Newsletter‎ > ‎

Newsletter - Volume 4 Issue 1 - July 2015

A bulletin of the IEEE Computer Society Special Technical Community on Sustainable Computing

Providing quick access to timely information on sustainable computing.

Sponsored by:

Message from the editor

Cristina Rottondi, Politecnico di Milano am grateful to the STC community for giving me the chance to serve as the newsletter chair and I thank the former editor Diwakar Krishnamurthy for his great efforts and valuable work in leading the newsletter. 

I also welcome Dr. Ozlem Bilgir Yetim as new Industry Chair, who have volunteered to curate a report on the May 2015 Greenpeace report “Clicking Green: A Guide to Building the Green Internet”. 

The other contributions of this issue are a discussion on the convergence of emerging networking infrastructures and the digitalization process by Dr. Vittorio Trecordi; a report by Prof. Patricia Lago and Prof. Daniela Damian on the first track on Software Engineering in Society held by the International Conference on Software Engineering; an overview on energy efficiency and resiliency issues in cloud computing by Carlos Colman-Meixner and an overview on research activities focused on energy efficiency issues in software defined networking by Dr. Paola Grosso, with particular reference to the conference paper “Green Routing in Software-Defined Data Center Networks Based On OpenNAAS” co-authored by Hao Zhu, Jose Baranda, and Cees De Laat and presented during the Fourth International Conference on Green IT Solutions (ICGREEN2015). I thank them all for their interesting insights!

Greenpeace report “Clicking Green: A Guide to Building the Green Internet”.

By Ozlem Bilgir Yetim, VMware

In May, Greenpeace has published their 2015 “Click Clean” report. The report shows that more than a half dozen major internet companies (including major players like Apple, Facebook and Google) are committed to use 100% renewable energy. Moreover, second tier of major internet companies have also started to investigate how to increase their renewable energy options.

In Table 1, the clean energy index of major Internet companies and colocation companies are shown.

Key findings of this scorecard are:

  • Despite its rapid growth, Apple still holds its leading position in renewable energy usage. Last year, it announced three new data center expansions of all of which will be powered with renewable energy. Moreover, Apple also pushes major colocation providers to maintain its 100% renewable energy usage.
  • Colocation companies are still far behind consumer-facing data center operators in terms of looking for renewable energy for their operations. Equinix took an important step in this area and committed to 100% renewable.
  • In terms of renewable energy deployment, Google matches Apple. However, monopoly utilities (especially the ones in North and South Carolina, Georgia, Singapore and Taiwan) seriously threaten its goal of using 100% renewable.
  • Unlike Apple, Facebook and Google, Amazon’s commitment to 100% renewable energy lacks transparency. Moreover, it does not guide Amazon’s investment decisions toward renewable energy yet.
  • Video streaming contributes significantly to our online footprint. 
  • Microsoft’s cloud footprint continues to grow drastically due to its attempt to catch up with Amazon in this sector. However, it stays behind of Apple and Google in terms of its renewable energy usage.
  • In order to reach their renewable energy goals, data center operators need to increase their efforts to push policy makers for changes to overcome the resistance of monopoly utilities. 

Cloud Power: Streaming Video on the Rise

Internet usage has been growing dramatically. By the year 2018, the total number of devices connected to the internet will be almost twice the world population. Major contributor to the internet data is the video streaming. By the year 2018, YouTube, Netflix, Hulu and other video streaming services are expected to contribute to 76% of the total consumer traffic.

Figure 2 shows the expected energy consumption breakdown for ICT sector.

Figure 2. Electricity consumption for the ICT sector

Renewable Power for the Cloud: Drivers and Barriers

Electricity costs are the major expenses in data centers. In addition to the brand and customer driven concerns about the environmental impact, high electricity costs motivate data center operators to consider renewable energy, as well as energy efficiency for their future energy needs. 

Renewable energy production has been rising in the recent years. However, in order to limit the amount of global warming to 2 degrees Celsius and prevent the catastrophic impacts of climate change, more investments are necessary. According to the report, in order to achieve this goal, clean energy investment need to exceed $500 billion per year by 2020, and reach $1 trillion per year by 2030.

Government renewable goals and private sector renewable purchases & investments are the key renewable energy drivers.

Monopoly utility markets are one of the barriers to renewable energy. Utilities such as North Carolina (Duke Energy), Virginia (Dominion Power) and Georgia (Southern Company) in the US, or Asian utilities such as Taipower in Taiwan, rely heavily on dirty resources to generate electricity. Up to 70% of global internet passes through Northern Virginia. However, due to Dominion Power’s high reliance on dirty sources, major data center operators, including Amazon and DuPont Fabros Technology face challenges in adapting to renewable energy.

Similarly, Duke Energy is the monopoly utility in North Carolina. Its heavy reliance on dirty sources causes a significant challenge for the top data center operators, including Apple, Facebook, and Google to achieve their 100% renewable energy goals. While Duke agreed to provide large costumers renewal energy option in 2013, due to the design and price structure for the Green Rider Tariff imposed, no company has agreed to purchase renewable energy. 

Similar to Duke and Dominion, Taipower in Taiwan is the monopoly utility and uses only 4% of renewable sources. Even though it recently established a renewable energy tariff program, it is designed with a built-in premium over current rates. In order to force these monopolies to improve their investments on renewable energy, legislators and data center operators need to work together.

Another significant barrier to renewable energy is its effect on utility companies' financial gains. Utilities in both the European Union and the United States position against renewable energy to prevent lost profits.

Data center hubs continue to grow in the European Union. Utilities use high percentages of renewable energy but their strategies since 2000 have aligned with extending conventional energy sources. Increasing electricity demand and their view that renewable energy would not be profitable reinforced these strategies. In 2014, 10 utilities formed the Margritte Group to lobby against renewable energy. On a positive note, Netherlands continues to further its support for wind farms, and companies like Apple and Google have successfully sourced renewable energy. However, a greater effort is necessary to remove the utility barriers to achieve 100% renewable energy.

US utilities, such as Edison Electric Institute, pose reluctance to renewable energy. Their concerns have centered around distributed solar power since centralized power generation is the major source of their profits. This goes against the IT companies', such as Facebook's, stand on using renewable energy because distributed power generation complementing the power from the grid is necessary to fulfill their power needs.

As shown in Figure 2, many big IT companies withdrew their collaboration with American Legislative Exchange Council [ALEC] due to its participation in attempts to stop clean energy and climate policies in US.

Figure 3. Timeline of IT companies that have left ALE

Report motives companies to not to limit their support for renewable energy solely to increasing their own options. Increasing the diversity of renewable energy sources is attractive to all customers, both in terms of long-term cost and energy security.

Road Map to a Green Internet 

Energy transparency is necessary for costumers to assess the environmental impact of their energy suppliers. In terms of energy performance reporting, some operators do not use Power Utilization Effectiveness (PUE) anymore since it fails to provide an understanding of the environmental impacts of a data center. Scope 2 Guidance update to the WRI/WBCSD Greenhouse Gas Protocol is a recent metric used in reporting. Scope 2 Guidance requires reporting of “(1) the energy mix associated with the local grid; (2) details on what market-based purchases of electricity have been utilized, including details on the resource type of the supplying facility, facility location, and facility age”. Apple is a good example for adapting energy transparency, whereas Amazon do not provide any details on its energy footprint.

The significant growth in renewable energy commitment forces energy utilities to offer renewable energy.  Moreover, location of a new data center is also a very important bargaining chip against energy utilities. Data center operators have the greatest amount of leverage with local energy utilities and policymakers prior to location selection and they can use this leverage for their own benefits. Apple, Google and Facebook are good examples of companies which committed to renewable energy recently. On the other hand, Amazon and Microsoft announced expansion in their brown data centers.

Energy efficiency and greenhouse gas mitigation is also important for the green internet roadmap. Akamai takes a right turn by providing its energy performance target and regularly updating it. On the other hand, Microsoft defends carbon neutrality and meets it claims by purchasing carbon.

Strategies for increasing renewable electricity supply should focus on three principles; additionality (i.e. help increasing the renewable energy that goes into the grid, thus reduce brown energy usage), using sustainable sources and being local so that companies can secure renewable 

supply of electricity.

Powering Data Centers with Renewable Energy: A User Manual

There are different options for companies to improve their green energy usage. Apple leads the onsite investments for renewable energy.  There are Power Purchase Programs (PPAs) which secure financial returns for green energy providers and provide guaranteed pricing for data center operators. Even though, Renewable energy credits (RECs) - or their European equivalent, guarantees of origin (GOOs) have also been used in renewable energy trading, many IT companies (notable exceptions are Intel and Microsoft), has been moving away from buying them. In many places in US and EU, Direct Access (DA) programs are available. Through these programs, companies can access to energy suppliers other than their local. Green energy tariffs, which sell 100% renewable energy, have also become popular in recent years. These options help increase the renewable energy usage.

Smart (networked) Everything: long way to the promise land of a widely digitalized world

By Dr. Vittorio Trecordi, CEO of ICT Consulting

Pervasive digital technology

Digital technology is progressively penetrating and transforming all areas of society and business.

It’s in the inherent nature of digital systems to break legacy processes and business models, mainly based on human intervention. The pervasive interlock of work and information flows happening across the immaterial world of digital systems and the material/natural world put all existing industries and social activities under heavy pressure. Digitalization is progressively changing the shape of the playing fields, breaking the barriers that kept each business in a segregated silos and blurring the boundaries of market segments and market players. The direction is towards cooperating distributed intelligence, based on pervasive and bandwidth rich networking capabilities. The hyperconnected web spanning large central cloud and miniaturized and dispersed computing/storage platforms is the digital ground where proper orchestration and management of massive amount of real time data (big data) is put in place to achieve business goals in a totally new way. Of course, digital units are equipped with sensor/actuator interfaces to bridge the finalization of business goals in the material/natural world. Smartphones/tablets and other personal or family/business equipment offer convenient gateways between digitally mediated material world and human beings. 

The new utility landscape

Information and communication is expected to be extensively, on-demand and by default available as other commodity utility services (e.g. water and power). However, it’s worth pointing out that the pervasive nature of digital is extensively affecting also utility industries and markets, such as power systems moving towards smart grids. The push towards widespread digital innovation has generated the rush to well-known emerging hypes such as Smart Cities and Internet of Things. As a consequence, utilities are widely undergoing  digital transformation building on distributed intelligence and widespread network systems primarily designed and deployed to achieve own business goals. 

Future-proof networking capabilities 

Networking capabilities are key to pave the way to the future widely digitalized world. The massive increase in data volumes exchanged by fixed and mobile, wired and wireless networks calls for and extensive upgrade of networks.  Emerging needs push for bandwidth increase and widespread network coverage, while preserving secure communication requirements and continuous service availability, at a reasonable cost. Wired networks are facing the development of extensive and deep fiber plants roll-outs, to progressively replace telephony network copper access (based on twisted pairs) and pave the way to deliver ultra-broadband speeds ( from 100 to 1000 Mbps and over to each network termination) in a stable and predictable manner. Wireless networks are struggling to increase efficiency in the exploitation of spectrum, a scarce resource by definition, in order to cope with increasing capacity and coverage requirements. Wireless carriers are complementing own core networks and macro-cell radio access coverage with heterogeneous access techniques, building on an integrated architecture considering small-femto cells, distributed antenna systems and unlicensed technology (based on WiFi evolutions). C.RAN (Cloud or Centralized Radio Access Network) technology is also being considered to exploit the benefits of centralization of baseband functionality, to be shared across several of radio nodes (performance and economic benefits are expected in terms of baseband pooling, enhanced coordination between cells, virtualization, network extensibility as well as energy efficiency). From the standpoint of spectrum usage, the migration of TV from analog to digital offers the opportunity to release spectrum regions below 1 GHz for mobile broadband communications, while whitespaces frequencies are being considered for long-range low rate/power communication suitable for Internet Of Things application (see LoRa and Weightless initiatives).  Beyond that, the progressive penetration of  video distribution in wired network redirects part of the traffic load of video to wires, thus leaving wireless for mobility needs as more appropriate.

Capital investments and market players

Large capital investments are  needed to develop the required network developments and the telecom industry is struggling to find the economic sense for the challenging undertaking. Telecom operators are facing the need to invest while revenues and margins from legacy businesses (voice, telco messaging) are shrinking and new service streams enabled by the future network platforms are being offered over the top (and by players that are not directly committed to network investments: well-known OTTs). The structure itself of telecom market is under scrutiny by policy makers and regulators to capture the implications of the emerging landscape and understand the new parameters of competition. Telecom market policy is facing uncertainty subject to decisions in relevant and influent topics such as: network neutrality, the appropriate number of market-players (i.e. limitations and remedies to face the effects of network consolidation on competition), the geographical scope of telecom markets (e.g. national vs EU-wide unique telecom market). 

Blurring the boundaries of markets and businesses

The overlap of businesses brought forward by digitalization drives a wider discussion on policy and regulation across different fields: telecom and power business overlap in the development of emerging smart worlds. 

Power companies plan the roll-out of own telecom platforms to serve own needs in terms of communications serving smart grid monitoring and controls and smart metering, as well as intelligent home and smart building, smart lighting and supporting smart transportations. Gas companies are planning to roll-out own smart control and metering networks. Here comes the need to consider the opportunity to explore synergies among different needs driving the roll-out of massive and costly wired and wireless networks. Synergies are possible in cross-utility sectors (electricity and gas), as well as between utilities and telecom industries.

The Italian telecommunication marketplace is an interesting lab of such opportunity-threat game brought forward by digitalization. The Italian Government, following EU recommendations,  finalized in march 2015 the plan for the development of ultra-broadband networks [1] aiming at achieving 2020 DAE objectives [2]. The recovery from late deployments has pushed the Italian Government to take actions to foster private investments, by complementing them with public funding, as well as to stimulate synergies with other utilities planning the roll-out of fiber networks. Consequently, the major Italian Electric Power distributor (ENEL) has announced the availability to deploy fiber in the access network [3] as part of its program to roll-out the new-generation power meter, and to make new fibers available to telecom operators. The process for the strategic plan to be finalized and implemented is in progress, however a number of discussions are emerging:

  • Both telecom operators and energy companies rolling out fiber plants deserve the binding and not negotiable need to exclusive intervention on own plants for maintenance, building on level of service commitments and on security;
  • Both telecom operators and energy companies are struggling to intercept public funds to contribute to own massive investments and are looking forward to extract benefits and revenues from fiber access networks (maintenance fees, rental fees, fees for services that can be built on the digital platform).  

The need for modern policy intervention

The scoping of discussions to create the ground for digital world should necessarily transcend the boundaries of existing markets, i.e. market definitions themselves should be reviewed. The cross-sectorial policy and regulatory action struggles with the need to face and re-compose many conflicting interests. 

Digitalization and technology convergence drive opportunities and threats at the same time: market players are scrutinizing the emerging landscape and putting under discussion from the foundation their positioning to find a role in the future. Also, the role of public direct intervention (last resort to face “digital divide” in a broader sense with respect to plain network reach) is being reconsidered as an option to build portions of the digital platform (back to network access public monopoly  is sometimes returning on the spot of policy makers). Market bottlenecks are changing: access networks are a well-known bottleneck and deserves attention of policy-makers, but also emerging “networking gateways”, such as search engine and app marketplaces currently in place as well as coming Internet of Things platforms, call for deep scrutiny of open minded and knowledgeable policymakers. 

The emerging scenario puts into evidence the need for policy-makers to establish fundamental principles for the emerging digital world to develop, considering the inputs coming for the wide range of market players impacted by the breakthrough transformation and defining a balanced, stable and flexible market ground. 


[1] “The Italian strategy for next generation access network”, march 2015, Presidenza del Consiglio dei Ministri, .

[2] “Digital Agenda in the Europe 2020 strategy”, EU Commission,

[3] “Enel CEO: Italy could have a high speed tlc network in 3-5 years”, may 28, 2015, Laura Serafini, Ilsole24ore,

Energy-efficient programmable networks.

Dr. Paola Grosso (University of Amsterdam)


Software-defined networking (SDN) is a novel paradigm that is changing the way networks operate. Traditionally, it is the protocols running in the switching and routing hardware that make decisions on packet forwarding, e.g. which outgoing link should each individual data packet be sent to. SDN enables individual applications to 'program' directly into the hardware the forwarding behavior, allowing more flexible and tailored network response. Concretely, the control plane (routing decision) is decoupled from data plane (data forwarding) and moved to a centralized controller that can be accessed directly by the end user software. In essence, networks can be programmed.

From the beginning SDN was seen as a very appealing architecture to increase the energy-efficiency of network operations. Network traffic could be sent more easily to devices powered with cleaner power sources, or exhibiting smaller power profiles. The assumption was that 'green routing' techniques, which rely on the precise traffic and topology information, could be easily accomplished leveraging the controllers in SDN networks. 

Still, to accomplish this goal there are some important features that need to be present. Firstly, it is necessary to provide easy-to-use interfaces for the end users to express the desired services to the underlying SDN-capable devices.  Secondly, there must be methods and models to gather and consume and reason on the information about power consumption of the devices. 

In our research we leveraged an existing framework, OpenNaaS, to prove the suitability of SDN as architecture for the delivery of sustainable network services. OpenNaaS was the outcome of a EU-funded project called Mantychore, and it has since been extensively extended thanks to the research program of the Pan-European National Research and Education network GEANT [ref-GEANT].  The advantage for us to rely on OpenNaaS is that it provides the unified interfaces to request energy-efficient network services. Furthermore it has a complete view of the entire network and it interacts directly with the data plane through its SDN capabilities. 

We augmented this framework with three necessary features: 1. Energy monitoring; 2. Energy description models; and 3. Green routing capabilities. 

Our energy monitoring relies on the data coming directly from power meters and from the metrics derived from other available information, e.g. CO2 emission can be calculated if the power sources are known. We measure the power usage of single devices, as well as the power usage of an entire network route.

Our energy models rely on an OWL ontology called EDL to categorize power and energy information. The use of a Semantic Web information model provides reasoning capabilities that can be exploited when making decision on how to program network routes.

Finally, our green routing algorithm selects the most efficient path for the minimization of the power consumption, or alternatively the reduction of electricity costs or CO2 emissions.

Thanks to our work, it will be now possible to engage in extensive studies in real networks, such as GEANT, of the actual energy efficiency of programmable networks.

Energy-Efficiency and Resiliency in Cloud-Computing 

Carlos Colman-Meixner, University of California, Davis. 

The adoption of cloud computing is growing very fast, at an estimated rate of 20 % each year, with prevision to reach a 160 billion US$ market by the year 2020 [1].  This market growth requires to be sustainable in terms of environmental and societal impact, and that means that energy efficiency as well as resiliency of cloud-computing services and infrastructures will be driving the evolution of cloud systems in the next year. . Energy efficiency is key for sustaining the increasing demand of cloud-computing capacity, and to minimize the operation expenditure (OPEX) of servers, racks, and, generically, datacenters. Cloud-computing sustainability not only requires energy efficiency, but also high resiliency, which is key for increasing user's trust in the cloud.

Several studies focused in providing solution for energy efficiency [2] and resiliency [3] in cloud computing, however most of them consider each objective separately given their trade-off (i.e., maximizing resiliency requires more energy consumption).  In fact, few approaches focused in cloud-computing sustainability considering joint support to energy efficiency and resiliency. In the following we overview two of them: a sustainable cloud communication approach, and a sustainable cloud processing approach

The sustainable cloud communication approach, called Energy-Aware Shared Backup Protection with Sleeping Mode [4], consists in the combined use of backup connections to provide resiliency and sleeping mode technique to save energy in a cloud infrastructure of inter-datacenter networks.  The approach reduces energy consumption especially under low traffic, because the primary routes are consolidated in active routers and backup routes tend to be placed in sleep mode in regular operation and be active when failure occurs. However, the increase capacity consumption is the price that we need to pay for the sustainability.

The sustainable cloud-processing approach, or dynamic power- and failure-aware approach introduced in [5] provides sustainability of cloud-computing virtualization.  This approach uses as core technique, "VM clustering or consolidation of working and backup VMs into PMs or servers to avoid idle equipment and/or put those idle equipment into sleep if is necessary to save energy".  To achieve this goal, Ref. [5] suggested three algorithms for VM allocation considering time constraint.  An extension of this approach is presented in [6]. 

One important aspect of cloud-computing sustainability is the multi-objective nature of the problem given the tradeoff between energy consumption and resiliency. In fact, some studies in VM placement in clustering introduce a multiple-objective optimization approach which is applicable for this type of problem [7].

In conclusion, joint approaches for cloud-computing energy efficiency and resiliency are a new and exciting topic for research. To deal with the intrinsic tradeoff nature of this problem, multi-objective optimization approaches are recommended as potential extensions of recent works.  


[1] Luis Colombus, "Roundup of Cloud Computing Forecasts and Market Estimates, 2015", Forbes Magazine, (Access on July 2015), link:

[2] Toni Mastelic, Ariel Oleksiak, Holger Claussen, Ivona Brandic, Jean-Marc Pierson, and Athanasios V. Vasilakos. "Cloud Computing: Survey on Energy Efficiency", ACM Computer Survey 47, 2, Article 33, Jan. 2015. Link:

[3] Ravi Jhawar and Vincenzo Piuri, "Fault Tolerance and Resilience in Cloud Computing Environments" R. Jhawar, V. Piuri, "Fault Tolerance and Resilience in Cloud Computing Environments", in Computer and Information Security Handbook, 2nd Edition, J. Vacca (ed.), Morgan Kaufmann, 2013. ISBN: 978-0-1239-4397-2.


[4] Bing Luo; Liu, W., "The Sustainability and Survivabiltiy Network Design for Next Generation Cloud Networking," In proc. IEEE Ninth International Conference on Dependable, Autonomic and Secure Computing (DASC' 11), Dec.2011, pp. 555-560.


[5] Sampaio, A.M.; Barbosa, J.G., "Dynamic Power- and Failure-Aware Cloud Resources Allocation for Sets of Independent Tasks", In Proc. IEEE International Conference on Cloud Engineering (IC2E’13), March 2013.


[6] Altino M. Sampaio, Jorge G. Barbosa, "Towards high-available and energy-efficient virtual computing environments in the cloud", Future Generation Computer Systems, Volume 40, Nov. 2014, pp. 30-43.


[7] Lopez Pires, F.; Baran, B., "Multi-objective Virtual Machine Placement with Service Level Agreement: A Memetic Algorithm Approach", In Proc. IEEE/ACM 6th International Conference on Utility and Cloud Computing (UCC’13), Dec. 2013, pp. 203-210.


Software Engineering in Society at ICSE.

By Patricia Lago, VU university of Amsterdam, and Daniela Damian, University of Victoria    

The ICSE 2015 program featured for the first time a new track titled Software Engineering in Society (SEIS). ICSE organization felt that it was time for the flagship conference in software engineering to recognize and tackle more explicitly the role and impact of software engineering in a sustainable society. This first SEIS received 39 submissions (24 long and 15 short), among which five long papers and six short papers have been selected for presentation.

Co-chaired by Patricia Lago (VU University Amsterdam) and Daniela Damian (Unversity of Victoria), the SEIS program kick-started with a panel (titled “Software Systems and Sustainability - Principles and Commitments”) and three regular conference sessions organized as short paper presentations with mini-panels engaging attendees in what turned into very lively and inspiring debates. We (Patricia and Dana) are very grateful to our session moderators, panelists, as well as Antonia Bertolino (ICSE 2015 General Chair), Gerardo Canfora and Sebastian Elbaum (ICSE 2015 Program Co-chairs) for their trust.

The Panel discussed the need for the community to be able to tell industry what they can do for developing software that supports sustainability: while sustainability is very much used and misused by organizations, it is still far away from their business agenda. In spite of the implicit (or sometimes evident) financial benefits of going green, the software industry does not invest (yet). Regulation and incentives from Governments is necessary to kick-start a fundamental change of focus. SE researchers can help industry toward outward thinking and influence society. Finally, teaching SE students social- and science responsibility is mentioned as being part of the role of SEIS.

The three mini-panel sessions focused on the relation between SE and: value and concerns, sustainability enabling techniques, and users. The following summarized some take-home messages:

  • In this intrinsically-interdisciplinary area, keeping customers (stakeholders) in the loop is key, and goes beyond simple involvement (cf. agile software development principles)
  • The economic impact of SE sustainability should be always addressed as a way to quantify the ROI and convince educators, practitioners, the public sector to dedicate attention and resources
  • SE should focus on both using computing resources for sustainability purposes; and not using resources that are not needed. Especially in relation to the current trend toward smart-systems and IoT, SE research should we aware of the risk that the more we optimize the least resilient society becomes. 

We thank all participants of the SEIS sessions: the room was always packed, the discussion lively and inspiring. The power of multi- and inter-disciplinary research was never more present. SEIS will continue its discussion at ICSE 2016, we look forward to it. Soon you will find online the ICSE 2015 SEIS proceedings (on the IEEE Digital Library).