability of a cluster, the cluster having a plurality of software components and at least one node, the computer program product adapted when run on a computer to effect steps including determining a plurality of component availability models using a repair model and a plurality of failure parameters, each of the plurality of component availability models corresponding to one of the plurality of software components; combining the plurality of component availability models; determining repair rates for node and cluster reboots; and constructing an availability model based on the repair rates and the combined plurality of component availability models. 
     Additional features and advantages of the invention are set forth in the description that follows, and in part are apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention are realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a representational diagram of a cluster whose availability may be modeled using the present invention; 
         FIG. 2  is a flowchart depicting one example of a set of operations in one embodiment of the present invention; 
         FIG. 3  is a representational diagram showing an example of an escalation graph in accordance with an embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating one example of operations that may be used in determining a component availability model of one embodiment of the present invention; 
         FIG. 5  is a flowchart showing one example of operations that may be used to aggregate availability information regarding components in a cluster in one embodiment of the present invention; and 
         FIG. 6  is a diagram illustrating one example of a state-space model in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made in detail to the preferred embodiments of the Present invention, examples of which are illustrated in the drawings. 
       FIG. 1  is a simplified representational diagram of a cluster whose availability may be modeled using the present invention. A cluster  100  includes nodes  102 ,  110 ,  120  and  130 . The nodes  102 ,  110 ,  120  and  130  may be computers or any platform that has hardware and software components. Preferably, the nodes  102 ,  110 ,  120  and  130  can execute instructions from a computer-readable medium and store data. 
     The cluster  100  may include any number of nodes. Nodes are connected to each other via a network. In  FIG. 1 , a network  142  connects the nodes  102  and  120 , a network  144  connects the nodes  120  and  130 , a network  146  connects nodes  130  and  110 , and a network  140  connects the nodes  110  and  102 . As such, the nodes  102 ,  110 ,  120 ,  130  in the cluster  100  can exchange information, such as messages, data packets and the like, using the networks  140 ,  146 ,  144  and  142 . 
     The node  102  includes operating system  104 , a set of hardware components  106  and a set of software applications  108 . The operating system  104  and the set of software applications  108  may be considered software components of the node  102 . Repairs to software components may include restarting an application, rebooting the node  102 , and other activities including those that may necessitate hardware fixes and/or repairs. 
     The operating system  104  may be a program that, after being initially loaded into the node  102  by a boot program, manages all the other programs on the node  102 , including the set of software applications  108 . The set of software applications  108  may make use of the operating system  104  by making requests for services through a defined application program interface, for example. In addition, users may interact directly with the operating system  104  through a user interface such as a command language or a graphical user interface. 
     The set of hardware components  106  may be logic circuits, a memory, a power supply, or any other types of hardware within the node  102 . The node  102  may include any number of hardware components  106 , and is not limited by the embodiment depicted in  FIG. 1 . A hardware component in the set of hardware components  106  may have a failure rate, such as a mean time between failures and a repair time. It should be appreciated that there are a variety of availability models for hardware components. Any suitable model may be used to model availability of the set of hardware components  106 . 
     Similarly, the nodes  110 ,  120  and  130  have operation systems  114 ,  124  and  134 , sets of hardware components  116 ,  126  and  136 , and sets of software applications  118 ,  128  and  138 , respectively. In other words, the nodes  110 ,  120  and  130  are similar to the node  102  in that each node has an operating system, a set of hardware components, and a set of software applications. 
     The operating systems  104 ,  114 ,  124  and  134  may be the same operating systems, or, alternatively, may be different operating systems that are able to exchange information. Messages, information, files and the like may pass through the nodes without obstruction by the operating systems. Further, the hardware and software components on the nodes  102 ,  110 ,  120  and  130  may differ. For example, applications in the set of software applications  108  may be different from applications in the set of software applications  138 . 
     Hardware components in the set of hardware components  106 ,  116 ,  126  and  136  may have different failure rates and repair times. In addition, software components in the nodes  102 ,  110 ,  120  and  130  may have different failures, failure resolution actions and recovery times. Thus, though the nodes  102 ,  110 ,  120  and  130  may be within the cluster  100 , they may not be configured identically. 
     In one embodiment, an availability model for the cluster  100  may incorporate availability information of the following: (1) the cluster itself; (2) its nodes  102 ,  110 ,  120  and  130 ; (3) the operating systems  104 ,  114 ,  124  and  134 ; (4) the set of hardware components  106 ,  116 ,  126  and  136 ; and (4) the set of software applications  108 ,  118 ,  128  and  138 . Further, such availability model is constructed in a computational feasible manner using a repair model and failure parameters, both of which are also described in detail below. 
       FIG. 2  shows example operations that may be performed in one embodiment of the present invention to construct a cluster availability model. At step  210 , a repair model to be used in modeling component availability is identified. For example, application developers who are familiar with the software component may make such identification. A repair model is used to categorize and distinguish software component failures on the basis of their service impact and types of repair actions that can be applied on failed software components. The repair model may include repair modes such as component soft-reset, component warm-restart, component cold-restart, component fail-over, node reboot and cluster reboot. It should be appreciated that the present invention in its broader context is not dependent on the use of specific repair modes in a repair model. 
     A component soft-reset mode may reflect those failures having a recovery action that is automatically initiated by a component manager, for example. Component soft-reset may include a warm restart of an application, or a warm restart only of a subset of the application. 
     A component warm-restart mode may include terminating an application and restarting it using saved information regarding the application. 
     A component cold-restart mode may reflect those failures resolved by terminating and restarting the application. When an application is restarted under the cold-restart mode, any previously saved information regarding the application may be ignored, potentially resulting in a greater service interruption. 
     A component fail-over mode may reflect those failures resolved by having all components on the affected node fail over to a hot standby. 
     A node reboot mode may occur when errors are not resolved after all components fail-over to another node. Node reboot may involve a complete reboot of the affected node and a restart of all components that reside on the affected node. Node reboot may occur after all the application specific recovery actions disclosed above fail. 
     Finally, if node reboot fails, a cluster reboot may be performed. A cluster reboot may involve a shutdown and reboot of all nodes in the cluster. In other words, it may involve restarting all hardware and software components running on the cluster. 
     For each software component, there typically are a number of repair actions that may be applied. At step  210 , an escalation graph may be defined to describe repair actions that may be applied to a software component, and the relationship among them (e.g., to describe what the next repair action will be if a prior action fails). Such an escalation graph may be defined for each of the software components in a cluster. For example, faced with a failed software component, one may first attempt component soft-restart. When this action fails, one may escalate to component cold-restart, or even to node reboot. In other words, a number of repair modes may apply to a software component, depending on factors such as a severity of a problem and effectiveness of a particular repair.  FIG. 3 , discussed infra, illustrates an example of an escalation graph. 
     To enhance the accuracy of a cluster availability model, it is desirable to accurately categorize failures of software components, or repair modes for software component failures. A failure mode can be described in terms of an expected failure rate and its impact on service. A repair mode can be described in terms of an expected repair time (though for modeling reasons, this is usually also expressed as a rate), and a probability of success (often referred to as its efficacy). 
     At step  220 , component failure parameters, which parameterize each class of component failures, are defined. These parameters are used to describe each mode of failure or repair. Examples of failure parameters include (1) a failure rate, or inversely, a mean-time-between-failure (“MTBF”); (2) a repair rate, or inversely, a mean-time-to-repair (“MTTR”); and efficacy or a fraction of repair attempts that are likely to succeed. It should be appreciated that the present invention is not dependent on the use of specific failure parameters. 
     At step  230 , failure parameter values are calculated. In estimating these values, one may rely on any number of measures, such as traditional techniques, measuring relevant variables and using estimations. For example, when using failure rate, repair rate, and efficacy as failure parameters, one may use traditional techniques to estimate failure rates. As to repair rates, one may have application developers specify or measure the times that an application requires for each component-level repair action. An efficacy may initially be estimated by application developers and then refined later with measured data. Platform developers and integrators may specify or measure the times that a bare platform requires for a node or cluster reboot, and the efficacy for node reboots. It should be appreciated that other methods may be used to calculate or estimate failure parameter values. Step  230  is described in more detail with regard to  FIG. 4 , discussed infra. 
     Turning to step  240 , for each class of component failure, values of state-space parameters to be used in an availability model are calculated. These parameters may be obtained, for example, by aggregating availability information of each software component, e.g., aggregating failure rates, aggregating repair rates, and aggregating repair efficacies. In calculating these values, one may assume that software components in a cluster are statistically independent. As such, it is assumed that there is no coupling of failures, i.e., an occurrence of one failure does not affect a likelihood of an occurrence of another failure. Step  240  is described in more detail with regard to  FIG. 5 , discussed infra. 
     At step  250 , for each class of system failure, values of state-space parameters to be used in an availability model are calculated. These parameters may be obtained, for example, by using aggregate failure rates, aggregate repair rates, and aggregate repair efficacies for node and cluster reboots. One of ordinary skill in the art will appreciate that independence between each class of system failure is not assumed. Step  250  is described in more detail with regard to  FIG. 5 , discussed infra. 
     An availability model of a cluster may be represented as a continuous-time Markov model. As such, the availability of the cluster may be represented as a steady-state occupancy rate for each repair mode in the repair model. At step  260 , a state-space model of an overall system is constructed using the state-space parameters calculated in steps  240  and  250 . In a state-space model, each repair mode may be assumed to have a well-specified impact on a service and may be represented as a discrete state in the model. State-space parameters are used to arrive at transition rates between the various states. In the model, imperfect recovery may be modeled by multiplying as two arcs (one to a successful repair state and the other to an escalation state), using the aggregate rates and aggregate efficacies computed in steps  240  and  250 . An example of a state-space model is illustrated in  FIG. 6 , discussed infra. 
     At step  270 , a state-space model is solved using any of traditional means for solving a continuous-time Markov model. This step produces a steady-state occupancy rate for each state in the model. Since each state represents a known level of services, one may estimate service availability of a cluster using appropriate state occupancy rates. 
       FIG. 3  shows one example of an escalation graph for a software component. Specifically,  FIG. 3  shows an escalation graph for a web server application. The web server is associated with four repair modes: “kill/restart web server” (node  302 ), “web server fail over” (node  303 ), “node reboot” (node  304 ) and “cluster reboot” (node  305 ). Node  301  represents a “web server OK” state, or the state in which a web server is functioning normally. 
     Directed edges in the escalation graph indicate an escalating series of repair actions that may be applied to the web server. When a web server fails, one may first attempt to “kill/restart web server” (node  302 ) or to “node reboot” (node  304 ). If the “kill/restart web server” option is successful, one would go back to “web server OK” (node  301 ). Otherwise, one may escalate to “web server fail over” (node  303 ). If this repair action fails, one may escalate to “node reboot” ( 304 ) and then to “cluster reboot” ( 305 ). Alternatively, one could skip “web server fail over” (node  303 ) and go to “node reboot” (node  304 ) instead. Thus, an escalation graph shows how responses may be escalated to deal with a software component failure. 
       FIG. 4  is a flowchart showing one example of modeling availability of OS and non-OS applications. Although the flow chart illustrates the steps serially, it should be appreciated that the present invention is not dependent on the steps being performed in any particular order. For a non-OS application, a value of each failure parameter for a component soft-reset, component warm-restart, component cold-restart and component fail-over may be determined by application developers, at steps  410 ,  420 ,  430  and  440 , respectively. Specifically, assuming that one uses failure rate, repair rate and efficacy as failure parameters, at step  410 , values of the failure rate, repair rate and efficacy for a component soft-reset are estimated or measured. At step  420 , values of the failure rate, repair rate and efficacy for a component warm-restart are estimated or measured. At step  430 , values of the failure rate, repair rate and efficacy for a component cold-restart are estimated or measured. Finally, at step  440 , values of the failure rate, repair rate and efficacy for a component fail-over are estimated or measured. 
     In OS applications, a value of each failure parameter for a node reboot and a cluster reboot may be determined by platform developers at steps  450  and  460 , respectively. Assuming that one uses a failure rate, repair rate and efficacy as failure parameters, at step  450 , values of the failure rate, repair rate and efficacy for a node reboot are estimated or measured. At step  460 , values of the failure rate, repair rate, and efficacy for a cluster reboot are estimated or measured. The rates determined in steps  450  and  460  are for the bare platform, and are exclusive of the time to restart all of the applications. 
       FIG. 5  shows one example of operations that may be performed in finding state-space parameters for a cluster availability model that uses a failure rate, a repair rate, and an efficacy as failure parameters. Although the operations are illustrated serially, it should be appreciated that the present invention is not dependent upon their performance in any particular order. Steps  510  through  530  relate to component failure modes and, as such, application developers may perform the computations. Step  540  relates to system failure and platform developers may perform the computations. Turning to step  510 , an aggregate failure rate for each failure mode is computed. For example, an aggregate failure rate may be a sum of failure rates of all software components that contribute to that failure mode. 
     At step  520 , an aggregate repair rate for each failure mode is computed. An aggregate repair rate may be a failure-rate weighted average of the repair rates for all components contributing to that failure mode. 
     At step  530 , an aggregate efficacy for each failure mode is computed. Like an aggregate repair rate, a failure-rate weighted average of the efficacies of all components for that failure mode may be used as an aggregate efficacy. 
     At step  540 , repair rates for node and cluster reboots are determined. For example, repair rates of node and cluster reboots may be obtained by first specifying the times that a bare platform or cluster requires for a node or cluster reboot and an efficacy for node reboots. Second, platform specific summation functions may be defined. Such functions may be used to recognize a possibility of parallel initialization of multiple applications, for example. An example of such functions includes a sum of cold restart times of a platform or cluster and operating system and software-component cold restart times. Another example includes a sum of reboot time of a bare platform or cluster and cold reboot times of relevant components. It should be understood that such functions need not be a summation, and that any function that is appropriate for the cluster to be modeled may be used. Finally, repair rates associated with node and cluster reboots can be calculated using the combined restart times in the event that these repair models are appropriate. 
     As the time to restart applications is non-negligible, the time to restart a node or a cluster must be considered a function of the applications that need to be started. This may be broken down into a basic cluster start-up time and the application cold-start time. To a first approximation, the node reboot time should be the basic platform reboot time plus the cold-start times for all of the configured applications. It is important to note, however, that there can be some parallelism in these start-up activities, and so there may be a platform-specific summation function that is more complex than a simple sum. 
       FIG. 6  shows one example of a steady-state model that is represented as a directed weighted graph. In the diagram of  FIG. 6 , there are six nodes corresponding to a “service OK” state (node  601 ), “application reset” state (node  602 ), “application fail over” state (node  603 ), “node reboot” state (node  604 ), “cluster reboot” state (node  605 ) and “node fail” state (node  606 ). The “service OK” and “node reboot” states indicate that services are available. 
     Directed edges between nodes show relationships between corresponding states. For example, one may go from the “service OK” state (node  601 ) to the “application reset” state (node  602 ) or the “node fail” state (node  606 ). Edge weights represent transition rates between two states. In the example shown in  FIG. 6 , transition rates are represented using following values: an aggregate application failure rate (lambda ar ), aggregate application-reset repair rate (mu ar ), aggregate application-reset efficacy (f ar ), aggregate application fail over repair rate (mu fo ), aggregate application fail over efficacy rate (f fo ), aggregate node reboot repair rate (mu nr ), aggregate node reboot efficacy (f nr ), aggregate cluster reboot repair rate (mu cr ), aggregate node failure rate (lambda nf ) and aggregate node-fail efficacy rate (f nf ). 
     Specifically, in  FIG. 6 , arc  610  represents the aggregate application failure rate (lambda ar ), which is the probability of transition from the “service OK” state (node  601 ) to the “application reset” state (node  602 ). Arc  611  represents the product of the aggregate application-reset repair rate (mu ar ) and the aggregate application-reset efficacy (f ar ), which is the probability of transition from the “application reset” state (node  602 ) back to the “service OK” state (node  601 ). Arc  612  represents the product of mu ar  and (1-f ar ), which is the probability of transition from the “application reset” state (node  602 ) to the “application fail over” state (node  603 ). Arc  613  represents the product of the aggregate application fail over repair rate (mu fo ) and the aggregate node reboot efficacy (f nr ), which is the probability of transition from the “application fail over” state (node  603 ) to the “node reboot” state (node  604 ). Arc  614  represents the product of mu fo  and (1-f fo ), which is the probability of transition from the “application fail over” state (node  603 ) to the “cluster reboot” state (node  605 ). Arc  615  represents the product of the aggregate node reboot repair rate (mu nr ) and (1-f nr ), which is the probability of transition from the “node reboot” state (node  604 ) to the “cluster reboot” state (node  605 ). Arc  616  represents the product of mu nr  and (1-f nr ), which is the probability of transition from the “node fail” state (node  606 ) to the “cluster reboot” state (node  605 ). Arc  617  represents the product of mu nr and f nr , which is the probability of transition from the “node fail” state (node  606 ) to the “service OK” state (node  601 ). Arc  618  represents the aggregate node failure rate (lambda nf ), which is the probability of transition from the “service OK” state (node  601 ) to the “node fail” state (node  606 ). Arc  619  represents the aggregate cluster reboot repair rate (mu cr ), which is the probability of transition from the “cluster reboot” state (node  605 ) to the “service OK” state (node  601 ). Arc  620  represents the product of mu nr  and f nr , which is the probability of transition from the “node reboot” state (node  604 ) to the “service OK” state (node  601 ). 
     The above description of embodiments of the present invention has been given by way of examples. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the embodiments. It is sought, therefore, to cover such changes and modifications as they fall within the spirit and the scope of the invention as defined by the appended claims and their equivalents.