I recently had to distribute a batch job among multiple workers; the way I finally did it was computing the xxhash over the _id and storing that in an extra field (_id_hash) as a Long.

Using this field, I could distribute the documents from the collection among N workers by computing the _id_hash % N and then having worker zero using modulus 0, worker 1 using modulus 1 and so on.

I figured that mongodb would use a similar approach internally to compute the db shard to store a document on; I tried to find a way to reuse that mechanism but could not.

Would it be possible to expose that capability of the shard key somehow?