To ensure that a request signed with a cluster time by the recipient can be processed by a stale donor node, the external validation keys copied from the recipient to the donor must have replicated to each donor node and been cached in-memory before the migration commits.

This can be done by waiting for the opTime of the last key insert to replicate to each voting node. Keys are copied at the beginning of a migration but this can be waited for near the end to improve performance.