Expiration of Persistent Data Structures That Satisfy Search Queries

[0001] The present application is a continuation of U.S. patent application Ser. No. 14/929,248, filed on Oct. 30, 2015; which is a continuation of Ser. No. 14/611,170, filed on Jan. 30, 2015; which is a continuation of U.S. patent application Ser. No. 13/353,135, filed on Jan. 18, 2012, issued as U.S. Pat. No. 9,002,854 on Apr. 7, 2015; which is a continuation of U.S. patent application Ser. No. 11/868,370, filed Oct. 5, 2007, issued as U.S. Pat. No. 8,112,425 on Feb. 7, 2012; which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/828,283, filed Oct. 5, 2006, the entire contents of the aforementioned are hereby incorporated by reference as if fully set forth herein, under 35 U.S.C. §120. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application(s) or the prosecution history thereof and advise the USPTO that the claims in this application may be broader than any claim in the parent application(s).

[0002] 1. Field of the Invention

[0003] This invention relates generally to information organization, search, and retrieval and more particularly to time series data organization, search, and retrieval.

[0004] 2. Description of the Related Art

[0005] Time series data are sequences of time stamped records occurring in one or more usually continuous streams, representing some type of activity made up of discrete events. Examples include information processing logs, market transactions, and sensor data from real-time monitors (supply chains, military operation networks, or security systems). The ability to index, search, and present relevant search results is important to understanding and working with systems emitting large quantities of time series data.

[0006] Existing large scale search engines (e.g., Google and Yahoo web search) are designed to address the needs of less time sensitive types of data and are built on the assumption that only one state of the data needs to be stored in the index repository, for example, URLs in a Web search index, records in a customer database, or documents as part of a file system. Searches for information generally retrieve only a single copy of information based on keyword search terms: a collection of URLs from a Website indexed a few days ago, customer records from close of business yesterday, or a specific version of a document.

[0007] In contrast, consider an example of time series data from a typical information processing environment, shown in FIG. 1. Firewalls, routers, web servers, application servers and databases constantly generate streams of data in the form of events occurring perhaps hundreds or thousands of times per second. Here, historical data value and the patterns of data behavior over time are generally as important as current data values. Existing search solutions generally have little notion of time-based indexing, searching or relevancy in the presentation of results and don't meet the needs of time series data.

[0008] Compared to full text search engines, which organize their indices so that retrieving documents with the highest relevance scores is most efficient, an engine for searching time series data preferably would organize the index so that access to various time ranges, including less recent time ranges, is efficient. For example, unlike for many modem search engines, there may be significantly less benefit for a time series search engine to cache the top 1000 results for a particular keyword.

[0009] On the other hand, given the repetitive nature of time series data, opportunities for efficiency of index construction and search optimization are available. However, indexing time series data is further complicated because the data can be collected from multiple, different sources asynchronously and out of order. Streams of data from one source may be seconds old and data from another source may be interleaved with other sources or may be days, weeks, or months older than other sources. Moreover, data source times may not be in sync with each other, requiring adjustments in time offsets post indexing. Furthermore, time stamps can have an almost unlimited number of formats making identification and interpretation difficult. Time stamps within the data can be hard to locate, with no standard for location, format, or temporal granularity (e.g., day, hour, minute, second, sub-second).

[0010] Searching time series data typically involves the ability to restrict search results efficiently to specified time windows and other time-based metadata such as frequency, distribution of inter-arrival time, and total number of occurrences or class of result. Keyword-based searching is generally secondary in importance but can be powerful when combined with time-based search mechanisms. Searching time series data requires a whole new way to express searches. Search engines today allow users to search by the most frequently occurring terms or keywords within the data and generally have little notion of time based searching. Given the large volume and repetitive characteristics of time series data, users often need to start by narrowing the set of potential search results using time-based search mechanisms and then, through examination of the results, choose one or more keywords to add to their search parameters. Timeframes and time-based metadata like frequency, distribution, and likelihood of occurrence are especially important when searching time series data, but difficult to achieve with current search engine approaches. Try to find, for example, all stories referring to the “Space Shuttle” between the hours of 10 AM and 11 AM on May 10, 2005 or the average number of “Space Shuttle” stories per hour the same day with a Web-based search engine of news sites. With a focus on when data happens, time-based search mechanisms and queries can be useful for searching time series data.

[0011] Some existing limited applications of time-based search exist in specific small-scale domains. For example, e-mail search is available today in many mainstream email programs and web-based email services. However, searches are limited to simple time functions like before, after, or time ranges; the data sets are generally small scale and highly structured from a single domain; and the real-time indexing mechanisms are append only, usually requiring the rebuilding of the entire index to interleave new data.

[0012] Also unique to the cyclicality of time series data is the challenge of presenting useful results. Traditional search engines typically present results ranked by popularity and commonality. Contrary to this, for time series data, the ability to focus on data patterns and infrequently occurring, or unusual results may be important. To be useful, time series search results preferably would have the ability to be organized and presented by time-based patterns and behaviors. Users need the ability to see results at multiple levels of granularity (e.g., seconds, minutes, hours, days) and distribution (e.g., unexpected or least frequently occurring) and to view summary information reflecting patterns and behaviors across the result set. Existing search engines, on the other hand, generally return text results sorted by key word density, usage statistics, or links to or from documents and Web pages in attempts to display the most popular results first.

[0013] In one class of time series search engine, it would be desirable for the engine to index and allow for the searching of data in real-time. Any delay between the time data is collected and the time it is available to be searched is to be minimized. Enabling real-time operation against large, frequently changing data sets can be difficult with traditional large-scale search engines that optimize for small search response times at the expense of rapid data availability. For example, Web and document search engines typically start with a seed and crawl to collect data until a certain amount of time elapses or a collection size is reached. A snapshot of the collection is saved and an index is built, optimized, and stored. Frequently accessed indices are then loaded into a caching mechanism to optimize search response time. This process can take hours or even days to complete depending on the size of the data set and density of the index. Contrast this with a real-time time series indexing mechanism designed to minimize the time between when data is collected and when the data is available to be searched. The ability to insert, delete and reorganize indices, on the fly as data is collected, without rebuilding the index structure is essential to indexing time series data and providing real-time search results for this class of time series search engines.

[0014] Other software that is focused on time series, e.g., log event analyzers such as Sawmill or Google's Sawzall can provide real-time analysis capabilities but are not search engines per se because they do not provide for ad hoc searches. Reports must be defined and built in advance of any analysis. Additionally, no general keyword-based or time-based search mechanisms are available. Other streaming data research projects (including the Stanford Streams project and products from companies like StreamBase Systems) can also produce analysis and alerting of streaming data but do not provide any persistence of data, indexing, time-based, or keyword-based searching.

[0015] There exists, therefore, a need to develop other techniques for indexing, searching and presenting search results from time series data.

[0016] Methods and apparatus consistent with the invention address these and other needs by allowing for the indexing, searching, and retrieval of time series data using a time series search engine (TSSE). In one implementation, one aspect of TSSEs is the use of time as a primary mechanism for indexing, searching, and/or presentation of search results. A time series search language (TSSL) specific to time-based search mechanisms is used to express searches in human readable form and results are presented using relevancy algorithms specific to time series data. Search expression and results presentation are based on key concepts important to searching time series data including but not limited to time windows, frequency, distribution, patterns of occurrences, and related time series data points from multiple, disparate sources.

[0017] In one aspect of the invention, multiple sources of time series data are organized and indexed for searching and results are presented upon user or machine initiated searches. In another aspect, a time series search engine (TSSE) includes four parts: (1) a time stamp process; (2) an indexing process; (3) a search process; and (4) a results presentation process.

[0018] In one aspect of the invention, a computer-implemented method for time searching data includes the following steps. Time series data streams are received. One example of time series data streams includes server logs and other types of machine data (i.e., data generated by machines). The time series data streams are time stamped to create time stamped events. The time stamped events are time indexed to create time bucketed indices, which are used to fulfill search requests. Time series search request are executed, at least in part, by searching the time bucketed indices.

[0019] In certain implementations, time stamping the time series data streams includes aggregating the time series data streams into events and time stamping the events. For example, the events may be classified by domain and then time stamped according to their domain. In one approach, for events that are classified in a domain with a known time stamp format, the time stamp is extracted from the event. However, for events that are not classified in a domain with a known time stamp format, the time stamp is interpolated.

[0020] In another aspect of the invention, time bucketed indices are created by assigning the time stamped events to time buckets according to their time stamps. Different bucket policies can be used. For example, the time buckets may all have the same time duration, or may have different time durations. In addition, time buckets may be instantiated using a lazy allocation policy. The time stamped events may also be segmented, and the segments used to determine time bucket indices. Various forms of indexing, including hot indexing, warm indexing and speculative indexing, may also be used.

[0021] The creation of time bucket indices facilitates the execution of time series searches. In one approach, a time series search request is divided into different sub-searches for the affected time buckets, with each sub-search executed across the corresponding time bucket index.

[0022] Other aspects of the invention include software, computer systems and other devices corresponding to the methods described above, and applications for all of the foregoing.

[0031] The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.

[0032] FIG. 1 illustrates different examples of time series data environments with potentially large numbers of data sources and streams of time series data across multiple domains. In this figure, the first picture represents an information-processing environment with time series data from web servers, application servers, and databases in the form of server logs. The second picture is a typical market-trading environment with transactions between multiple buyers and sellers and between two or more markets. Time series data is generated in the form of transaction records representing the intention or trade or the final settlement of the trade as examples. In the third picture, a real-time monitoring environment is depicted with multiple sensors producing time series data in the form of recorded measurements. All three of these environments are examples of potential applications for the TSSE.

[0033] Aspects of the invention will be described with respect to the first picture in FIG. 1, the information-processing environment, but the invention can also be used with other time series data environments and applications including the other environments shown in FIG. 1.

[0034] FIG. 2 illustrates one approach 200 to architecting a TSSE. Time series data streams 205 arrive synchronously or asynchronously from multiple sources, multiple searches 255 are expressed by users and/or other systems, and results sets 275 are presented through a variety of mechanisms including, for example, application programming interfaces and web-based user interfaces.

[0035] The arrival of time series data streams 205 at the TSSE 200 can be effected by having the TSSE gather them directly or by having a user-supplied script collect, preprocess, and deliver them to a default TSSE collection point. This architecture preferably tolerates data arriving late and temporally out of order. Currently, most sources of time series data are not designed for sophisticated processing of the data, so the TSSE typically will collect or be fed raw time series data that are close to their native form. The TSSE can be situated in different locations so long as it has access to the time series data. For example, one copy of the TSSE can be run on a single central computer or multiple copies can be configured in a peer-to-peer set-up with each copy working on the same time series data streams or different time series data streams.

[0036] FIG. 2 depicts an example TSSE 200 with four major processes: time stamp process 210, index process 220, search process 230 and presentation process 240. The time stamp process 210 turns raw time series data 205 into time stamped events 215 to be fed to the indexing process 220. Following our information processing example, raw logs 205 from multiple web servers, application servers and databases might be processed by the time stamp process 210 to identify individual events 215 within the various log formats and properly extract time and other event data. The event data 215 is used by the index process 220 to build time bucketed indices 225 of the events. These indices 225 are utilized by the search process 230 which takes searches 255 from users or systems, decomposes the searches, and then executes a search across a set of indices.

[0037] For example, a user might want to locate all the events from a particular web server and a particular application server occurring within the last hour and which contain a specific IP address. In addition, the search process 230 may choose to initiate the creation of meta events 237 at search time to handle time-based and statistical summary indices useful in searching through repetitive, temporal data. For example, meta events 237 may represent averages, means, or counts of actual events or more sophisticated pattern based behavior. In this case a user might want to search to find all the events occurring with a frequency of three per minute.

[0038] Upon completion, the search process 230 hands results from the selected indices 235 to the presentation process 240 which merges result sets, ranks results, and feeds the results 275 to an API or user interface for presentation.

[0039] Process 210 shown in FIG. 2 of an exemplary implementation 200 of a TSSE is to acquire streaming time series data, identify individual events within the stream, and assign time stamps to each event. An example time stamp process 210 block diagram is shown in FIG. 3 and includes several steps including event aggregation 310, domain identification 320, time extraction 330, and time interpolation 340. Time series data streams 205 are received as input to the time stamp process 210 and then processed into individual time stamped events 215.

[0040] Step 310 in the time stamp process 210 of FIG. 3 aggregates the streaming time series data 205 into individual events 315. In our information-processing example, web server time series data streams may have a single line per event and be easy to identify. However, an application server time series data stream may contain single events with a large number of lines making identification of individual events within the stream difficult.

[0041] In one implementation, event aggregation 310 uses feature extraction (e.g., leading punctuation, significant words, white space, and breaking characters) and machine learning algorithms to determine where the event boundaries are. FIG. 4 is a diagram of an event aggregation process suitable for use with the time stamp process of FIG. 3.

Source Identification—Classification into Domains

[0042] Given the repetitive, yet dynamic, nature of the time series data 205 in our information processing example (which data will be referred to as machine data 205 or MD 205), an effective aggregation process 310 (such as shown in FIG. 4) preferably will learn about data formats and structure automatically. In one implementation, learning is separated into different domains based on the source of MD 205. Domains can be general system types, such as log files, message bus traffic, and network management data, or specific types, such as output of a given application or technology—Sendmail logging data, Oracle database audit data, and J2EE messaging.

[0043] In this example event aggregation process 310, the domain for a given source of MD is identified 415 so that domain specific organization methods can be applied. Domains are determined through a learning process. The learning process uses collections of MD from well-known domains as input and creates a source signature 412 for each domain. In one implementation, source signatures 412 are generated from representative samples of MD 205 by creating a hash table mapping punctuation characters to their frequency. While tokens and token values can change in MD collection, in this particular implementation, the signature 412 generated by the frequency of punctuation is quite stable, and reliable within a specific domain. Other implementations could use functions of the punctuation and tokens, such as the frequencies of the first punctuation character on a line, or the first capitalized term on a line. Given that source signatures 412 can be large and hard to read, signatures can have a corresponding label in the form of a number or text that can be machine generated or human assigned. For example, the source signature 412 for an Apache web server log might be programmatically assigned the label “205”, or a user can assign the label “Apache Server Log”.

[0044] In one embodiment, clustering is used to classify 415 collected MD 205 into domains according to their source signatures 412. As collections of MD 205 are encountered, each collection's signature is matched to the set of known source signatures 412 by performing a nearest-neighbor search. If the distance of the closest matching signature 412 is within a threshold, the closest matching signature 420's domain is assumed to be the domain of the source. If no best match can be found, a new source signature 412 can be created from the sample signature and a new source domain created. Alternatively, a default source domain can be used. In one implementation, the distance between two signatures is calculated by iterating over the union of attributes of the two signatures, with the total signature distance being the average of distances for each attribute. For each attribute A, the value of A on Signature I and Signature2, VI and V2, are compared and a distance is calculated. The distance for attribute A is the square of (VI−V2)*IDF, where IDF is the log(N I IAI), where N is the number of signatures, and IAI is the number of signatures with attribute A.

[0045] Some MD 205 sources are non-textual or binary and cannot be easily processed unless a known process is available to convert the binary MD into textual form. To classify a source as textual or binary, a sample MD collection is analyzed. Textual MD can also have embedded binary MD, such as a memory dump, and the classification preferably identifies it as such. In one implementation, the textual/binary classification works as follows. The sample is a set of lines of data, where a line is defined as the data between new lines (i.e., ‘\n’), carriage returns (i.e., ‘\r’), or their combination (i.e., ‘\r\n’). For each line, if the line's length is larger than some large threshold, such as 2k characters, or if the line contains a character with an ASCII value of zero (0), a count of Binary-looking lines is incremented. Otherwise, if the line's length is shorter than a length that one would expect most text lines to be below, such as 256 characters, a count of Text-looking lines is incremented. If the number of Text-looking lines is twice as numerous as the Binary-looking lines (other ratios can be used depending on the context), the source is classified as text. Otherwise, the source is classified as binary.

Aggregation of Machine Data into Raw Events

[0046] When the source signature 420 for a collection of MD has been identified 415, the corresponding aggregation rules are applied 425 to the MD collection. Aggregation rules describe the manner in which MD 205, from a particular domain, is organized 425 into event data 315 by identifying the boundaries of events within a collection of MD, for example, how to locate a discrete event by finding its beginning and ending. In one implementation, the method of aggregation 425 learns, without prior knowledge, by grouping together multiple lines from a sample of MD 205. Often MD 205 contains events 315 that are anywhere from one to hundreds of lines long that are somehow logically grouped together.

[0047] The MD collection may be known a priori, or may be classified, as single-line type (i.e., containing only single-line events) or multi-line type (i.e., possibly containing multi-line events) prior to performing aggregation. For those MD collections that are classified as single line type, aggregation 425 is simple—single-line type MD collections are broken on each line as a separate event. Multi-line type MD collections are processed 425 for aggregation. In one implementation, a MD collection is classified as a multi-line type if 1) there is a large percentage of lines that start with spaces or are blank (e.g., if more than 5% of the lines start with spaces or are blank), or 2) there are too many varieties of punctuation characters in the first N punctuation characters. For example, if the set of the first three punctuation characters found on each line has more than five patterns (e.g., ‘:::’, ‘!:!’, ‘,,,’, ‘:..’, ‘( )*’), the collection might be classified as multi-line.

[0048] Another aspect of aggregation methods 425 is the ability to learn, and codify into rules, what constitutes a break between lines and therefore the boundary between events, by analyzing a sample of MD. For example, in one implementation, an aggregation method 425 compares every two-line pair looking for statistically similar structures (e.g., use of white space, indentation, and time-stamps) to quickly learn which two belong together and which two are independent. In one implementation, aggregation 425 works as follows. For each line, first check if the line starts with a time-stamp. If so, then break. Typically, lines starting with a time-stamp are the start of a new event. For lines that do not start with a time-stamp, combine the current line with the prior line to see how often the pair of lines occurs, one before the other, as a percentage of total pairs in the MD sample. Line signatures are used in place of lines, where a line signature is a more stable version of a line, immune to simple numeric and textual changes. In this implementation, signatures can be created by converting a line into a string that is the concatenation of leading white space, any punctuation on the line, and the first word on the line. The line “10:29:03 Host 191.168.0.1 rebooting:normally” is converted to “::..:Host.”

[0049] Now this current line signature can be concatenated with the previous line signature (i.e., signature1 combined with signature2) and used as a combined key into a table of break rules. The break rule table maps the combined key to a break rule, which determines whether there should be a ‘break’, or not, between the two lines (i.e., whether they are part of different events or not). Break rules can have confidence levels, and a more confident rule can override a less confident rule. Break rules can be created automatically by analyzing the co-occurrence data of the two lines and what percent of the time their signatures occur adjacently. If the two line signatures highly co-occur, a new rule would recommend no break between them. Alternatively, if they rarely co-occur, a new rule would recommend a break between them. For example, if line signature A is followed by line signature B greater than 20% of the time A is seen, then a break rule might be created to recommend no break between them. Rules can also be created based on the raw number of line signatures that follow/proceed another line signature. For example, if a line signature is followed by say, ten different line signatures, create a rule that recommends a break between them. If there is no break rule in the break rule table, the default behavior is to break and assume the two lines are from different events. Processing proceeds by processing each two-line pair, updating line signature and co-occurrence statistics, and applying and learning corresponding break rules. At regular intervals, the break rule table is written out to the hard disk or permanent storage.

[0050] Once the incoming time series stream 205 has been aggregated 310 into individual events 315, the events and their event data are input into a time stamp identification step 320 which determines whether or not the time series event data contains tokens that indicate a match to one of a collection of known time stamp formats. If so, the event is considered to have a time stamp from a known domain and extraction 330 is performed. Otherwise, interpolation 340 is performed.

[0051] If a known domain has been identified for an event, the event 315 is taken as input to a time stamp extraction step 330 where the time stamp from the raw event data is extracted and passed with the event to the indexing process 220. In an exemplary implementation, this timestamp extraction 330 occurs by iterating over potential time stamp format patterns from a dynamically ordered list in order to extract a time to be recorded as the number of seconds that have passed since the Unix epoch (0 seconds, 0 minutes, 0 hour, Jan. 1, 1970 coordinated universal time) not including leap seconds. Additionally, the implementation takes into account time zone information and normalizes the times to a common offset. To increase performance, the ordering of this list is determined using a move-to-front algorithm, wherein whenever a match is found the matching pattern is moved to the beginning of the list. In such an implementation, the most frequently occurring patterns are checked earliest and most often, improving performance. The move-to-front lists may be maintained either for all time series data sources together, on a per-source basis (to take advantage of the fact that the formats in a single source are likely to be similar), or in some other arrangement.

[0052] In the case where the event did not contain a time stamp from a known domain, then a timestamp is assigned to the event based on its context. In one implementation, the time stamp is linearly interpolated 340 from the time stamps of the immediately preceding and immediately following events 315 from the same time series data stream. If these events also contain no time stamps from a known domain, further earlier and/or later events can be used for the interpolation. The time stamp extraction module 330 automatically stores the time stamp of every hundredth event (or some other configurable period) from each time series data stream in order to facilitate time stamp interpolation 340. In another implementation, time stamps are interpolated 340 based on the time associated with the entire time series data stream 205 including acquisition time, creation time or other contextual meta time data.

[0053] Returning to FIG. 2, in the indexing process 220, indexes are created based on incoming event data 215. The indexing process 220 organizes and optimizes the set of indices in an online fashion as they are extended with more events. An example TSSE indexing process 220 is shown in FIG. 5 and includes, in one implementation, several steps including bucketing 510, segmenting 520, archival 530, allocation 540, insertion 550, committing to secondary storage 560, merging buckets in secondary storage 570, and expiring buckets in secondary storage 580.

[0054] Events indexed by the TSSE are often queried, updated, and expired using time-based operators. By hashing the components of the index over a set of buckets organized by time, the efficiency and performance of these operators can be significantly improved. The final efficiency of the bucketing will, of course, depend on the hardware configuration, the order in which the events arrive, and how they are queried, so there is not a single perfect bucketing policy.

[0055] In one implementation, buckets with a uniform extent are used. For example, each time bucket can handle one hour's worth of data. Alternate policies might vary the bucket extents from one time period to another. For example, a bucketing policy may specify that the buckets for events from earlier than today are three hour buckets, but that the buckets for events occurring during the last 24 hours are hashed by the hour. In the information processing example, a bucket might cover the period Jan. 15, 2005 12:00:00 to Jan. 15, 2005 14:59:59. In order to improve efficiency further, buckets are instantiated using a lazy allocation policy (i.e., as late as possible) in primary memory (i.e., RAM). In-memory buckets have a maximum capacity and, when they reach their limit, they will be committed to disk and replaced by a new bucket. Bucket storage size is another element of the bucketing policy and varies along with the size of the temporal extent. Finally, bucket policies typically enforce that buckets (a) do not overlap, and (b) cover all possible incoming time stamps.

[0056] Step 510 in indexing an event by time is to identify the appropriate bucket for the event based on the event's time stamp and the index's bucketing policy. Each incoming event 215 is assigned 510 to the time bucket where the time stamp from the event matches the bucket's temporal criteria. In one implementation, we use half-open intervals, defined by a start time and an end time where the start time is an inclusive boundary and the end time is an exclusive boundary. We do this so that events occurring on bucket boundaries are uniquely assigned to a bucket. Following our example in the information processing environment, a database server event with the time stamp of Jan. 15, 2005 12:00:01 might be assigned to the above-mentioned bucket.

[0057] Once an appropriate bucket has been identified 510 for an event, the raw event data is segmented 520. A segment (also known as a token) is a substring of the incoming event text and a segmentation 520 is the collection of segments implied by the segmentation algorithm on the incoming event data. A segment sub string may overlap another substring, but if it does, it must be contained entirely within that substring. We allow this property to apply recursively to the containing substring, so that the segment hierarchy forms a tree on the incoming text.

[0058] In one implementation, segmentation 520 is performed by choosing two mutually exclusive sets of characters called minor breakers and major breakers. Whenever a breaking character, minor or major, is encountered during segmentation of the raw data, segments are emitted corresponding to any sequence of bytes that has at least one major breaker on one end of the sequence. For example, if, during segmentation, a minor breaking character is found, then a segment corresponding to the sequence of characters leading from the currently encountered minor breaker back to the last major breaker encountered is recorded. If a major breaker was encountered, then the sequence of characters leading back to either the last major breaker or the last minor breaker, whichever occurred most recently, determines the next segment to be recorded.

[0059] Segmentation 520 rules describe how to divide event data into segments 525 (also known as tokens). In one implementation a segmentation rule examines possible separators or punctuation within the event, for example, commas, spaces or semicolons. An important aspect of segmentation is the ability to not only identify individual segments 525, but also to identify overlapping segments. For example, the text of an email address, “[email protected]”, can be broken into individual and overlapping segments; <bob.smith>, <@> and <corp.com> can be identified as individual segments, and <<bob.smith><@><corp.com>> can also be identified as an overlapping segment. As described above, in one implementation, segmentation 520 uses a two-tier system of major and minor breaks. Major breaks are separators or punctuation that bound the outer most segment 525. Examples include spaces, tabs, and new lines. Minor breaks are separators or punctuation that break larger segments into sub segments, for example periods, commas, and equal signs. In one implementation, more complex separators and punctuation combinations are used to handle complex segmentation tasks 520, for example handling Java exceptions in an application server log file.

[0060] An example of segmentation in our information-processing example, IP addresses could be broken down using white space as major breakers and periods as minor breakers. Thus, the segments for the raw text “192.168.1.1” could be:

[0061] “192”

[0062] “192.168”

[0063] “192.168.1”

[0064] “192.168.1.1”

[0065] In another implementation, certain segments may represent known entities that can be labeled and further understood algorithmically or by human added semantics. For example, in the above representation, “192.168.1.1” may be understood to be an IP address. Named entity extraction can be algorithmically performed in a number of ways. In one implementation, the segment values or segment form from the same segment across multiple events is compared to an entity dictionary of known values or known forms.

[0066] In another implementation, entity extraction techniques are used to identify semantic entities within the data. In one implementation, search trees or regular expressions can be applied to extract and validate, for example, IP addresses or email addresses. The goal of extraction is to assist the segmentation process 520 and provide semantic value to the data.

[0067] At this point in the process, incoming events have time stamps 215, segments 525, and a time bucket 515 associated with them. To create the persistent data structures that will be used later to perform lookups in the search process, we store the raw data of the event with its segmentation, create indices that map segments and time stamps to offsets in the event data store, and compute and store metadata related to the indices.

[0068] Because the TSSE tolerates, in near real time, both the arrival of new events and new searches, the system preferably is careful in managing access to disk. For the indexes, this is accomplished by splitting index creation into two separate phases: hot indexing and warm indexing. Hot indexes are managed entirely in RAM, are optimized for the smallest possible insert time, are not searchable, and do not persist. “Warm” indexes are searchable and persistent, but immutable. When hot indexes need to be made searchable or need to be persistent, they are converted into warm indexes.

[0069] In the implementation shown in FIG. 5, a hot index 555 contains a packed array of segments, a packed array of event addresses and their associated time stamps, and a postings list that associates segments with their time stamped event addresses. For performance reasons, the packed arrays can have hash tables associated with them to provide for quick removal of duplicates. When incoming events are being indexed, each segment of the event is tested for duplication using the segment array and its associated hash. The event address is also tested for duplication, against the event address array and its associated hash. If either of the attributes is a duplicate, then the instance of that duplicate that has already been inserted into the packed array is used. Otherwise, the new segment or event address is copied into the appropriate table 550 and the associated hash table is updated. As events are inserted into the hot index, the space associated with each of the packed arrays gets used. A hot slice is considered to be “at capacity” when one of its packed arrays fills up or when one of its hash tables exceeds a usage threshold (e.g., if more than half of the hash table is in use). Once a hot index reaches capacity 540, it cannot accept more segments for indexing. Instead it is converted to a warm index, committed to disk 560, and replaced with a new empty hot index.

[0070] Another feature of this particular system is speculative indexing. Based on earlier indexing processes, new time buckets can be initialized using all or part of a representative, completed bucket as an exemplar. In other words, by keeping around copies of data that may reasonably be expected to occur in a time bucket, we can improve indexing performance by speculatively initializing parts of the hot index. In one embodiment, the speculative indexing is performed by copying the packed array of segments and its associated hash table from an earlier hot index. The hot slice is then populated as usual with the exception that the segment array is already populated and ready for duplicate testing. Because of the highly regular language and limited vocabulary of machines, the hit rate associated with this speculation can be very good.

[0071] The searching process (as described in the next section) allows the user to search on segments, segment prefixes, and segment suffixes. To accommodate these search types, in one implementation, the segments array can be sorted and then stored as a blocked front coded lexicon (hereafter called “the forward lexicon”). This data structure makes it possible to perform segment and segment prefix lookups efficiently while still achieving a reasonable amount of compression of the segment text. When a search is being performed on a particular segment, the offset of the segment in the forward lexicon is used as an efficient way to look up metadata associated with the queried-for segment in other associated tables.

[0072] To handle suffix lookups, a blocked front coded lexicon can be created on the same collection of segments after they have been string-reversed (hereafter called “the reverse lexicon”). Also, a map is populated that converts the offset of a reversed segment in the reverse lexicon to the equivalent non-reversed segment's offset in the forward lexicon (hereafter called “the reverse-forward map”). When performing suffix lookups, the offset in the reverse lexicon is used as an offset into the reverse-forward map. The value stored at that position in the map is the appropriate offset to use for the other metadata arrays in the warm index.

[0073] The warm index provides a list of event offsets for each segment indexed, preferably in an efficient manner. In one implementation, this can be done by maintaining an array of compressed postings lists and an associated array of offsets to the beginning of each of those compressed postings lists. The postings lists are maintained in segment offset order, so when a lookup is performed, the segment ID can be used to find the appropriate entry of the postings lists offsets array. The values in the postings lists entries are the offsets that should be used to look up events in the packed array of event addresses.

[0074] Finally, statistical metadata can be provided for each indexed segment (e.g., the first and last time of occurrence of the segment, the mean inter-arrival time, and the standard deviation of the inter-arrival time).

[0075] During the course of the indexing process, it is possible that a single time bucket will be filled and committed to disk 560 several times. This will result in multiple, independently searchable indices in secondary storage for a single time span. In an exemplary implementation, there is a merging process 570 that takes as input two or more warm indices and merges them into a single warm index for that time bucket. This is a performance optimization and is not strictly required for searching.

[0076] Furthermore, over a long period of time, it is possible that applying the indexing process 220 to time series data will cause a large amount of persistent data to accumulate. The indexing process, therefore, preferably contains an expiration process 580 that monitors the database for time buckets to be deleted based on user-provided preferences. In one implementation, these preferences might include a trailing time window (“events older than 3 months need not be returned in search results”), a time range (“events earlier than January 1 of this year need not be returned in search results”), a maximum number of events (“no more than 1 million events need be returned in search results”), or a maximum total size for the index (“return as many useful search results as possible while consuming no more than 100 GB of Disk”). A process periodically wakes up and tests the collection of warm slices for any slices that meet the expiration criterion. Upon expiration, a warm index file and its associated raw event data and segmentation is moved out of the active index. The index file need not necessarily be deleted. In one implementation, the index file could be streamed to less expensive offline storage.

[0077] An example TSSE search process is shown in FIG. 6 and includes several methods for parsing 610 a search phrase, issuing multiple sub-searches 625 in order to satisfy the incoming parent search, using sub-search results 635 to prune searches, and merging 640 sub-search results into a final set of search results for presentation to the user.

[0078] During search processing, incoming search phrases 255 are parsed 610 according to a time series search language (TSSL) in order to generate annotated parse trees 615. An exemplary TSSL language syntax includes a series of modifiers or commands taking the format name::value. Some modifiers may have default values and some can only be used once, while some can appear several times in the same search with different values. Examples include the following:

• • [0079] average::value—calculate the average number of events using the value time frame.
  • [0080] page::value—present search results by value. Value can be seconds, minutes, hours, days, weeks or months or any other metadata element, for example, source or event type.
  • [0081] count::—calculate the total number of for events.
  • [0082] daysago::value—search for events within the last value days.
  • [0083] index::value—the index to search-main, default, history, or another index defined by the TSSE.
  • [0084] hoursago::value—search for events within the last value hours. eventtype::value—search for events with an event type or tag that matches the specified value.
  • [0085] host::value—search for events whose hostname was set to the specified value. This is the host that logged the event, not necessarily the host that generated the event.
  • [0086] maxresults::value—the maximum number ofresults to return. minutesago::value—search for events within the last value minutes.
  • [0087] related::value—search for events with segment values (e.g., 404 or username) matching one or more in the current event.
  • [0088] similar::value—search for events with a similar event type to the current event. sourcetype::value—search for events with a given sourcetype of value.
  • [0089] unexpected::value—search for events that lie outside observed patterns in the index by the specified value of 0 (expected) to 9 (most unexpected).

[0090] Modifiers can be combined with keywords, wildcard characters, literal strings, quoted phrases and Boolean operators, such as AND, OR, NOT. Parentheses can be used to nest search and sub-search phrases together. An example search phrase might be “sourcetype::mysql*sock*NOT (started OR (host::foo OR host::BAR)) maxresults::IO (eventtype::baddb OR eventtype::?8512-3) daysago::30”.

[0091] In one implementation, a custom parser 610 handles the Boolean operators “NOT” and “OR” and defaults to “AND”. This implementation also handles using parentheses to disambiguate the language when there are several operators. Otherwise, it associates left-to-right. The implementation also supports special search operators that are indicated using a domain specifier followed by a demarcation element. For example, searching for “source::1234”, might indicate that the searcher (human or system) wants to restrict results to events that were received from a particular source ID.

[0092] Incoming search phrases may also trigger ad hoc computation 612 based on a map of special keywords. For example, a special search string might be used to indicate that a search is to be stored and reissued on a periodic basis or to request a list of sources. In this case, the search string would be stored in a table on disk along with a schedule specifying the schedule on which the search should be reissued. Depending on the results of the search when executed, additional actions may be triggered. For example, an email alert might be sent, an RSS feed might be updated, or a user-supplied script might be executed. Another example of a search that triggers ad hoc computation 612 is one that is indicated to be saved for later use, but not to be reissued on a periodic basis.

[0093] Assuming that the search parser 610 determined that an annotated syntax tree 615 should be created for the search string, the next component, the search execution engine 620 will use the annotated syntax tree 615 to issue sub-searches 625 to the time bucketed indices 565. Each sub-search 625 is targeted at an individual time bucket 565. Time buckets are queried in the order that is most advantageous to pruning given the sort order for the results. For example, if search results are sorted in reverse chronological order, then the sub-search for the most recent time bucket will be issued first. This allows the search execution engine 620 to examine the results 635 of the sub-search before proceeding with additional (expensive) sub-searches 625. For example, if a particular sub-search returns enough results 635, then it is not necessary to proceed with additional sub-searches 625.

[0094] Once enough results sets 637 have been accumulated to satisfy the parent search, another module will take the results and merge 640 them into a single result set 235, 237 that satisfies the search. This merging process, in one implementation, performs a merge sort on the results from each of the buckets to keep them in the order required for the presentation process.

[0095] The final process in an exemplary implementation of our example TSSE is the preparation of search results for presentation 240, as shown in FIG. 7. Unlike current large-scale search engines that present non-interactive results ordered by keyword relevance ranking, this example TSSE can present results organized by time, event relationships, and keyword relevance ranking.

[0096] Unique to the challenge of indexing and searching time series data is the presentation of results using time as a primary dimension 710. Because existing large-scale search engines do not organize information by time, the presentation of time-based results is not a consideration. However, a primary benefit of a TSSE is the ability to index, search and present time series data chronologically. Results can be presented by aggregating and summarizing search results based on discrete time ranges or based on statistical calculations.

[0097] For example, the example TSSL can specify to see results for only a particular time frame and/or to see results presented by seconds, minutes, hours, days, weeks or months. In this way the search window can be limited to a timeframe and the results can be constructed for optimal viewing based on the density of the expected result set returned from a search. The search “192.168.169.100 hoursago::24 page::seconds”, will return time series events including the keyword “192.168.169.100” that occurred within the last 24 hours and will summarize the display results by seconds. In an exemplary implementation of a TSSE, summarization can include both aggregated display lines summarizing the events for the summary window and/or paging the results by the summary window. In the example above, each page of the search results presentation may include one second in time. Examples include but are not limited to:

• • [0098] Ability to scroll/page through the data (n) results at a time by count.
  • [0099] Ability to scroll/page through the data by time: next/previous second, minute, hour, day, year.
  • [0100] Ability to specify max count per timeframe.
  • [0101] Ability to get next (n) results within a paged time frame—(within a second) get next 100.

[0102] In addition to time-based presentation 710, an example TSSE preferably is able to present additional aggregation and summarization of results by metadata characteristics 720, such as, data source, data source type, event type, or originating host machine. In this way, results can be not only organized by time, but also refined by metadata aggregation and summarization. The search “192.168.169.100 page::source” will present all the results with “192.168.169.100” and put each data source containing results on a separate page. Examples include but are not limited to:

• • [0103] Original physical location of the data source.
  • [0104] Original physical machine, sensor etc. generating the data.
  • [0105] Type of data source as dynamically assigned by the indexing process.
  • [0106] Type of event as dynamically assigned by the indexing process.

[0107] Because time and certain metadata parameters (e.g., machine IP addresses) can be continuous, an example TSSE user interaction model can include the ability to move from small increments of time (seconds or minutes) or metadata parameters (different classes of IP addresses) using a zoom control 730. This zoom control can be combined with other metadata search parameters to enable the rapid movement through large amounts of data. Examples include but are not limited to:

• • [0108] Ability to zoom in and out around a given time from any second(s) to minute(s), hour(s), etc.
  • [0109] Ability to zoom in to second resolution around 12:I SAM Jun. 3, 2005, for a specific data source type and physical machine location.

[0110] Given the different types of users (humans and systems) and the varying types of time series data and events (e.g., single line events a few bytes in size, to multiple line events several megabytes in size) it is useful to be able to specify the density of the results. In one implementation the presentation density can be controlled 740 to return and/or display only the raw data without any metadata in a simple ASCII text format. Alternatively the same results can be returned and or displayed with full metadata as rich XML.

[0111] The TSSE can be implemented in many different ways. In one approach, each box shown in the various figures is implemented in software as a separate process. All of the processes can run on a single machine or they can be divided up to run on separate logical or physical machines. In alternate embodiments, the invention is implemented in computer hardware, firmware, software, and/or combinations thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method steps of the invention can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention 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. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include 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 disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits) and other forms of hardware.

[0112] Therefore, although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. Various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.