# A Survey on Blockchain-Based Search Engines
## 3 Search Engines and Their Approaches
### Centralized Search Engines and Their Problems  
The primary approach for conventional search systems is centralized. The centralization in this context refers to the controlling methods. The centralized are controlled by a single group [18]. Large commercial search engines (such as Google, Yahoo, and Bing) use this paradigm. According to the architecture of search systems, almost all of them consist of three parts, including a web crawler for collecting information, a file system or  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **File Sharing** & **Video** & **Cloud, Fog** & & & **Messaging** & & **Database** & **Web Hosting** \\  & [6; 21] & **Streaming** & **Computing** & **[22; 30]** & **Computing** & **[29; 30]** & **System** & **[31; 32]** & **[33; 34; 35]** & **[36; 37]** \\  & & & & **[8; 24; 25; 26] & & & & & \\ \hline
**Security** & & \(\surd\) & \(\surd\) & \(\surd\) & \(\surd\) & & & \(\surd\) \\
**Privacy** & & & \(\surd\) & \(\surd\) & \(\surd\) & & & \(\surd\) \\
**Access** & & & & \(\surd\) & & & & \(\surd\) & \(\surd\) \\
**control** & \(\surd\) & & & \(\surd\) & & & & \(\surd\) & \(\surd\) \\
**Anonymity** & \(\surd\) & \(\surd\) & & \(\surd\) & & & & \(\surd\) \\
**Data** & \(\surd\) & & & \(\surd\) & & & & \\
**Integity** & \(\surd\) & & & & & & & \\
**Visibility** & \(\surd\) & & & & & & & \\
**Data** & & & & & & & & \\
**protection** & & & & \(\surd\) & & & & \\
**Safety** & & & & & & & & \\
**Trusted** & & & & & & & & \\
**Computing** & & & & & & & & \\
**Scalability** & & & & & & & & \\
**Imunutability** & & & & & & & \(\surd\) \\
**Fault** & & & & & & & \(\surd\) \\
**tolerance** & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Applications and blockchain capabilities.  
database to store information, and a searching subsystem to find the relevant document to a query [40; 41].  
The following is a brief description of two primary subsystems, including the crawler and the searcher:  
* The crawler: This offline subsystem retrieves web pages and creates a text-based index. A web crawler is a type of software that, given one or more seed URLs, downloads the web pages associated with these URLs, extracts any hyperlinks, and then downloads the web pages recursively [42; 43].
* The searcher: This subsystem, which is an online component, processes a stream of queries. The search algorithm ranks documents depending on various parameters, including the quantity of anchor text hits and the document's PageRank. Counts of hits of different types at various proximity levels are computed for each matching document. These counts are then routed through many lookup tables before being translated into a rank. The purpose of searching is to deliver high-quality results quickly.  
From the point of view of architectural structure, these subsystems can be distributed. For example, as stated in [44], Google uses several distributed crawlers to handle hundreds of millions of web pages and save the retrieved data in multiple distributed servers. Although these companies may use distributed architecture, they have a centralized/monopolistic control that leads to many problems such as privacy violation and monopoly, as explained below.  
* Privacy: user privacy violation is one of the most critical problems of popular search engines, with the typical track user searches and other online activities. They make user profiles by storing user search information and analyzing it to improve their [45; 46; 47], processes.
* Monopoly: Monopolization about search engines means that they manipulate the search results. In other words, monopoly means the deviations from some kind of \"objective\" or \"unbiased\" standard for delivering search results. The enormous market power of companies such as Google provides monopolization for these companies [48]. This monopoly results in information distortion (disinformation) [49], loss of social welfare, and, for sites not covered, the loss of economic value [50].