内部分布式时间服务器实现

Question

我已经为即将到来的分布式NoSQL数据库系统开发了一个内部分布式时间服务器(没有主服务器)。它应该处理拜占庭时钟和时钟倾斜问题，只要2/3在分布式系统中的时钟是正确的。

不过，我想看看其他人是如何实现这种模式的(对基于IEEE 1588的主/从模式实现不感兴趣)--更好的是一些已经在使用的开放源代码--断言我已经正确地实现了它，因为很难为它编写单元测试。

有人知道这样的开源实现吗？我们使用的编程语言是C++，所以我更喜欢C/C++引用，不过只要代码具有可读性，它可能就不那么重要了。

下面是到目前为止我的实现的代码(为了简单起见部分伪代码)：

/*!
\brief Maximum allowed clock skew in milliseconds
\details A network node that has a clock skew greater than this value will be ignore
* and an error message will be logged
\note Maximum divergence (drift) between two clocks on the network nodes will be 3x this amount if we 
* have a worst case Byzantium clock issue
*/
#define MAX_ALLOWED_CLOCK_SCEW_MS 333

/*!
\class CTimeServer
\brief Internal distributed time server
\details The time server frequently recieves the time from all the other master server nodes
* in the DBMS and calculates the current time by averaging all of the recieves timestamps.
\note If a server node has a greater clock skew than \c MAX_ALLOWED_CLOCK_SCEW_MS then it its
* timestamp is ignored and an error message is logged
\note Clocks are accurately synchronized until more than 1/3 of the nodes have Byzantium clock issues
\author Inge Eivind Henriksen
\date February 2014
*/
class CTimeServer
{
    private:
        /** System offset in milliseconds */
        std::atomic<int> offsetAverageMs;

        /*!
        \brief Node offsets type
        \par key Node ID
        \par value Offset in milliseconds
        */
        typedef std::map<int, int> nodeOffset_t;

        /*!
        \brief Iterator type for \c nodeOffset_t
        \relates nodeOffset_t
        */
        typedef nodeOffset_t::iterator nodeOffsetIter_t;

        /** Node offsets */
        nodeOffset_t nodeOffsets;

        /*!
        \brief Calculates the offset time in milliseconds between all the nodes in the distributed system
        */
        int CalculateOffsetMs() {
            bool exists;
            nodeOffsetIter_t offsets_iter(&nodeOffsets);
            int offsetMs = offsets_iter.first(&exists);
            int averageMs = 0;

            while (exists)
            {
                averageMs += offsetMs;
                averageMs /= 2;

                // Get the next file manager in the map
                offsetMs = offsets_iter.next(&exists);
            }

            return averageMs;
        }
    public:
        CTimeServer() {
            offsetAverageMs = 0;
        }

        /*!
        \brief Register the time of a node
        \param nodeHostName [in] Network node host name or IP address
        \param nodeId [in] Network node ID
        \param timestamp [in] Network node timestamp
        */
        void RegisterNodeTime(const wchar_t *nodeHostName, int nodeId, time_t timestamp) {
            int now = (int)time(NULL);
            int offset = (int)timestamp - now;

            // Make sure the node clock is within the permitted values
            if (abs(offset) > MAX_ALLOWED_CLOCK_SCEW_MS)
            {
                // Node clock skew was outside the permitted limit, so remove it from the list of valid time offsets
                nodeOffsets.erase(nodeId);

                // Throw an error
                std::wstringstream err;
                err << L"Network node " << nodeHostName << L" exceeded the maximum allowed clock skew of " 
                    << MAX_ALLOWED_CLOCK_SCEW_MS << L" ms by " << offset << " ms. Set the clock to correct this problem.";
                throw err.str().c_str();
            }

            nodeOffsets.update(nodeId, offset);

            // Recalculate the offset average
            offsetAverageMs.store(CalculateOffsetMs());
        }

        /*!
        \brief Get the distributed system time
        \returns The distributed system time
        */
        time_t GetTime() {
            int now = (int)time(NULL);
            return (time_t)(now + offsetAverageMs.load()));
        }

Answer 1

在时间同步协议方面有相当多的文献，特别是在无线传感器网络中，在这种网络中，部署环境不适合时间主设备。关于this page的主题有一个很好的介绍。似乎最受关注的协议是洪泛时间同步协议(FTSP)，它来自于马洛蒂、库西、西蒙和莱德奇( Lédeczi )的paper。我在TinyOS的wiki上描述了它的一个实现，它有您正在寻找的代码类型。

对于任何没有主从的系统，都要提出一个警告:没有“正确”时间的概念。您可以得到的最佳结果是节点收敛到公共时间引用。这是一个共识的时间，但不应该被认为是一个权威的“正确”的时间。