web app introduction

Rewritten-docs.md

@@ -238,7 +238,7 @@ addons (mostly administrative features).
   assignment); otherwise, the solution with most points is used
 - teacher can edit student solution and privately resubmit it; optionally saving
   all results (including temporary ones)
-- localization of all texts (UI and exercises)
+- localization of all texts (user interface (UI) and exercises)
 - Markdown support for creating exercise texts
 - tagging exercises in database and search by these tags
 - comments, comments, comments (exercises, tests, solutions, ...)
@@ -1666,7 +1666,56 @@ be implemented in next releases.
 
 ### Web-app
 
-@todo: what technologies can be used on client frontend side, why react was used
+The web application is one of the possible client applications of the ReCodEx
+system. Creating a web application as a client has several advantages:
+
+- no installation or setup is required on the user's device
+- works on all platforms including mobile platforms
+- when a new version is rolled out all the clients will use this version without
+any need for installing an update manually
+
+One of the downsides is the large number of different web browsers (including
+the older versions of a specific browser) and their different interpretation
+of the code (HTML, CSS, JS). Some features of the latest specifications of HTML5
+are implemented in some browsers which are used by a subset of the Internet
+users. This has to be taken into account when choosing apropriate tools
+for implementation of a website.
+
+There are two basic ways how to create a website these days:
+
+- **server-side approach** - user's actions are processed on the server and the
+HTML code with the results of the action is generated on the server and sent back
+to the user's Internet browser. The client does not handle any logic (apart from
+rendering of the user interface and some basic user interaction) and is therefore
+very simple. The server can use the API server for processing of the actions so
+the business logic of the server can be very simple as well. A disadvantage of
+this approach is that a lot of redundant data is transferred across the requests
+although some parts of the content can be cached (e.g., CSS files). This results
+in longer loading times of the website.
+- **server-side rendering with asynchronous updates (AJAX)** - a slightly different
+approach is to render the page on the server as in the previous case but then
+execute user's actions asynchronously using the `XMLHttpRequest` JavaScript
+functionality. Which creates a HTTP request and transfers only the part of the
+website which will be updated.
+- **client-side approach** - the opposite approach is to transfer the communication
+with the API server and the rendering of the HTML completely from the server directly
+to the client. The client runs the code (usually JavaScript) in his/her web browser
+and the content of the website is generated based on the data received from the API
+server. The script file is usually quite large but it can be cached and does not
+have to be downloaded from the server again (until the cached file expires).
+Only the data from the API server needs to be transfered over the Internet and
+thus reduce the volume of payload on each request which leads to a much more
+responsive user experience, especially on slower networks. Since the client-side
+code has full control over the UI and a more sophisticated user interactions
+with the UI can be achieved.
+
+All of these approaches are used in production by the web developers and all
+of them are well documented and there are mature tools for creating websites
+using any of these approaches.
+
+We decided to use the third approach -- to create a fully client-side application
+which would be familiar and intuitive for a user who is used to modern web
+applications.
 
 @todo: please think about more stuff about api and web-app... thanks ;-)