On “prepare” command, an oplog entry is written for each operation in the transaction with the new root field

to denote it is a no-op. Then the primary follows the current “prepare” procedure: reserving an OpTime for Prepare Oplog Entry; preparing the WriteUnitOfWork; and writing down Prepare Oplog Entry, with the reserved OpTime. However, the format of Prepare Oplog Entry will include a new “prepareTransaction” command as its “o” root field rather than an “applyOps” command.

{
    "ts": Timestamp(1538534640, 10),
    "t": NumberLong(1),
    "op": "c",
    "ns": "admin.$cmd",

    // The new prepare command.
    "o": { "prepareTransaction": 1 },

    "lsid": < lsid > ,
    "txnNumber": NumberLong("1"),
    // Pointing to the previous operation in the transaction.
    // Assuming there exists an operation at this OpTime for this transaction.
    "prevOpTime": { "ts": Timestamp(1538534630, 2), "t": NumberLong(1) }
}