com.swirlds.merkledb.Snapshotable Maven / Gradle / Ivy
/*
* Copyright (C) 2021-2024 Hedera Hashgraph, LLC
*
* 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 com.swirlds.merkledb;
import java.io.IOException;
import java.nio.file.Path;
/**
* Interface for classes that can be snapshotted.
*
* Only one snapshot can happen at a time!
*
*
* IMPORTANT, after this is completed the caller owns the directory. It is responsible for deleting it when it
* is no longer needed.
*
*/
public interface Snapshotable {
/**
* Perform a snapshot, saving data contained in this object to the specified directory.
*
* Snapshots are done in the transaction handler thread, so they have to be synchronized with flushes
* (running in virtual pipeline thread) and compactions (running in dedicated background threads). To sync
* up with flushes, this method is run either from VirtualRootNode.detach(), which is called while pipeline
* is paused, or on learner reconnect, while no data is stored to the virtual map. Synchronization with
* compaction processes is slightly more complicated. Before virtual map snapshot is started, it takes a
* lock (semaphore), which is monitored by compaction threads. As soon as the semaphore is locked,
* compaction is immediately stopped. If compaction thread is in the middle of index update, it updates a
* single index entry and then stops until the snapshot is done. Compaction thread may also be in the
* middle of deleting compacted files. In this case, files are deleted first, and snapshot will not take
* them into consideration.
*
* @param snapshotDirectory
* Directory to put snapshot into, it will be created if it doesn't exist.
* @throws IOException
* If there was a problem snapshotting
*/
void snapshot(Path snapshotDirectory) throws IOException;
}