Expiring links and where to store them

I currently have a website setup where unique links are generated every time a person 开发者_如何学Cloads a page. When a user follows a link that link should expire. That is, they shouldn't be able to hit back on their browser and click that link again.

While I can do this now, I'm not sure where I should be storing the unique link IDs.

There are going to be a lot of links generated since the website involves a lot of clicking through to get to other sections of the site, but the links themselves will expire after 30 minutes.

Should I store them on the database (MySQL) or cache them in memory with xcache?

If you do not need to ever reference the links again (ie an audit trail) storing them in memory should be fine, even storing them in session() should be acceptable. If you want an audit trail, I would store them in a more permanent matter, IE a database.