If the field chosen as the shard key is incremented or decremented, or changes monotonically in some form (for example AA, AB, AC, AD ... AZ, BA, BB, and so on) as new documents are added to the collection, you are at risk of having all the new documents being inserted into the last chunk if the shard key value increases, or the first chunk where the value of the shard key decreases. The reason this happens is that when the collection is initially sharded, shard key ranges are established and documents are inserted into chunks. As new data is added to the collection, the values of minKey and maxKey are continually upgraded, but the chunk shard key ranges remain the same.
To illustrate this point, in the following diagram, the initial sharding process produces shard key boundaries that range from 101 to 130. As the client application continues to write data, the value of X increments, so the value of maxKey is continuously updated. However, as the boundaries have already...
