io.atomix.copycat.server.storage.compaction.MinorCompactionTask Maven / Gradle / Ivy
/*
* Copyright 2015 the original author or authors.
*
* Licensed 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 io.atomix.copycat.server.storage.compaction;
import io.atomix.catalyst.util.Assert;
import io.atomix.copycat.server.storage.Segment;
import io.atomix.copycat.server.storage.SegmentDescriptor;
import io.atomix.copycat.server.storage.SegmentManager;
import io.atomix.copycat.server.storage.entry.Entry;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.util.Collections;
/**
* Removes {@link io.atomix.copycat.server.storage.Log#clean(long) cleaned} entries from an individual
* log {@link Segment} to reclaim disk space.
*
* The minor compaction task is a lightweight process that rewrites an individual segment to remove entries for
* which {@link Entry#isTombstone()} is {@code false}. Tombstones are entries that represent the deletion of
* state from the application, and tombstones are not removed during minor compaction since the safety of removing
* tombstones relies on expensive context. For more information on the implementation for removing tombstones,
* see the {@link MajorCompactionTask}.
*
* When a segment is rewritten by the minor compaction task, a new compact segment is created with the same starting
* index as the segment being compacted and the next greatest version number. The version number allows the
* {@link SegmentManager} to account for failures during log compaction when recovering the log from disk. If a failure
* occurs during minor compaction, the segment manager will attempt to load the segment with the greatest version
* for a given range of entries from disk. If the segment with the greatest version did not finish compaction, it
* will be discarded and the old segment will be used. Once the minor compaction task is done rewriting a segment,
* it will {@link SegmentDescriptor#lock()} the segment to indicate that the segment has completed compaction and
* is safe to read, and the compacted segment will be made available to the {@link io.atomix.copycat.server.storage.Log}.
*
* @author