HOSN-app/thesis/content/04-concept/restrictions.tex

When designing the hybrid \ac{OSN}, there are a few limitations that need to be considered, for which appropriate solutions have to be found. These restrictions include:

\begin{itemize}
	\item \textbf{\ac{API} of the \ac{OSN}}: Ideally the \ac{OSN} offers a public \ac{API} with full functionality. An API consists of multiple endpoints that can be used to retrieve, add, or modify specific data. Since the user's data should be protected as best as possible, access via the \ac{API} is usually restricted. The restriction may be due to a limited number of requests per time interval or a limited range of offered functions.
	\item \textbf{Crawling the \ac{OSN} web pages}: If there is no official \ac{API} or if it is sharply restricted, the contents can theoretically also be extracted by crawling. However, this brings with it several challenges. Modern web pages load many contents asynchronously so that the initial \ac{HTML} does not yet contain these contents. Furthermore, there are sophisticated mechanisms that notice crawling and lock out crawlers. Likewise, it may be difficult to add data to the \ac{OSN}. For security reasons, in most cases, special tokens are sent along with each request to detect and prevent abuse and fake requests.
	\item \textbf{Development, operation and licensing costs}: Costs for the development, operation and licensing of third-party software may incurred. At best, conscious decisions lead to the avoidance of expenses.
	\item \textbf{Operating system or runtime environment}: Nowadays \acp{OSN} can be used on almost all devices; independent of their operating system. In order to achieve the same user experience, the hybrid \ac{OSN} should be usable on the same variety of operating systems. Any restrictions imposed by the operating system (user and application rights, connectivity) must be taken into account during development.
	\item \textbf{Resources}: The mostly mobile devices (Twittter is used by more than 80\% of users on mobile \cite{google2017twitter-mobile}) running the hybrid \ac{OSN} may have limited resources (storage space, processing power, Internet connection/data volume, battery). When making design decisions, it is important to plan as resource-conserving as possible and to find scalable solutions. Overall, the overhead for the hybrid extension should be as low as possible compared to the original application.
	\item \textbf{Availability of data}: The data that is exchanged securely and not via the \ac{OSN}'s servers must always be available. Whether a user is offline or how old the data is must not affect its availability.
\end{itemize}

While the restrictions on the hybrid client itself can be actively influenced and resolved, the restrictions on the \ac{OSN} cannot be controlled. If the \ac{OSN} does not provide any interfaces and the hurdle of data exchange with the servers is insurmountable, this can completely prevent the development of a hybrid client.