Unique ID Generation in Distributed Systems

public class SnowflakeIdGenerator {
    private static final long EPOCH = 1704067200000L;  // 2024-01-01 00:00:00 UTC
    private static final long MACHINE_ID_BITS = 10;
    private static final long SEQUENCE_BITS = 12;
    private static final long MAX_MACHINE_ID = (1L << MACHINE_ID_BITS) - 1;  // 1023
    private static final long MAX_SEQUENCE = (1L << SEQUENCE_BITS) - 1;       // 4095

    private final long machineId;
    private long lastTimestamp = -1;
    private long sequence = 0;

    public SnowflakeIdGenerator(long machineId) {
        if (machineId < 0 || machineId > MAX_MACHINE_ID) {
            throw new IllegalArgumentException("Machine ID must be 0-" + MAX_MACHINE_ID);
        }
        this.machineId = machineId;
    }

    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();

        if (timestamp == lastTimestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0) {
                // Sequence exhausted in this millisecond — wait for next ms
                timestamp = waitNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }

        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards!");
        }

        lastTimestamp = timestamp;

        return ((timestamp - EPOCH) << (MACHINE_ID_BITS + SEQUENCE_BITS))
             | (machineId << SEQUENCE_BITS)
             | sequence;
    }

    private long waitNextMillis(long lastTs) {
        long ts = System.currentTimeMillis();
        while (ts <= lastTs) {
            ts = System.currentTimeMillis();
        }
        return ts;
    }
}

// Generated IDs are:
// 1. Unique — machineId prevents collisions across servers
// 2. Roughly time-sorted — later IDs have higher timestamps
// 3. 64-bit — fits in a long, efficient for indexing
// 4. Decentralized — no coordination between machines

// Example IDs (decimal):
// 7189234567890001  (server 1, time T)
// 7189234567890002  (server 1, time T, next sequence)
// 7189234572340001  (server 2, time T+5ms)

// Using ulid-creator library
import com.github.f4b6a3.ulid.UlidCreator;

String ulid = UlidCreator.getMonotonicUlid().toString();
// "01HXYZ3NDEKTSV4RRFFQ69G5FAV"
// Lexicographically sortable — string comparison = time ordering!

// UUID v4 — random, no ordering
UUID id = UUID.randomUUID();  // "550e8400-e29b-41d4-a716-446655440000"

// UUID v7 (Java 21+ draft) — time-ordered, random tail
// Not yet in standard Java, but coming