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Asked: 2026-05-20T00:49:07+00:00

What is the best way to describe a commenting and voting process as we

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What is the best way to describe a commenting and voting process as we have it here on SO in UML? I need a clear way to describe that one user asks a question, multiple users submit answers and each user can upvote / downvote answers and the question as he likes.

I tried to model that using an activity diagram that starts on top, has to branches, one branch that goes to a node that says “comment”, the other one goes in a “voting” action. Each action then has a flow to itself (representing that the user can upvote / downvote and comment as often as he wants) and then both join in a final node.

I do not think that this is in any way comprehensible so I want to ask: what is the correct way to describe this using UML?

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1 Answer

  1. Editorial Team

    If you want to describe the rules (i.e. relationships) then a class diagram is probably more appropriate than an activity diagram. Here’s a pretty crude but nevertheless representative example:
    class diagram: question-answer-vote

    Activity diagrams are appropriate for describing chained activities (processes). In this case however, the behaviour is pretty simple; a User can submit Posts (Question or Answer) and also vote on Posts.

    The diagram captures the following rules (reading directly from the relationships):

    Each User may submit many Posts
A User need not submit any Posts
Each Post must be submitted by exactly one User
Each User may vote on many Posts
A User need not vote on any Posts
Each Post may be voted on by many Users
A Post need not be voted on by any Users
Each Vote has a type which is either up or down
A Post is either a Question or an Answer
A Question may have many Answers
A Question need not have any Answers
Each Answer must be a response to exactly one Question

    Note the model is missing some important rules (e.g. can’t up/down vote your own posts). Also missing Comments (another subtype of Post) and other salient attributes (e.g. User name, Post text, etc.). Hopefully representative enough for purpose though.

    hth.

    PS: I took liberties with syntax for Vote.type. It should really be defined as an enumeration, but for these purposes the way it’s shown is sufficient. Apologies to any lurking UML police 🙂

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