The ULIDs are not constantly growing and are not sequential within the same millisecond.

[vmelamed]

I generated and printed a bunch of ULIDs.

Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());
Console.WriteLine(Ulid.NewUlid().ToString());

And I got this:

01K4P8T8PW:2NRPGWK4VC9H38PS
01K4P8T8PX:SFM8VGRCFTSZ1STZ
01K4P8T8PX:HWSQB413GQ18FQM8
01K4P8T8PX:43FGEJ4WX30YSCGG
01K4P8T8PX:M8BW2DH4BW3BS2XS
01K4P8T8PX:XB4Q98K7V1PVX1A9
01K4P8T8PX:711NFBQ7GBSR33R4
01K4P8T8PX:JMEQXX10P5V1GYZM
01K4P8T8PX:3DK2XNSG46KVGZR0
01K4P8T8PX:HVPHQ4P1H1Z8QMF1
01K4P8T8PX:TJDPST8MCT6GJ3W1

(I inserted the colon after the 10th letter for ease of separating the parts. I understand that technically this is not quite accurate but it helps.)

1. The second ULID is lexicographically greater than the third: 01K4P8T8PX:SFM8VGRCFTSZ1STZ > 01K4P8T8PX:HWSQB413GQ18FQM8.
2. According to the description shouldn't the third ULID be something like 01K4P8T8PX:SFM8VGRCFTSZ1SV0?

I understand that this is not a standard, but it seems to me that these are important issues, esp. for relational DB-s. No?

Thank you!

[xinyi04]

The default method (Ulid.NewUlid()) does not guarantee monotonic order for Ulids generated at the same time (with millisecond precision). In such cases, a custom random number algorithm that inherently possesses monotonic order is required.For example, in the following sample, I use the first four bytes for cumulative increment and the last six bytes for generating random numbers, thus achieving both global uniqueness and monotonic order.

static public class UlidUtil
{
    private static uint ms_MonotonicCounter = 0;
    private static readonly object ms_LockObject = new object();
    private static long ms_LastCreateTimeMs = 0;

    public static string CreateUlid()
    {
        lock (ms_LockObject)
        {
            var currentTime = DateTimeOffset.Now;
            var currentTimeMs = currentTime.ToUnixTimeMilliseconds();

            // Reset counter only when time actually changes
            if (currentTimeMs != ms_LastCreateTimeMs)
            {
                ms_LastCreateTimeMs = currentTimeMs;
                ms_MonotonicCounter = 0;
            }

            // Check if counter is approaching overflow
            if (ms_MonotonicCounter >= uint.MaxValue - 1000) // Reserve 1000 positions
            {
                // Wait for next millisecond to prevent overflow
                while (DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() == ms_LastCreateTimeMs)
                {
                    Thread.Sleep(1); // Wait 1 millisecond
                }
                // Time has changed, counter will reset in next iteration
                return CreateUlid(); // Recursive call
            }

            ms_MonotonicCounter++;

            var randomness = new byte[10];
            // First 4 bytes: monotonically increasing counter (lock-free)
            var counterBytes = BitConverter.GetBytes(ms_MonotonicCounter);
            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(counterBytes); // Convert to big-endian
            }
            Array.Copy(counterBytes, 0, randomness, 0, 4);

            // Last 6 bytes: high-quality random numbers
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(randomness, 4, 6);
            }

            return Ulid.NewUlid(currentTime, randomness).ToString();
        }
    }
}

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 30 days.

[xinyi04]

您好，我已收到您给我发的邮件，thank you!