Abstract:
To retrieve a sequence of associated events in log data, a request expression is parsed to retrieve types of dependencies between events which are searched, and the constraints (e.g., keywords) which characterize each event. Based on the parsing results, query components can be formed, expressing the constraints for individual events and interrelations (e.g., time spans) between events. A resultant span query comprising the query components can then be run against an index of events, which encodes a mutual location of associated events in storage.

Description:
TECHNICAL FIELD 
     The subject matter of this application is generally related to computer log management. 
     BACKGROUND 
     Log management is a process of collecting, aggregating, analyzing, alerting on and archiving data from various computer network devices, such as servers, routers, switches, firewalls, etc. Log search is a process of identifying individual log messages associated with events. Log search, as a part of log management, can assist in achieving corporate compliance goals and reduces the risk of legal exposure from security breaches. Log search can also significantly reduce network downtime by helping information technology (IT) departments identify particular problems in a network, and fix those problems by identifying and analyzing log messages potentially associated with those problems. 
     An important aspect of log analysis is the ability to search for associated log messages (e.g., associated pairs or triples of log messages). Conventional search approaches use an index which allows retrieval of a sequence of search items. For example, web search and desktop search provide means to access individual entities, such as documents or database records. These searches, however, are of limited value in log management because groups of associated log messages cannot be retrieved. 
     SUMMARY 
     To retrieve a sequence of associated events in log data, a request expression is parsed to retrieve types of dependencies between events which are searched, and constraints (e.g., keywords) which characterize the events. Based on the parsing results, query components can be formed, expressing constraints for individual events and interrelations (e.g., time spans) between events. A resultant span query comprising query components can be run against an index of events, which encodes a mutual location of associated events in a searchable data structure (e.g., log file storage). 
     The disclosed implementations provide quicker access to log events, allow better means for retrieval of associated messages, and automate the analysis of log data by making the search process more interactive. Relationships between messages (events) can be specified as following in time, belonging to an entity such as a server, being performed by an agent such as a user, and a conjectured causal relationship between events. 
     Other event retrieval scenarios of multi-message search are disclosed. For example, a Which scenario includes a search of all occurrences of intermediate query subjects, followed by a search of final query subjects, which are related to the intermediate query subjects by an association of terms specified in a user query. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of an example log management system. 
         FIG. 2  is a schematic diagram of an example method for forming a query for associated events in log data. 
         FIG. 3  is a schematic diagram of an example method for performing a query for associated events in log data. 
         FIG. 4  is a block diagram of examples of components within sequence of events queries, a multiple restriction query, and a causation query. 
         FIG. 5  is a flow diagram of an example process for searching for associated events in log data using a sequence of events query. 
         FIG. 6  is a flow diagram of an example process for searching for associated events in log data using a multiple restriction query. 
         FIG. 7  is a flow diagram of an example process for searching for associated events in log data using a causation query. 
         FIG. 8  is a schematic diagram of an example computing system that can be used in connection with processes described in reference to  FIGS. 1-7 . 
     
    
    
     DETAILED DESCRIPTION 
     Log Management System Overview 
       FIG. 1  is a block diagram of an example log management system  100 . In some implementations, the log management system  100  includes one or more collectors  102   a  through  102   n , a pre-parser  104 , persistent storage  106 , a parser/mapper  108 , a database  110 , a reporter  112  and a tag system  114 . The tag system  114  further includes a tag indexer  116  and a tag reporter  118 . Such implementations allow one or more users to use the reporter  112  and the tag reporter  118  to generate new reports and customized reports on data collected by the collectors  102   a  through  102   n . The event data (e.g., log messages) for such reports can be collected and processed by the remaining components of the log management system  100 . Hereinafter, the term “tag” and “attribute” are used interchangeably. Tags/attributes are entities that define properties or characteristics of objects or elements. Tags/attributes usually, but not always, consist of a name and value. 
     In some implementations, the collectors  102   a  through  102   n  are operationally coupled with the pre-parser  104 . In some implementations, the various components of the system  100  can be co-located on the same hardware platform, or located on multiple hardware platforms connected by one or more networks (e.g., Ethernet, Internet, intranet). Each of the collectors  102   a  through  102   n  can be any device that collects event data from one or more other computing devices or applications. Such information collection can be active or passive, and can include information such as error messages, user transactions, system events, application milestones, and data access attempts, etc. For example, the collector  102   a  can be a LogLogic LX 500 appliance (manufactured by loglogic, Inc. of San Jose, Calif.), which can be configured to passively collect and analyze information, such as, for example, messages, errors and transactions as they occur on one or more data servers. Other event information collected actively from the same servers may include availability status, current storage capacities, resource usage, etc. In another example, the collector  102   b  can be a LogLogic LX 2000 appliance, which can be configured to actively and passively collect and analyze information from e-mail systems, such as, for example, message/attachment statistics, failed login attempts, password changes, etc. 
     The information collected by collectors  102   a  through  102   n  is transmitted to the pre-parser  104 . Such information can be transmitted via “flat files” produced by sequentially logging events as they occur, or as periodic messages representing snippets of the flat files. In some implementations, transmission of log data can follow one or more standard networking protocols, such as Transmission Control Protocol (TCP), for transmitting data as a stream of bytes. The collectors also append information to each log message which describe the context of the message (e.g. time the log message was received by the collector, the source device where the message originated). 
     The pre-parser  104  receives raw log information, together with the context header generated by the collectors, from the one or more collectors  102   a  through  102   n . During the process, the pre-parser  104  determines the log source type and version, using pre-defined signatures for each log source (e.g. Cisco® Firewall version 7, Oracle® Database version 10). The pre-parser  104  stores this information with the unaltered log information in persistent storage  106  for subsequent use by the log management system  100 . 
     The system  100  uses the persistent storage  106  to store log information received from the pre-parser  104 . In some implementations, the storage method can employ one or more “flat files” containing individual logs messages. For example, a “row” in a flat file containing log messages from an email system may contain a user&#39;s name (e.g., “Matt”), IP address, location, and event occurrence, such as a failed login attempt. Such an entry may include a timestamp for identifying the time (e.g., down to the minute or fraction of a second) that the entry was made. In some implementations, the system  100  may be configured to retain certain logs in the persistent storage  106  for a user-specified period of time. For example, log files containing event data pertinent to Health Insurance Portability and Accountability Act (HIPAA) regulations may be retained for several years. 
     The parser/mapper  108  uses log information contained in the persistent storage  106  to generate tags, or indices, representing the data. The process includes the action of parsing lines of log information stored in the persistent storage  106  while searching for pertinent event data. Such parsing processes may employ pre-defined parsing rules and keyword indices. For example, parsing a log file from an email application may employ parsing rules that specify that the system  100  is to search for user-related event data containing “USER_ID” and the user&#39;s name in brackets. In addition to parsing, the parser/mapper  108  maps the information it locates to the corresponding log file and associated event data in the log file. In such a way, a specific log file&#39;s event data, such as that containing “USER_ID[MATT],” can be mapped. In another example, parsing rules and keywords may exist such that a daily application milestone, for example “CERES END OF DAY,” is extracted from a system log along with a timestamp. Such information can be used to compare (and report over time) regular event occurrences, such as the “END OF DAY” event for an application named “CERES.” 
     In some implementations, the operations performed by the parser/mapper  108  can occur in near real-time as log file entries are available in the persistent storage  106 . In other implementations, the parser/mapper  108  can be scheduled to execute at pre-determined intervals or thresholds, such as those based on elapsed time or log file size. 
     The system  100  can store indices for mapped log information in one or more databases. For example, the system  100  can use the database  110  to store an inverted index representing mapped log information in the persistent storage  106 . In some implementations, entries in the database  110  can be created by the parser/mapper  108 . A user employing the reporter  112  can access the database  110  to aid in executing standard text searches using regular expressions. 
     One possible implementation of the attribute/value generation is done through regular expression rules. First a set of regular expression rules, which constitute the preparser rules, detect the “signature” of different message types. Once the log message type is identified by its signature, a set of rules, which constitute the parser rules, specific to the message types are applied to extract the different attribute/value sets from each message. After the initial extraction of the attribute/value sets, there may be additional rules which add additional attributes to the message. These latter set of rules essentially segment the space of the initial attribute/value set (generated by the regular expression rules) into regions, and label each region with a new attribute/value. We refer to these latter set of rules as mapping or classification rules. 
     An example of the process described above is as follows. First, a signature of the message identifies it as a “typeA” log. Next, rules for a “typeA” log are applied. This step could generate, for example, the attributes: user=uuu, device=ddd, action=aaa and result=rrr. If a user is in set {a, b, c} and an action is in set {a1, a2, a3}, then criticality=ccc. The overall attribute/value set for the message which gets stored will be the union of steps 1, 2 and 3 above, i.e., the message is transformed in the following set of attribute/value pairs: (message type=typeA, user=uuu, device=ddd, action=aaa, result=rrr, criticality=ccc). 
     The process of generating a report using the reporter  112  can begin when a query originating on the reporter  112  is sent to the database  110 . For example, the query&#39;s purpose may be to search for email event data associated with a user named “MATT.” Upon receipt of the query, the database  110  transmits the corresponding indices to the reporter  112 . Using these indices, the reporter  112  requests the corresponding event data from the persistent storage  106 . The reporter  112  receives the corresponding event data, completing the process. In some implementations, the database  110  and the reporter  112  can be implemented using open source database or search technologies, for example MySQL® or Lucene®. Using such technologies can facilitate token-based searches such as “find me all the event data entries that contain the phrase ‘failed login attempt’.” Such searches can utilize Boolean functions, wildcard characters, and other special query tools. However, the predefined schemas and structured reports available by using the database  110  and the reporter  112  alone may limit the complexity and scope of queries performed on the system  100 . In particular, users may want to search on raw log messages with more sophisticated queries, particularly queries that are based upon relationships between attributes of related event data in the log files. Such reporting capabilities can be provided by the tag system  114 , as described below. 
     Tag System Overview 
     The tag system  114  includes the tag indexer  116  and the tag reporter  118 . The tag indexer  116  receives tags, or indices, from the parser/mapper  108  based on the raw log data it processes in the persistent storage  106 . The tag indexer  116  represents particular instances of information in the log files as unordered attribute/value pairs. In some implementations, the tag indexer  116  stores these attribute/value pairs as an inverted log index using a list of pointers to the raw log messages corresponding to the attribute/value. For example, an attribute/value pair may represent the name of a user (e.g., user=Matt) or the result of an action (e.g., action=failed login) on a particular server (e.g., server=email). The tag indexer  116  maintains a count of each attribute/value pair for each time period (e.g., user=Matt: 10, 12, 24 means that user=Matt occurs 10, 12, and 24 times in three consecutive hours). The tag indexer  116 , may also maintain counts of attribute/value tuples, where a tuple is a set of attribute/value pairs that co-occur together. For example one such tuple may represent: user=Matt, action=failed login and server=email. In this case, this tuple represents the number of failed logins by user Matt on the email server for different time periods. In addition to the count representing the user&#39;s number of failed login attempts, the tag indexer  116  can maintain pointers (explained in more detail below) to the corresponding raw entries in the log files. Using the information contained in the tag indexer  116 , the tag reporter  118  can be used to generate reports relating to attributes and values. Such reports can be more sophisticated than the standard token-based reports provided by the reporter  112  because the tag reporter  118  can utilize relationships among two or more related attribute/value pairs, as will be described in more detail below. 
     For example, while the reporter  112  may be used to generate a standard list of email system events, the tag reporter  118  may be used to plot the number of email messages over the last day, and group them by SPAM result. In another example, the tag reporter  118  may be used to generate a report of the highly confidential email messages, grouped by the top n senders. 
     In some implementations, a user interface (not shown) for the log management system  100  may contain separate tabs for the reporter  112  and the tag reporter  118 . The user may further select functionality under each tab corresponding to the user&#39;s report requirements at the time. For example, controls under the tag reporter  118  tab may permit the user to view information such as attribute pairs (described below) that share relationships and can be used to generate reports that exploit such relationships. Other controls under the tag reporter  118  may facilitate the formulation and launching of reports using event data in the tag indexer  116 . 
     Query Formation and Processing Overview 
       FIG. 2  is a schematic diagram of an example method  200  for forming a query for associated events in log data. The method  200  accepts a query string  202  as an input. The query string  202  includes search terms, such as a sequence of keywords. The query string  202  may form a natural language query in English or another language. The query string  202  may produce search results as an ordered set of events (e.g., log messages) or collections of events that satisfy the search terms. Hereinafter, the term “event” and “message” are used interchangeably, as well as “terms” and “keywords” to describe content of messages. 
     In some implementations, events in the log data may be indexed. The index of events is a system for storing events which is optimized for their search, and could be, for example, an inverse index, which stores for every log message term all occurrences of the term in the log messages. To make retrieval efficient, log messages may be split into multiple index systems, such as after a particular time interval elapses (e.g., every hour) or after the index reaches a particular size. Occurrences of a log message in an index can be specified as a pair of integers, the first being an index document identifier (ID) and the second being a message ID within the index document. In one example, individual log messages in the indices may be made searchable by representing each log message as a sequence of terms (e.g., keywords) including start and end terms. An index query may include the terms from the input search query  202  embedded within the start and end terms. 
     Other inputs may include a list of devices  204  at which events may occur and a sort order  206  for the search results. Inputs may, for example, be provided by a user or retrieved from a storage device. 
     The method  200  parses and transforms  208  the query string  202 . The parsing and transforming  208  may include a pre-formatting of the query string  202 . For example, the query string  202  may be changed to lower case letters and separated into words. The query string  202  may be parsed to identify clauses, reserved words, attributes, and values. The parsing and transformation  208  may further include processing special occurrences, such as wild cards and word stems. The query string  202  may be rewritten in a canonical form. The query may be syntactically analyzed to identify and remove clauses that have no effect on the search results. Wildcards in the query may be expanded into disjunctions (e.g., logical “or” statements). For example, in the case of a word stem wildcard such as “play*” a disjunction may include “play or player or playing or played.” 
     The method  200  builds  210  a span query. Boolean operators, phrases, and terms are combined to form the span query. The method  200  may process  212  nested queries. For example, the query may include multiple clauses where one or more clauses are subordinate to another clause. The method  200  groups  214  the span query. The span query may be grouped by required values, prohibited values, and disjunctive values. The method  200  may add  216  clauses to the query for devices. 
     The method  200  generates as its output queries  218 ,  220 , and  222  based on the query string  202 . The method  200  may output pairs of span queries  218  for positive and negative query clauses. The method  200  may output a span query  220  including a SpanNot (e.g., a query having one or more terms with attribute values for inclusion and one or more terms with attribute values for exclusion). The method  200  may output a raw Boolean query  222 , such as a result of parsing the query string  202 . 
       FIG. 3  is a schematic diagram of an example method  300  for performing a query for associated events in log data. In some implementations, the method  300  begins with estimating  302  a number of search results and with performing  304  a Boolean query. The method  300  builds  306  a HitsCollector to receive the results of a span query. The method  300  performs  308  a span query. If there are results from the Boolean query and the span query, and there are results that match, then the method  300  performs  310  a SpanNot query. Otherwise if there are results from both the Boolean query and the span query, and there are no matching results, and a negative component exists, then the method  300  performs  312  a span query for the negative component. The method  300  retrieves  314   a - c  file name and file location pairs for results from the span query, the span query for the negative component, and the SpanNot query. If there are results from the span query for the negative component or the SpanNot query, then the method  300  subtracts  314   c  negative pairs of file names and file locations. The method  300  retrieves  316  the log messages associated with the remaining file name and file location pairs. The method  300  may output results  318   a - c , such as the log messages, a count of the log messages, and user feedback regarding the condition of the search results. 
     Query Templates Overview 
       FIG. 4  is a block diagram of examples of components within sequence of events queries  402  and  404 , a multiple restriction query  406 , and a causation query  408 . The sequence of events queries  402  and  404  include the constraints for each event, such as “user john123”  402   a  and  404   a  and a temporal relations, such as “after”  402   b  and “then”  404   b  and  404   d , followed by the constraints for the following event, such as “denied access”  402   c  and  404   c . The query  402  for two events may be input by a user as, “Did user ‘john123’ successfully logged in to ‘1.2.3.4’ after he was denied access to ‘5.6.7.8’.” Keywords, such as “user john123,” “logged,” “1.2.3.4,” “denied,” “access,” and “5.6.7.8” are identified. Stop words, such as articles, pronouns, and other specified common words are eliminated from the query. The template includes the components for constraint for each event and the component for temporal relation between them. 
     In some implementations more than two events in a sequence of events can be searched. For example, the query  404  input by a user as, “Did user ‘john123’ successfully logged in to ‘1.2.3.4’ then he was denied access to ‘5.6.7.8’ then he attempted login to ‘15.16.17.18’ within 40 seconds” includes components for constraints for individual events  404   a  and  404   c , interchanged with the components for temporal relationships  404   b  and  404   d , and also an overall time span component  404   f.    
     The system processes multiple restriction queries, such as query  406 , using a two-step search. The query  406  includes an intermediate component  406   c , where the search keywords are indicated explicitly (e.g., “logged” and “1.2.3.4”), and a final component  406   a , which includes keywords specified initially (e.g., “devices,” “accessed,” and “users”) and the terms obtained as a result of running the intermediate query  406   c . There is also a separator component  406   b  which includes reserved words, such as the “which” or “who” keywords. 
     The system processes causation queries, such as query  408 , using a multi-step search which identifies common keywords in log messages, which may be causes of the searched events. The query  408  includes a constant component  408   a , which reduces the set of events under consideration. The query  408  includes a variable component  408   c , which specifies the keywords whose correlation with preceding events will be searched for. The query  408  includes a separator component  408   b , which includes reserved words, such as the “why” keyword. 
     Searching for a Sequence of Events Within a Time Span Overview 
       FIG. 5  is a flow diagram showing an example of a process  500  for searching for associated events in log data using a sequence of events query. The process  500  begins with forming  502  a query for individual events. For example, the system  100  may form the query components  402   a  and  402   c , and the query components  404   a ,  404   c , and  404   e.    
     The process  500  forms  504  a query for a sequence of events based on the queries for individual events. The individual queries may implicitly include a time span between a first event (e.g.,  402   c ) and a last event (e.g.,  402   a ) in a sequence of events. Alternatively or in addition, the sequence of events query may include an explicit time span constraint, such as  404   f.    
     The process  500  performs  506  the query for the sequence of events. If there are results in multiple index files ( 508 ), then the process  500  filters ( 510 ) results in the sequence of events to maintain the time span constraint. For example, the system  100  may calculate the time span between index files to maintain the overall time span constraint. The process  500  forms ( 512 ) the set of results as sequences of associated events within a time span. 
     Performing a Multiple Restriction Query Overview 
       FIG. 6  is a flow diagram showing an example of a process  600  for searching for associated events in log data using a multiple restriction query. The process  600  begins with parsing ( 602 ) a query to identify one or more intermediate components and a final component. For example, the system  100  may receive a query string including “Which devices were accessed by users who successfully logged in into ‘1.2.3.4’.” The system  100  parses the query to locate a reserved word, such as “which” or “who.” The system  100  identifies the clause preceding the reserved word  406   b  as the final component  406   a  and the clause after the reserved word  406   b  as the intermediate component  406   c.    
     The process  600  forms ( 604 ) a query for the identified intermediate component. For example, the system  100  may form a query, such as “SELECT user from this table WHERE device=‘1.2.3.4’.” 
     The process  600  performs ( 606 ) the query for the intermediate component to identify keywords to be included in a query for the final component. For example, the system  100  performs the query for the intermediate component to determine the users associated with the device “1.2.3.4.” 
     The process  600  forms ( 608 ) the query for the final component using results of the query for the intermediate component. For example, the system  100  may form a query, such as “SELECT device from this table WHERE user=[results of intermediate query].” 
     The process  600  performs ( 610 ) the query for the final component. For example, the system  100  performs the “SELECT device from this table WHERE user=”query for each of the results determined from the intermediate query. 
     The process  600  merges ( 612 ) the results from the intermediate query and the final queries. For example, the system  100  may build a user interface view that presents an individual event from the intermediate query along with a set of events from the final query associated with the individual event. The values of “user” from the intermediate query form a SpanOr component for the final query. 
     Each query part may have additional constraints. For example, the system  100  may receive a query string, such as “Which devices ‘dst:255.*;service:67;s_port:68;rule:0’ were accessed by users who successfully logged in to ‘1.2.3.4’ product ‘product:VPN-1&amp;Firewall-1’.” The system  100  forms and performs the intermediate query “SELECT user from this table WHERE device=‘1.2.3.4’ AND ‘product:VPN-1&amp;Firewall-1’.” The system  100  forms and performs the final query “SELECT device from this table WHERE device=‘dst:255.*;service:67;s_port:68;rule:0’ AND user=[results of intermediate query].” 
     Performing a Causation Query Overview 
       FIG. 7  is a flow diagram showing an example of a process  700  for searching for associated events in log data using a causation query. The process  700  parses ( 702 ) a query to identify constant and variable components. For example, the system  100  may locate a reserved word, such as “why,” to identify the constant component  408   a  preceding the reserved word and the variable component  408   c  following the reserved word. Alternatively, another reserved word, such as “when,” may be used as well as another rule for locating the constant and variable components. 
     The process  700  forms ( 704 ) a query by merging the constant and variable components. For example, the system  100  may form a query such as “SELECT * from this table where user=‘john234’ and device=‘1.2.3.4’.” 
     The process  700  performs ( 706 ) the merged query to identify events to be explained. For at least one of the identified merged events, the process  700  retrieves ( 708 ) events preceding the identified merged event within a particular time span. For example, the system  100  may perform a query for each of the results from the merged query, such as “SELECT * from his able WHERE time=[time result of merged query]−minutes.” The time span may be received from the user or retrieved from a storage device. 
     The process  700  performs ( 710 ) a query based on the constant component. For example, the system  100  may perform a constant query, such as “SELECT * from this table where device=‘1.2.3.4’.” For at least one of the identified constant events, the process  700  retrieves ( 712 ) events preceding the identified constant event within a particular time span. 
     The process  700  builds ( 714 ) a lattice for the identified merged events. The process  700  builds ( 716 ) a lattice for the identified constant events. A lattice is the set of event intersections between two or more sets of events. An order can be defined on the intersections. The process  700  performs ( 718 ) a set subtraction to determine differences between the merged event intersection lattice and the constant event intersection lattice. The process  700  outputs the list of differences to the user as possible answers to the causation question posed by the user. 
     For example, the query for users who accessed the device “1.2.3.4” may include user “john234” as well as seven other users (user 1, user 2, user 3, user 4, user 5, user 6, and user 7). The users 1, 2, 3, and 4 have sequences of events that ended in granting access to the device “1.2.3.4” and the users 5, 6, 7, and “john234” have sequences of events that ended in denying access. The events include an interrupt event, an exchange event, a connect event, a reset event, and a start event represented hereafter as “i,” “e,” “c,” “r,” and “s,” respectively. The events for each of the users are shown in the following table: 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Events 
                 Result 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 User 1 
                 i 
                 e 
                 c 
                   
                   
                 Granted 
               
               
                   
                 User 2 
                 i 
                 e 
                 c 
                   
                 s 
                 Granted 
               
               
                   
                 User 3 
                 i 
                   
                   
                 r 
                   
                 Granted 
               
               
                   
                 User 4 
                 i 
                 e 
                   
                   
                 s 
                 Granted 
               
               
                   
                 User 5 
                 i 
                 e 
                 c 
                 r 
                   
                 Denied 
               
               
                   
                 User 6 
                 i 
                   
                 c 
                 r 
                   
                 Denied 
               
               
                   
                 User 7 
                   
                   
                 c 
                 r 
                   
                 Denied 
               
               
                   
                 john234 
                   
                   
                 c 
                 r 
                 s 
                 Denied 
               
               
                   
                   
               
             
          
         
       
     
     The intersections of sets of events for users that were granted access to the device “1.2.3.4” includes “i e c,” “i,” “i e,” and “i e s.” The intersections of sets of events for users that were denied access to the device “1.2.3.4” includes “i c r” and “cr.” In this example, there are no sets of events in common between the intersections for granted access and the intersections for denied access. Where one or more sets of events are in common between the results of the intersections, the common sets of events are removed. The remaining sets of events from the intersections for denied access are then compared to the events for the user “john234” to determine the events “c r” as a possible cause of the user “john234” being denied access to the device “1.2.3.4.” 
     In some implementations, the example above may be used to predict a result of a set of events. For example, to determine if the user “john234” is denied access to the device “1.2.3.4” based on the set of events “c r s,” the set of events “c r s” may be compared to the remaining sets of events from intersections for granted access and intersections for denied access. If the set of events “c r s” includes a set of events from the intersections for granted access, then granted access is a possible result of the set of events “c r s.” Otherwise, if the set of events “c r s” includes a set of events from the intersections for denied access, then denied access is a possible result of the set of events “c r s.” 
       FIG. 8  is a schematic diagram showing an example of a generic computer system  800 . The system  800  can be used for the operations described in association with any of the computer-implement methods described previously, according to one implementation. The system  800  includes a processor  810 , a memory  820 , a storage device  830 , and an input/output device  840 . Each of the components  810 ,  820 ,  830 , and  840  are interconnected using a system bus  850 . The processor  810  is capable of processing instructions for execution within the system  800 . In one implementation, the processor  810  is a single-threaded processor. In another implementation, the processor  810  is a multi-threaded processor. The processor  810  is capable of processing instructions stored in the memory  820  or on the storage device  830  to display graphical information for a user interface on the input/output device  840 . 
     The memory  820  stores information within the system  800 . In one implementation, the memory  820  is a computer-readable medium. In one implementation, the memory  820  is a volatile memory unit. In another implementation, the memory  820  is a non-volatile memory unit. 
     The storage device  830  is capable of providing mass storage for the system  800 . In one implementation, the storage device  830  is a computer-readable medium. In various different implementations, the storage device  830  may be a floppy disk device, a hard disk device, an optical disk device, or a tape device. 
     The input/output device  840  provides input/output operations for the system  800 . In one implementation, the input/output device  840  includes a keyboard and/or pointing device. In another implementation, the input/output device  840  includes a display unit for displaying graphical user interfaces. 
     The features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The features can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device or in a propagated signal, for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations by operating on input data and generating output. The described features can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. 
     Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Generally, a computer will also include, or be operatively coupled to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     To provide for interaction with a user, the features can be implemented on a computer having a display device such as a CRT (cathode ray tube) or LCD (liquid crystal display) monitor for displaying information to the user and a keyboard and a pointing device such as a mouse or a trackball by which the user can provide input to the computer. 
     The features can be implemented in a computer system that includes a back-end component, such as a data server, or that includes a middleware component, such as an application server or an Internet server, or that includes a front-end component, such as a client computer having a graphical user interface or an Internet browser, or any combination of them. The components of the system can be connected by any form or medium of digital data communication such as a communication network. Examples of communication networks include, e.g., a LAN, a WAN, and the computers and networks forming the Internet. 
     The computer system can include clients and servers. A client and server are generally remote from each other and typically interact through a network, such as the described one. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. 
     Although a few implementations have been described in detail above, other modifications are possible. In addition, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the following claims. Accordingly, other implementations are within the scope of the following claims.