# Software Engineering Applications Enabled by Blockchain Technology: A Systematic Mapping Study
## 4 Results  
### Trend of Studies That Use Blockchain in Software Engineering (RQ1)  
The interrelation between blockchain and SE, in particular, the applications of blockchain in SE have been overlooked according to [PERSON] and [PERSON] [30]. This is also supported by the low number of studies identified in our systematic mapping. We retrieved a total of 999 studies by searching four online databases. These studies went through a two-stage assessment process against inclusion/exclusion criteria and as a result, 18 studies were selected. This indicates a high number of irrelevant studies retrieved (98% noise), which is common in database searches [39]. We observed some of the irrelevant studies to understand the main reasons for exclusion. Studies were excluded in the first assessment phase mainly because they explore blockchain in other more mature fields, such as supply chain and healthcare. Additionally, studies were excluded in the second phase mainly because they focus on software engineering issues in blockchain-oriented software, which is outside the scope of our study. We also carried out snowballing, as complementary to the database search. In total, we selected 22 studies. A plausible explanation of the low number of studies could be related to the novelty of blockchain technology itself. While it is true that Bitcoin, the first blockchain application, was invented in 2008 by [PERSON], smart contracts became popular with the release of Ethereum in 2015, as a Turing-complete blockchain platform. A recent systematic mapping study on blockchain-based smart contracts carried out by [PERSON] et al. [40] found out an increasing trend of publications since 2016. The first study that we identified was published in 2015 and it uses cryptocurrency blockchain technology for decentralized software license validation [41]. As expected, this study does not make use of smart contracts. The first study that implements smart contracts was published in 2017 with the goal to develop a non-stoppable virtual organization for software development communities [29]. The remaining studies (91%, 20/22) were published during the last three years (2018-2020) and they were devoted to addressing issues in collaborative software development, by means of blockchain technology. This trend indicates growing research efforts in blockchain-based software engineering (see Figure 3). The growing trend seems to respond to [PERSON]'s [8] call for more studies in the 1 st International Workshop on Blockchain Oriented Software Engineering (BOSE). Moreover, the selected studies were published in conferences and workshops (see details in Appendix A), which suggest a conference-driven publication culture. \begin{table}
\begin{tabular}{c c} \hline \hline
**Contribution Type*** & **Explanation** \\ \hline Model & Studies that propose a novel model or improve existing ones. \\ Framework & Studies that propose a conceptual framework to guide the development of a solution. \\ Interviews & Studies that aim to explore a particular technology such as blockchain in the field of SE, by means of interviews. \\ Platform & Studies that propose platforms that add value to existing business models or establish new ones. \\ \hline \hline \end{tabular}  
* The contribution type of the studies is based on what the authors of these studies claim to contribute (in cases where they specify their contribution type).  
\end{table} 
Table 4: Contribution Types. Figure 3: Distribution of studies based on publication year and publication venue. Figure 4 shows that all the studies except [42] propose solutions or validate them in experimental settings, while 17 out of 22 studies contribute by means of models or frameworks. Many of the studies (10 out of 22) propose solutions that need to be implemented and tested to assess their strengths and weaknesses. We identified only one evaluation study [42]. The authors of this study investigate the impact of blockchain in global software development, by conducting interviews with industry practitioners.