以服務為基礎的軟體系統可靠度模型及容錯設計

Reliability Modeling and Fault-Tolerant Design of Service-Based Software Systems

彭冠力(Kuan-Li Peng)

指導教授 ： 黃慶育

國立清華大學/電機資訊學院/資訊工程學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00536

摘要

時至今日，以服務為基礎的軟體系統（Service-based Software Systems；簡稱 SBSS）大量部署在雲端服務等資訊領域。SBSS 系統藉由標準的通訊協定，使得不同廠商所開發的軟體功能（服務）能快速整合起來。藉著整合各種不同的服務，整個資訊系統就能有效率地建構雛型、開發、正式上線，並且能快速反應新的用戶需求，增加營運收益。除此之外，在 SBSS 系統上還可以藉由整合多方類似功能的服務，實現軟體容錯機制，提升服務的穩定性與可靠性。

SBSS 系統除了要考量功能性，還要考量服務品質，因為如果系統服務容易中斷、品質低落，會流失許多用戶。然而，SBSS 系統先天存在許多不穩定性，諸如伺服器忙碌、網路壅塞，皆會影響服務的品質與其可靠度評估的準確性。值得注意的是，在 SBSS 系統運作的每個層級，從服務供應商、連外網路，到外部函式庫、資料庫，皆普遍存在服務的相依性。一旦所共享的資源失效，所有相依的服務會同時受到影響，這使得 SBSS 系統的可靠度及其評估愈加嚴峻、複雜。

為此，本篇論文著眼以服務為基礎的軟體系統之可靠度評估，率先詳細考量各種不穩定性、相依性，以及軟體容錯的內部機制對於 SBSS 系統可靠度的影響。我們以馬可夫過程（Markov process）等理論模型為基礎，提出 SBSS 系統可靠度評估與分析的框架。這套框架結合了 SBSS 容錯系統的失效相依性模型建構、系統可靠度評估、系統可靠度分析。本文以一套在業界使用過的旅遊業服務系統，稍加簡化為範例系統，在此評估所提出方法的可行性與準確性。除此之外，我們以 ns-3 網路模擬系統為基礎，針對 SBSS 容錯系統開發出模擬架構，能夠考量從網路底層到服務層中許多影響可靠度的因素。本論文也針對 recovery blocks （RB）與 n-version programming （NVP）兩套典型的軟體容錯機制，以隨機過程的觀點詳加分析兩者在 SBSS 系統運作的效能與可靠度。

實驗結果顯示所提出的方法，在不同的服務流程、容錯機制、網路狀態、服務狀態的設置下，皆能夠準確地評估出 SBSS 系統的可靠度，並且也總結歸納出 RB 與 NVP 在 SBSS 系統中所呈現的特性。此外，本論文所呈現的結果也顯示相依性對 SBSS 系統的可靠度影響甚鉅，需要審慎考量。實驗中也實作了針對各別服務、各別相依性對 SBSS 系統可靠度影響程度的諸多分析方法，綜合比較他們的運算量以及分析能力，可供 SBSS 系統工程師致力改善系統品質時參考。

關鍵字

以服務為基礎的軟體系統 ； 系統可靠度模型 ； 敏感度分析 ； 服務相依性 ； 共因失效 ； 軟體容錯

並列摘要

Presently, service-based software systems (SBSSs) have been heavily deployed to fulfill the functionalities of cloud computing and widely used in many other application fields. The core technology of SBSSs, namely service-oriented computing (SOC), enables faster integration of existing software components (services) from different parties through standard protocols. By combining varied services, a system may be prototyped, built, and adapted quickly to fulfill the constant changing demands of the customers. SBSSs also make software fault tolerance (FT) feasible to provide continuous and reliable system service delivery when failures occur, by leveraging several functionally equivalent services in the repositories.

In addition to the functional requirements, maintaining the quality of service (QoS) becomes increasingly important for the SBSSs, since failures for the systems to operate at expected service levels may cause great financial loss to an organization. However, the uncertainty nature of SBSSs, such as the server overloads and the network problems, actually introduces new challenges for system reliability evaluation. It is also noted that resource sharing commonly exists in different levels of the SBSS operations, such as the service providers, interconnected networks, or external libraries. Correlated failures due to the shared resources further complicate the SBSS reliability modeling, and they threaten the performance of existing FT designs.

This dissertation presents a first study that considers the instability and dependency factors as well as the internal FT mechanisms in analyzing the reliability in SBSSs. We propose a reliability framework based upon the Markov process and varied theoretical tools. It incorporates the failure dependence modeling, the system reliability modeling, as well as the reliability analysis for the SBSSs with the FT designs. The proposed method is applied to a travel agency system based upon a real-world practice for verifying its accuracy of reliability modeling and effectiveness of varied reliability measures. In addition, we design a discrete-event SBSS simulation framework based upon the widely adopted ns-3 network simulator; it is capable of modeling the conditions of the underlying networks and could be also combined with various error models. Stochastic reliability and performance modeling for the enhanced SBSS FT schemes (ERB and ENVP) are also presented, discussed, and compared with the experimental results.

Extensive results show that the proposed method could accurately model the SBSS reliability with varied workflow structures, FT designs, as well as the service and networking conditions. Various characteristics for the FT performance on SBSSs are uncovered. It is also confirmed that failure dependency of the services is an essential factor for analyzing any valuable SBSS. A set of reliability measures with different capabilities and complexities are available for assisting SBSS engineers with system improvements.

並列關鍵字

Service-Based Software Systems ； System Reliability Modeling ； Sensitivity Analysis ； Service Correlation ； Common Cause Failures ； Software Fault Tolerance

參考文獻

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