[论文笔记] An End-to-End Approach for QoS-Aware Service Composition (EDOC, 2009)

Time:  5.0 hours
Florian Rosenberg, Predrag Celikovic, Anton Michlmayr, Philipp Leitner, Schahram Dustdar, "An End-to-End Approach for QoS-Aware Service Composition," edoc, pp.151-160, 2009 IEEE International Enterprise Distributed Object Computing Conference (edoc 2009), 2009

    这篇论文出自维也纳技术大学的Distributed Systems Group, Rosenberg师从Schahram Dustdar, 毕业后留下来继续做研究. 他的博士毕业论文题目是"QoS-Aware Composition of Adaptive Service-Oriented Systems", 主要的publication包括:

Top-Down Business Process Development and Execution using Quality of Service Aspects. Enterprise Information Systems, November 2008, 2(4), 459-475, Taylor & Francis. 
Composing RESTful Services and Collaborative Workflows: A Lightweight Approach. IEEE Internet Computing, Sept/Oct 2008, 12(5), 24-31, IEEE. 
End-to-End Security for Enterprise Mashups. ICSOC'09, 24.-27. November 2009, Stockholm, Sweden.
An End-to-End Approach for QoS-Aware Service Composition. EDOC'09, 31. August - 4. September 2009, Auckland, New Zealand. 
Towards Composition as a Service - A Quality of Service Driven Approach. WISS'09, co-located with ICDE'09, 29. March 2009, Shanghai, China. 
Integrated Metadata Support for Web Service Runtimes. MWS'08, co-located with EDOC'08, 15-19. September 2008, Munich, Germany. 
Integrating Qualtiy of Service Aspects in Top-Down Business Process Development using WS-CDL and WS-BPEL. EDOC'07, 15-19. October 2007, Annapolis, Maryland, USA. 
Towards a Distributed Service-Oriented Business Rules System. ECOWS'05, 14-16. November 2005, Vaxjo, Sweden. 

1.
背景:
    当前服务组合多是以静态方式进行, 具有inflexible和non-adaptable的缺点, 不能满足动态环境的要求;
    当前的解决方案缺乏连贯的框架可支持从specifiction, optimization到executable composition generation这些功能.
本文要解决的问题: 
    基于QoS服务组合的端到端的解决方案.
本文工作:
   提出了一种基于domain-specific language(DSL)的组合方法, DSL可以表达服务的功能性和QoS需求; 并且开发了VRESCo系统, 可以半自动化地找到最优解.

2. (S3) 作者提出了 "Composition as a Service (CaaS)"的概念, 迎合时下最热的cloud computing.
VRESCo环境如下:
 
输入文件是采用作者开发的Vienna Composition Language(VCL)来描述. 可以指定功能性和QoS需求(IOPE以及QoS约束).
本文主要在讨论基于QoS的服务组合(step b, c), 对于abstract service composition如何生成并没有做细节描述.
从Listing 1(S4)的VCL sample来看, 服务组合的流程逻辑是要硬编码在文件里的, 并非自动组合生成.

3. (S5) 这一章讲QoS aware service composition, 这个过程不关注IOPE了, 只关注QoS, 本文涉及到的QoS有８个:
        Response Time, Latency, Price, Availability, Accuracy, Throughput, Reliable Messaging, Security

(S5.2) Feature(即service class ) resolution: 就是指基于QoS的服务选择过程.
Prefiltering: 根据required feature constraint(即局部限制)进行筛选

(S5.3) 讨论服务组合的建模形式
本文中采用structured composition(基于structured process model). 作者在(S5.3)第一段阐述了采用这个模型的理由.
structured composition的生成过程如下:
        abstract dependency graph (ADG) –> structured composition –> annotating the control flow decision in the structured composition with AND/XOR
具体结构定义见文中描述.
 

(S5.5) 将基于QoS的服务组合问题建模成Constraint Optimization Problem (COP)来求解
将constraint分成以下几类: global/feature constraints, required/optional constraints
required constraints是必须要满足的, 同其他论文中一样.
而对于optional constraints, 则是用来构建目标函数的.
下图是目标函数, cjk是optional feature constraint ( for feature j, kth QoS quality)(满足条件是值取1, 否则取0, 比如对于positive dimension, 定义为  ), strength是用户定义的权重.
ck是optional global constraint (for kth QoS quality).

COP问题的建模和IP问题的建模有点像, 都要定义variable, constraint和objective function. 
我现在的理解是: COP的建模能力比IP强, 但是求解的效率比IP弱. 

(S5.6)建模成IP问题来求解
总体过程大同小异. objective function与其他论文里的IP方法不同:

这个函数是将feature QoS进行scale后乘上权重而得.

4. Constraint-based approaches有scalability方面的问题. (S6)实验中提到, 当number of features > 15 (5 candidates per feature), 求解的时间久超过了120秒. 而类似规模的问题用IP方法求解, 耗时远小于1秒.

 

5. 相关的两篇论文
F. Rosenberg, P. Leitner, A. Michlmayr, P. Celikovic, and S. Dustdar, "Towards Composition as a Service - A Quality of Service Driven Approach," in Proceedings of the First IEEE Workshop on Information and Software as Service (WISS’09), co-located with the 25th International Conference on Data Engineering (ICDE’09), 29. March 2009, Shanghai, China. IEEE Computer Society, Mar. 2009.
Y. Guan, A. K. Ghose, and Z. Lu, "Using constraint hierarchies to support QoS-guided service composition," in Proceedings of the IEEE International Conference on Web Services (ICWS’06), Chicago, IL, USA. IEEE Computer Society, 2006, pp. 743–752.

6. 总的来说, 这篇论文介绍了VRESCo系统和VCL语言, 讨论了用CP和IP方法来求解QoS-aware service composition问题(在建模的时候, 把constraint分成hard和soft两种, objective function由soft constraint来表达), 这些是跟其他论文里不同的地方. 其他的也没什么特别的东西.