Imagine the following scenario: you are the operator of a small website that has a private user’s area which requires visitors to login to view. A few months later, you want to add a gallery application which also requires users to login and, finally, you want users to be able to edit a community wiki, but not allow non-members to make changes.

We can see that we have two choices: force the users to login to each separate application separately, or somehow combine all the applications under a single login.

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