Preventing Multiple Votes from a single user, with anonymity

Question

I have an Android application that allows users to post information to a database, via a java web server that I have code on.

Users are allowed to up/down vote said post. How do I prevent the same user from voting a plethora of times on the same post?

A few ideas I had:

1. Disabling locally via adding a local storage "key" for the post ID Unique Key. For instance, when the user votes up or down. It writes a key with the information of the type of vote and the Unique Identifier of the post on the user's local storage. Is this feasible with a lot of votes? Would it cause any sort of hard drive consumption/lag?

2. Storing a uniquely-generated key on the user's phone generated at application install. This key would be submitted to a new table in the database that associates posts with votes and the "user-key".

Answer 1

If you're expecting lots of information, a database is probably the way to go. Obviously this will use storage space, but not much. Writing to the db tends to be very fast in my experience.

The unique key would work fine too. That way would be more secure (depending on how you do it) since it would be harder to spoof it from the client. If you generate the key based on the user's Google account, or the device ID, you're in good shape. If you do it by some pseudorandom method then the user could still cast multiple votes by just clearing the app data and getting a new key.

If it's really important to prevent multiple votes, do it on the server. If it's not particularly important, do whichever method seems easier to you. You can also consider doing both, for the best of both worlds: immediate feedback to the user on a duplicate vote attempt (or just altering the UI to make it impossible), plus a backup validation on the server in case the user tries to get around the client check.