VectorClock.java
/*
* Syncany, www.syncany.org
* Copyright (C) 2011-2016 Philipp C. Heckel <philipp.heckel@gmail.com>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package org.syncany.database;
import java.util.TreeMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/**
* Implements a vector clock that records the time stamps of all send and receive
* events. It contains functions to compare and merge two vector clocks.
*
* <p>Vector clocks are a mechanism to track events/actions across multiple distributed
* clients. Using vector clocks, one can determine relationships between different events.
* In particular:
*
* <ul>
* <li>Event A happened before / was caused by event B (cause)</li>
* <li>Event B happened after / caused event B (effect)</li>
* <li>Event A and B happened simultaneously (no cause/effect relationship)</li>
* </ul>
*
* @author Frits de Nijs
* @author Peter Dijkshoorn
* @author Philipp C. Heckel (philipp.heckel@gmail.com)
*/
public class VectorClock extends TreeMap<String, Long> {
private static final long serialVersionUID = 109876543L;
private static final Pattern CLOCK_PATTERN = Pattern.compile("\\(([^)]*)\\)");
private static final int CLOCK_PATTERN_GROUP_CONTENT = 1;
private static final Pattern ENTRY_PATTERN = Pattern.compile("([a-zA-Z]+)(\\d+)");
private static final int ENTRY_PATTERN_GROUP_NAME = 1;
private static final int ENTRY_PATTERN_GROUP_TIME = 2;
public static final Pattern MACHINE_PATTERN = Pattern.compile("[a-zA-Z]+");
public enum VectorClockComparison {
SMALLER, GREATER, EQUAL, SIMULTANEOUS;
}
/**
* Increases the component of a unit by 1.
*
* @param unit The identifier of the vector element being increased
* @return Returns the new clock value for the given unit
*/
public Long incrementClock(String unit) {
validateUnitName(unit);
Long newValue = (containsKey(unit)) ? get(unit).longValue() + 1 : 1L;
put(unit, newValue);
return newValue;
}
private void validateUnitName(String unit) {
if (!MACHINE_PATTERN.matcher(unit).matches()) {
throw new RuntimeException("Machine name cannot be empty and must be only characters (A-Z).");
}
}
/**
* Set the component of a unit.
*
* @param unit The identifier of the vector element being set
* @param value The new value of the unit being set
*/
public void setClock(String unit, long value) {
validateUnitName(unit);
put(unit, value);
}
/**
* Retrieve the unit's value
*
* @param unit The identifier of the vector element being retrieved
* @return Returns the value of the unit (if existent), or <code>null</code> if it does not exist
*/
public Long getClock(String unit) {
return get(unit);
}
@Override
public Long get(Object unit) { // TODO [low] This should not be used, or shoul it? Why inherit from TreeMap?
Long lResult = super.get(unit);
if (lResult == null) {
lResult = 0L;
}
return lResult;
}
@Override
public VectorClock clone() {
return (VectorClock) super.clone();
}
@Override
public String toString() {
/*
* Please note that this is an incredibly important method.
* It is used in hundreds of tests! Don't mess with it!
*/
Object[] lIDs = keySet().toArray();
Object[] lRequests = values().toArray();
StringBuilder builder = new StringBuilder();
builder.append('(');
for (int i = 0; i < lRequests.length; i++) {
builder.append(lIDs[i]);
builder.append(lRequests[i].toString());
if (i + 1 < lRequests.length) {
builder.append(',');
}
}
builder.append(')');
return builder.toString();
}
/**
* Converts a serialized vector clock back into a {@link VectorClock} object.
* @see #toString()
*/
public static VectorClock parseVectorClock(String serializedVectorClock) {
VectorClock vectorClock = new VectorClock();
Matcher clockMatcher = CLOCK_PATTERN.matcher(serializedVectorClock);
if (clockMatcher.matches()) {
String clockContents = clockMatcher.group(CLOCK_PATTERN_GROUP_CONTENT);
String[] clockEntries = clockContents.split(",");
for (String clockEntry : clockEntries) {
Matcher clockEntryMatcher = ENTRY_PATTERN.matcher(clockEntry);
if (clockEntryMatcher.matches()) {
String machineName = clockEntryMatcher.group(ENTRY_PATTERN_GROUP_NAME);
Long clockValue = Long.parseLong(clockEntryMatcher.group(ENTRY_PATTERN_GROUP_TIME));
vectorClock.put(machineName, clockValue);
}
else {
throw new IllegalArgumentException("Not a valid vector clock, entry does not match pattern: " + clockEntry);
}
}
return vectorClock;
}
else {
throw new IllegalArgumentException("Not a valid vector clock: " + serializedVectorClock);
}
}
/**
* VectorClock compare operation. Returns one of four possible values
* indicating how clock one relates to clock two:
*
* VectorComparison.GREATER If One > Two. VectorComparison.EQUAL If One =
* Two. VectorComparison.SMALLER If One < Two. VectorComparison.SIMULTANEOUS
* If One != Two.
*
* @param clock1 First Clock being compared.
* @param clock2 Second Clock being compared.
* @return VectorComparison value indicating how One relates to Two.
*/
public static VectorClockComparison compare(VectorClock clock1, VectorClock clock2) {
// Initially we assume it is all possible things.
boolean isEqual = true;
boolean isGreater = true;
boolean isSmaller = true;
// Go over all elements in Clock one.
for (String lEntry : clock1.keySet()) {
// Compare if also present in clock two.
if (clock2.containsKey(lEntry)) {
// If there is a difference, it can never be equal.
// Greater / smaller depends on the difference.
if (clock1.get(lEntry) < clock2.get(lEntry)) {
isEqual = false;
isGreater = false;
}
if (clock1.get(lEntry) > clock2.get(lEntry)) {
isEqual = false;
isSmaller = false;
}
}
// Else assume zero (default value is 0).
else if (clock1.get(lEntry) != 0) {
isEqual = false;
isSmaller = false;
}
}
// Go over all elements in Clock two.
for (String lEntry : clock2.keySet()) {
// Only elements we have not found in One still need to be checked.
if (!clock1.containsKey(lEntry) && (clock2.get(lEntry) != 0)) {
isEqual = false;
isGreater = false;
}
}
// Return based on determined information.
if (isEqual) {
return VectorClockComparison.EQUAL;
}
else if (isGreater && !isSmaller) {
return VectorClockComparison.GREATER;
}
else if (isSmaller && !isGreater) {
return VectorClockComparison.SMALLER;
}
else {
return VectorClockComparison.SIMULTANEOUS;
}
}
}