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;
19 import static java.util.Objects.requireNonNull;
21 import java.io.IOException;
22 import java.nio.ByteBuffer;
23 import java.nio.channels.FileChannel;
24 import java.nio.file.Path;
25 import org.checkerframework.checker.nullness.qual.NonNull;
28 * A {@link StorageLevel#DISK} implementation of {@link FileReader}. Maintains an internal buffer.
30 final class DiskFileReader extends FileReader {
31 private final FileChannel channel;
32 private final ByteBuffer buffer;
34 // tracks where memory's first available byte maps to in terms of FileChannel.position()
35 private int bufferPosition;
37 DiskFileReader(final Path path, final FileChannel channel, final int maxEntrySize) {
39 this.channel = requireNonNull(channel);
40 buffer = ByteBuffer.allocate(chooseBufferSize(maxEntrySize)).flip();
44 private static int chooseBufferSize(final int maxEntrySize) {
45 return (maxEntrySize + SegmentEntry.HEADER_BYTES) * 2;
49 void invalidateCache() {
50 buffer.clear().flip();
55 ByteBuffer read(final int position, final int size) {
56 // calculate logical seek distance between buffer's first byte and position and split flow between
57 // forward-moving and backwards-moving code paths.
58 final int seek = bufferPosition - position;
59 return seek >= 0 ? forwardAndRead(seek, position, size) : rewindAndRead(-seek, position, size);
62 private @NonNull ByteBuffer forwardAndRead(final int seek, final int position, final int size) {
63 final int missing = buffer.limit() - seek - size;
65 // fast path: we have the requested region
66 return buffer.slice(seek, size).asReadOnlyBuffer();
69 // We need to read more data, but let's salvage what we can:
70 // - set buffer position to seek, which means it points to the same as position
71 // - run compact, which moves everything between position and limit onto the beginning of buffer and
72 // sets it up to receive more bytes
73 // - start the read accounting for the seek
74 buffer.position(seek).compact();
75 readAtLeast(position + seek, missing);
76 return setAndSlice(position, size);
79 private @NonNull ByteBuffer rewindAndRead(final int rewindBy, final int position, final int size) {
80 // TODO: Lazy solution. To be super crisp, we want to find out how much of the buffer we can salvage and
81 // do all the limit/position fiddling before and after read. Right now let's just flow the buffer up and
84 readAtLeast(position, size);
85 return setAndSlice(position, size);
88 private void readAtLeast(final int readPosition, final int readAtLeast) {
91 bytesRead = channel.read(buffer, readPosition);
92 } catch (IOException e) {
93 throw new StorageException(e);
95 verify(bytesRead >= readAtLeast, "Short read %s, expected %s", bytesRead, readAtLeast);
99 private @NonNull ByteBuffer setAndSlice(final int position, final int size) {
100 bufferPosition = position;
101 return buffer.slice(0, size).asReadOnlyBuffer();