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feat: handle clock drift #18 

Added drift tolerance of 10 seconds to preserve monotonicity when the
system clock is adjusted by NTP after a small clock drift or when the
system clock jumps back by 1 second due to leap second.

The random component is incremented when the current time:
- is the same as the previous time;
- has moved backwards up to 10 seconds.

The time component is also incremented if the random component is
exceeded, which is quite rare.
This commit is contained in:
Fabio Lima 2022-04-21 14:53:06 -03:00
parent 5abd3f46ff
commit 745bd55ffc
7 changed files with 250 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
CHANGELOG.md
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@ -6,7 +6,11 @@ All notable changes to this project will be documented in this file.
Nothing unreleased.
## [4.1.1] - 2021-11-06
## [4.2.0] - 2022-04-21
Handle clock drift. #18
## [4.1.2] - 2021-11-06
Compare internal fields as unsigned integers.
@ -278,7 +282,8 @@ Project created as an alternative Java implementation of [ULID spec](https://git
- Added `LICENSE`
- Added test cases
[unreleased]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.1.2...HEAD
[unreleased]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.2.0...HEAD
[4.2.0]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.1.2...ulid-creator-4.2.0
[4.1.2]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.1.1...ulid-creator-4.1.2
[4.1.1]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.1.0...ulid-creator-4.1.1
[4.1.0]: https://github.com/f4b6a3/ulid-creator/compare/ulid-creator-4.0.0...ulid-creator-4.1.0

2
README.md
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@ -39,7 +39,7 @@ Add these lines to your `pom.xml`.
<dependency>
<groupId>com.github.f4b6a3</groupId>
<artifactId>ulid-creator</artifactId>
<version>4.1.2</version>
<version>4.2.0</version>
</dependency>
```
See more options in [maven.org](https://search.maven.org/artifact/com.github.f4b6a3/ulid-creator).

13
src/main/java/com/github/f4b6a3/ulid/Ulid.java
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@ -1,7 +1,7 @@
/*
* MIT License
*
* Copyright (c) 2020-2021 Fabio Lima
* Copyright (c) 2020-2022 Fabio Lima
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
@ -584,12 +584,12 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
* millisecond;
*
* (2) This method can generate monotonic increasing ULIDs 99.999999999999992%
* ((2^80 - 10^9) / (2^80)) of the time, considering an unrealistic rate of
* 1,000,000,000 ULIDs per millisecond.
* ((2^80 - 10^9) / (2^80)) of the time within a single millisecond interval,
* considering an unrealistic rate of 1,000,000,000 ULIDs per millisecond.
*
* Due to (1) and (2), it does not throw the error message recommended by the
* specification. When an overflow occurs in the last 80 bits, the random
* component simply wraps around.
* specification. When an overflow occurs in the random 80 bits, the time
* component is simply incremented.
*
* @return a ULID
*/
@ -599,8 +599,7 @@ public final class Ulid implements Serializable, Comparable<Ulid> {
long newLsb = this.lsb + 1; // increment the LEAST significant bits
if (newLsb == INCREMENT_OVERFLOW) {
// carrying the extra bit by incrementing the MOST significant bits
newMsb = (newMsb & 0xffffffffffff0000L) | ((newMsb + 1) & 0x000000000000ffffL);
newMsb += 1; // increment the MOST significant bits
}
return new Ulid(newMsb, newLsb);

2
src/main/java/com/github/f4b6a3/ulid/UlidCreator.java
View File

@ -1,7 +1,7 @@
/*
* MIT License
*
* Copyright (c) 2020-2021 Fabio Lima
* Copyright (c) 2020-2022 Fabio Lima
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal

53
src/main/java/com/github/f4b6a3/ulid/UlidFactory.java
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@ -1,7 +1,7 @@
/*
* MIT License
*
* Copyright (c) 2020-2021 Fabio Lima
* Copyright (c) 2020-2022 Fabio Lima
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
@ -25,6 +25,7 @@
package com.github.f4b6a3.ulid;
import java.security.SecureRandom;
import java.time.Clock;
import java.util.Random;
import java.util.function.LongFunction;
import java.util.function.Supplier;
@ -42,14 +43,20 @@ import java.util.function.Supplier;
*/
public final class UlidFactory {
private final Clock clock; // for tests
private final LongFunction<Ulid> ulidFunction;
public UlidFactory() {
this.ulidFunction = new UlidFunction();
this(new UlidFunction(), null);
}
private UlidFactory(LongFunction<Ulid> ulidFunction) {
this(ulidFunction, null);
}
private UlidFactory(LongFunction<Ulid> ulidFunction, Clock clock) {
this.ulidFunction = ulidFunction;
this.clock = clock != null ? clock : Clock.systemUTC();
}
/**
@ -85,6 +92,19 @@ public final class UlidFactory {
return new UlidFactory(new UlidFunction(randomSupplier));
}
/**
* Returns a new factory.
*
* The given random supplier must return an array of 10 bytes.
*
* @param randomSupplier a random supplier that returns 10 bytes
* @param clock a custom clock instance for tests
* @return {@link UlidFactory}
*/
protected static UlidFactory newInstance(Supplier<byte[]> randomSupplier, Clock clock) {
return new UlidFactory(new UlidFunction(randomSupplier), clock);
}
/**
* Returns a new monotonic factory.
*
@ -116,13 +136,26 @@ public final class UlidFactory {
return new UlidFactory(new MonotonicFunction(randomSupplier));
}
/**
* Returns a new monotonic factory.
*
* The given random supplier must return an array of 10 bytes.
*
* @param randomSupplier a random supplier that returns 10 bytes
* @param clock a custom clock instance for tests
* @return {@link UlidFactory}
*/
protected static UlidFactory newMonotonicInstance(Supplier<byte[]> randomSupplier, Clock clock) {
return new UlidFactory(new MonotonicFunction(randomSupplier), clock);
}
/**
* Returns a UUID.
*
* @return a ULID
*/
public Ulid create() {
return create(System.currentTimeMillis());
return create(clock.millis());
}
/**
@ -168,9 +201,14 @@ public final class UlidFactory {
*/
protected static final class MonotonicFunction implements LongFunction<Ulid> {
private long lastTime = -1;
private long lastTime = 0;
private Ulid lastUlid = null;
// Used to preserve monotonicity when the system clock is
// adjusted by NTP after a small clock drift or when the
// system clock jumps back by 1 second due to leap second.
protected static final int CLOCK_DRIFT_TOLERANCE = 10_000;
// it must return an array of 10 bytes
private Supplier<byte[]> randomSupplier;
@ -189,13 +227,16 @@ public final class UlidFactory {
@Override
public synchronized Ulid apply(final long time) {
if (time == this.lastTime) {
// Check if the current time is the same as the previous time or has moved
// backwards after a small system clock adjustment or after a leap second.
// Drift tolerance = (previous_time - 10s) < current_time <= previous_time
if ((time > this.lastTime - CLOCK_DRIFT_TOLERANCE) && (time <= this.lastTime)) {
this.lastUlid = lastUlid.increment();
} else {
this.lastUlid = new Ulid(time, this.randomSupplier.get());
}
this.lastTime = time;
this.lastTime = lastUlid.getTime();
return new Ulid(this.lastUlid);
}
}

95
src/test/java/com/github/f4b6a3/ulid/UlidFactoryDefaultfTest.java
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@ -8,7 +8,11 @@ import com.github.f4b6a3.ulid.UlidFactory;
import static org.junit.Assert.*;
import java.time.Clock;
import java.time.Instant;
import java.time.ZoneId;
import java.util.Random;
import java.util.function.Supplier;
public class UlidFactoryDefaultfTest extends UlidFactoryTest {
@ -29,6 +33,97 @@ public class UlidFactoryDefaultfTest extends UlidFactoryTest {
checkCreationTime(list, startTime, endTime);
}
@Test
public void testClockDrift() {
long diff = UlidFactory.MonotonicFunction.CLOCK_DRIFT_TOLERANCE;
long time = Instant.parse("2021-12-31T23:59:59.000Z").toEpochMilli();
long times[] = { time, time + 0, time + 1, time + 2, time + 3 - diff, time + 4 - diff, time + 5 };
Clock clock = new Clock() {
private int i;
@Override
public long millis() {
return times[i++ % times.length];
}
@Override
public ZoneId getZone() {
return null;
}
@Override
public Clock withZone(ZoneId zone) {
return null;
}
@Override
public Instant instant() {
return null;
}
};
Supplier<byte[]> randomSupplier = UlidFactory.getRandomSupplier(new Random());
UlidFactory factory = UlidFactory.newInstance(randomSupplier, clock);
long ms1 = factory.create().getTime(); // time
long ms2 = factory.create().getTime(); // time + 0
long ms3 = factory.create().getTime(); // time + 1
long ms4 = factory.create().getTime(); // time + 2
long ms5 = factory.create().getTime(); // time + 3 - 10000 (CLOCK DRIFT)
long ms6 = factory.create().getTime(); // time + 4 - 10000 (CLOCK DRIFT)
long ms7 = factory.create().getTime(); // time + 5
assertEquals(times[0], ms1);
assertEquals(times[1], ms2);
assertEquals(times[2], ms3);
assertEquals(times[3], ms4);
assertEquals(times[4], ms5);
assertEquals(times[5], ms6);
assertEquals(times[6], ms7);
}
@Test
public void testLeapSecond() {
long second = Instant.parse("2021-12-31T23:59:59.000Z").getEpochSecond();
long leapSecond = second - 1; // simulate a leap second
long times[] = { second, leapSecond };
Clock clock = new Clock() {
private int i;
@Override
public long millis() {
return times[i++ % times.length] * 1000;
}
@Override
public ZoneId getZone() {
return null;
}
@Override
public Clock withZone(ZoneId zone) {
return null;
}
@Override
public Instant instant() {
return null;
}
};
Supplier<byte[]> randomSupplier = UlidFactory.getRandomSupplier(new Random());
UlidFactory factory = UlidFactory.newInstance(randomSupplier, clock);
long ms1 = factory.create().getTime(); // second
long ms2 = factory.create().getTime(); // leap second
assertEquals(times[0] * 1000, ms1);
assertEquals(times[1] * 1000, ms2);
}
@Test
public void testGetUlidInParallel() throws InterruptedException {

93
src/test/java/com/github/f4b6a3/ulid/UlidFactoryMonotonicTest.java
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@ -8,8 +8,12 @@ import com.github.f4b6a3.ulid.UlidFactory;
import static org.junit.Assert.*;
import java.time.Clock;
import java.time.Instant;
import java.time.ZoneId;
import java.util.Arrays;
import java.util.Random;
import java.util.function.Supplier;
public class UlidFactoryMonotonicTest extends UlidFactoryTest {
@ -31,6 +35,95 @@ public class UlidFactoryMonotonicTest extends UlidFactoryTest {
checkCreationTime(list, startTime, endTime);
}
@Test
public void testClockDrift() {
long diff = UlidFactory.MonotonicFunction.CLOCK_DRIFT_TOLERANCE;
long time = Instant.parse("2021-12-31T23:59:59.000Z").toEpochMilli();
long times[] = { time, time + 0, time + 1, time + 2, time + 3 - diff, time + 4 - diff, time + 5 };
Clock clock = new Clock() {
private int i;
@Override
public long millis() {
return times[i++ % times.length];
}
@Override
public ZoneId getZone() {
return null;
}
@Override
public Clock withZone(ZoneId zone) {
return null;
}
@Override
public Instant instant() {
return null;
}
};
Supplier<byte[]> randomSupplier = UlidFactory.getRandomSupplier(new Random());
UlidFactory factory = UlidFactory.newMonotonicInstance(randomSupplier, clock);
long ms1 = factory.create().getTime(); // time
long ms2 = factory.create().getTime(); // time + 0
long ms3 = factory.create().getTime(); // time + 1
long ms4 = factory.create().getTime(); // time + 2
long ms5 = factory.create().getTime(); // time + 3 - 10000 (CLOCK DRIFT)
long ms6 = factory.create().getTime(); // time + 4 - 10000 (CLOCK DRIFT)
long ms7 = factory.create().getTime(); // time + 5
assertEquals(ms1 + 0, ms2); // clock repeats.
assertEquals(ms1 + 1, ms3); // clock advanced.
assertEquals(ms1 + 2, ms4); // clock advanced.
assertEquals(ms1 + 2, ms5); // CLOCK DRIFT! DON'T MOVE BACKWARDS!
assertEquals(ms1 + 2, ms6); // CLOCK DRIFT! DON'T MOVE BACKWARDS!
assertEquals(ms1 + 5, ms7); // clock advanced.
}
@Test
public void testLeapSecond() {
long second = Instant.parse("2021-12-31T23:59:59.000Z").getEpochSecond();
long leapSecond = second - 1; // simulate a leap second
long times[] = { second, leapSecond };
Clock clock = new Clock() {
private int i;
@Override
public long millis() {
return times[i++ % times.length] * 1000;
}
@Override
public ZoneId getZone() {
return null;
}
@Override
public Clock withZone(ZoneId zone) {
return null;
}
@Override
public Instant instant() {
return null;
}
};
Supplier<byte[]> randomSupplier = UlidFactory.getRandomSupplier(new Random());
UlidFactory factory = UlidFactory.newMonotonicInstance(randomSupplier, clock);
long ms1 = factory.create().getTime(); // second
long ms2 = factory.create().getTime(); // leap second
assertEquals(ms1, ms2); // LEAP SECOND! DON'T MOVE BACKWARDS!
}
private void checkOrdering(Ulid[] list) {
Ulid[] other = Arrays.copyOf(list, list.length);
Arrays.sort(other);