ZooKeeper如何保证事务原子性?

// PrepRequestProcessor
private void pRequestHelper(Request request) throws RequestProcessorException {
	switch (request.type) {
	case OpCode.multi:
		MultiOperationRecord multiRequest = new MultiOperationRecord();
		try {
			ByteBufferInputStream.byteBuffer2Record(request.request, multiRequest);
		} catch (IOException e) {
			request.setHdr(new TxnHeader(request.sessionId, request.cxid, zks.getNextZxid(), Time.currentWallTime(), OpCode.multi));
			throw e;
		}
		List<Txn> txns = new ArrayList<Txn>();
		long zxid = zks.getNextZxid();
		KeeperException ke = null;

		Map<String, ChangeRecord> pendingChanges = getPendingChanges(multiRequest);
		request.setHdr(new TxnHeader(request.sessionId, request.cxid, zxid,
				Time.currentWallTime(), request.type));

		for (Op op : multiRequest) {
			Record subrequest = op.toRequestRecord();
			int type;
			Record txn;
			if (ke != null) {
				type = OpCode.error;
				txn = new ErrorTxn(Code.RUNTIMEINCONSISTENCY.intValue());
			} else {
				try {
					pRequest2Txn(op.getType(), zxid, request, subrequest, false);
					type = op.getType();
					txn = request.getTxn();
				} catch (KeeperException e) {
					ke = e;
					type = OpCode.error;
					txn = new ErrorTxn(e.code().intValue());

					if (e.code().intValue() > Code.APIERROR.intValue()) {
						LOG.info("Got user-level KeeperException when processing {} aborting"
								 + " remaining multi ops. Error Path:{} Error:{}",
								 request.toString(),
								 e.getPath(),
								 e.getMessage());
					}

					request.setException(e);

					/* Rollback change records from failed multi-op */
					rollbackPendingChanges(zxid, pendingChanges);
				}
			}

			try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
				BinaryOutputArchive boa = BinaryOutputArchive.getArchive(baos);
				txn.serialize(boa, "request");
				ByteBuffer bb = ByteBuffer.wrap(baos.toByteArray());
				txns.add(new Txn(type, bb.array()));
			}
		}
		request.setTxn(new MultiTxn(txns));
		if (digestEnabled) {
			setTxnDigest(request);
		}
		break;
}