URL Shortening Service -- Incrementing Short URL

I was trying to noodle out how to make the short URL guaranteed unique, since all of my ideas would have caused potential issues for simultaneous submissions. I thought about creating a new collection with just a single value, COUNTER, which I would findAndUpdate() when I needed to get a new value. However, I ended up instead just using the mongoose-sequence plugin. Does anyone have a better solution that I just missed?