The idea to use md5 hash in the first place was to have a unique values with very low collision for urls. So when we are actually using the first 6/7 chars of the base64 encoded version of this hash, theoretically there's a good chance of the key collision with the first 6/7 chars. In case of the collsion for 2 urls (having the same first 6 chars), what is the best way to generate a unique key?