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Header.java
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Header.java
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/*
* Copyright (c) 2002-2017 "Neo Technology,"
* Network Engine for Objects in Lund AB [http://neotechnology.com]
*
* This file is part of Neo4j.
*
* Neo4j is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package org.neo4j.index.internal.gbptree;
import java.util.function.Consumer;
import org.neo4j.io.pagecache.PageCache;
import org.neo4j.io.pagecache.PageCursor;
/**
* Defines interfaces and common implementations of header reader/writer for {@link GBPTree}.
*/
public class Header
{
/**
* Writes a header into a {@link GBPTree} state page during
* {@link GBPTree#checkpoint(org.neo4j.io.pagecache.IOLimiter)}.
*/
public interface Writer
{
/**
* Writes header data into {@code to} with previous valid header data found in {@code from} of {@code length}
* bytes in size.
*
* @param from {@link PageCursor} positioned at the header data written in the previous check point.
* @param length size in bytes of the previous header data.
* @param to {@link PageCursor} to write new header into.
*/
void write( PageCursor from, int length, PageCursor to );
}
final Consumer<PageCursor> CARRY_OVER = cursor -> {};
static final Writer CARRY_OVER_PREVIOUS_HEADER = (from,length,to) ->
{
from.copyTo( from.getOffset(), to, to.getOffset(), length );
};
static Writer replace( Consumer<PageCursor> writer )
{
// Discard the previous state, just write the new
return (from,length,to) -> writer.accept( to );
}
/**
* Reads a header from a {@link GBPTree} state page during opening it.
*/
public interface Reader
{
/**
* Called when it's time to read header data from the most up to date and valid state page.
* Due to the nature of the underlying {@link PageCache} this method may be called several times,
* some times with invalid data in the {@link PageCursor}. Because of this there mustn't be any
* exceptions thrown or decisions made based on the read data until the GBPTree constructor has been
* completely executed.
*
* @param from {@link PageCursor} positioned at beginning of the header data to read.
* @param length number of bytes available to read in the header data.
*/
void read( PageCursor from, int length );
}
}