Employing the Once-Only Principle in the domain of the
electronic public procurement
Maria Siapera1, Konstantinos Douloudis1, Gerasimos Dimitriou1, and Andriana
1 Department of Digital Systems, University of Piraeus, Greece
Abstract. This paper presents a case study for the application of the “Once Only
Principle” (OOP) for cross-border public services where citizens and businesses
provide data only once in contact with public administrations. In order to support
the European Union (EU)’s vision of a Single Digital Market, a generic federated
OOP architecture has been developed, within the context of the European Union
funded “The Once-Only Principle Project” (TOOP). The federated architecture
is showcased in a real-life use case scenario that demonstrates in practical ways
how OOP can be implemented to aid the elimination of administrative burdens
in the domain of electronic public procurement. More specifically, the scenario
focuses on the case of the Greek “European Single Procurement Document”
(ESPD) public service, acting as a system that requests and receives data from
the TOOP infrastructure. Finally, the paper presents the results of testing connec-
tivity scenarios between the Greek and other Member State systems.
Keywords: Once-Only Principle · interoperability · eProcurement · data reuse
· interconnection · cross-border public digital services
There is no doubt, that Information and Communication Technology (ICT) systems are
nowadays the core of government processes, and enterprise business as well. In addi-
tion, one of the core strategies of the European Commission is that of a Digital Single
Market , meaning an internal European Union (EU) market that people are able to
move freely and conduct their business outside of their home country, in any Member
State. Thus, there is a significant need for citizens, enterprises and organizations to
carry out their business seamlessly and easily in any Member State of the EU.
There are many practical difficulties though, for businesses engaging in cross-border
activities. Things like, for example, complying with different national regulations or
with different administrative requirements of foreign public agencies, form serious im-
pediments. On the other hand, public agencies also face many difficulties when it comes
to collecting, but also verifying the validity of foreign official information on compa-
nies and their legal representatives due to multiple data sources and the cost of manual
back-office procedures. In this context, the interaction with public services and the gov-
ernments within the EU needs to be also reformed and digitally “transformed” to keep
up with the new digital economic pace and reality. There are several steps to be taken
though, in order to ensure the improvement of the delivery of services. These steps are
made not only by providing tools, utilizing eGovernment practices and rethinking in-
ternal processes , but also by making them a significant priority through political
commitment at the EU level. This kind of political commitment is showcased by the
signing of the Tallinn Declaration on eGovernment at the ministerial meeting during
Estonian Presidency of the Council of the EU on 6 October 2017 . As part of this
digital reform, the European Commission is taking concrete actions for the develop-
ment of Cross-Border Digital Public Services , such as the creation of European
interoperable platforms that implement the “Once-Only Principle”  (OOP) and the
provision of guidelines for the better use of ICT systems of public authorities. Some-
thing that aims at aiding the plans of the EU regarding the Digital Single Market, by
reducing the administrative burden in the Member States.
According to the “Once Only Principle”, sharing information and data across the
public administration systems is cheaper than collecting them more than once from cit-
izens and businesses. Thus, the OOP is based on the simplification of administrative
processes and the notion that information is collected only once and then can become
available and shared across the different systems that request for this information, re-
specting constraints that are imposed by regulations. This kind of digital information
availability, not only increases the efficiency of the public administrations but also re-
duces the time and cost of the administrative processes, improving the quality of public
services, both at national levels and cross-border .
To address the OOP related challenges and ease the implementation of OOP in cross-
border settings, the Once-Only Principle Project (TOOP)  was selected and funded
under the scope of the “EU Horizon 2020 Research and Innovation Funding Pro-
gramme” . TOOP aims to facilitate the cross-border application of OOP by demon-
strating it in practice through multiple real wide-scale pilots  using a federated ar-
chitecture that is designed on a pan-European collaboration, enabling the connection of
different registries and architectures in different countries and domains for better ex-
change of information across the public administrations . TOOP’s main goal is to
demonstrate the feasibility of its proposed federated architecture in real-world pilots.
This means that the aforementioned pilots are systems that are either already in produc-
tion, conducting real transactions connected to TOOP or are starting technical develop-
ment and will reach production level within the lifetime of TOOP .
In order to explore the viability of TOOP’s proposed federated architecture, there
are three pilot areas selected that serve as proof of feasibility across Europe. These pilot
areas are the following:
1. Cross-border eServices for Business Mobility
2. Updating Connected Company Data
3. Online Ship and Crew Certificates
Each pilot area consists of different pilot use cases and usage scenarios that are of
interest to the participating Member States. Such a particular use case scenario, of in-
terest, is the Greek ESPD (European Single Procurement Document) system  which
demonstrates the use of OOP in the eProcurement domain within the first pilot area and
it is further examined in this paper.
2 Background and Motivation for the study
2.1 Digital Single Market Strategy and Once Only Principle
In October 2015, the “Single Market Strategy” was introduced by the European Com-
mission (EC) . According to the EC, this strategy is “at the heart of the European
project” , foreseeing that people, services, and goods are freely moving within the
EU. The vision of the Single Market Strategy is backed by Initiatives and Action Plans,
ensuring that citizens/businesses can fairly access goods and services, irrespective of
their nationality and place of residence. One of the strategy’s backing Action Plans is
the “eGovernment Action Plan 2016-2020” .
The eGovernment Action Plan 2016-2020 focuses “on the wide-scale implementa-
tion of eGovernment”, making sure that the citizens and businesses in the EU can ex-
perience the tangible benefits of technological enablers being used in public services.
According to that, there are three policy priorities identified:
• Public administration modernization using digital enablers such as Con-
necting Europe Facility (CEF)’s  building blocks (i.e. eID, eSignature,
eDelivery, and eInvoicing);
• High-quality public services that ease the digital interaction between ad-
ministrations and citizens/businesses;
• The facilitation of mobility of citizens/businesses by cross-border interop-
To enhance cross-border activities within the EU, the introduction of the “Once Only
Principle” (OOP) was made by the European Commission. According to the eGovern-
ment Action Plan 2016-2020, the EC defines the Once Only Principle as follows:
“public administrations should ensure that citizens and businesses supply the same
information only once to a public administration. Public administration offices take
action if permitted to internally re-use this data, in due respect of data protection rules,
so that no additional burden falls on citizens and businesses” .
In October 2017, the Tallinn Declaration on eGovernment was signed by the minis-
ters of 32 Member States of the EU. The declaration focuses on the benefits and poten-
tial savings to be achieved, through political commitment to “take steps to identify re-
dundant administrative burden in public services and introduce once-only options for
citizens and businesses in digital public services” . Meaning that both the Tallinn
Declaration and the eGovernment Action Plan 2016-2020 establish the “Once Only
Principle” and data re-use, as political priority and commits to reduce administrative
burdens in public services.
2.2 Once Only Initiatives and Implementations
As part of the eGovernment Action Plan 2016-2020, two projects were funded under
the “EU Horizon 2020” research and innovation program in order to address the OOP
challenges: “The Once-Only Principle Project” (TOOP) and the “Stakeholder Commu-
nity Once-Only Principle for Citizens” (SCOOP4C) . TOOP, which was launched
in January 2017 and is being coordinated by the Tallinn University of Technology in
Estonia, focuses on reducing the administrative burden for businesses by employing the
OOP. TOOP’s proposed solution is based on already existing working systems and
building blocks in the Member States implementing “multiple sustainable pilots by us-
ing a federated IT architecture on cross-border, pan-European scale.” . The
SCOOP4C project, launched in November 2016 and coordinated by the University of
Koblenz-Landau in Germany, aims to “investigate, discuss, and disseminate how co-
creation and co-production in public service provisioning for citizens can be achieved
by implementing the once-only principle”. .
Another case of an initiative employing the OOP is the Business Registers Intercon-
nection System (BRIS)  providing cooperation across European business registers.
It provides to the citizens and the businesses an interface for accessing information
about companies and their branches across all Europe.
There are also many examples of using OOP in national implementations. For ex-
ample, in the Netherlands, the Dutch Tax office provides pre-filled tax reports. Citizens
don’t have to manually fill in tax forms, but only check and change things in case of an
2.3 eProcurement and the European Single Procurement Document
eProcurement refers to the exchange of supplies, services, goods and work through
the Internet or any other electronic channels  and is the focus of the 2014/24/EU
Directive . According to the EC , “public procurement is undergoing a digital
transformation. The EU supports the process of rethinking of public procurement pro-
cesses with digital technologies in mind”. eProcurement is considered at the heart of
EU Directives (2014/24/EU  on public procurement, 2014/23/EU  on the award
of concession contracts). The purposes of these directives are enhancing the free move-
ment of goods and services during procurement processes by making them simpler and
easier for the Small Medium-sized Enterprises (SMEs) to gain access to procurement
processes and bid for public contracts. Taking into consideration the lessening of ad-
ministrative burdens and the continuation of the public procurement reform across the
EU, the EC established the standard form for the ESPD  issuing an implementing
regulation at January 5, 2016.
An ESPD “is a self-declaration of the business’s financial status, abilities, and suit-
ability for a public procurement procedure . The access and participation in cross-
border tendering opportunities is simplified since the tenderers no longer need to pro-
vide full documentary evidence and different forms previously used in the EU procure-
ment. In addition, the service is available in all EU languages and can be used as
preliminary evidence of condition fulfillment required in public procurement proce-
dures across the EU” . With the introduction of the ESPD, companies can partici-
pate in procurement processes without having to submit various documents as evi-
dences satisfying certain criteria in order to participate. Only the winner of a procure-
ment process needs to provide the actual documents and evidences. Almost all EU
countries provide national ESPD services, since the use of the ESPD is obligatory. Also,
since all Contracting Authorities are legally obliged to accept the standard layout of the
ESPD, the Economic Operators can easily and safely qualify for any public tender in
Europe increasing the competitiveness.
3.1 The Generic Federated OOP Architecture
The aim of TOOP’s generic federated OOP architecture is to support the intercon-
nection and interoperability between national registries at the EU level. The architec-
ture consists of loosely coupled service components which utilize already existing
available building blocks, such as the CEF  eID and eDelivery infrastructures and
standards. The main benefit of re-using such components is the fact that they have al-
ready been used by the Member States and proven successful, avoiding the imposition
of any technological specific solutions on citizens, businesses and public administra-
tions. Fig. 1 shows a component architecture overview. A legal entity that can be either
a natural or legal person is the user of a Public eService provided by a Member State.
The Member State Authority providing the public service that requests and receives
data from a registry in another Member State, acts in the role of Data Consumer (DC),
whereas the Member State Authority providing the data acts in the role of Data Provider
(DP). The TOOP infrastructure fetches information from the Member State Authority
who acts as a DP making it available to the foreign Public Authority who acts as a DC.
The end-user only needs to provide minimum information in order to be identified,
authenticated and authorized allowing the TOOP infrastructure to discover what infor-
mation is available for the needs of the DC Service. The cross-border interoperability
is guaranteed via retrieving information in machine-readable format, packaged in a spe-
cific TOOP message format that uses the TOOP semantic building block in order to
preserve meaning. In addition, an eDelivery Gateway is used providing secure transport
between the DC and the DP and trust establishment between the participants.
As shown in Fig. 1, green components are deployed centrally either at TOOP or CEF
infrastructure. Components in purple consist the TOOP connector, which is provided
by TOOP as a facilitation, but Member States are free to replicate its functionalities
using their own development. Components in blue are the responsibility of the Member
State systems. It is worth to mention that since TOOP’s architecture is a reference ar-
chitecture, there is no fixed way of how to deploy TOOP services. TOOP is providing
components and guidelines as a facilitation but does not impose their usage.
The central components include the following:
• TOOP Directory: provides global search functionality over all participants,
• CEF BDMSL (Business Document Metadata Service Location): facilitates
the retrieval of participant identifiers.
• Semantic Mapping Service: maintains mappings between national data
models and the TOOP Common Semantic Model.
The services that need to be deployed at a Member State (MS) level are the follow-
• Service Metadata Publisher (SMP) Service: decentralized registry that links
identifiers to endpoint URLs and certificates for document exchange.
• eIDAS Node: Optional component that oversees identification and authen-
tication of the user.
• TOOP Connector: Optional component that encapsulates several functions
that support the data exchange from one participant to another. Some of its
main functions are the transformation of data input into a proper TOOP
message, routing metadata discovery and endpoint discovery. It is provided
by TOOP as facilitation, but its usage is not imposed to the Member States.
• AS4 (Applicability Statement 4 standard for the secure and payload-agnos-
tic exchange of business-to-business documents) Gateway: eDelivery Gate-
way that provides secure transport between the participants.
The DC User Interface, DC Business Processing and DP Business Processing com-
ponents can vary and be specific to each Member State’s eService.
Fig. 1 Component Architecture
3.2 ESPD Service as TOOP Data Consumer
3.2.1 Use case description
A typical usage scenario of the ESPD service with the TOOP integration is the follow-
ing: Firstly, a Contracting Authority (CA) has published a call for tender containing
among others the ESPD template for a specific call. The legal representative of an Eco-
nomic Operator (EO) that wishes to take part in the tender process, gets the ESPD re-
quest from the public call for tender and is being authenticated to the ESPD service,
providing information for the identification of the company for which creates an ESPD
response and the country of origin. The ESPD service requests via the TOOP Infra-
structure the requested information (i.e. company details) by the designated DP of the
country of origin. The TOOP DP receives the request, processes it and sends a response
back to the ESPD service. Upon a successful TOOP response reception from a DP, the
ESPD service validates the response, extracts the available data from the response and
after getting the user’s consent, automatically fills the part of the ESPD form that is
relevant with the requested information. Afterwards, the ESPD system asks of any ad-
ditional information and generates the requested ESPD. The legal representative pre-
views the generated ESPD response, completes and submits it and finally, proceeds
with the tender process.
The EOs can easily and safely qualify for any public tender of a CA in Europe since
all CAs are legally obliged to accept the standard layout of the ESPD. This fact makes
the use case a perfect instance of demonstrating the elimination of redundant provision
of data, by the legal representatives of the business’s during the fulfillment process of
an ESPD, through the TOOP infrastructure.
3.2.2 Advantages integrating TOOP with an ESPD Service
The provision of company details, as well as the provision of evidences to selection
and exclusion criteria defined by the CAs is cumbersome for EOs, making them hesitant
on participating in cross-border procurement procedures. Provision of a pre-filled ver-
sion of the ESPD, through the TOOP infrastructure, alleviates some of the challenges
that the EOs face, since it reduces the time and hassle of providing valid company data.
Traditionally, these would have to be provided either in a form of a document (i.e. a
certificate) or filled in by a business representative manually. Τhe required data can be
pulled directly from verified sources, such as the Business Registries that the EOs are
registered, by connecting the Business Registries as TOOP DPs and the National
ESPD/eProcurement services as TOOP DCs, thus avoiding information being submit-
ted by the EOs more than once.
Furthermore, the eIDAS Regulation  is put into practice providing a predictable
regulatory environment to enable secure and seamless electronic interactions between
businesses, citizens and public authorities. People and businesses can use their own
national electronic identification schemes (eIDs) to access public services in other
Member States since companies that have their information in Business Registries that
act as TOOP DPs, can access those data by using their eIDAS credentials. The ESPD
service that acts as a TOOP DC asks its users for their eIDAS credentials and consent
for requesting any data required, respecting and taking into consideration regulations
about the protection of personal data like the General Data Protection Regulation
Data exchange is implemented in a way that reduces human interaction during the
provision and consumption of the exchanged information, aiming as much as possible
to machine-processable information sharing. In addition, the services are open to all
Member States, avoiding bilateral agreements that do not scale EU-wide and thus, elim-
The ESPD service that acts a TOOP DC can connect and fetch data from every
TOOP DP that is available in the TOOP eDelivery Network. There is no need for spe-
cific point-to-point connections due to TOOP’s federated architecture. The TOOP DC
requests for data and then the TOOP Infrastructure is responsible for the discovery and
retrieval of the data.
Finally, the messages sent and received across the TOOP infrastructure are pack-
aged, signed and encrypted in a certain way before transmission, ensuring their integ-
rity, establishing trust between the TOOP DC and the TOOP DP.
3.2.3 The Greek ESPD Service as TOOP Data Consumer
Greece is participating in the TOOP infrastructure as a DC with the Greek ESPD
service called “Promitheus”. Each of TOOP’s pilots deploy the required subset of
TOOP service components necessary for the specific application scenario. Likewise,
only the TOOP components necessary for the ESPD service scenario are deployed.
The implementation architecture, as shown in Fig 2, follows the TOOP architecture
as described previously. The existing ESPD application is used without modification to
its core components. A component called “TOOP Interface” acts as a bridge between
the ESPD application and the rest of the TOOP infrastructure. The ESPD application
can request data in its own format while the “DC Interface” (instance of TOOP Inter-
face) converts that to a “TOOP Data Request” which is the proper request that can be
made through the TOOP network. The “TOOP Response” that comes back can also
then be converted to data that the ESPD application can work with.
Fig. 2 TOOP architecture of the Greek Data Consumer
TOOP has defined a multi-step process to conduct tests that check the readiness and
maturity of the pilot MS implementations. The last step of these series of tests is called
a “Connectathon” which consists of thorough connectivity tests between a MS DC and
a MS DP following the pilot scenario. When enough MSs are ready, a “Connectathon”
can take place. In each “Connectathon”, every MS uses their implementation and at-
tempt to exchange data with other MSs through a set of specified test scenarios. These
include example sets of data and some test cases for the DP and DC software. The
results of these exchanges are logged and discussed in order to address any issues that
may come up. This method of testing was selected because it simulates a real-life sce-
nario of exchanging data within the TOOP premises and aids in discovering issues with
the TOOP components and the MS’ implementations.
Table 1. Successful Connections made with the Greek ESPD service acting as TOOP DC
Currently, the Greek ESPD Service acting as a TOOP DC has been successfully inte-
grated and exchanged information with the Swedish, Slovakian, Slovenian, Italian, Ro-
manian, Austrian, Polish and Norwegian systems acting as TOOP DPs, as it is shown
in Table 1. The Greek ESPD service managed to exchange information with eight (8)
different Member State systems by only connecting once to the TOOP infrastructure.
There is no need for point-to-point connection between the Member States due to the
way the TOOP federated architecture is designed.
5 Conclusions and Future Work
To sum up, this paper has focused on the ongoing work done in the General Business
Mobility pilot area within the context of the TOOP project, especially the use case of
the Greek ESPD Service, acting as a TOOP DC in the eProcurement domain. It is an-
ticipated that more Member States will join, acting as TOOP DPs and demonstrate more
cross-border exchanges. Finally, regarding the Greek ESPD service, there is ongoing
work with the purpose of using the TOOP infrastructure to discover and retrieve Evi-
dences and Documents, needed for the fulfillment of a CA’s criteria requirements at the
time of awarding or when is needed and not only fetching e.g. company data, thus elim-
inating the need for the businesses to upload documents from their local IT-
Acknowledgments. This work is supported financially by the TOOP (The Once-Only
Principle) Project, which is partially funded by the European Union’s Horizon 2020
research and innovation program under G.A. No. 737460. We acknowledge the work
and contributions of the TOOP project partners.
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