how many use case for forum system

i'm creating a forum system for my website and i want to model it first using uml. here are the tasks that need to be modeled:

1. reading forum, threa开发者_如何学Pythond, post
2. editing forum, thread, post
3. deleting forum, thread, post
4. creating forum, thread, post

i found out that the best actors for this system are poster, reader, manager and admin. now i want to know how should i model the above tasks in use case diagram??? should i have an use case for each task?? like one use case for reading a thread, another one for reading forum and another for reading posts??

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A use case diagram is used for managing a collection of use-cases, for grouping them into similar sets of tasks, discovering relationships between use-cases, relationships to actors etc...

In the diagram itself, you typically have no more detail than the name of each use case, e.g. 'post new message'.

You might find it useful to split use-cases into multiple related use-cases, for example, you might want a 'logic' use case, which is used by your 'post new message' use case (and others).

Your actors sound fine to me, but your list of use-cases is very limited. You should think about how the forum will be used by each actor, and discover the tasks that each wish to perform.