# Challenges and Solution Directions of Microservice Architectures: A Systematic Literature Review
## 3 Research Methodology  
This article aims to identify the state of the art of MSA and describe the challenges in applying MSA and the corresponding solution directions. To this end, a systematic literature review (SLR) or systematic review was applied following the guidelines by [PERSON] [8]. This is a well-known protocol for performing systematic literature reviews in the software engineering community. The basic activities of the review are shown in Figure 1. The SLR starts with defining the research questions followed by a definition of the search strategy and the identification of the study selection and elimination criteria. Subsequently, study quality assessment criteria are defined and the data extraction form is developed. Once these steps are ready, the data synthesis method is developed. The research questions (RQs) of this SLR are given below:  
Figure 1: Activities under the review protocol. RQ1. What are the identified challenges of microservice architectures? RQ2. What are the proposed solution directions? The studies published between January 2014 (which is the date when MSA was first defined by [PERSON] and [PERSON] (2016)) and February 2022 were included in the SLR due to the fact that MSA was introduced in 2014. The electronic digital libraries included in the search were (in alphabetical order): ACM Digital Library, IEEE Xplore, Science Direct, Springer, and Wiley Inter-Science (see Table 1). Journal papers, conference papers, workshop papers, and books were considered potential search items. We used both automatic and manual search. The automatic search was performed by defining search strings using the APIs of the corresponding search databases. This was complemented with a manual search in which we used snowballing techniques. Hereby, the reference list of the selected primary studies is analyzed and the relevant studies are selected as primary studies (forward snowballing). On the other hand, we have looked at papers that cite the selected primary studies (backward snowballing). For selecting the primary studies, the following query was used:  
((\"micro service\" OR \"microservice\") AND (\"challenge\" OR \"obstacle\" OR \"difficulty\" OR \"difficulties\" OR \"problem\"))  
We identified and used the exclusion criteria listed below to eliminate the studies that were irrelevant for this SLR:  
EC 1: Studies with abstracts/titles that do not discuss MSA. EC 2: Studies with abstracts/titles that do not bring an approach to MSA. EC 3: Studies without a full text. EC 4: Duplicate studies retrieved from different digital libraries. EC 5: Studies that are not in English. EC 6: Studies that do not explicitly discuss the challenges of MSA. EC 7: Studies that relate to MSA but are experience and survey papers. EC 8: Studies that present the application of MSA and do not critically reflect on MSA concepts. The application of the exclusion and quality criteria eventually resulted in the selection of 85 papers from the initial set of 3842 papers. The subsequent step included the quality assessment of the resulting primary studies, for which we used the quality checklists as defined in ([PERSON], 2016). Accordingly, for the quality assessment, we used the checklist in Table 2. For the assessment scale we adopted a three-point scale, (i.e., yes = 1, somewhat = 0.5, no = 0). The scores for the assessment of the primary studies are provided in Appendix B. The authors performed a quality assessment. In case of conflicts, the results have been discussed, and a final conclusion has been provided. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Source**} & **\# Studies** & **\# Studies** \\  & **Initially Retrieved** & **After Applying Exclusion/Quality Criteria** \\ \hline IEEE Xplore & 233 & 48 \\ \hline ACM & 755 & 12 \\ \hline Springer & 1619 & 10 \\ \hline Science Direct & 978 & 11 \\ \hline Wiley & 174 & 4 \\ \hline
**Total** & 3842 & 85 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Publication sources searched.