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基于日志的大规模分布式软件系统可信保障技术研究
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摘要
随着计算技术和通讯技术的不断发展,以通信、存储和计算为核心的信息技术服务已经广泛渗透于社会生活的各个层面,7′24小时持续可信提供服务成为此类服务的基本需求。作为承载此类信息服务的软件基础设施通常以分布式方式组成,具备用户规模巨大、数据规模巨大以及计算设施规模巨大等规模化特征。规模化特征会导致此类软件系统在运行时的行为和环境复杂:一方面,软件自身的缺陷很难完全排除;另一方面,软件也不可能在开发阶段预期运行时面临的所有问题。因此,大规模分布式软件系统往往需要在系统组成构件持续失效的环境下提供服务,严重威胁7′24持续运行的高可信需求。
     “监视-执行”作为一种有效的系统运行时可信保障机制,依赖于“故障定位”技术准确、高效地判断当前系统发生的故障,从而及时执行相应的可信确保操作。日志信息作为系统行为的有效抽象被广泛应用于故障定位的两类方法:对已知故障特征行为的建模和对系统正常运行特征的建模。然而,在软件行为以及运行环境复杂的大规模分布式软件系统中,现有基于日志信息的故障定位工作仍面临巨大挑战,突出表现在:对已知故障建模的过程中存在大量噪声日志,影响模型的有效性;对已知故障建模的过程中会遗漏故障的持续性特征,导致故障定位结果产生大量误报、漏报;对系统复杂的正常运行行为难以提取稳定的特征模型,异常行为检测效率低下,难以运用于在线环境。
     本文针对上述挑战,以企业级大规模分布式软件系统为研究对象,从基于故障特征的故障定位方法以及基于正常行为特征的故障定位方法两方面展开研究,通过使用系统日志信息的时间序列属性、统计属性以及事件属性,结合小波变换、相似度计算以及聚类索引等相关技术,针对噪声日志过滤、故障持续特征建模以及异常行为检测等关键问题展开研究。本文的主要贡献包括:
     (1)故障特征提取中基于时间序列相似度的噪声日志过滤方法
     针对故障特征提取中的噪声日志过滤问题,本文结合Haar小波变换提出了一种基于时间序列相似度的噪声日志过滤方法。该方法将每类日志信息建模为时间序列,用于提取日志信息在特定时间窗口内的数量变化规律。通过逐一提取候选噪声日志的特征模板,与故障特征建模阶段该类日志的行为进行比对,从而将与故障无关的日志删除。在阿里巴巴云计算公司内部生产集群环境的实验中,本方法将故障模型精确率提高至96%(检测窗口为50秒时),召回率提高至94%(检测窗口为100秒时)。
     进一步,为了降低噪声日志过滤方法的时间复杂度高,本文研究了噪声模板的聚类索引问题。通过使用时间序列相似度对噪声模板进行聚类,并使用高效的索引结构skip-list对类簇内的样本模板进行索引,能够将噪声日志过滤方法的性能提高43%。
     (2)故障特征提取中基于日志项状态变化的故障持续状态建模方法
     针对故障持续性特征的建模问题,本文提出了一种基于伴随状态追踪机制的方法。由于不同故障特征其持续时间和持续状态变化不同,在检测窗口之外的故障特征信息往往会被错误识别或者遗漏。基于伴随状态追踪机制的故障监测方法通过将故障特征建模为日志类型项状态变化模型,能够区分当前日志项是由新故障所引起还是之前故障的残留特征。在阿里巴巴云计算公司内部生产集群环境中的实验中,本方法将系统对故障特征识别的有效性(F-Measure均值)提高至90%(监视窗口为6秒时),从而能够有效地消除系统故障报警的误报与漏报。
     (3)正常行为特征提取中基于线程级日志序列的异常检测方法
     针对日志序列的异常检测问题,本文使用正常运行状态下的线程级日志序列作为系统正常行为模板,从而将异常日志检测问题转换为相似序列模板匹配问题。为了解决原始日志序列模板相似度进行聚类的稳定性及异常检测的时效性缺陷,本文使用日志类型特征向量的余弦相似度方法来近似计算序列相似性,并使用Top-K搜索限定待比对的模板集合,同时结合日志类型倒排索引对初始模板集合进行快速筛选,在Hadoop平台中将相似模板匹配的效率提高了8.6倍(相似度阈值为0.95,取Top50的情况下)。
     进一步,为了提高日志模板检测精确度,本文提出了基于子序列特征向量聚类索引的序列模板匹配方法。由于日志类型特征向量缺乏时序信息,导致其对线程级日志序列相似度计算的“近似”效果较差。本文在重复连续子串分析的基础上,将线程级日志序模板建模为子序列集合,以子序列标识为粒度构建子序列特征向量,从而对日志序列模板进行聚类。该方法在保持一定匹配效率的基础上,同时考虑了日志序列的局部时序关系,在Hadoop平台中将对比日志类型特征向量方法将匹配精确度提高了15%(相似度阈值为0.90,取Top40的情况下)。
     此外,为了辅助上述研究工作,本文进一步构造了日志监视、分析、展示工具集LogAnalyzer,实现了对在线日志行为实时展现以及对不同场景下系统行为的离线分析,从而帮助系统维护与管理人员从日志信息的角度理解系统行为。
With the rapid progress of computing techniques and communication techniques,the information service has been widely used in regular life. It persistently providestrusted service for7′24hours a week. The infrastructure of such service is usuallydistributed systems that are constructed with large amount of computing resources,handle large amount of user requests and storage large amount of user data. In order tobuild a large-scale trusted distributed system, the complexity of system behavior andsystem running environment would increase dramatically for the follow two reasons:software bugs are difficult to get rid of, and it is difficult to anticipate all the situationsthat could encountered at runtime. A direct result of such complexity is that the systemshould be persistently providing trusted service while the system composing componentpersistently fails, which seriously affects the7′24high trusted service requiremt.
     Monitoring-Action is a widely used runtime trust grantee mechanism. It uses thefault localization techniques to accurately diagnosis the root cause of system failure, andthen doing the corresponding actions to grantee the runtime trust property of the system.Event log has been widely used in fault localization techniques as an effectiveabstraction of system behavior. There are mainly two ways to localize a fault: use faultmodel to identify a known fault; use normal behavior model to identify the abnormalbehavior. However, due to the complexity system behavior and system environment,there still exist great challenges to localize fault by event logs: there exist large amountnoisy logs through the fault modeling process, which could lead to false positives andfalse negatives in the fault localization process; it is difficult to extract the normalbehavior model under distributed environment, and the low detection efficiency makesit difficult to detect runtime abnormal system behavior.
     In order to tame those challenges, we research the fault localization problems inboth fault model and normal behavior model method in typical large-scale distributedsoftware systems. By using the Haar wavelet transform techniques, similaritymeasurement and cluster index techniques, we try to solve the noisy event log filtering,continues fault behavior tracing and normal behavior modeling problem throughdifferent properties of event logs, including time series property, statistical property andevent property. Our main works including:
     (1) Time series similarity-based noisy event logs filtering
     To filter the noisy event logs during fault feature extraction process, we propose atime series similarity-based noisy log filtering method to filter out the event logs that arenot related to the injected fault. By modeling a specific type of event logs into timeseries, and use Haar wavelet transform to extract the occurring pattern of the time series.By comparing the similarity between noisy log templates and the target log time series, we successfully identify the event logs that are not related to the injected fault andincrease the effectiveness of the fault model. By conducting the experiment in an innerlarge-scale distributed software system of alibaba cloud computing company, we canimprove the fault accuracy to96%(when setting detection time window to50seconds)and the fault recall to94%(when setting the detection time window to100seconds).
     To improve the efficiency of the filtering process, we propose a skip-list basedcluster index to shorten the filtering time. By clustering the noisy temples with timeseries similarity measurement and use skip-list to index the noisy temples, wesuccessfully improve the filtering efficiency by43%.
     (2) Event log item state-based continues fault tracing
     In order to model the fault features outside the detection time window, we proposea companion state tracing mechanism to extract the fault feature. Traditional faultmodeling method uses the event logs that are observed within a fixed fault feature timewindow to model a fault and ignore the fault feature outside the time window. Sincedifferent types of fault are rather different in fault propagation time period and faultpropagation patterns, the fault feature could be mistakenly identified as false positivesand false negatives. By modeling fault into companion state machines, we can identifythe current event log pattern is caused by the previously detected fault or caused by anew fault. By conducting the experiment in an inner cluster of alibaba cloud computingcompany, we successfully increase the effectiveness of fault model up to90%(whensetting detection time window to6seconds).
     (3) Thread level event log sequence-based abnormal behavior detection
     In order to detect abnormal behavior from event log templates, we use the eventlogs within a thread as the system normal behavior templates, and model the abnormalevent log detection problem into sequence matching problem. To improve the efficiencyof traditional cluster-based sequence matching method, we use cosine similarity ofevent type feature vector to anticipate the sequence similarity, use Top-K searchingtechniques to limit the size of comparing set, and use invert index techniques to furtherfilter the target temples set. By conducting the experiment on Hadoop cluster, weincrease the comparing efficiency for8.6times (when setting similarity threshold to0.95and use Top50).
     To further improve the effectiveness of the thread level event log behavior model,we propose a sub-sequence feature vector-based cluster index method for sequencematching. Since the event type feature vector doesn’t contains temporal information, itmakes the similarity measurement doesn’t similar well to the original sequencesimilarity measurement. We use the repeat sub-sequence analyzing techniques to dividethe original sequence into sub-sequence. Then, we use the sub-sequence id to formfeature vector and use the cosine similarity method to cluster the original sequence. Thesub-sequence is a reasonable abstraction of localized temporal information of the original sequence, so we can acquire a better matching effectiveness than event typefeature vector-based method. By conducting the experiment in Hadoop cluster, weincrease the sequence matching actuary for15%(when setting the similarity thresholdto0.90and use Top40).
     Except for the above3contributions, we also built a set of tools namedLogAnalyzer for gathering and analyzing the event logs in large-scale distributedsoftware systems, which can help the system administrator to better understand thesystem runtime behavior through event logs.
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