Backend of rest api as a seperate application or not?

I am currently building a Rest Api. I am wrestling with the following issue.

Let's say I have an Api which registers Users. So I can make Rest calls to my User resource Like:

www.mysite.com/user, http://ift.tt/1BA0Btb, http://ift.tt/1BA0Btd}

Now somewhere I (as an owner of the API) need to have a backend of the API. For example I want to have a list of users and want to be able to manage my users.

My Question is about common practice. Should the backend be a seperate application? Or can I put this in the same application as the API itself? Are there any open source examples?

Thanks very much.