in src/main/java/software/amazon/qldb/Session.java [158:166]
CommitTransactionResult sendCommit(String txnId, ByteBuffer transactionDigest) {
final CommitTransactionRequest request = CommitTransactionRequest.builder()
.transactionId(txnId)
.commitDigest(SdkBytes.fromByteBuffer(transactionDigest))
.build();
SendCommandResponse result = send(SendCommandRequest.builder().commitTransaction(request));
return result.commitTransaction();
}