Croatian Fiscalization 2.0 Technically Ready for Even Higher Loads

Fiscalization 2.0 in Croatia entered production under strong pressure of deadlines, expectations, and differing levels of digital readiness among businesses, but the first results point to the stable operation of the system core. In an interview for ICTbusiness, Saša Bilić, President of the Management Board of APIS IT, clearly distinguishes between two things that are often conflated in public discourse: the exchange of eInvoices, which is carried out through information intermediaries, and the fiscalization of eInvoices as a separate state service.

In this context, APIS IT is a key player as the designer and maintainer of the fiscalization system, and as a participant through MIKROeRAČUN, a free application of the Tax Administration intended for the smallest entities. Bilić states that in a short period, the system processed more than 2.2 million eInvoices without major disruptions and that the architecture was designed from the outset to be robust and highly scalable.

The key lies in a container-based platform, high availability in an active-active mode, and the ability for horizontal and vertical scaling, combined with testing at load levels comparable to the peak loads of Fiscalization 1. In production, he claims, peak loads of up to 45 requests per second were recorded without significant resource strain, which is particularly important during accounting peaks. Simultaneously, Bilić emphasizes that MIKROeRAČUN is intentionally functionally limited and is not a competitor to the market of information intermediaries, but rather a safety net for those without IT support and who are not under a full obligation to send eInvoices in 2026.

According to APIS IT, the most common initial problems were not infrastructural, but semantic and integration-related: different interpretations of fields, certificates, and adaptation of existing ERP systems. In such a model, the Tax Administration remains the holder of business interpretations, intermediaries are the first line towards users, and APIS IT is the guarantor of system stability and technological evolution.

The system processed more than 2.2 million eInvoices in a very short period without major disruptions. How was scalability addressed through the IT architecture so that it could handle such a transaction volume from the first day of production? What are the key technical elements that ensure high availability and system resilience under peak loads, for example, at the end of accounting periods?

Here, it is important to emphasize that within the Fiscalization 2.0 project, two separate processes are taking place: the exchange of eInvoices and the fiscalization of eInvoices. The exchange of eInvoices is carried out through information intermediaries, while APIS IT participates only in the context of MIKROeRAČUN, a free application that the Tax Administration has made available to small and micro entrepreneurs. Therefore, in this case, we are talking about the characteristics of the eInvoice fiscalization system, which we at APIS IT designed as a robust and highly scalable solution based on a container platform. All key components operate in a highly available active-active mode and support horizontal and vertical scaling, ensuring continuous system operation even under conditions of pronounced peak loads.

The system architecture was designed and extensively tested for loads comparable to the peak loads of Fiscalization 1, which, according to estimates, are several times higher than the expected loads within Fiscalization 2.0. Production experience to date further confirms this, with recorded peak loads of up to 45 requests per second without significant load on dedicated system resources.

Such an architectural approach ensures high availability, resilience, and stable system performance, including during critical periods such as the end of accounting cycles.

More than 300,000 taxpayers timely selected an access point, with the possibility to choose between 33 information intermediaries, their own solutions, and MIKROeRAČUN. How do you assess the balance between the state’s free access point and market-based information intermediaries with commercial solutions?

The balance between the state’s free access point and market-based information intermediaries is based on clearly delineated roles. MIKROeRAČUN is a functionally limited solution intended for the receipt and fiscalization of incoming eInvoices and exclusively for a specific group of users – taxpayers outside the VAT system who in 2026 are not obliged to send eInvoices in their regular business operations. On the other hand, market information intermediaries offer advanced, commercial solutions tailored to the needs of entrepreneurs subject to full application of the Fiscalization Act, including integrations with business systems, process automation, and additional services.

Within the implementation of the Act, the Tax Administration has provided the free access point MIKROeRAČUN in order to enable the fulfilment of legal obligations for all taxpayers. Namely, this approach allows all VAT taxpayers who are obliged to exchange eInvoices to conduct their business processes smoothly, regardless of the digital readiness of their business partners.

Is there a risk that the free application could, in the long term, affect market dynamics and investments by private intermediaries?

Given the fact that MIKROeRAČUN users are small taxpayers with a significantly lower volume of invoices and no need for integration with accounting software because they are not obliged to keep business books, there is no real risk that the free application could, in the long term, disrupt market dynamics or negatively affect investments by private information intermediaries. The role of MIKROeRAČUN is exclusively supportive, not competitive to market solutions.

How is it ensured that all participants have a technically equal position in the eInvoice exchange system?

Technical equality of all participants in the eInvoice exchange system is ensured through uniform technical and security standards prescribed by the Fiscalization Act and accompanying documentation. All information intermediaries, including the state access point, are subject to the same rules of exchange, validation, and data security, thereby ensuring a neutral and interoperable system for all participants.

More than 95,000 taxpayers chose MIKROeRAČUN in the first days, and more than 130,000 eInvoices were received through it. What were the key functional criteria in the design of MIKROeRAČUN to make it acceptable for micro and small entrepreneurs without IT support?

In designing MIKROeRAČUN, the key functional criteria were ease of use, availability without technical integration, and a clear focus on basic legal obligations. The application was, I emphasize again, developed with micro and small entrepreneurs outside the VAT system in mind, who do not have their own IT support, with an emphasis on an intuitive user interface, a minimal number of steps required to receive and review eInvoices, and the absence of any need to install additional software or adapt existing business systems.

Is further functional expansion of the application planned, or will it remain a strictly basic access point?

MIKROeRAČUN is conceived as an access point intended for small business entities, providing them with a clear and simple record of incoming invoices and payment obligations. At present, no significant functional expansion towards advanced business capabilities is planned, as its purpose is to ensure fulfillment of legal obligations for users who are not subject to full application of the Fiscalization Act. This maintains a clear distinction between the free state access point and the commercial solutions of information intermediaries.

How is the actual business impact of MIKROeRAČUN on reducing the administrative burden of small entrepreneurs measured?

The actual business impact of MIKROeRAČUN on reducing the administrative burden of small entrepreneurs is monitored through operational indicators such as the number of active users, the number of received eInvoices, a reduction in the need for alternative invoice delivery methods, and the number of user inquiries and technical issues. According to announcements by the Tax Administration, the functionalities of MIKROeRAČUN will continue to be monitored and the need for enhancements assessed.

How does APIS IT address situations where different information intermediaries interpret optional fields in the national extension differently, which can cause problems in automatic posting for recipients?

Different interpretations of optional fields in the national eInvoice extension are primarily addressed through clearly defined and uniform technical rules published by the Tax Administration and available to all information intermediaries. Observed differences to date relate to deviations that do not affect tax processes or automatic postings – in most cases, we see situations where one party leaves a field empty while another, for example, enters 0.00. In cooperation with the Tax Administration, we plan further adjustments that will ensure such interpretations are not treated as differences, because in essence they are not.

Is there a centralized validator available to users for pre-checking the correctness of the structure before submission for fiscalization, to reduce the number of errors in production?

For the purpose of verifying technical correctness, the Tax Administration provides validation mechanisms that check the structure and basic compliance of eInvoices with the standards prescribed by the EU norm, with the Croatian extension. This reduces the number of technical errors in actual system operation, while information intermediaries and business solution vendors are responsible for additional internal checks and adaptations to the specific business needs of their users.

You emphasize that the system was developed and tested in cooperation with accountants, information intermediaries, and the business sector, but that certain initial difficulties were observed. What types of technical or business misalignments most frequently appeared in the first days of production?

The eInvoice fiscalization system was developed and tested in multiple phases, with active participation of representatives of professional associations of accountants, information intermediaries, and the business sector. Despite extensive testing, in the first days of production operation, expected initial difficulties were observed, primarily arising from different levels of technical readiness among participants and differences in the interpretation of certain elements of the specification.

The most frequently observed misalignments related to semantic errors in data, especially in the use of optional and nationally extended eInvoice fields, as well as incorrect or improperly configured certificates, and occasional issues in integrating existing business systems with new services. There were no technical infrastructure problems that would have had a significant impact on overall availability. I would also note here that the Tax Administration continuously publishes instructions and answers to frequently asked questions on its official website, and taxpayers are directed to these sources as the central and authoritative place for all relevant information.

How is the operational support model organized between APIS IT, the Tax Administration, and information intermediaries?

The operational support model is organized through clearly defined cooperation between the Tax Administration, APIS IT, and information intermediaries. APIS IT ensures the stability and technical maintenance of the central infrastructure, the Tax Administration provides regulatory and business interpretations, while information intermediaries are the first line of support towards end users of their solutions. Such a model enables rapid separation of technical and business issues and more efficient resolution of reported problems.

Are there clearly defined SLA indicators for incident resolution, and how are they monitored in practice?

Internal service level indicators (SLAs) are defined for the system and continuously monitored through monitoring tools and operational reports. Although individual SLA values are not publicly communicated, system availability and responsiveness are monitored in real time, and incidents are classified by impact and priority, with clearly defined escalation procedures.

Given that this is critical infrastructure, does the state guarantee a certain SLA for service availability (for example, 99.9 percent), and are there protocols for handling longer system outages (so-called business continuity plans for entrepreneurs)?

Given that this is critical state infrastructure, the system is designed for the highest level of availability and resilience. There are defined protocols for handling extraordinary situations, including technical failures or temporary service interruptions, as well as recommendations for taxpayers on how to proceed in such circumstances, ensuring business continuity and legal certainty for taxpayers.

Which types of errors dominate in support tickets: certificate issues, server timeouts, or semantic data errors?

Analysis of received tickets shows that the largest share consists of semantic data errors and structural inconsistencies in messages, while a smaller portion relates to certificate issues and rare communication delays. This confirms that challenges in the initial phase are more related to the adaptation of business processes and interpretation of standards than to the technical stability of the system.

APIS IT has a key role as the state’s technological partner. What did cooperation with private ERP vendors and information intermediaries look like during system testing and development?

Cooperation between APIS IT and information intermediaries during development and testing was intensive and continuous. Through workshops, technical consultations, testing on the Compliance Testing Portal, and the exchange of feedback, the system was adapted to real business scenarios, which significantly contributed to stability in production operations.

How open is the system to upgrades and participation of external technology partners — is there an API or sandbox environment for integrating new solutions?

From the outset, the system was designed to be open and upgradeable. Standardized APIs and the Compliance Testing Portal are available to information intermediaries and solution vendors, enabling integration of new solutions, gradual introduction of upgrades, and long-term system sustainability.

How does APIS IT assess the impact of this project on future digital state services, such as automation of tax processes or e-documentation?

For us at APIS IT, this project is part of a complex program of Digital Transformation of the Tax Administration and represents one of the new foundations for further development of digital public services. The acquired experience and built infrastructure create the prerequisites for further automation of tax processes and – most importantly – simplification of administrative obligations for citizens and entrepreneurs, and a key step in modernizing business operations.