/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.lucene.facet.taxonomy.writercache;
import org.apache.lucene.facet.taxonomy.FacetLabel;
import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyWriter;
/**
* TaxonomyWriterCache is a relatively simple interface for a cache of
* category->ordinal mappings, used in TaxonomyWriter implementations (such as
* {@link DirectoryTaxonomyWriter}).
* <p>
* It basically has put() methods for adding a mapping, and get() for looking a
* mapping up the cache. The cache does <B>not</B> guarantee to hold everything
* that has been put into it, and might in fact selectively delete some of the
* mappings (e.g., the ones least recently used). This means that if get()
* returns a negative response, it does not necessarily mean that the category
* doesn't exist - just that it is not in the cache. The caller can only infer
* that the category doesn't exist if it knows the cache to be complete (because
* all the categories were loaded into the cache, and since then no put()
* returned true).
* <p>
* However, if it does so, it should clear out large parts of the cache at once,
* because the user will typically need to work hard to recover from every cache
* cleanup (see {@link #put(FacetLabel, int)}'s return value).
* <p>
* <b>NOTE:</b> the cache may be accessed concurrently by multiple threads,
* therefore cache implementations should take this into consideration.
*
* @lucene.experimental
*/
public interface TaxonomyWriterCache {
/**
* Let go of whatever resources the cache is holding. After a close(),
* this object can no longer be used.
*/
public void close();
/**
* Lookup a category in the cache, returning its ordinal, or a negative
* number if the category is not in the cache.
* <P>
* It is up to the caller to remember what a negative response means:
* If the caller knows the cache is <I>complete</I> (it was initially
* fed with all the categories, and since then put() never returned true)
* it means the category does not exist. Otherwise, the category might
* still exist, but just be missing from the cache.
*/
public int get(FacetLabel categoryPath);
/**
* Add a category to the cache, with the given ordinal as the value.
* <P>
* If the implementation keeps only a partial cache (e.g., an LRU cache)
* and finds that its cache is full, it should clear up part of the cache
* and return <code>true</code>. Otherwise, it should return
* <code>false</code>.
* <P>
* The reason why the caller needs to know if part of the cache was
* cleared is that in that case it will have to commit its on-disk index
* (so that all the latest category additions can be searched on disk, if
* we can't rely on the cache to contain them).
* <P>
* Ordinals should be non-negative. Currently there is no defined way to
* specify that a cache should remember a category does NOT exist.
* It doesn't really matter, because normally the next thing we do after
* finding that a category does not exist is to add it.
*/
public boolean put(FacetLabel categoryPath, int ordinal);
/**
* Returns true if the cache is full, such that the next {@link #put} will
* evict entries from it, false otherwise.
*/
public boolean isFull();
/**
* Clears the content of the cache. Unlike {@link #close()}, the caller can
* assume that the cache is still operable after this method returns.
*/
public void clear();
}