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:
- When marking the start point for dumping the oplog, start from the oldest active transaction timestamp, not from the latest oplog timestamp
- Oldest active transaction timestamp is found by scanning the config.transactions collection.
- See this server code for details