Framing Sustainability as a Property of Software Quality

Sustainability is defined as the “capacity to endure”³⁴ and “preserve the function of a system over an extended period of time.”¹³ Discussing sustainability consequently requires a concrete system (such as a specific software system) or a specific software-intensive system. Analysis of the sustainability of a specific software system requires software developers weigh four major dimensions of sustainability—economic, social, environmental, and technical—affecting their related trade-offs.³²

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The first three stem from the Brundtland report,⁴ whereas technical is added for software-intensive systems²⁷ at a level of abstraction closer to implementation. The economic dimension is concerned with preserving capital and value. The social dimension is concerned with maintaining communities. The environmental dimension seeks to improve human welfare by protecting natural resources. And the technical dimension is concerned with supporting long-term use and evolution of software-intensive systems. Sustainability is achievable only when accounting for all dimensions. Including the environmental dimension makes it possible to aim at dematerializing production and consumption processes to save natural resources.¹² Connections among the four dimensions involve different dependencies and stakeholders.^28,31 Potential conflicts among stakeholder interests means software developers must understand the relationships among goals of the four dimensions.

The shortcoming of current software engineering practice with regard to sustainability is that the technical and economic dimensions are taken into account while the environmental and social dimensions are not. The question we address here is how these concepts relate to software and how to break down the respective concerns into software-quality requirements. We focus on the (currently neglected) environmental dimension and its relation to the other dimensions. While most efforts in environmental sustainability through software have focused on energy efficiency, we tie the concept of environmental sustainability to other sustainability dimensions of a software system, particularly to address second-order effects,¹³ or those of a software system in its operational context, as with, say, how a car-sharing service used by many users over a number of years affects the surrounding environment.

Our contribution is a sustainability analysis framework that aids practitioners exploring software qualities related to the four dimensions and explicitly representing dependencies among the dimensions. To illustrate the application of this framework we offer two case-study examples from different domains.

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Sustainability Analysis Framework

The framework aims to capture the relevant qualities that characterize sustainability concerns of software systems, helping identify how these qualities influence each other with respect to the different aspects of sustainability (see the sidebar “Software Sustainability“). Software qualities as nonfunctional properties have been studied and adopted in software engineering. In particular, various methods for quality evaluation in software architecture have been defined to support holistic reasoning and decision making that involve software, hardware, human, and system elements. We exploited this holistic approach, defining our framework by extending an existing model, the Third Working Draft of ISO/IEC 42030 Architecture Evaluation,¹⁴ as outlined in Figure 1. The blue boxes denote generalized pre-existing components from the working draft. While the draft specifically targets evaluations, the potential context of the framework is broader, embracing any activity that relies on a sound representation of qualities, including requirements engineering, design decision making, trade-off analyses, and quality assessment.

The following paragraphs describe the dimensions used in the framework to characterize sustainability in the context of software-intensive systems:

Social sustainability. Social sustainability focuses on ensuring current and future generations have the same or greater access to social resources by pursuing generational equity. For software-intensive systems, it encompasses the direct support of social communities in any domain, as well as activities or processes that indirectly create benefits for social communities;

Environmental sustainability. Environmental sustainability aims to improve human welfare while protecting natural resources; for software-intensive systems, this means addressing ecological requirements, including energy efficiency and creation of ecological awareness; and

Technical sustainability. Technical sustainability addresses the long-term use of software-intensive systems and their appropriate evolution in a constantly changing execution environment; and

Economic sustainability. Economic sustainability focuses on preserving capital and financial value.

An evaluation criterion can be a quality requirement, as in Figure 1. In particular, as we focus on characterizing sustainability-related software qualities, we address how quality requirements relate to sustainability, or “sustainability quality requirements.” In this context, requirements could include both traditional quality requirements (such as performance, usability, security, and maintainability) and sustainability-related requirements (such as energy efficiency).

Whenever we specifically target sustainability, as in Figure 1, where the association aims to link the evaluation objective to the sustainability dimension, software developers must resolve trade-offs among the various qualities classified as belonging to each of the four dimensions. In particular, traditional software decision making considers trade-offs either between different technical sustainability criteria (such as performance versus availability) or between technical sustainability criteria and economic sustainability criteria (such as performance versus costs). In contrast, sustainability-related software decision making involves trade-offs between environmental sustainability criteria (such as energy efficiency) and social, economic, and technical sustainability criteria.

To frame software qualities this way we position them in the four sustainability dimensions and relate them to the concerns of the relevant stakeholders. For the sake of simplicity, this information is not included in the case-study examples, though the description of a paper-mill control system refers to three main stakeholders: surrounding community and society at large (concerned about environmental sustainability like forest sustainability); customers (concerned about economic sustainability like production savings expressing productivity and economic value creation); and producing organization, including managers and engineers (concerned about technical sustainability like optimization of configurability and performance).

Moreover, interdependent quality requirements may influence one another, as in association/association-class influences among sustainability quality requirements; for example, in the paper-mill control system (see Figure 2), performance and energy savings could influence each other, while increasing performance could demand more resources that consume more power and ultimately have a negative effect on energy savings. Using our framework to make these influences explicit helps designers of software-intensive systems appreciate the importance of the various qualities.

In addition, the trade-offs software developers make among qualities change depending on stakeholders (such as business customers, vendors, and society at large). If a company’s main stakeholder is a vendor, performance probably wins over energy savings; the opposite is probably true if the stakeholders are consumers. Keeping track of the elements captured by the framework is thus crucial for reasoning about the trade-offs to be made among the various qualities in the four sustainability dimensions.

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Examples

We show the applicability of the sustainability analysis framework through examples. For each, we briefly introduce the domain, then discuss its sustainability qualities and their interdependencies. We illustrate the framework’s added value with various aspects of business sustainability: stakeholders (in the first case) and specialized influences relations between qualities (in the second case). The granularity of requirements ranges from coarse-grain high-level goals to fine-grain detailed system requirements. These case-study examples are at the high-level end of this spectrum (see van Lamsweerde²⁰). Figures 2 and 3 emphasize several framework elements: sustainability quality requirements (for which we detail parameters and metrics to capture quality levels); their influences and interdependencies; and the sustainability dimension they belong to (represented as “swimlanes”). In the figures we do not propose a new notation but the approach we suggest for capturing the relations among the four sustainability dimensions. For formalizing and modeling in more detail, the notations proposed by Chung et al.⁶ are also useful. Here, we use a simple notation based on Unified Modeling Language class diagrams.

Paper-mill control system. The worldwide paper-production industry is an example of successful sustainability improvement through advances in technical and economic solutions.⁸ A typical plant control system (PCS) some 30 years ago would have involved a paper-production cycle of several days. The energy consumption would have been very high (though the cost of electricity was lower, the energy costs were three times more per ton of pulp than today); so was the pollution, mostly in a form of water polluted by chlorine compounds (water pollution at the time had just started to be an public-policy issue). A PCS would manage the entire process through a few hundred sensors and actuators. A typical plant would employ from 2,000–3,000 people, with a considerable number of them relatively uneducated, and several tens of experts who would optimize the process with respect to production quality through their experience. A PCS today can handle several hundred thousand signals while reducing the production cycle to an hour while lowering the environmental impact significantly; for example, water consumption of 200–300 cubic meters per ton of pulp in 1970 decreased to less than 50 cubic meters per ton and in some mills below even 10 cubic meters per ton. The number of employees in Swedish plants (Sweden is a major pulp and paper producer) decreased over 75%, though their qualifications increased; today, over 50% of employees are highly qualified engineers and technical specialists. Production in such plants has increased dramatically, by at least 10 times in the past 30 years.^a The main concern for mill owners today is energy savings, including energy for the technological process (such as in cooking paper pulp) and energy for the PCS. This gives environmentally sustainable software a double role: decrease energy consumption of the PCS itself, which is distributed and complex, with many devices, and decrease energy consumption of the entire production system through smart algorithms and energy-efficient technologies controlled by software. Consequently, the survival of paper-mill companies in Sweden (and worldwide) depends on all four sustainability dimensions, driven primarily by customers and competitors but also by society, including towns, cities, and municipalities, as well as the entire country.

Figure 2 includes example sustainability quality requirements, sorted by sustainability dimensions and the relations among them. We distinguish between vertical (within a dimension) and horizontal (between dimensions) relations. The social dimension refers to the changes in the infrastructure in the companies and in society needed to support requirements for employee skills. A company would need highly educated people, putting demand on their supply from society. The company would need to make a short- and long-term plan for requalification of employees, and the local society (typically a municipality or county) would need to take responsibility for retraining people. Increased education level would improve environmental sustainability awareness. Such awareness is an example of a horizontal relation. An example of a vertical relation in the environmental dimension involves the following operating environment. A company might deploy new technologies that leads to less water pollution and greater effectiveness of the process that leads to increased environment sustainability (in terms of cleaner water, less energy, reduced forest resources, and forest regeneration). However, such results would require a wise trade-off between increased production, in terms of scalability, performance, and configurability, and economic and environmental requirements; for example, increased productivity could undermine environmental demands, and addressing them would require new technologies, as well as changes in the process, including direct and indirect changes (such as selective tree cutting, paper recycling, and planting new trees) requiring changes in the technology of the control system.

The horizontal relations also reflect a balancing of stakeholder interests; trade-offs are typically required between economic and social sustainability requirements or between economic and environmental sustainability requirements. In contrast, technical requirements provide the solutions that improve economic and environmental dimensions.

This case example illustrates how the sustainability analysis framework can be applied in development processes of large, long-lived systems that require public investment and feature significant profit margins. Economic and technical sustainability are customer-driven. The environmental and social sustainability requirements do not come from the customers of the paper mill but from the surrounding community and society at large, including region and state. Due to the large public investment, society can impose requirements. Since environmental and social sustainability requirements do not come from customers, they tend to be overlooked by managers and engineers. Integrating our four-dimensional sustainability analysis framework into the engineering processes of such long-lived industrial systems provides valuable support to managers and engineers trying to satisfy not only economic and technical but also environmental and social sustainability requirements.

Car-sharing platform. In a 2013 study, we analyzed the sustainability impact of DriveNow, a München-based car-sharing platform²⁷ created to serve users who do not otherwise have access to a car for short-distance inner-city trips (see Figure 3). The primary quality requirement is significant use of the platform in the economic sustainability dimension. It is supported by a well-designed application that in turn supports (in the social sustainability dimension) strong public acceptance of the application. The focus was on the different types of influences affecting framework relations. As with any kind of requirement or goal, sustainability can be linked through various types of influence relationships, as in goals.²⁰ We focus here on support and conflict. In the following paragraphs, we discuss one requirement and its interrelations, illustrating outcomes due to direct and indirect effects on quality requirements. Environmental sustainability, in terms of energy savings, is affected in at least three ways:

GPS. For a well-designed application, reliable GPS functionality is needed, and adding it will, in turn, negatively affect energy savings in the application;

Energy. DriveNow aims to get people to share cars, leading to reduced car production, hence energy savings in production; and

Marketing. DriveNow generates revenue not only through the platform itself but also through the marketing value created by driving new cars around the city; they will be seen by potential customers who may be motivated to buy them, leading in turn to more emissions and less energy savings due to increased car production.

The result is a well-known phenomenon known as first-, second-, and third-order effects.¹³ While use of the app leads to more energy consumption due to GPS use, or a first-order effect (the direct effect of a software system), it also facilitates sharing more cars and thus reduces total energy use, or a second-order effect, the indirect effects triggered by use of a software system in its operational context. On a larger scale, the effect might turn around yet again and lead to a completely different result, or a third-order effect, systemic effects triggered by long-term, widespread use.

The original development of DriveNow did not consider all four dimensions or all these effects. The primary dimension was economic, and the secondary dimension was technical. Both social and environmental were not considered, yielding several consequences:

Social. When the service was available for only a few months and analyzed by the project team, it turned out a user community was developing in which individual users had established an understanding of themselves as part of a larger community supporting shared mobility services. Had the company’s founders and developers considered the social dimension in advance, the system’s user interface could have been developed to make it easier to form carpools among users;

Environmental. DriveNow uses mostly environmentally friendly hybrid and electric cars, providing a good basis for environmental sustainability. However, as the company’s founders and developers did not consider the environmental aspect of the service during their initial business case analysis, no green IT options were explored for the server side. Likewise, they did not do a comparative (simulation) study of how the long-term widespread use of the service would affect München traffic and parking. Consequently, the environmental sustainability of the system still needs improvement; and

Interrelation of dimensions. One example of often-underestimated relations among the dimensions our framework helps analyze is the use of electric cars, which must be driven in the right way to ensure they produce less pollution (environmental dimension). There is thus a need to offer training (social dimension) for that type of driving, including or leading to further investment (economic dimension).

While simplified, this case illustrates the importance of understanding the interdependencies among qualities by business managers and software developers alike. Our framework is useful for understanding and addressing them, avoiding dangerous pitfalls like negative business consequences and environmental effects.

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Observations

These case studies illustrate how our approach to sustainability analysis links the four sustainability dimensions that are seemingly unrelated to software qualities. Determining and analyzing the relations among the qualities, as outlined in Figure 2 and Figure 3, give decision makers a blueprint for analyzing sustainability qualities and gaining insight into sustainability stewardship. By addressing all four dimensions, the framework enables software practitioners to make trade-offs across different dimensions; for example, in the case of the paper-mill control system, a manager using the framework can easily identify not only technical and environmental but also social and economic trade-offs. The framework also helps capture the influence of various stakeholders on the various qualities regarding the four dimensions. Both studies show sustainability quality relations potentially carry positive or negative influences. Moreover, they reveal that when evaluating a system’s sustainability quality, all aspects of the system’s performance should be taken into consideration; for example, in the case of DriveNow, environmental and social dimensions were originally not included, hindering potential positive effects on the environment. The framework allows management to draw a more comprehensive picture of the relevant quality aspects and help make more-informed decisions.

Figure 2 and Figure 3 are snapshots at the time of the case studies and do not characterize the systems’ overall life cycles. The case studies, four dimensions, and relations have their own life cycles. In particular, the relations and their quantification will likely change over time; the initial deployment of infrastructure for a PCS requires a substantial energy investment up front, but situation-aware systems accrue significant benefits over time. While first- and second-order effects could indicate one trajectory in the assessment of sustainability, the effects on global goals can change or even reverse the trend. Moreover, the effect of software systems on the environment could differ dramatically depending on the framework conditions. Any concerns related to sustainability requirements must be prioritized and traded off against business requirements and financial constraints.

The notion of sustainability entails a long chain of (possibly circular) consequences across all the dimensions. When identifying the concerns pertaining to a software system, management must define the sustainability concerns directly influencing the system, the boundaries outside the scope (but that could be useful for decision making), and the boundaries too remote to be considered. The ISO/IEC 42030 working draft models the environment in which a system is situated. In our understanding of the draft, part of such an environment is within the system’s scope, while part is outside it. However, sustainability requirements and concerns likely increase system scope.

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Due to the large public investment in such an industry [paper production], society can impose requirements.

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There are also limitations as to what the sustainability-analysis framework can provide. The influences among the sustainability quality requirements must be determined by developers and/or stakeholders, as the framework can provide only the means for linking them but not the analysis itself. Constraints and parameters must be chosen by the developers, as it is not possible to list them in a way that is generic enough to be applicable in all circumstances and at the same time specific enough to be useful. The best guidance we can provide with this framework is through examples showing how to apply it and its potential benefits. Part of our own future work is to extend this guidance with further examples.

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Conclusion

This article has presented a framework for trading off sustainability quality requirements from the various dimensions of sustainability. It is based on the Third Working Draft of ISO/IEC 42030 Systems and Software Engineering Architecture Evaluation¹⁴ and a first attempt at understanding the multidimensional effect of software on its environment. It can assist software practitioners in making trade-offs, not only among technical and economic aspects of business sustainability but also in relation to society and the environment. We focus on classifying sustainability quality requirements as the first step toward sound decision making, trade-off analyses, and quality evaluation. Applying the framework enables software developers to specifically consider the neglected environmental and social dimensions in relation to the technical and economic dimensions. Using the framework, practitioners are better able to determine their sustainability goals and see the potential outcomes of the criteria.

We hope to help provide new research directions and a foundation for discussing the integration of the various ISO quality models. Our own future research will focus on how the framework’s sustainability quality requirements can be systematically deduced from a goal model while considering the effects of software on its environment. These requirements include how to refine such information in the form of constraints on design and implementation. Moreover, the resulting models could be useful for cost estimation, specifically in terms of how software design decisions affect architecture and infrastructure. Another open challenge we hope to address is “scoping,” or distinguishing sustainability concerns outside the software system but directly influencing it, so the information about such concerns could help take optimal decisions. Finally, as there are no standardized metrics for software sustainability, applying the framework can help establish sound metrics that would serve as as a basis for building satisfactory tool support.

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Acknowledgments

This work was partially sponsored by the European Fund for Regional Development under project RAAK MKB Greening the Cloud, the Deutsche Forschungsgemeinschaft under project EnviroSiSE (grant PE2044/1-1) and the Swedish Foundation for Strategic Research via project RALF3. Thanks, too, to the participants of the GREENS Workshop at the 35^th International Conference on Software Engineering in San Francisco, CA, in 2013 who contributed thoughts and ideas, especially Henning Femmer and Hausi Muller.

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