Prerequisites

## Link-Service - Specification
Create a RESTful JAVA service which is able to transform a http link into a minimized and trackable link. 
The service should be able to track a click on such a generated link and also redirect the user to the initial URL.

Example: https://www.happycar.de/info/versicherung/document/some/very/long/path will become https://link.happycar.de/abc123

### Technical Requirements
- JAVA8
- Spring MVC
- Maven or Gradle (Gradle preferred)
- JUnit4

### Code Requirements
- Google Java Style https://google.github.io/styleguide/javaguide.html
- Proper Documentation

### Functional Requirements
- RESTful API (CRUD)
- Link Tracking (e.g. referral) and Redirection

Used frameworks and technologies

• JDK 1.8
• IntelliJ IDEA 2016.1 Community Edition
• Spring MVC (REST API provider)
• Spring Boot (Used for correct packaging purposes)
• Gradle (Project build tool)
• JUnit (Unit-testing framework)
• CheckStyle (Code analyzer)
• FindBugs (Code analyzer)

API spec

• POST: "/link/register" body: String link - registering simple link
• POST: "/link/register/trackable" body: String link - registering link with UTM params for trackability
• GET: "/lnk/**" - redirecting from short link to initial link

Application workflow

1. Register link using one of two provided endpoints
   1. Register simple link using - POST: "/link/register"
   2. Register trackable link using - POST: "/link/register/trackable"
2. Access one of short already registered links in order to redirect to initial long link

Assumptions made

• Trackable links - as per (https://support.google.com/analytics/answer/1033867?hl=en), trackable links are the links which have special mandatory UTM fields in order to enable tracking of those links in Google Analytics. However, trackable links should be created by issuer and that is why the only API endpoint was created in order to receive already trackable links from 3rd-party issuers, check that all UTM fields are presented and minify them as is. Service https://goo.gl/ uses similar approach.
• Minified links storage - the solution for link minimisation and storing was used very similar to https://goo.gl/ service. First of all, unique ID got generated out of Alphabet, which contains latin lower case and upper case chars as well as arabic digits (62 characters in total). After that we check if this ID was not presented before and store the ID together with big initial link if it was not. The ID is 6 chars length and is capable of having 62^6 unique values which covers up enormous amount of links. In current implementation <ID, Link> pairs are stored in simple HashMap container which is not persisted after application shutdown. In real application, there would probably be a key-value database (maybe something like Redis).
• Mocking frameworks - additional advantage for testing this application could be a mocking framework like Mockito. It would be useful, for example, when testing LinkController#redirect() method in order to mock HttpServletRequest and HttpServletResponse objects. As mocking frameworks are not presented in technical requirements, they were not used.

How to start application

gradlew clean build
java -jar build/libs/tryout-1.0.0.jar