1 /***
2 * Copyright 2003-2010 Terracotta, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 package net.sf.ehcache.store;
18
19 import net.sf.ehcache.Element;
20
21 /***
22 * An eviction policy.
23 * <p/>
24 * The Cache will use a policy at startup. There are three policy implementations provided in ehcache:
25 * LRU, LFU and FIFO. However many other policies are possible. That the policy
26 * has access to the whole element enables policies based on the key, value, metadata, statistics, or a combination of
27 * any of the above.
28 *
29 * @author Greg Luck
30 */
31 public interface Policy {
32
33 /***
34 * @return the name of the Policy. Inbuilt examples are LRU, LFU and FIFO.
35 */
36 String getName();
37
38 /***
39 * Finds the best eviction candidate based on the sampled elements. What distinguishes
40 * this approach from the classic data structures approach is that an Element contains
41 * metadata (e.g. usage statistics) which can be used for making policy decisions,
42 * while generic data structures do not. It is expected that implementations will take
43 * advantage of that metadata.
44 *
45 * @param sampledElements this should be a random subset of the population
46 * @param justAdded we probably never want to select the element just added.
47 * It is provided so that it can be ignored if selected. May be null.
48 * @return the selected Element
49 */
50 Element selectedBasedOnPolicy(Element[] sampledElements, Element justAdded);
51
52 /***
53 * Compares the desirableness for eviction of two elements
54 *
55 * @param element1 the element to compare against
56 * @param element2 the element to compare
57 * @return true if the second element is preferable for eviction to the first element
58 * under ths policy
59 */
60 boolean compare(Element element1, Element element2);
61 }