1 /*
2 * Copyright (C) 2016, Google Inc.
3 * and other copyright owners as documented in the project's IP log.
4 *
5 * This program and the accompanying materials are made available
6 * under the terms of the Eclipse Distribution License v1.0 which
7 * accompanies this distribution, is reproduced below, and is
8 * available at http://www.eclipse.org/org/documents/edl-v10.php
9 *
10 * All rights reserved.
11 *
12 * Redistribution and use in source and binary forms, with or
13 * without modification, are permitted provided that the following
14 * conditions are met:
15 *
16 * - Redistributions of source code must retain the above copyright
17 * notice, this list of conditions and the following disclaimer.
18 *
19 * - Redistributions in binary form must reproduce the above
20 * copyright notice, this list of conditions and the following
21 * disclaimer in the documentation and/or other materials provided
22 * with the distribution.
23 *
24 * - Neither the name of the Eclipse Foundation, Inc. nor the
25 * names of its contributors may be used to endorse or promote
26 * products derived from this software without specific prior
27 * written permission.
28 *
29 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
30 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
31 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
32 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
33 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
34 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
35 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
36 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
37 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
38 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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40 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
41 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
42 */
43
44 package org.eclipse.jgit.util.time;
45
46 import java.time.Duration;
47
48 /**
49 * A provider of time.
50 * <p>
51 * Clocks should provide wall clock time, obtained from a reasonable clock
52 * source, such as the local system clock.
53 * <p>
54 * MonotonicClocks provide the following behavior, with the assertion always
55 * being true if
56 * {@link org.eclipse.jgit.util.time.ProposedTimestamp#blockUntil(Duration)} is
57 * used:
58 *
59 * <pre>
60 * MonotonicClock clk = ...;
61 * long r1;
62 * try (ProposedTimestamp t1 = clk.propose()) {
63 * r1 = t1.millis();
64 * t1.blockUntil(...);
65 * }
66 *
67 * try (ProposedTimestamp t2 = clk.propose()) {
68 * assert t2.millis() > r1;
69 * }
70 * </pre>
71 *
72 * @since 4.6
73 */
74 public interface MonotonicClock {
75 /**
76 * Obtain a timestamp close to "now".
77 * <p>
78 * Proposed times are close to "now", but may not yet be certainly in the
79 * past. This allows the calling thread to interleave other useful work
80 * while waiting for the clock instance to create an assurance it will never
81 * in the future propose a time earlier than the returned time.
82 * <p>
83 * A hypothetical implementation could read the local system clock (managed
84 * by NTP) and return that proposal, concurrently sending network messages
85 * to closely collaborating peers in the same cluster to also ensure their
86 * system clocks are ahead of this time. In such an implementation the
87 * {@link org.eclipse.jgit.util.time.ProposedTimestamp#blockUntil(Duration)}
88 * method would wait for replies from the peers indicating their own system
89 * clocks have moved past the proposed time.
90 *
91 * @return a {@link org.eclipse.jgit.util.time.ProposedTimestamp} object.
92 */
93 ProposedTimestamp propose();
94 }