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/*
* Copyright (C) 2016, Google Inc. and others
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Distribution License v. 1.0 which is available at
* https://www.eclipse.org/org/documents/edl-v10.php.
*
* SPDX-License-Identifier: BSD-3-Clause
*/
package org.eclipse.jgit.util.time;
import java.time.Duration;
/**
* 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 org.eclipse.jgit.util.time.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 org.eclipse.jgit.util.time.ProposedTimestamp#blockUntil(Duration)}
* method would wait for replies from the peers indicating their own system
* clocks have moved past the proposed time.
*
* @return a {@link org.eclipse.jgit.util.time.ProposedTimestamp} object.
*/
ProposedTimestamp propose();
}
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