Advertisement
Research Article

Decision Support for Personalized Cloud Service Selection through Multi-Attribute Trustworthiness Evaluation

  • Shuai Ding mail,

    dingshuai@hfut.edu.cn (SD); xialooking@163.com (CYX)

    Affiliations: School of Management, Hefei University of Technology, Hefei, P.R. China, Key Laboratory of Process Optimization and Intelligent Decision-Making, Ministry of Education, Hefei, P.R. China

    X
  • Chen-Yi Xia mail,

    dingshuai@hfut.edu.cn (SD); xialooking@163.com (CYX)

    Affiliation: Tianjin Key Laboratory of Intelligence Computing and Novel Software Technology and Key Laboratory of Computer Vision and System (Ministry of Education), Tianjin University of Technology, Tianjin, P.R. China

    X
  • Kai-Le Zhou,

    Affiliations: School of Management, Hefei University of Technology, Hefei, P.R. China, Key Laboratory of Process Optimization and Intelligent Decision-Making, Ministry of Education, Hefei, P.R. China

    X
  • Shan-Lin Yang,

    Affiliations: School of Management, Hefei University of Technology, Hefei, P.R. China, Key Laboratory of Process Optimization and Intelligent Decision-Making, Ministry of Education, Hefei, P.R. China

    X
  • Jennifer S. Shang

    Affiliation: The Joseph M. Katz Graduate School of Business, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America

    X
  • Published: June 27, 2014
  • DOI: 10.1371/journal.pone.0097762
Corrections

3 Sep 2014: The PLOS ONE Staff (2014) Correction: Decision Support for Personalized Cloud Service Selection through Multi-Attribute Trustworthiness Evaluation. PLoS ONE 9(9): e107821. doi: 10.1371/journal.pone.0107821 | View correction

Abstract

Facing a customer market with rising demands for cloud service dependability and security, trustworthiness evaluation techniques are becoming essential to cloud service selection. But these methods are out of the reach to most customers as they require considerable expertise. Additionally, since the cloud service evaluation is often a costly and time-consuming process, it is not practical to measure trustworthy attributes of all candidates for each customer. Many existing models cannot easily deal with cloud services which have very few historical records. In this paper, we propose a novel service selection approach in which the missing value prediction and the multi-attribute trustworthiness evaluation are commonly taken into account. By simply collecting limited historical records, the current approach is able to support the personalized trustworthy service selection. The experimental results also show that our approach performs much better than other competing ones with respect to the customer preference and expectation in trustworthiness assessment.

Introduction

Cloud computing has become the driver for innovation in the recent years, from startups (e.g. Dropbox, Instagram) to established enterprises (Samsung). They are all using cloud computing to better serve their customers around the world [1]. Cloud service is also gaining wide acceptance and becoming popular to individuals as it reduces hardware and licensing costs, and it is scalable and allows users to work from any computer anywhere.

Several leading IT enterprises including Google, IBM, Microsoft, and Amazon have started to offer cloud services to their customers [2][4]. While many small and medium-sized enterprises (SMEs) and individual customers prefer to apply cloud services to build their business system or personal applications, they are often facing two major challenges at the selection time: (1) multiple cloud services are often available by different venders providing similar functional properties (i.e., “functionally-equivalent”). Customers usually lack appropriate, qualified, sufficient information and benchmarks to assess cloud services with regard to individual preferences and market dynamics [5]; (2) although cloud service vendors are struggling to improve service quality and performance, cloud computing are not necessarily trustworthy – unhandled exceptions and crashes may cause cloud service to deviate dramatically from the expectation [6], [7]. Therefore, there is an increasing demand to help the non-expert customers with the selection of trustworthy cloud service.

The trustworthiness of cloud service affects customers' perception towards service quality, which has significant bearing on customer satisfaction and royalty. The trustworthy attributes include reliability, scalability, availability, safety, security, etc [8][10]. Designing a general and comprehensive analytical model for trustworthiness evaluation is challenging, as the model needs the assessor to achieve, in reasonable time, useful results to determine the best service option. Due to their commercial value (similar to online recommendation system), several evaluation models [11][14] have been proposed by academia and industry lately. These models focus on quantitative analysis and evaluate trustworthiness through a collectively exhaustive dataset.

Except for some specific cases, the assessment dataset remains very sparse due to the costly and time-consuming nature of cloud service invocation. Intuitively, without sufficient data, fair review of cloud services cannot be achieved by existing evaluation methods [9], [15]. Fortunately, cloud vendors can collect historical records (QoS values, customer ratings, etc) from different cloud applications in cloud computing environment. With the vast amount of collaborative filtering (CF) technologies available in the field of online recommendation system, we believe there is a strong theoretical foundation to derive a generic trustworthiness model to support the evaluation of cloud service.

There have been some attempts to improve the accuracy of cloud service assessment by a CF process. However, very little attention is paid to the trustworthiness of cloud service, and no interest is given to the case when significant attribute values are missing. The lack of general and formal methodology can be attributed to the large process gap between the cloud service recommenders and trustworthiness researchers. To deal with this challenge, we propose a new CF approach to make use of hidden information (i.e. experience usability, value distribution) to measure the similarity between different services. Moreover, to support personalized selection of cloud services, we also provide a natural treatment for multi-attribute aggregation taking into account customer's preference and expectation.

Background

In the current market, multiple cloud services of similar functions are often available for specific domains. For example, in cloud storage service (e.g. data service, online file system, online backup plan), over 100 functionally-equivalent cloud services are offered by vendors. Some typical examples can be found in Table 1. Given the lack of cloud computing experience of non-expert customers, it is tedious to manually select an appropriate candidate from a set of functionally-equivalent services. Therefore, cloud service evaluation through quality analysis has gained much attraction among service-oriented computing and cloud computing communities over the past two decades.

thumbnail

Table 1. Online cloud storage services.

doi:10.1371/journal.pone.0097762.t001

Given the intricate interactions among QoS (Quality of Service) attributes, customer preferences and market dynamics that jointly influence the perceived quality of cloud services, developing a market-relevant analytical model is crucial to cloud service selection [16][18]. Due to their commercial value and the associated research challenges, many researchers and practitioners have studied the topics. Two types of service selection models are widely examined: evaluation-focused service selection models and prediction-focused service selection models.

By achieving market-relevant evaluations, customers can identify risks and benefits of each cloud service application and choose the best for adoption. The most employed evaluation models include: AHP-based cloud service ranking [19], reputation-aware service rating [20], trust-aware service selection [21], brokerage-based selection [22], SLA-based cloud trustworthiness estimation [11], trustworthy service selection [23]. Although these techniques can accurately and exhaustively estimate service quality, their implementation is time-consuming and costly.

Instead of real-world cloud service invocations, the prediction-focused service selection models can produce QoS values or service ranking using collaborative filtering (CF). The CF approaches for cloud service selection can be categorized as: item-based approaches [24], customer-based approaches [25], their fusion approaches [26], model-based approaches [27], and ranking-oriented approaches [28], where the first three categories are rating-oriented approaches. These approaches help assessors predict the missing attribute values by exploiting neighbors' usage experiences. Several collaborative filtering approaches for cloud service selection have been studied, but they did not consider customer preference and expectation in trustworthiness assessment.

In the prediction process, similar neighbors (customers or services) are identified to generate useful collaborative information. Popular choices for similarity estimation include Pearson correlation coefficient (PCC) [29] and vector similarity (VS) [30]. Since these measures only consider the numerical relationship between different ratings, they remain imprecise and confusing for estimating the neighbor similarity to support missing value prediction. Concerned that PCC may overestimate the similarities of negative services, Zheng et al. [26] propose a significance weight and modify PCC to improve the accuracy of similarity computation in service recommendation. However, the significance weight affects the similarity computation of positive services with more usage experiences. To address this problem, Ding et al. [31] define a convex function (usage structure factor) to reflect the usability of customer experience.

While a great number of researchers have focused on the trust-aware service selection and recommendation, little attention has been devoted to the role of customer preference and expectation in multi-attribute trustworthiness evaluation [32]. In addition, large quantities of works offer some valuable clues to discern between different services, the significances arising from value distribution is seldom considered. Thus, we will here combine evaluation-focused and prediction-focused approaches to propose a novel trustworthiness evaluation method which will fully utilize the information of similar services and customer's experience, and take into account both the missing attribute value prediction and the multi-attribute trustworthiness evaluation at the same time.

Methods and Materials

Based on the fact that the size and rate of growth in customers outweigh the expansion of delivered services in the cloud computing market, we employ item-based CF approach rather than the user-based or their fusion approach to produce the missing attribute values in trustworthiness evaluation. Motivated by the observation that experience usability and value distribution could provide valuable insight and distinctive information in the CF process, we create a new similarity measure for enhancing the prediction performance.

Pearson Correlation Coefficient

To make an accurate prediction, we first estimate the similarity between different cloud services. Given a service selection problem consisting of M customers and N services, the customer-service matrix for missing value prediction is denoted as(1)
where the entry qm,n denotes a historical record (QoS value or customer rating) of cloud service csn made by customer um, “qm,n = null” states that um didn't invoke csn yet.

Pearson Correlation Coefficient (PCC).

[29] Taking use of numerical distance to estimate the correlation between different services, PCC has been successfully adopted for recommendation system evaluations. Let csn and csv be two services, Un,y be the subset of customers who have invoked both csn and csv, then PCC is applied to calculate the similarity between csn and csv by(2)
where Sim(csn, csv) is in the interval of [−1, 1], and stand for the average values of csn and csv made by different customers. However, as noted in Ref. [26], PCC always overestimate the similarities of negative services, which are actually not similar but happen to have similar usage experience made by few customers. Table 2 shows a simple customer-service matrix which contains six customers (u1 to u6) and ten cloud services (cs1 to cs10). When utilizing Eq. (2), we calculate the PCC values between the services, and get the following relation: Sim(cs1, cs3)>Sim(cs1, cs4)>Sim(cs1, cs2), which indicates cs3 is more similar to cs1 than cs2 and cs4. It is clearly contrary to the reality due to the limited usage experience. Therefore, it is necessary to reinforce the similarity information in the CF process.

thumbnail

Table 2. A simple customer-service matrix.

doi:10.1371/journal.pone.0097762.t002

Significance estimation

It seems logical to believe that some cloud services in customer-service matrix may have high significances in making recommendations [33], [34]. For instance, a cloud service, which has more useful historical records, may be regarded as more important compared with a negative service. PCC is only related to the numerical distance between different services, but it has nothing to do with the statistical features of historical records. For this case, we introduce two types of significances arising from the experience usability and value distribution of historical records, respectively.

Estimating the experience usability.

To determine the significance of neighbors in a CF process, one often assumes a linear relationship between usage experiences and neighbor significances [26], [31]. One difference of our work from traditional CF approaches is that we apply a distance measurement method to estimate the experience usability in customer-service matrix. During the distance measurement, Jaccard's coefficient [35] is frequently employed to estimate the discrimination of asymmetric information on binary variables. Before integrating Jaccard's coefficient into our similarity measure, we map the original customer-service matrix into a rectangular binary matrix as follows:(3)
where the entry bm,n = 1 denotes the customer um has invoked the service csn previously, whereas bm,n = 0 denotes that um didn't invoke csn. Let |Un| be the number of customers who has invoked csn before, and |Un,y| be the number of customers who invoked both csn and csv. We use the Jaccard's coefficient Jn,y to reflect the rise of significance due to the experience usability, which can be expressed mathematically as:(4)
where Jn,y is in the interval of [0, 1], and a larger Jn,y value indicates that the historical records made over csv is more useful in the CF process. Jn,y = Jy,n holds for all services, which is consistent with the intuition that the similarity between csv and csn is only related to the subset of historical records made by the customers who have invoked both csn and csv.

Based on the customer-service matrix in Table 2, we get the significances arising from the experience usability for each service, as shown in Table 3. The values shown in grey are calculated for the negative service cs3. As observed from Table 2, cs3 has only been invoked twice. Consequently, his experience usability values are significantly lower than other services (e.g. J1,3<<J1,2). We can infer that integrating Jn,y into similarity measure will notably reduce the influence of negative service. It is worth noting that neither PCC nor Jn,y can distinguish between cs2 and cs4, since they do not have sufficient power to detect the crucial difference in value distributions.

thumbnail

Table 3. Significances arising from the experience usability.

doi:10.1371/journal.pone.0097762.t003
Estimating the value distribution.

The neighbors which have the same PCC similarity may have different value distributions. It is necessary to detect more hidden information in the customer-service matrix for significance estimation. For this case, we propose a method to discriminate neighbors' significances arising from their unique value distributions. In practice, the customer-service matrix is very sparse due to limited usage experiences. Therefore, we will ignore the historical records made by the customer u, where u Un,y. Let Dn = {qm,n | um Un,y} and Dy = {qm,y | um Un,y} be the historical records in similarity computation made over csn and csv, and |Dy| be the cardinality of Dn, and dom(Dn) be the domain of Dn subject to the following constraints:(5)
Following dom(Dn), the dataset Dy can be grouped into three categories:(6)
Since Dy is a finite discrete dataset, the probability of each category can be computed as:(7)
where , and . From the information entropy aspect, we use the following expression to detect the difference between the value distributions of csn and csy:(8)
where denotes the information entropy of Dy, and Hmax denotes the maximal entropy in customer-service matrix, respectively. VDn,y is a linear function defined in [0, 1]. From the maximum entropy principle [36], we have Hmax = log2(3). Thus, Eq.(8) can be rewritten as(9)
where VDn,y attains its unique global minimum if ; otherwise it attains global maximum when .

We can thus calculate the significances VDn,y arising from the value distribution using Eq.(9) over the customer-service matrix in Table 2. Table 4 shows the values of these significances. The values shown in grey are calculated for the cloud services cs2 and cs4.

thumbnail

Table 4. Significances arising from the value distribution.

doi:10.1371/journal.pone.0097762.t004
Similarity measurement adopting significance.

After we have defined the two types of significance for each service, we can then create the similarity measure, Sims(csn, csv), which takes into account the significance previously defined. To estimate the significance as accurately as possible, we identify the significance of csv with respect to csn as a linear combination of Jn,y and VDn,y, such that:(10)
where α is defined to determine how much our significance relies on experience usability and value distribution. If α = 0, we only extract the experience usability for conducting significance estimation, and if α = 1, we consider only the value distribution. Hence, the similarity measure can be written in standard form:(11)
where SIGn,y denotes the significance of csv with respect to csn, and Sim(csn, csv) denotes the PCC value between csn and csv. Different from existing similarity measures, our approach employs not only numerical distance but also usage experience as well as value distribution to determine the similarity between different services. With the definition of similarity measure defined in Eq.(11), for every cloud service in customer-service matrix, we rank their neighbors and select the top-k most similar services to make missing value prediction. Following the top-k similar service defined in [26], we get(12)
where CSn denotes the neighbor set of csn in customer-service matrix, and SimS(csn, csv) denotes the similarity between csn and csv. For the customer-service matrix in Table 2, we set α to 0.8 to obtain the similarity measures between different services (see Table 5). The top 3 neighbors of each service are marked in grey areas as seen in each column.

thumbnail

Table 5. Similarities between different services.

doi:10.1371/journal.pone.0097762.t005
Missing value prediction.

With the exponential growth of cloud service on the Internet, service recommendation techniques like QoS-aware CF approaches have become increasingly important and popular [37]. Based on our similarity measure, we propose an enhanced item-based CF approach (named as JV-PCC) to reinforce the prediction performance. To predict the missing value of service csn for customer um, we first determine the objective weight of each similar neighbor:(13)
where csv denotes a similar neighbor of csn, while SimS(csn, csv) denotes the similarity between csn and csv. The objective weights define the relative importance of each similar neighbor in the CF process. Next, we attain a prediction by a classic aggregation function:(14)
where qm,y denotes the historical record of csy made by customer um. In practice, local runtime environment (e.g. network bandwidth) and customer's rating style may significantly influence the historical records over delivered services. However, the above function deems inappropriate as it is calculated through only one customer. To address this problem, JV-PCC predicts the missing attribute value by the following equation:(15)
where ωn,y denotes the objective weight of csy with respect to csn, while and denote the minimum and maximum values of service csy, respectively. Table 6 displays the values estimated for the missing records in Table 2. In the experimental examples, both customer-based and service-based neighborhood information were adopted for approximating the missing value.

thumbnail

Table 6. Predicted attribute values.

doi:10.1371/journal.pone.0097762.t006

Trustworthiness-aware service selection

Several models, focusing on the quantitative measurement of service trustworthiness, have been proposed in Refs. [9], [18], [38]. However, different customers have different preference and expectation in service selection. A thorough understanding into these factors is essential to ensure effective evaluation finding. Here, we introduce a cloud service evaluation model, which helps aggregate trustworthy attributes by considering customer's preference and expectation.

Attribute utility determination.

To make use of observed or estimated values, we need to know that different attributes may have inconsistent dimensions. The results in [32] show that utility can be used to identify an entity's trustworthiness. Therefore, we first derive the utility from the customer-service matrix so as to ensure their values are in the range of [0, 1]. Trustworthy attributes are often divided into quantitative and qualitative attributes, of which the former are objective measures (e.g. QoS value), and the latter are subjective customer ratings. In addition, quantitative trustworthy attributes can be grouped into two classes: “benefit” and “cost”. For “benefit” (“cost”) attribute, e.g. throughput (response-time), the higher (lower) its value is, the greater the possibility that a customer would choose it becomes. In our model, qualitative attributes are also considered as “benefit” attributes. Let qm,n be the attribute value of csn, and the attribute utility (risk-neutral) Hm,n has the following form:(16)
where and denote the minimum and maximum attribute values for customer um, and they are subject to the following constraints:(17)
The attribute utility Hm,n is in the range of [0, 1], where a larger Hm,n indicates that customer um is more satisfied with the service csn.

Customer satisfaction estimation.

From influential theory in marketing science, we consider that the perception of cloud service trustworthiness is a customer satisfaction function, which includes customer preference and expectation attributes. In general, customer satisfaction function should exhibit two characteristics: (1) given the same expectation, a trustworthy cloud service is weighed much more heavily than an untrustworthy service. This effect is reflected in the derivation of attribute utility; (2) customer satisfaction slightly increases when attribute utility surpasses a certain value (expectation), and significantly decreases when attribute utility falls below expectation [39]. We formalize this interaction as a piecewise linear function:(18)
where Cm,n is constrained to 0≤Cm,n≤1; the parameter δ regulates the impact of customer preference on perceived trustworthiness; and Hexp denotes the customer expectation with regard to selecting trustworthy cloud service. As shown in Fig. 1, Cm,n is continuous (i.e. the piecewise function converges at Hm,n = Hexp).

thumbnail

Figure 1. Customer satisfaction function Cm,n.

(a) and (b) depict the distributions of customer satisfaction as recorded at the fixed expectation Hexp = 0.7 and Hexp = 0.9, where the parameter δ is varied from 2 to 6 in increment of 2. It can be observed that the rate of change in customer satisfaction differs significantly when Hm,n falls below and exceeds the expectation.

doi:10.1371/journal.pone.0097762.g001

The severity and rate of satisfaction (controlled by Hexp and δ, respectively) reflect different customer's tolerance to untrustworthy candidates. Let Hexp = 0.7, and δ = 2. Table 7 shows the customer satisfaction for each attribute value (historical record or predicted value), which corresponds to the original customer-service matrix in Table 2 and the predicted missing values given in Table 6.

thumbnail

Table 7. Customer satisfaction.

doi:10.1371/journal.pone.0097762.t007
Trustworthy attribute aggregation.

After estimating customer satisfaction and ensuring the value of Cm,n in the interval of [0, 1], the degree of trustworthiness (alias “trust value” [40]) of each cloud service in customer-service matrix can be achieved by aggregating trustworthy attributes. Let be the customer satisfaction on a set of specified attributes A1..AJ, then the trust value of csn is computed as:(19)
where denotes the weight of trustworthy attribute Aj, .

The trust value gives the comprehensive perception of cloud service trustworthiness, while the weights modify this trust value based on the relative importance of trustworthy attributes. Actually, a set of specified trustworthy attributes can be easily weighted by applying existing technologies such as those discussed in [32]. We omit the details for brevity.

Decision support for personalized service selection.

Multi-attribute trustworthiness evaluation is an important step for making accurate service selection. We suppose that um is the active customer, who requires trustworthy cloud service. While the evaluation results have arrived, a set of appropriate service candidates can be identified for um by:(20)
where trustn denotes the trust value of csn, denotes the selection threshold determined by um. We aim to remedy the shortcomings of evaluation-focused selection methods by avoiding the costly and time-consuming real-world service invocations. Note that when the service selection for the active customer um needs to be degrade by decreasing the parameter .

Let , a set of trustworthy cloud services can be recommended for u1u6 as(21)
where the customer satisfaction for each attribute value is presented in Table 7. In practice, our approach makes it possible to deal with various types of trustworthiness-aware cloud service selections by combing the evaluation-focused and the prediction-focused methods. Note that if trustworthiness is not the only issue that affects customer's decision making, it is necessary to extend the selection process of our approach, e.g., price-oriented service filtering, into other attributes or indexes.

Results

In this section, abundance of experiments are conducted to show how to recommend trustworthy cloud service in the context of large sparse assessment dataset, and to verify the efficiency of our CF approach.

Prototype implementation and results

To demonstrate the effectiveness of the proposed service selection approach, we use Microsoft C# .NET to develop a prototype system. Based on literature [8], [41], [42], we find Availability and Performance are two commonly used trustworthy attributes. We utilize them to conduct trustworthiness-aware service selection, by including two types of historical records: response-time and throughput. Their evaluation styles and weights are summarized as Table 8. We employ an open QoS research dataset [43] to simulate the historical records of Performance and Availability in cloud service market. The QoS values for response-time and throughput were collected from 339 users over 5825 web services in a real-world environment. Since it is impractical to discover and distinguish all functionally-equivalent services at the selection time, we randomly select 100 services' QoS records, and construct two 339×100 customer-service matrices for our experiment. Figure 2 shows the value distribution of response-time and throughput in user-service matrix. We cannot simply utilize these QoS records to analyze and rank the cloud services since these customer-service matrices are sparse assessment datasets, and cannot accurately interpret the trustworthiness status of all services. Suppose u339 is the active customer. The historical records made by u339 contains 9 and 7 missing values (on response-time and throughput, respectively) which will potentially affect his cloud service selection decision. Therefore, the proposed CF approach is employed to predict the missing attribute values. At this simulation experiment, the similarity parameter α is set to 0.8 and remains so until the trust values for u339 are reported. Once the prototype system obtains the customer satisfactions by utilizing Eqs.(12)(14), where the parameter δ = 2 and the expectation Hexp = 0.7, the active customer will receive the trust values of each service. We vary the selection parameter from 0 to 1 in increment of 0.1, and count the cloud services whose trust values surpass (the number of recommended services for u339, i.e. |CS339|). The experiment results are shown in Figure 3. Although we only study two trustworthy attributes in the experiment, the proposed approach can be easily extended to other trustworthiness-aware service selection problems. When selecting the optimal trustworthy services from a set of functionally-equivalent candidates, the entry data of our approach are the corresponding historical records (i.e., QoS values or customer ratings), the active customer's preference and expectation towards service trustworthiness, and the selection parameter.

thumbnail

Figure 2. QoS value distributions.

(a) and (b) depict the value distributions of response-time and throughput in our customer-service matrices, where “−1” indicates that the service invocation failed due to an http error. The ranges of response-time and throughput are 0–16.053 seconds and 0–541.546 kbps, respectively.

doi:10.1371/journal.pone.0097762.g002
thumbnail

Figure 3. The number of recommended services for u339.

Results are presented for the proposed cloud service selection approach, where the parameter is varied from 0 to 1 in increment of 0.1.

doi:10.1371/journal.pone.0097762.g003
thumbnail

Table 8. Trustworthy attributes of cloud service.

doi:10.1371/journal.pone.0097762.t008

Impact of δ and Texp

Different customers have different preference and expectation in trustworthy service selections. Instead of risk-neutral attribute utility, we use the customer satisfaction Cm,n to identify the perceived trustworthiness of delivered services. To evaluate the impact of customer's preference and expectation, we have conduct additional experiments with variable parameters δ and Hexp. In these experiments, we first vary δ from 2 to 6 in increment of 2, where the expectation Hexp is fixed at 0.7 first. Later, we set δ to 2, and vary Hexp from 0.7 to 0.9 in increment of 0.1. Figure 4 (a) shows the experimental results of preference parameter δ and Figure 4 (b) shows the experimental results of expectation Hexp. The parameters δ and Hexp jointly determine how to derive the customer satisfaction from attribute utility to approximate the active customer's attitude towards profit and risk.

thumbnail

Figure 4. Impact of preference and expectation.

(a) and (b) depict the experimental results of preference parameter δ and expectation Hexp, respectively. They indicates that δ regulates the elimination rate of untrustworthy cloud services, whereas Hexp controls the degree of customer's tolerance to untrustworthy service.

doi:10.1371/journal.pone.0097762.g004

Performance comparison of CF approaches

In this work, we present an enhanced item-based CF approach (i.e., JV-PCC) to predict the missing attribute values for cloud service selection. Our approach engages the significances (Jn,y and VDn,y) to improve the accuracy of similarity estimation. To study the prediction performance, we compare JV-PCC with two existing item-based approaches: Item-based CF adopting PCC (IPCC) [44], and Extended PCC approach (f-PCC) [31].

Evaluation metric.

We use Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) to evaluate the prediction performance of our approach in comparison with other approaches. MAE and RMSE are defined as:(22)
where and are the predicted QoS value and the actual value, respectively.

Experimental setup and results.

The size of top-k similar service set plays an important role in CF approach, which determines how many neighbors' historical records are employed to generate predictions. To study the impact of neighborhood size k, we separate the customer-service matrices into two parts: training set (80% historical records in the matrix) and test set (the remaining 20% records). We set the density to 50%, the significance parameter α to 0.7, and vary k from 5 to 30 in increment of 5. Figure 5 shows the experimental results for response-time and throughput. Under the same simulation condition, JV-PCC and f-PCC significantly outperform IPCC. The observations also suggest that better accuracy can be achieved by our model when more historical records are available in the service selection study.

thumbnail

Figure 5. Impact of neighborhood size k.

(a) and (b) depict the MAE fractions of JV-PCC, f-PCC and IPCC for response-time and throughput, while (c) and (d) depict the RMSE fractions. It can be observed that JV-PCC achieves smaller MAE and RMSE consistently than f-PCC for both response-time and throughput. Regardless of JV-PCC or f-PCC, as k increases, MAE and RMSE drop at first, indicating that better performance can be achieved by employing more similar services' records to generate the predictions. However, when k surpasses a specific level (i.e. k = 25), they fail to drop with a further increase in k, which were caused by the limited number of similar neighbors.

doi:10.1371/journal.pone.0097762.g005

Conclusions

Trustworthiness-aware service selection is a critical issue among cloud computing and service-oriented architecture communities. In this paper, we propose a personalized service selection approach which takes into account the missing value prediction and the multi-attribute evaluation requirements. We find that the proposed approach can tackle various types of trustworthiness-aware selection problems in cloud service market. Meanwhile, the experimental results demonstrate that the proposed CF approach significantly improves the prediction performance as compared with other competing item-based approaches.

Employing untrustworthy cloud service will expose users to high-risk IT structure, resulting in a host of intra-organizational hazards that detriment the organization and disrupt the normal operations [45]. In the present work, we can only look into the static approach for trustworthy service selection, and we will investigate more types of trustworthiness evaluation models (e.g. probability model, dynamic model, etc) in the future since different cloud service applications may have different selection criteria and data structures.

Acknowledgments

We would like to thank Prof. Youtao Zhang, from University of Pittsburgh, for giving detailed and valuable comments on the paper.

Author Contributions

Conceived and designed the experiments: SD CYX KLZ SLY JSS. Performed the experiments: SD. Analyzed the data: SD JSS CYX. Contributed reagents/materials/analysis tools: SD CYX. Contributed to the writing of the manuscript: SD CYX.

References

  1. 1. Katzan H (2011) Cloud software service: concepts, technology, economics. Service Science 1(4): 256–269. doi: 10.1287/serv.1.4.256
  2. 2. Stolyar AL (2013) An infinite server system with general packing constraints. Operations Research 61(5): 1200–1217. doi: 10.1287/opre.2013.1184
  3. 3. Chen PY, Wu SY (2013) The impact and implications of on-demand services on market structure. Information Systems Research 24(3): 750–767. doi: 10.1287/isre.1120.0451
  4. 4. Sunyaev A, Schneider S (2013) Cloud services certification. Communications of the ACM 56(2): 33–36. doi: 10.1145/2408776.2408789
  5. 5. Sanjukta D, Anna YD (2011) Risk management and optimal pricing in online storage grids. Information Systems Research 22(4): 756–773. doi: 10.1287/isre.1100.0288
  6. 6. Marston S, Li Z, Bandyopadhyay S, Zhang JH, Ghalsasi A (2011) Cloud computing – The business perspective. Decision Support Systems 51: 176–189. doi: 10.1016/j.dss.2010.12.006
  7. 7. Cusumano M (2010) Cloud computing and SaaS as new computing platforms. Communications of the ACM 53(4): 27–29. doi: 10.1145/1721654.1721667
  8. 8. Alhamad M, Dillon T, Chang E (2011) A trust-evaluation metric for cloud applications. International Journal of Machine Learning and Computing 1(4): 416–421. doi: 10.7763/ijmlc.2011.v1.62
  9. 9. Yao JH, Chen SP, Wang C, Levy D, Zic J (2011) Modelling collaborative services for business and QoS compliance. IEEE International Conference on Web Services (ICWS), pp.299–306.
  10. 10. Tao F, Zhao DM, Hu YF, Zhou ZD (2010) Correlation-aware resource service composition and optimal-selection in manufacturing grid. European Journal of Operational Research 201(1): 129–143. doi: 10.1016/j.ejor.2009.02.025
  11. 11. Grag SK, Versteeg S, Buyya R (2013) A framework for ranking of cloud computing services. Future Generation Computer Systems 29(4): 1012–1023. doi: 10.1016/j.future.2012.06.006
  12. 12. Limam N, Boutab R (2010) Assessing software service quality and trustworthiness at selection time. IEEE Transactions on Software Engineering 36(4): 559–574. doi: 10.1109/tse.2010.2
  13. 13. Hang CW, Singh MP (2011) Trustworthy service selection and composition. ACM Transactions on Autonomous and Adaptive Systems 6(1): 1–17. doi: 10.1145/1921641.1921646
  14. 14. Chakraborty S, Roy K (2012) An SLA-based framework for estimating trustworthiness of a cloud. IEEE 11th International Conference on Trust, Security and Privacy in Computing and Communications, pp. 937–942.
  15. 15. Mehdi M, Bouguila N, Bentahar J (2012) Trustworthy web service selection using probabilistic models. IEEE 19th International Conference on Web Services (ICWS), Honolulu, HI, pp.17–24.
  16. 16. Hackney R, Xu HN, Ranchhod A (2006) Evaluating web services: Towards a framework for emergent contexts. European Journal of Operational Research 173(3): 1161–1174. doi: 10.1016/j.ejor.2005.07.010
  17. 17. Ding S, Yang SL, Zhang YT, Liang CY, Xia CY (2014) Combining QoS prediction and customer satisfaction estimation to solve cloud service trustworthiness evaluation problems. Knowledge-Based Systems 56(1): 216–225. doi: 10.1016/j.knosys.2013.11.014
  18. 18. Lecue F (2010) Combing collaborative filtering and semantic content-based approaches to recommend web services. IEEE Fourth International Conference on Semantic Computing, pp. 200–205.
  19. 19. Tserpes K, Aisopos F, Kyriazis D, Varvarigou T (2012) A recommender mechanism for service selection in service-oriented environments. Future Generation Computer Systems 28(8): 1285–1294. doi: 10.1016/j.future.2011.11.003
  20. 20. Zheng ZB, Ma H, Lyu MR, King I (2011) QoS-aware web service recommendation by collaborative filtering. IEEE Transactions on Services Computing 4(2): 140–152. doi: 10.1109/tsc.2010.52
  21. 21. Chen X, Liu XD, Huang ZC, Sun HL (2010) Region KNN: A scalable hybrid collaborative filtering algorithm for personalized web service recommendation. IEEE International Conference on Web Service (ICWS), pp.9–16.
  22. 22. Zheng ZB, Wu XM, Zhang YL, Lyu MR, Wang JM (2013) QoS ranking prediction for cloud services. IEEE Transactions on Parallel and Distributed Systems 24(6): 1213–1222. doi: 10.1109/tpds.2012.285
  23. 23. Pal R, Hui P (2013) Economic models for cloud service markets: Pricing and capacity planning. Theoretical Computer Science 496(22): 113–124. doi: 10.1016/j.tcs.2012.11.001
  24. 24. Sundarewaran S, Squicciarini A, Lin D (2012) A brokerage-based approach for cloud service selection. IEEE Fifth International Conference on Cloud Computing, pp.558–565.
  25. 25. Resnick P, Iacovou N, Suchak M, Bergstrom P, Riedl J (1994) Grouplens: An open architecture for collaborative filtering of networks. Proceeding of ACM Conference on Computer Supported Cooperative Work, pp.175–186.
  26. 26. Breese JS, Heckerman D, Kadie C (1998) Empirical analysis of predictive algorithms for collaborative filtering. Proceedings of 14th Annual Conference on Uncertainty in Artificial Intelligence, 43–52.
  27. 27. Ding S, Ma XJ, Yang SL (2012) A software trustworthiness evaluation model using objective weight based evidential reasoning approach. Knowledge and Information Systems 33(1): 171–189. doi: 10.1007/s10115-011-0442-0
  28. 28. Jin R, Chai J, Si L (2005) An automatic weighting scheme for collaborative filtering. SIGIR, 337–344.
  29. 29. Bobadilla J, Hernando A, Ortega F, Gutierrez A (2012) Collaborative filtering based on significances. Information Sciences 185: 1–17. doi: 10.1016/j.ins.2011.09.014
  30. 30. Seifoddini H, Djassemi M (1991) The production data-based similarity coefficient versus Jaccard's similarity coefficient. Computer & Industrial Engineering 21(1–4): 263–266. doi: 10.1016/0360-8352(91)90099-r
  31. 31. Cheng HD, Chen JR (1997) Automatically determine the membership function based on the maximum entropy principle. Information Sciences 96(4): 163–182. doi: 10.1016/s0020-0255(96)00141-7
  32. 32. Shao LS, Zhang J, Wei Y, Zhao JF, Xie B, et al.. (2007) Personalized QoS Prediction for Web Services via Collaborative Filtering. IEEE International Conference on Web Services (ICWS), pp.439–446.
  33. 33. Dozelli P, Basili V (2006) A practical framework for eliciting and modeling system dependability requirements: Experience from the NASA high dependability computing project. Journal of Systems and Software 79(1): 107–119. doi: 10.1016/j.jss.2005.03.011
  34. 34. Hwang SY, Lee CH (2013) Reliable web service selection in choreographed environments,. Decision Support Systems 54(3): 1463–1476. doi: 10.1016/j.dss.2012.12.017
  35. 35. Anderson EW, Sullivan MW (1993) The antecedents and consequences of customer satisfaction of firms. Marketing Science 12(2): doi: 10.1287/mksc.12.2.125
  36. 36. Karaoglanoglou K, Karatza H (2011) Resource discovery in a Grid system: Directing requests to trustworthy virtual organizations based on global trust values. Journal of Systems and Software 84(3): 465–478. doi: 10.1016/j.jss.2010.10.043
  37. 37. Cruz JM, Liu ZG (2012) Modeling and analysis of the effects of QoS and reliability on pricing, profitability, and risk management in multiperiod grid-computing networks. Decision Support Systems 52(3): 562–576. doi: 10.1016/j.dss.2011.10.012
  38. 38. Rosario S, Benveniste A, Haar S, Jard C (2008) Probabilistic QoS and soft contracts for transaction-based web services orchestrations. IEEE Transactions on Services Computing 1(4): 187–200. doi: 10.1109/tsc.2008.17
  39. 39. Zheng ZB, Zhang YL, Lyu MR (2010) Distributed QoS evaluation for real-world web services. Proceedings of the 8th International Conference on Web Services (ICWS), Miami, Florida, USA, pp. 83–90.
  40. 40. Deshpande M, Karypis G (2004) Item-based top-N recommendation. ACM Transactions on Information System 22(1): 143–177. doi: 10.1145/963770.963776
  41. 41. Benlian A, Hess T (2011) Opportunities and risks of software-as-a-service: Findings from a survey of IT executives. Decision Support Systems 52: 232–246. doi: 10.1016/j.dss.2011.07.007
  42. 42. Demirkan H, Cheng HK, Bandyopadhyay S (2010) Coordination strategies in a SaaS supply chain. Journal of Management Information Systems 26(4): 119–143. doi: 10.2753/mis0742-1222260405
  43. 43. Tsesmetzis D, Roussaki I, Sykas E (2008) QoS-aware service evaluation and selection. European Journal of Operational Research 191: 1101–1112. doi: 10.1016/j.ejor.2007.07.015
  44. 44. Wang Q, Wang C, Ren K, Lou KJ, Li J (2011) Enabling public auditability and data dynamics for storage security in cloud computing. IEEE Transactions on Parallel and Distributed Services 22(5): 847–859. doi: 10.1109/tpds.2010.183
  45. 45. Iosup A, Ostermann S, Yigitbasi MN, Prodan R, Fahringer T, et al. (2011) Performance analysis of cloud computing services for many-tasks scientific computing. IEEE Transactions on Parallel and Distributed Systems 22(6): 931–945. doi: 10.1109/tpds.2011.66