## 6 Conclusions
### Limitations of the Approach  
_Pros._ The proposed tag-based datastore aimed to provide the right tradeoff for query flexibility, data confidentiality, smart contract security, and minimizing the financial cost of the datastore. The advantages of the proposed datastore includes its simplicity, flexibility, user access control, and user management. The simplicity of the smart contract design minimizes the likelihood of introducing security vulnerabilities and reduces the gas consumption of its operations. The flexibility of the design supports various applications (such as sensing data logging and genome analysis) and use cases (such as storing plain or encrypted tag and data).  
_Cons._ The obvious limitation of the proposed datastore is that it relies on Ethereum. Additionally, the emphasis on simplicity would inevitably force the users to perform additional operations, such as encryption and extraction of the needed answer from the returned result of a datastore read. The current system does not yet offer a graphical user interface. Furthermore, the number of applications and use cases is relatively limited.  
In the future, we plan to improve the current design and system implementation by addressing these limitations. First, we will compile more applications of the proposed secure datastore beyond sensing data logging and genome analysis. Second, we will enhance the datastore frontend with more wrapper functions to reduce the burden on the users (i.e., data producers and data consumers). For each type of application, some wrapper functions could be developed such that the datastore resembles more a traditional database system. For example, the data insertion and query functions similar to what has been used in the genome analysis could be offered by the database frontend. Internally, the calls would be transformed so that our tag-based smart contract can be used. Third, we will port the tag-based smart contract from Ethereum to other blockchain platforms (such as Hyperledger) that support Turing-Complete smart contracts. Finally, we will enhance the research prototype with a graphic user interface and a suite of examples in Python and JavaScript. We plan to share our implementation source code as a public GitHub project.  
Conceptualization, W.Z., H.U. and L.L.; methodology, W.Z.; software, I.M.A. and W.Z.; validation, I.M.A., W.Z., H.U. and L.L.; formal analysis, W.Z.; investigation, I.M.A. and W.Z.; resources, W.Z., H.U. and L.L.; data curation, I.M.A. and W.Z.; writing--original draft preparation, I.M.A. and W.Z.; writing--review and editing, W.Z.; visualization, W.Z.; supervision, L.L.; project administration, H.U. and L.L.; funding acquisition, H.U. and L.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by US Department of Energy grant number DE-FE0031745.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** We would like to thank the anonymous reviewers for their highly constructive criticisms and invaluable suggestions. This work was supported by the United States Department of Energy Award DE-FE0031745.  
**Conflicts of Interest:** The authors declare no conflict of interest.