Shard key frequency refers to how many documents have the same shard key value. As an example, if you choose the ISO 3 country code as a shard key, let's say that 67% of your customers are in India and the other 33% are evenly spread across the collection. When the sharding process completes, you might end up with a skewed distribution that, visually, might appear like this:
In this example, the value of minKey would be ABW (Aruba), while the value of maxKey would be ZWE (Zimbabwe). To avoid the effect of uneven frequency, combining the ISO 3 country code field with another field such as telephone area dialing code might produce a more even frequency. Finally, we'll examine how the rate of change affects the sharding process.
