Stack Overflow

FR

1.User can add question which will have a title , body and a list of tag
2.User can add a answer for a particular question
3.one vote by a user for an item
4.author of the question can mark any answer as accepted
4.search a question by tag and optionally by a keyword in title

NFR

1.scale : system should be able handler increase number of question, answer and votes
2.Concurrency : system to be able to handle concurrent request on vote up and down
3.Extensible: system should handle different kinds of search

Entities

Tag :

-id
-name

User
-userID
-name
-emailID

Question extends Votable**
**-questionId: int
-tags : Set<Tags>
-user : User
-title : String

-creationTime: timestamp
-body : string
-answers : List<Answer>
-Answer acceptedAnswer

+addAnswer(Answer)
+acceptedAnswer(Answer)
+getAuthor()

Answer extends Votable**
**-answerID : int
-user : User
-body : String
-isAccepted : boolean
-question : Question

+markAsAccepted()
+getAuthor()

Vote:
-User vote
-VoterType

VoterType Enum
UPVOTE (1) , DOWNVOTE (-1)

Votable
-HashMap<String, Vote> votes; (based on userid)
+addVote(vote) - synchronized
Check if author is doing the vote return false
If existing vote revert that and add the new vote

+retractVote(vote) - synchronized
Remove vote

+getScore()

QuestionRepository - interface
+save()
+findByTitleKeyword()
+findByTag()

InmemoryQuestionRepository - implements QuestionRepository
ConcurrentHashMap<String , Set<Question>> tagBaseDQuestions
ConcurrentHashMap<String , Set<Question>> titleBaseDQuestions

+save()
+findByTitleKeyword() - split on words and make it as a key
+findByTag()

StackOverflowSystem
-QuestionRepository repository

-ConcurrentHashMap<String , User> users;

+createUser(userid , name , emailid)

+addQuestion(title, author , List<Tag> tags, body)

+postAnswer(question, author, body)

+vote(userid, Voterable, voteype)

+searchByTag()

+searchByTitleKeyword()

+acceptedAnswer(questionId, answerID)