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New cache for in-flight Raft entries
A highly efficient cache which only can keep a consecutive range of raft log entries cached, allowing for a very simple and efficient circular buffer to be used. The cache also allows putting a maximum bound on the amount of entries as well as their total combined size. The cache also exposes metrics so that cache hits, misses and total data can be observed.
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...ing/src/main/java/org/neo4j/causalclustering/core/consensus/log/cache/CircularBuffer.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 Affero 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 Affero General Public License for more details. | |||
| * | |||
| * You should have received a copy of the GNU Affero General Public License | |||
| * along with this program. If not, see <http://www.gnu.org/licenses/>. | |||
| */ | |||
| package org.neo4j.causalclustering.core.consensus.log.cache; | |||
|
|
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| import static java.lang.Math.floorMod; | |||
| import static java.util.Arrays.fill; | |||
|
|
|||
| /** | |||
| * <pre> | |||
| * Design | |||
| * | |||
| * S: start index | |||
| * E: end index | |||
| * | |||
| * When S == E the buffer is empty. | |||
| * | |||
| * Examples: | |||
| * | |||
| * S | |||
| * v | |||
| * Empty [ | | | | | | ] | |||
| * ^ | |||
| * E | |||
| * | |||
| * | |||
| * S | |||
| * v | |||
| * Size 2 [ | | | | | | ] | |||
| * ^ | |||
| * E | |||
| * | |||
| * | |||
| * S | |||
| * v | |||
| * Full [ | | | | | | ] | |||
| * ^ | |||
| * E | |||
| * | |||
| * New items are put at the current E, and then E is moved one step forward (circularly). | |||
| * The item at E is never a valid item. | |||
| * | |||
| * If moving E one step forward moves it onto S | |||
| * - then it knocks an element out | |||
| * - and S is also moved one step forward | |||
| * | |||
| * The S element has index 0. | |||
| * Removing an element moves S forward (circularly). | |||
| * | |||
| * @param <V> type of elements. | |||
| */ | |||
| public class CircularBuffer<V> | |||
| { | |||
| private final int arraySize; // externally visible capacity is arraySize - 1 | |||
| private Object[] elementArr; | |||
|
|
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| private int S; | |||
| private int E; | |||
|
|
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| CircularBuffer( int capacity ) | |||
| { | |||
| if ( capacity <= 0 ) | |||
| { | |||
| throw new IllegalArgumentException( "Capacity must be > 0." ); | |||
| } | |||
|
|
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| this.arraySize = capacity + 1; // 1 item as sentinel (can't hold entries) | |||
| this.elementArr = new Object[arraySize]; | |||
| } | |||
|
|
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| /** | |||
| * Clears the underlying buffer and fills the provided eviction array with all evicted elements. | |||
| * The provided array must have the same capacity as the circular buffer. | |||
| * | |||
| * @param evictions Caller-provided array for evictions. | |||
| */ | |||
| public void clear( V[] evictions ) | |||
| { | |||
| if ( evictions.length != arraySize - 1 ) | |||
| { | |||
| throw new IllegalArgumentException( "The eviction array must be of the same size as the capacity of the circular buffer." ); | |||
| } | |||
|
|
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| int i = 0; | |||
| while ( S != E ) | |||
| { | |||
| //noinspection unchecked | |||
| evictions[i++] = (V) elementArr[S]; | |||
| S = pos( S, 1 ); | |||
| } | |||
|
|
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| S = 0; | |||
| E = 0; | |||
|
|
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| fill( elementArr, null ); | |||
| } | |||
|
|
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| private int pos( int base, int delta ) | |||
| { | |||
| return floorMod( base + delta, arraySize ); | |||
| } | |||
|
|
|||
| /** | |||
| * Append to the end of the buffer, possibly overwriting the | |||
| * oldest entry. | |||
| * | |||
| * @return any knocked out item, or null if nothing was knocked out. | |||
| */ | |||
| public V append( V e ) | |||
| { | |||
| elementArr[E] = e; | |||
| E = pos( E, 1 ); | |||
| if ( E == S ) | |||
| { | |||
| //noinspection unchecked | |||
| V old = (V) elementArr[E]; | |||
| elementArr[E] = null; | |||
| S = pos( S, 1 ); | |||
| return old; | |||
| } | |||
| else | |||
| { | |||
| return null; | |||
| } | |||
| } | |||
|
|
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| public V read( int idx ) | |||
| { | |||
| //noinspection unchecked | |||
| return (V) elementArr[pos( S, idx )]; | |||
| } | |||
|
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| public V remove() | |||
| { | |||
| if ( S == E ) | |||
| { | |||
| return null; | |||
| } | |||
| //noinspection unchecked | |||
| V e = (V) elementArr[S]; | |||
| elementArr[S] = null; | |||
| S = pos( S, 1 ); | |||
| return e; | |||
| } | |||
|
|
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| public V removeHead() | |||
| { | |||
| if ( S == E ) | |||
| { | |||
| return null; | |||
| } | |||
|
|
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| E = pos( E, -1 ); | |||
| //noinspection unchecked | |||
| V e = (V) elementArr[E]; | |||
| elementArr[E] = null; | |||
| return e; | |||
| } | |||
|
|
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| public int size() | |||
| { | |||
| return floorMod( E - S, arraySize ); | |||
| } | |||
| } | |||
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