Haven't started scoping yet but suspect this will need drivers changes.

Summary

This project proposes making updateMany, without a shard key in the filter, work correctly against a sharded collection, for all combinations of ordered:true/false and upsert:true/false. We will consider several ways to implement this - using transactions against all concurrently targeted shards or using two-phase updates where the first phase establishes cursors and the second phase performs the actual updates.

Motivation

The current implementation of updateMany against a sharded collection doesn’t work correctly, because it doesn’t synchronise with chunk migrations and doesn’t perform chunk filtering. As a result, it has the potential of applying updates to the same document between zero and multiple times. Furthermore, it the upsert:true option requires that a shard key be specified so that a unique shard can be targeted.

Documentation

Scope Document
Technical Design Document