雪花算法，网上随随便便都能搜到很多，然后无一例外，都说明了算法的一些场景限制，可就是没有一篇文章直接给出一个已经解决或部分解决了这些限制，可以直接使用的算法……
因为我们面向的用户不是互联网企业，所以服务器不需要支持到25，数据中心不需要支持到25，毫秒内最大的序列数量不需要支持到212，反倒是时间戳，默认241只支持69年不太够，如果按1970年开始，那么2039年Id就要到最大值了，算算那时候我还没退休呢（我可是希望能够在当前公司做到退休的，毕竟老东西在外面已经没市场了 😛）！另外目前接触到的客户，基本服务器都是在域控内，所以NTP时间回拨问题肯定避免不了……
算法实现原始参考：snowflake-net
与原始算法的区别：
- 构造函数去除：sequence
- 构造函数增加：基准时间戳，默认为UTC时间2020/01/01；NTP回拨时间支持，默认2000ms；默认服务器3个Bit；默认数据中心2个Bit；默认序列12Bit，也就是说默认构造函数中，从服务器位和数据中心位扣除了5个Bit填补到时间戳，这样时间戳最大支持241+5（69*32=2208年，差不多22个世纪了 😛）
public class IdWorker
{
/// <summary>
/// UTC-Time 2020/01/01 0:00:00 +00:00
/// </summary>
const long DefaultBenchmarkTimestamp = 1577836800000L;
const byte IdMaxBits = 64;
const int TimestampMinBits = 41;
const int WorkerIdAndDatacenterIdMinBits = 2;
const int WorkerIdAndDatacenterIdMaxBits = 6;
const int SequenceMinBits = 10;
const int SequenceMaxBits = 14;
const int ExtraClockCallbackMillisToCleanUp = 1000;
const int AllowedNTPClockCallbackMaxMillis = 60000;
public long BenchmarkTimestamp { get; private set; }
public long WorkId { get; private set; }
public long DatacenterId { get; private set; }
public int TimestampBits { get; private set; }
public int WorkerIdBits { get; private set; }
public int DatacenterIdBits { get; private set; }
public int SequenceBits { get; private set; }
public int AllowedNTPClockCallbackMillis { get; private set; } //https://blog.youkuaiyun.com/a2279338659/article/details/143637143
private readonly int _sequenceIdMaxValue;
private readonly int _workerIdShift;
private readonly int _datacenterIdShift;
private readonly int _timestampShift;
private readonly object _lock = new();
private readonly int _maxCountMustBeCleanUp;
protected long _sequence;
protected long _firstTimestamp = -1L;
protected long _lastTimestamp = -1L;
/// <summary>
/// key: timestamp value: sequence
/// </summary>
protected readonly Dictionary<long, long> _storageTimestamps = new Dictionary<long, long>();
public IdWorker(int workerId, int datacenterId, int allowedNTPClockCallbackMillis = 2000,
long benchmarkTimestamp = DefaultBenchmarkTimestamp,
int workerIdBits = 3, int datacenterIdBits = WorkerIdAndDatacenterIdMinBits, int sequenceBits = 12)
{
ValidNumberRange(allowedNTPClockCallbackMillis, AllowedNTPClockCallbackMaxMillis, nameof(allowedNTPClockCallbackMillis));
var nowTimeStamp = this.TimeGen();
if (benchmarkTimestamp >= nowTimeStamp)
{
throw new ArgumentException($"{nameof(benchmarkTimestamp)} is greater than the current timestamp");
}
ValidBitsRange(workerIdBits, WorkerIdAndDatacenterIdMinBits, WorkerIdAndDatacenterIdMaxBits, nameof(workerIdBits));
ValidBitsRange(datacenterIdBits, WorkerIdAndDatacenterIdMinBits, WorkerIdAndDatacenterIdMaxBits, nameof(datacenterId));
ValidBitsRange(sequenceBits, SequenceMinBits, SequenceMaxBits, nameof(sequenceBits));
TimestampBits = IdMaxBits - workerIdBits - datacenterIdBits - sequenceBits - 1;
if (TimestampBits < TimestampMinBits)
{
throw new ArgumentException($"The number of bits used for Timestamp must be greater than or equal to {TimestampMinBits}, and currently it is only {TimestampBits}");
}
var workIdMaxValue = GetNumberWithBits(workerIdBits);
ValidNumberRange(workerId, workIdMaxValue, nameof(workerId));
var datacenterIdMaxValue = GetNumberWithBits(datacenterIdBits);
ValidNumberRange(datacenterId, datacenterIdMaxValue, nameof(datacenterId));
this._sequenceIdMaxValue = GetNumberWithBits(sequenceBits);
this.AllowedNTPClockCallbackMillis = allowedNTPClockCallbackMillis;
this.WorkerIdBits = workerIdBits;
this.DatacenterIdBits = datacenterIdBits;
this.SequenceBits = sequenceBits;
this.BenchmarkTimestamp = benchmarkTimestamp;
this.WorkId = workerId;
this.DatacenterId = datacenterId;
this._workerIdShift = this.SequenceBits;
this._datacenterIdShift = this.SequenceBits + this.WorkerIdBits;
this._timestampShift = this.SequenceBits + this.WorkerIdBits + this.DatacenterIdBits;
this._maxCountMustBeCleanUp = this.AllowedNTPClockCallbackMillis + ExtraClockCallbackMillisToCleanUp;
}
private static int GetNumberWithBits(int bits)
{
return -1 ^ (-1 << bits);
}
private static void ValidNumberRange(int inputValue, int maxValue, string numberName)
{
if (inputValue < 0 || inputValue > maxValue)
{
throw new ArgumentException($"The allowed range for {numberName} is {0}-{maxValue}, and {inputValue} is out of the range");
}
}
private static void ValidBitsRange(int inputBit, int minBits, int maxBits, string bitName)
{
if (inputBit < minBits || inputBit > maxBits)
{
throw new ArgumentException($"The allowed range for {bitName} is {minBits}-{maxBits}, and {inputBit} is out of the range");
}
}
public virtual DateTimeOffset GetDateTimeOffset(long snowflakeId)
{
if (snowflakeId < 0)
{
throw new ArgumentException($"{nameof(snowflakeId)} must be greater than or equals to 0");
}
var timestamp = (snowflakeId >> this._timestampShift) + this.BenchmarkTimestamp;
if (timestamp <= 0)
{
throw new Exception("Conversion failed");
}
return DateTimeOffset.FromUnixTimeMilliseconds(timestamp);
}
public virtual long NextId()
{
lock (_lock)
{
var timestamp = 0L;
var prevTimestamp = -1L;
var compareTimestamp = this._lastTimestamp - this.AllowedNTPClockCallbackMillis;
do
{
timestamp = this.TimeGen();
if (prevTimestamp == timestamp)
{
continue;
}
if (timestamp < compareTimestamp)
{
throw new Exception("Serious clock callback, requiring manual handling");
}
if (!this._storageTimestamps.ContainsKey(timestamp))
{
this._sequence = 0;
this._storageTimestamps[timestamp] = this._sequence;
break;
}
else
{
this._sequence = this._storageTimestamps[timestamp];
this._sequence = (this._sequence + 1) & this._sequenceIdMaxValue;
if (this._sequence > 0)
{
this._storageTimestamps[timestamp] = this._sequence;
break;
}
prevTimestamp = timestamp;
}
}
while (true);
UpdateAndCleanUp();
return ((timestamp - this.BenchmarkTimestamp) << this._timestampShift)
| (this.DatacenterId << this._datacenterIdShift)
| (this.WorkId << this._workerIdShift)
| _sequence;
void UpdateAndCleanUp()
{
if (this.AllowedNTPClockCallbackMillis > 0)
{
if (this._lastTimestamp < timestamp)
{
this._lastTimestamp = timestamp;
}
if (this._firstTimestamp < 0)
{
this._firstTimestamp = timestamp;
}
if (this._storageTimestamps.Count >= this._maxCountMustBeCleanUp)
{
var cleanTimestamp = timestamp - this.AllowedNTPClockCallbackMillis;
while (this._firstTimestamp < cleanTimestamp)
{
this._storageTimestamps.Remove(this._firstTimestamp);
this._firstTimestamp++;
}
}
}
else
{
this._lastTimestamp = timestamp;
if (this._storageTimestamps.Count >= this._maxCountMustBeCleanUp)
{
var allowedDicCount = this.AllowedNTPClockCallbackMillis + 1;
var i = -1;
do
{
this._storageTimestamps.Remove(timestamp + i);
i--;
}
while (this._storageTimestamps.Count > allowedDicCount);
}
}
}
}
}
protected virtual long TimeGen()
{
return DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
}
}
使用起来的代码还是与参考算法实现没区别，我可是保留了所有的命名 😛
var idWorker = new IdWorker(0, 0);
var id = idWorker.NextId();
除默认的NextId方法外，我还增加了方法GetDateTimeOffset，根据生成的Id来解析出该Id生成时的时间戳，方便反向测试时间戳是否正确，测试代码如下
Console.WriteLine($"Id: {long.MaxValue} Long Max");
var idWorker = new IdWorker(0, 0, sequenceBits: 10, allowedNTPClockCallbackMillis: 0);
Console.WriteLine("Test in single thread, press any key to start...");
Console.ReadKey();
var idList = GenerateId();
PrintIdList(idList);
Console.WriteLine("Test in multi-thread, press any key to start...");
Console.ReadKey();
var threadCount = 5;
var listArr = new List<long>[threadCount];
Parallel.For(0, threadCount, i =>
{
listArr[i] = GenerateId();
});
for (var i = 0; i < listArr.Length; i++)
{
Console.WriteLine($"List index: {i}");
PrintIdList(listArr[i]);
}
Console.WriteLine($"All Count: {listArr.SelectMany(_ => _).Distinct().Count()}");
List<long> GenerateId(int count=1000)
{
var tmpList = new List<long>();
for (var j = 0; j < count; j++)
{
var id = idWorker.NextId();
tmpList.Add(id);
}
return tmpList;
}
void PrintIdList(IList<long> idList)
{
Console.WriteLine($"Id Total Count:{idList.Count}");
foreach (var id in idList)
{
var time = idWorker.GetDateTimeOffset(id);
Console.WriteLine($"Id: {id} Time: {time}");
}
}
算法参考资料:分布式唯一ID生成算法——雪花算法（Snowflake）
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