org.eclipse.jgit/src/org/eclipse/jgit/util/time/MonotonicClock.java

/*
 * Copyright (C) 2016, Google Inc.
 * and other copyright owners as documented in the project's IP log.
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package org.eclipse.jgit.util.time;

import java.time.Duration;
import java.util.concurrent.TimeUnit;

/**
 * A provider of time.
 * <p>
 * Clocks should provide wall clock time, obtained from a reasonable clock
 * source, such as the local system clock.
 * <p>
 * MonotonicClocks provide the following behavior, with the assertion always
 * being true if {@link ProposedTimestamp#blockUntil(Duration)} is used:
 *
 * <pre>
 *   MonotonicClock clk = ...;
 *   long r1;
 *   try (ProposedTimestamp t1 = clk.propose()) {
 *   	r1 = t1.millis();
 *   	t1.blockUntil(...);
 *   }
 *
 *   try (ProposedTimestamp t2 = clk.propose()) {
 *   	assert t2.millis() > r1;
 *   }
 * </pre>
 *
 * @since 4.6
 */
public interface MonotonicClock {
	/**
	 * Obtain a timestamp close to "now".
	 * <p>
	 * Proposed times are close to "now", but may not yet be certainly in the
	 * past. This allows the calling thread to interleave other useful work
	 * while waiting for the clock instance to create an assurance it will never
	 * in the future propose a time earlier than the returned time.
	 * <p>
	 * A hypothetical implementation could read the local system clock (managed
	 * by NTP) and return that proposal, concurrently sending network messages
	 * to closely collaborating peers in the same cluster to also ensure their
	 * system clocks are ahead of this time. In such an implementation the
	 * {@link ProposedTimestamp#blockUntil(Duration)} method would wait for
	 * replies from the peers indicating their own system clocks have moved past
	 * the proposed time.
	 *
	 * @return "now". The value can be immediately accessed by
	 *         {@link ProposedTimestamp#read(TimeUnit)} and friends, but the
	 *         caller must use {@link ProposedTimestamp#blockUntil(Duration)} to
	 *         ensure ordering holds.
	 */
	ProposedTimestamp propose();
}