class MvccTransactionCoordinator…

​

  public long commit(TransactionRef txn) {

          long commitTimestamp = prepare(txn);

​

          TransactionMetadata transactionMetadata = transactions.get(txn);

          transactionMetadata.markPreparedToCommit(commitTimestamp, this.transactionLog);

​

          sendCommitMessageToAllTheServers(txn, commitTimestamp, transactionMetadata.getParticipatingKeys());

​

          transactionMetadata.markCommitComplete(transactionLog);

​

          return commitTimestamp;

  }

​

​

  public long prepare(TransactionRef txn) throws WriteConflictException {

      TransactionMetadata transactionMetadata = transactions.get(txn);

      Map<MvccTransactionalKVStore, List<String>> keysToServers = getParticipants(transactionMetadata.getParticipatingKeys());

      List<Long> prepareTimestamps = new ArrayList<>();

      for (MvccTransactionalKVStore store : keysToServers.keySet()) {

          List<String> keys = keysToServers.get(store);

          long prepareTimestamp = store.prepare(txn, keys);

          prepareTimestamps.add(prepareTimestamp);

      }

      return prepareTimestamps.stream().max(Long::compare).orElse(txn.getStartTimestamp());

  }