**6. Conclusions**

The distributed ledger technology has the potential of being a game-changer in many domains, its recent developments being triggered not only by technology expectations but also by social ones. DLTs have enabled the development of decentralized applications but this process is still complicated due to the high availability of new and rather immature technological solutions that address different implementation issues. DLTs and especially blockchain are an effervescent innovation area. Therefore, the review of technological solutions and implementation guidelines are needed to improve understanding and to ease the development of decentralized applications.

Most of the similar initiatives found in the literature are focusing on the blockchainbased distributed ledger and pay little attention to other DLT implementations. In this paper, we have provided a review of the existing DLT solutions. We highlight their applicability, advantages, and disadvantages concerning other technologies. A significant number of solutions have been reviewed to provide a holistic image of the DLT implementation variations. In the technology review process, we have considered references from both academia and the private sector.

At the same time, our review was driven by a decentralized application architecture that has been used and validated in previous publications. The DLT solutions are categorized according to the 3-tiers of the architecture thus the criteria for selecting the technologies and organizing the review are strictly related to the development issues that may be encountered and need to be addressed in each tier.

Finally, we provide a guideline for decentralized application development defining specific steps for decentralizing a system design and business model using DLT and for implementing and launching it as a decentralized application. No similar guideline could be found in the reviewed literature. The guideline can be used to streamline the nowadays efforts for investigating how the DLT can be applied, integrated, and used for decentralizing systems such as medical systems, electricity grids, financial sector, etc., and for implementing new decentralized applications.

**Author Contributions:** Conceptualization, T.C. and I.S.; methodology, T.C. and I.A.; investigation, C.A. and M.A.; writing—original draft preparation, C.A. and M.A.; writing—review and editing, I.S. and T.C.; visualization, I.A.; supervision, I.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the European Union's Horizon 2020 research and innovation program gran<sup>t</sup> number 957816 (BRIGHT) and gran<sup>t</sup> number 774478 (eDREAM).

**Data Availability Statement:** Not Applicable, the study does not report any data.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
