# Query Processing in Blockchain Systems: Current State and Future Challenges
## 3 Application Domains Identified through Literature Review
### Registries  
A registry is an authoritative data source of records, usually maintained by a government agency. For instance, a land registry specifies who is permitted to use land, for how long, and on which conditions. Although the registry is maintained by a central authority, several other parties have to have access to the data in order to enable an economic and healthy business environment for the sale and purchase of property [62]. Only a few countries maintain a functioning land registry, which is still often based on paper-based documents, leaving them vulnerable to loss, misuse, or corruption. As a result, delays in ownership transfer or tampering with the land register are possible and bound to happen on a regular basis [63]. Another problem is that some registries exist duplicated in siloed entities so that this fragmentation might cause possible data conflicts and therefore, no single view of truth [64].  
It is obvious that the use of blockchain technology can also provide a solution to all of these problems. On the one hand, blockchain technology ensures that documents are available to all participants almost immediately after they have been added to the blockchain. This eliminates unnecessary delays in processing that occur when paper-based documents are shipped. As a result, all participants always have the latest state of a document at their disposal and conflicting copies of one and the same document cannot exist [65]. On the other hand, the use of blockchains reliably prevents the forgery of documents due to the characteristics of a blockchain, i.e., its immutability and tamper-resistance. Since no central authority can gain full control over the blockchain, corruption is also not a problem as long as the majority of the participants are honest [66]. Obviously, it must be ensured that insights from the documents are not made public. However, this can be achieved by means of access policies and tailored permissions restricting the access of individual parties to the data. Such an approach is acceptable in terms of fraud protection as long as the blockchain itself is still governed by multiple entities [67].  
This benefit is also demonstrated by many research papers for other registries, e.g., [PERSON] et al. [68], [PERSON] et al. [69], [PERSON] and [PERSON] [70], [PERSON] et al. [71], and [PERSON] and [PERSON] [72].  
Based on this research, we can conclude that there is only one type of data in registries:  
* The data entered into registries are usually documents (i.e., semi-structured data) that are modified over time. Typically, the latest state of a document is of importance, but in cases of conflicts, its history is also required (e.g., in court). In the context of registries, queries regarding the latest of a certain document (as well as its history) are particularly relevant. Moreover, a data subject can be part of multiple registries, e.g., one registry containing all house owners and one containing all vehicle owners. In order to determine all properties of a certain data subject, a join between all available registries is required. Typical queries therefore include, but are not limited to:
* Retrieve the latest state of a specific document.
* Retrieve the latest state and a prior state of a specific document to highlight changes in the latest state.
* Join two or more registries on a certain attribute to get a holistic view of all stored documents.