2 * Copyright (c) 2024 PANTHEON.tech, s.r.o. and others. All rights reserved.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
16 package io.atomix.storage.journal;
18 import static com.google.common.base.Verify.verify;
20 import java.io.IOException;
21 import java.nio.ByteBuffer;
22 import java.nio.channels.FileChannel;
23 import org.eclipse.jdt.annotation.NonNull;
26 * A {@link StorageLevel#DISK} implementation of {@link FileReader}. Maintains an internal buffer.
28 final class DiskFileReader extends FileReader {
30 * Just do not bother with IO smaller than this many bytes.
32 private static final int MIN_IO_SIZE = 8192;
34 private final FileChannel channel;
35 private final ByteBuffer buffer;
37 // tracks where memory's first available byte maps to in terms of FileChannel.position()
38 private int bufferPosition;
40 DiskFileReader(final JournalSegmentFile file, final int maxEntrySize) {
41 this(file, allocateBuffer(file.maxSize(), maxEntrySize));
44 // Note: take ownership of the buffer
45 DiskFileReader(final JournalSegmentFile file, final ByteBuffer buffer) {
47 channel = file.channel();
48 this.buffer = buffer.flip();
52 static ByteBuffer allocateBuffer(final int maxSegmentSize, final int maxEntrySize) {
53 return ByteBuffer.allocate(chooseBufferSize(maxSegmentSize, maxEntrySize));
56 private static int chooseBufferSize(final int maxSegmentSize, final int maxEntrySize) {
57 if (maxSegmentSize <= MIN_IO_SIZE) {
58 // just buffer the entire segment
59 return maxSegmentSize;
62 // one full entry plus its header, or MIN_IO_SIZE, which benefits the read of many small entries
63 final int minBufferSize = maxEntrySize + SegmentEntry.HEADER_BYTES;
64 return minBufferSize <= MIN_IO_SIZE ? MIN_IO_SIZE : minBufferSize;
68 void invalidateCache() {
69 buffer.clear().flip();
74 ByteBuffer read(final int position, final int size) {
75 // calculate logical seek distance between buffer's first byte and position and split flow between
76 // forward-moving and backwards-moving code paths.
77 final int seek = bufferPosition - position;
78 return seek >= 0 ? forwardAndRead(seek, position, size) : rewindAndRead(-seek, position, size);
81 private @NonNull ByteBuffer forwardAndRead(final int seek, final int position, final int size) {
82 final int missing = buffer.limit() - seek - size;
84 // fast path: we have the requested region
85 return buffer.slice(seek, size).asReadOnlyBuffer();
88 // We need to read more data, but let's salvage what we can:
89 // - set buffer position to seek, which means it points to the same as position
90 // - run compact, which moves everything between position and limit onto the beginning of buffer and
91 // sets it up to receive more bytes
92 // - start the read accounting for the seek
93 buffer.position(seek).compact();
94 readAtLeast(position + seek, missing);
95 return setAndSlice(position, size);
98 private @NonNull ByteBuffer rewindAndRead(final int rewindBy, final int position, final int size) {
99 // TODO: Lazy solution. To be super crisp, we want to find out how much of the buffer we can salvage and
100 // do all the limit/position fiddling before and after read. Right now let's just flow the buffer up and
103 readAtLeast(position, size);
104 return setAndSlice(position, size);
107 private void readAtLeast(final int readPosition, final int readAtLeast) {
110 bytesRead = channel.read(buffer, readPosition);
111 } catch (IOException e) {
112 throw new StorageException(e);
114 verify(bytesRead >= readAtLeast, "Short read %s, expected %s", bytesRead, readAtLeast);
118 private @NonNull ByteBuffer setAndSlice(final int position, final int size) {
119 bufferPosition = position;
120 return buffer.slice(0, size).asReadOnlyBuffer();