MongoDB 4.2 will add large transactions exceeding 16mb as well as prepared transactions. These will appear in the oplog differently than 4.0 transactions and tools will need to handle these in new ways:

• mongodump: (Split out to TOOLS-2304)
  • When marking the start point for dumping the oplog, start from the oldest active transaction timestamp, not from the latest oplog timestamp. Details on how to find this value are still in flux

• mongorestore
  • When restoring oplog entries, do not apply oplog entries that are part of a transaction in progress (large or prepared), but index them by transaction ID. However, do apply oplog entries that are neither large nor prepared.
  • When a transaction commit oplog entry is seen for a large or prepared transaction, find all related oplog entries and apply them serially to the destination (but not as part of a transaction)