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,
39 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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 import java.util.concurrent.TimeUnit;
48
49 /**
50 * A provider of time.
51 * <p>
52 * Clocks should provide wall clock time, obtained from a reasonable clock
53 * source, such as the local system clock.
54 * <p>
55 * MonotonicClocks provide the following behavior, with the assertion always
56 * being true if {@link ProposedTimestamp#blockUntil(Duration)} is used:
57 *
58 * <pre>
59 * MonotonicClock clk = ...;
60 * long r1;
61 * try (ProposedTimestamp t1 = clk.propose()) {
62 * r1 = t1.millis();
63 * t1.blockUntil(...);
64 * }
65 *
66 * try (ProposedTimestamp t2 = clk.propose()) {
67 * assert t2.millis() > r1;
68 * }
69 * </pre>
70 *
71 * @since 4.6
72 */
73 public interface MonotonicClock {
74 /**
75 * Obtain a timestamp close to "now".
76 * <p>
77 * Proposed times are close to "now", but may not yet be certainly in the
78 * past. This allows the calling thread to interleave other useful work
79 * while waiting for the clock instance to create an assurance it will never
80 * in the future propose a time earlier than the returned time.
81 * <p>
82 * A hypothetical implementation could read the local system clock (managed
83 * by NTP) and return that proposal, concurrently sending network messages
84 * to closely collaborating peers in the same cluster to also ensure their
85 * system clocks are ahead of this time. In such an implementation the
86 * {@link ProposedTimestamp#blockUntil(Duration)} method would wait for
87 * replies from the peers indicating their own system clocks have moved past
88 * the proposed time.
89 *
90 * @return "now". The value can be immediately accessed by
91 * {@link ProposedTimestamp#read(TimeUnit)} and friends, but the
92 * caller must use {@link ProposedTimestamp#blockUntil(Duration)} to
93 * ensure ordering holds.
94 */
95 ProposedTimestamp propose();
96 }