Lucene 索引删除策略源码解析
Lucene
从今天开始，我们要开始介绍Lucene中索引构建的流程。因为索引构建的逻辑涉及到的东西非常多，如果从构建入口IndexWriter来开始介绍，是很难说清楚的。所以接下来按化零为整的方式 ，从构建相关的各个组件开始介绍，尽量每一篇文章都是可以独立阅读，依赖的前置知识都是我已经介绍的内容。不管就算如此，还是会有部分内容可能需要结合整体流程才能明白，对于这部分的内容，大家可以先留个印象，以后介绍相关联的内容时，我会再重新指出。
今天我们一起来看看索引文件删除相关的。
IndexCommit
Lucene中，需要持久化的索引信息都要进行commit操作，然后会生成一个segments_N的索引文件记录此次commit相关的索引信息。
一次commit生成segments_N之后，就对应了一个IndexCommit，IndexCommit只是一个接口，它定义了可以从IndexCommit中获取哪些信息：
public abstract class IndexCommit implements Comparable<IndexCommit> {
// commit对应的segments_N
public abstract String getSegmentsFileName();
// commit关联的所有的索引文件
public abstract Collection<String> getFileNames() throws IOException;
// 索引所在的Directory
public abstract Directory getDirectory();
// 删除commit，后面会看到，删除其实减少commit关联的索引文件的引用计数
public abstract void delete();
// commit是否被删除了
public abstract boolean isDeleted();
// commit关联了几个segment
public abstract int getSegmentCount();
// segments_N文件中的N
public abstract long getGeneration();
// commit可以记录一些用户自定义的信息
public abstract Map<String, String> getUserData() throws IOException;
// 用来读取commit对应的索引数据
StandardDirectoryReader getReader() {
return null;
}
}
IndexCommit有三个实现类：
- CommitPoint
- ReaderCommit
- SnapshotCommitPoint
这个三个实现类都有对应的使用场景，在用到的时候我会再详细介绍，本文中会涉及到SnapshotCommitPoint，后面会详细介绍它。
IndexDeletionPolicy
在索引的生命周期中，可以有多次的commit操作，因此也会生成多个segments_N文件，对于这些文件是否要保留还是删除，lucene中是通过IndexDeletionPolicy来管理的。我们先来看下IndexDeletionPolicy的接口定义：
public abstract class IndexDeletionPolicy {
protected IndexDeletionPolicy() {}
// 重新打开索引的时候，对所有commit的处理
public abstract void onInit(List<? extends IndexCommit> commits) throws IOException;
// 有新提交时对所有commit的处理
public abstract void onCommit(List<? extends IndexCommit> commits) throws IOException;
}
从上面我可以看到，索引的删除策略其实只在两个地方进行应用，一个是加载索引的时候，打开一个旧索引时，根据当前设置的IndexDeletionPolicy进行处理。另一个是有新的commit产生时，借这个机会处理所有的commit。Lucene中提供的索引删除策略一共有四种，不过可以分为三类：
NoDeletionPolicy
NoDeletionPolicy索引删除策略就是保留所有的commit信息，效果就是你有多少次commit就多少个segments_N文件，看个例子：
public class DeletionPolicyTest {
private static final Random RANDOM = new Random();
public static void main(String[] args) throws IOException {
Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath());
WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer();
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);
indexWriterConfig.setUseCompoundFile(true);
indexWriterConfig.setIndexDeletionPolicy(NoDeletionPolicy.INSTANCE);
IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig);
indexWriter.aDDDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第一次commit，生成segments_1
indexWriter.commit();
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第二次commit，生成segments_2
indexWriter.commit();
indexWriter.close();
}
private static Document getDoc(int... point) {
Document doc = new Document();
IntPoint intPoint = new IntPoint("point", point);
doc.add(intPoint);
return doc;
}
}
上面的例子中有两次commit，下图是NoDeletionPolicy策略进行了两次commit的索引目录结构，可以看到生成了两个segments_N文件：
NoDeletionPolicy的代码实现非常简单，单例实现，并且在onCommit和onInit的时候都是空操作：
public final class NoDeletionPolicy extends IndexDeletionPolicy {
public static final IndexDeletionPolicy INSTANCE = new NoDeletionPolicy();
private NoDeletionPolicy() {
}
public void onCommit(List<? extends IndexCommit> commits) {}
public void onInit(List<? extends IndexCommit> commits) {}
}
KeepOnlyLastCommitDeletionPolicy
KeepOnlyLastCommitDeletionPolicy是Lucene默认的索引删除策略，只保留最新的一次commit，从索引目录看不管执行多少次commit只保留了N最大的segments_N文件，下图是KeepOnlyLastCommitDeletionPolicy策略进行了两次commit的结果，KeepOnlyLastCommitDeletionPolicy删除策略只保留了segments_2。把上面示例代码中的删除策略替换成KeepOnlyLastCommitDeletionPolicy，即可得到，注意需要先清空索引目录：
KeepOnlyLastCommitDeletionPolicy代码实现也比较简单，除了最后一个commit之外，其他的commit都删除：
public final class KeepOnlyLastCommitDeletionPolicy extends IndexDeletionPolicy {
public KeepOnlyLastCommitDeletionPolicy() {}
public void onInit(List<? extends IndexCommit> commits) {
onCommit(commits);
}
// commits是从旧到新排序的
public void onCommit(List<? extends IndexCommit> commits) {
// 只保留最新的一个
int size = commits.size();
for (int i = 0; i < size - 1; i++) {
commits.get(i).delete();
}
}
}
两个快照相关的删除策略
快照相关的删除策略有两个，SnapshotDeletionPolicy和PersistentSnapshotDeletionPolicy，分别对应了不可持久化和可持久化的模式。不管是SnapshotDeletionPolicy还是PersistentSnapshotDeletionPolicy，他们都封装了其他的IndexDeletionPolicy来执行删除策略，他们只是提供了为当前最新的commit生成快照的能力。只要快照存在，则跟快照相关的所有索引文件都会被无条件保留。
SnapshotDeletionPolicy
例子
public class SnapshotDeletionPolicyTest {
private static final Random RANDOM = new Random();
public static void main(String[] args) throws IOException, InterruptedException {
Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath());
WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer();
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);
indexWriterConfig.setUseCompoundFile(true);
SnapshotDeletionPolicy snapshotDeletionPolicy = new SnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy());
indexWriterConfig.setIndexDeletionPolicy(snapshotDeletionPolicy);
IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig);
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第一次commit，生成segments_1
indexWriter.commit();
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第二次commit，生成segments_2
indexWriter.commit();
// segments_2当做快照，无条件保留
snapshotDeletionPolicy.snapshot();
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第三次commit，生成segments_3
indexWriter.commit();
indexWriter.close();
}
private static Document getDoc(int... point) {
Document doc = new Document();
IntPoint intPoint = new IntPoint("point", point);
doc.add(intPoint);
return doc;
}
}
在上面的例子中，我们使用SnapshotDeletionPolicy，SnapshotDeletionPolicy底层封装的是KeepOnlyLastCommitDeletionPolicy，我们进行了三次commit，理论上KeepOnlyLastCommitDeletionPolicy只会保留最后一次，但是因为我们对第一次的commit进行了快照，所以第一次commit也被保留了：
接下来我们看看SnapshotDeletionPolicy是怎么实现。SnapshotDeletionPolicy保证生成快照的commit不会被删除的原理就是引用计数，SnapshotDeletionPolicy会记录每个commit生成快照的次数，在删除的时候，只会删除引用计数为0的commit。
成员变量
// key是IndexCommit的generation，value是对应的IndexCommit有多少个快照
// 需要注意的是，有被快照引用的才会记录在refCounts中，也就是只要被记录在refCounts中，引用次数至少是1
protected final Map<Long, Integer> refCounts = new HashMap<>();
// key是IndexCommit的generation，value是对应的IndexCommit
protected final Map<Long, IndexCommit> indexCommits = new HashMap<>();
// SnapshotDeletionPolicy只是增加了支持快照的功能，删除的逻辑是由primary参数对应的删除策略提供的
private final IndexDeletionPolicy primary;
// 最近一次提交的commit，只会对这个IndexCommit生成快照
protected IndexCommit lastCommit;
// 是否初始化的标记，实例化后，必须先调用onInit方法
private boolean initCalled;
生成快照
生成快照只会对当前最新的一个commit进行快照：
public synchronized IndexCommit snapshot() throws IOException {
if (!initCalled) {
throw new IllegalStateException(
"this instance is not being used by IndexWriter; be sure to use the instance returned from writer.getConfig().getIndexDeletionPolicy()");
}
if (lastCommit == null) {
throw new IllegalStateException("No index commit to snapshot");
}
// 新增lastCommit的引用计数
incRef(lastCommit);
return lastCommit;
}
protected synchronized void incRef(IndexCommit ic) {
long gen = ic.getGeneration();
Integer refCount = refCounts.get(gen);
int refCountInt;
if (refCount == null) { // 第一次被引用
indexCommits.put(gen, lastCommit);
refCountInt = 0;
} else {
refCountInt = refCount.intValue();
}
// 引用计数加+1
refCounts.put(gen, refCountInt + 1);
}
释放指定的快照
public synchronized void release(IndexCommit commit) throws IOException {
long gen = commit.getGeneration();
releaseGen(gen);
}
protected void releaseGen(long gen) throws IOException {
if (!initCalled) {
throw new IllegalStateException(
"this instance is not being used by IndexWriter; be sure to use the instance returned from writer.getConfig().getIndexDeletionPolicy()");
}
Integer refCount = refCounts.get(gen);
if (refCount == null) {
throw new IllegalArgumentException("commit gen=" + gen + " is not currently snapshotted");
}
int refCountInt = refCount.intValue();
assert refCountInt > 0;
refCountInt--;
if (refCountInt == 0) { // 引用计数为0，直接从refCounts中移除
refCounts.remove(gen);
indexCommits.remove(gen);
} else {
refCounts.put(gen, refCountInt);
}
}
删除commit
public synchronized void onCommit(List<? extends IndexCommit> commits) throws IOException {
// 把commits中的所有IndexCommit都封装成SnapshotCommitPoint，再使用primary执行onCommit方法
primary.onCommit(wrapCommits(commits));
// 更新最新的commit
lastCommit = commits.get(commits.size() - 1);
}
@Override
public synchronized void onInit(List<? extends IndexCommit> commits) throws IOException {
// 设置初始化的标记
initCalled = true;
primary.onInit(wrapCommits(commits));
for (IndexCommit commit : commits) {
if (refCounts.containsKey(commit.getGeneration())) {
indexCommits.put(commit.getGeneration(), commit);
}
}
if (!commits.isEmpty()) {
lastCommit = commits.get(commits.size() - 1);
}
}
private List<IndexCommit> wrapCommits(List<? extends IndexCommit> commits) {
List<IndexCommit> wrappedCommits = new ArrayList<>(commits.size());
for (IndexCommit ic : commits) {
// 把IndexCommit都封装成 SnapshotCommitPoint
wrappedCommits.add(new SnapshotCommitPoint(ic));
}
return wrappedCommits;
}
前面我们列出了SnapshotCommitPoint是IndexCommit的一个实现类，但是没有详细介绍，SnapshotCommitPoint除了能够提供IndexCommit接口所提供的信息之外，最核心的是在删除的时候，会先判断IndexCommit是否被快照引用，只有没有任何快照引用的IndexCommit才能删除：
public void delete() {
synchronized (SnapshotDeletionPolicy.this) {
if (!refCounts.containsKey(cp.getGeneration())) {
cp.delete();
}
}
}
存在的问题
需要注意的是SnapshotDeletionPolicy的快照信息是没有持久化，我们重新打开SnapshotDeletionPolicyTest例子中生成的索引：
public class SnapshotDeletionPolicyTest2 {
public static void main(String[] args) throws IOException, InterruptedException {
Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath());
WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer();
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);
indexWriterConfig.setUseCompoundFile(true);
SnapshotDeletionPolicy snapshotDeletionPolicy = new SnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy());
indexWriterConfig.setIndexDeletionPolicy(snapshotDeletionPolicy);
// 重新打开索引
IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig);
indexWriter.close();
}
}
可以发现segments_1被删除了，因为没有持久化快照信息，所以根据KeepOnlyLastCommitDeletionPolicy的删除策略，只保留了最新的一个commit：
PersistentSnapshotDeletionPolicy
例子
PersistentSnapshotDeletionPolicy主要是为了解决SnapshotDeletionPolicy无法持久化的问题。PersistentSnapshotDeletionPolicy持久化的时候会生成snapshots_N的索引文件，我们看个例子：
public class PersistentSnapshotDeletionPolicyTest {
private static final Random RANDOM = new Random();
public static void main(String[] args) throws IOException, InterruptedException {
Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath());
WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer();
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);
indexWriterConfig.setUseCompoundFile(true);
PersistentSnapshotDeletionPolicy persistentSnapshotDeletionPolicy = new PersistentSnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy(), directory);
indexWriterConfig.setIndexDeletionPolicy(persistentSnapshotDeletionPolicy);
IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig);
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第一次commit，生成segments_1
indexWriter.commit();
// segments_1当做快照，无条件保留
persistentSnapshotDeletionPolicy.snapshot();
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第二次commit，生成segments_2
indexWriter.commit();
indexWriter.addDocument(getDoc(RANDOM.nextInt(10000),RANDOM.nextInt(10000)));
// 第三次commit，生成segments_3
indexWriter.commit();
indexWriter.close();
}
private static Document getDoc(int... point) {
Document doc = new Document();
IntPoint intPoint = new IntPoint("point", point);
doc.add(intPoint);
return doc;
}
}
上面的例子和我们在介绍SnapshotDeletionPolicy的时候逻辑一样，只是把SnapshotDeletionPolicy换成了PersistentSnapshotDeletionPolicy，我们看结果：
从上面结果图中可以看到，segments_1和segments_3同样被保留了，但是多了一个持久化的快照信息的文件snapshots_0，有了这个文件，索引重新打开的时候就可以恢复快照信息，segments_1还是会被保留，用下面的例子我们重新打开索引，可以发现segments_1还是被保留了：
public class PersistentSnapshotDeletionPolicyTest2 {
public static void main(String[] args) throws IOException, InterruptedException {
Directory directory = FSDirectory.open(new File("D:\\code\\lucene-9.1.0-learning\\data").toPath());
WhitespaceAnalyzer analyzer = new WhitespaceAnalyzer();
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(analyzer);
indexWriterConfig.setUseCompoundFile(true);
PersistentSnapshotDeletionPolicy persistentSnapshotDeletionPolicy = new PersistentSnapshotDeletionPolicy(new KeepOnlyLastCommitDeletionPolicy(), directory);
indexWriterConfig.setIndexDeletionPolicy(persistentSnapshotDeletionPolicy);
IndexWriter indexWriter = new IndexWriter(directory, indexWriterConfig);
indexWriter.close();
}
}
接下来我们看看PersistentSnapshotDeletionPolicy的实现，主要就是持久化和恢复快照信息的逻辑。
成员变量
// 持久化快照信息的文件名snapshots_N中的N，从0开始
private long nextWriteGen;
// 持久化的文件所在的目录
private final Directory dir;
构造函数
public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary, Directory dir)
throws IOException {
this(primary, dir, OpenMode.CREATE_OR_APPEND);
}
public PersistentSnapshotDeletionPolicy(IndexDeletionPolicy primary, Directory dir, OpenMode mode)
throws IOException {
super(primary);
this.dir = dir;
if (mode == OpenMode.CREATE) { // 新建索引的模式，则需要清除所有的快照信息，索引模式以后再介绍
clearPriorSnapshots();
}
// 加载快照信息
loadPriorSnapshots();
if (mode == OpenMode.APPEND && nextWriteGen == 0) {
throw new IllegalStateException("no snapshots stored in this directory");
}
}
生成快照
public synchronized IndexCommit snapshot() throws IOException {
// 使用SnapshotDeletionPolicy来生成快照
IndexCommit ic = super.snapshot();
// 标记持久化是否成功，不成功的话需要删除快照
boolean success = false;
try {
// 持久化最新的快照信息
persist();
success = true;
} finally {
if (!success) { // 持久化失败，删除快照
try {
super.release(ic);
} catch (
@SuppressWarnings("unused")
Exception e) {
// Suppress so we keep throwing original exception
}
}
}
return ic;
}
释放快照
public synchronized void release(IndexCommit commit) throws IOException {
// 使用SnapshotDeletionPolicy来释放快照
super.release(commit);
// 持久化快照信息是否成功
boolean success = false;
try {
// 持久化最新的快照信息
persist();
success = true;
} finally {
if (!success) { // 持久化失败，重新加回快照信息
try {
incRef(commit);
} catch (
@SuppressWarnings("unused")
Exception e) {
// Suppress so we keep throwing original exception
}
}
}
}
持久化快照信息
private synchronized void persist() throws IOException {
// 快照文件名
String fileName = SNAPSHOTS_PREFIX + nextWriteGen;
boolean success = false;
try (IndexOutput out = dir.createOutput(fileName, iocontext.DEFAULT)) {
CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT);
out.writeVInt(refCounts.size());
for (Entry<Long, Integer> ent : refCounts.entrySet()) { // 持久化所有的引用信息
out.writeVLong(ent.geTKEy());
out.writeVInt(ent.getValue());
}
success = true;
} finally {
if (!success) {
IOUtils.deleteFilesIgnoringExceptions(dir, fileName);
}
}
dir.sync(Collections.singletonList(fileName));
if (nextWriteGen > 0) {
String lastSaveFile = SNAPSHOTS_PREFIX + (nextWriteGen - 1);
// 删除前一个快照文件，因为每次持久化都是把当前的快照信息全量持久化，所以只需要保留最新的一个就可以
// 这里有可能删除失败，所以在启动加载的时候会再次尝试把旧版本的文件都删掉
IOUtils.deleteFilesIgnoringExceptions(dir, lastSaveFile);
}
nextWriteGen++;
}
加载快照信息
private synchronized void loadPriorSnapshots() throws IOException {
long genLoaded = -1;
IOException ioe = null;
List<String> snapshotFiles = new ArrayList<>();
for (String file : dir.listAll()) {
if (file.startsWith(SNAPSHOTS_PREFIX)) { // 找到快照文件
long gen = Long.parseLong(file.substring(SNAPSHOTS_PREFIX.length()));
if (genLoaded == -1 || gen > genLoaded) { // 找到gen最大的快照文件
snapshotFiles.add(file);
Map<Long, Integer> m = new HashMap<>();
IndexInput in = dir.openInput(file, IOContext.DEFAULT);
try {
CodecUtil.checkHeader(in, CODEC_NAME, VERSION_START, VERSION_START);
int count = in.readVInt();
for (int i = 0; i < count; i++) {
long commitGen = in.readVLong();
int refCount = in.readVInt();
m.put(commitGen, refCount);
}
} catch (IOException ioe2) {
// 保存第一个捕获到的异常
if (ioe == null) {
ioe = ioe2;
}
} finally {
in.close();
}
genLoaded = gen;
// 清除旧数据
refCounts.clear();
// 保留最新的
refCounts.putAll(m);
}
}
}
if (genLoaded == -1) { // 没有加载快照文件
if (ioe != null) { // 加载过程中捕获到异常了，直接抛出
throw ioe;
}
} else { // 把旧版本的快照文件都删掉
if (snapshotFiles.size() > 1) {
String curFileName = SNAPSHOTS_PREFIX + genLoaded;
for (String file : snapshotFiles) {
if (!curFileName.equals(file)) {
IOUtils.deleteFilesIgnoringExceptions(dir, file);
}
}
}
nextWriteGen = 1 + genLoaded;
}
}
总结
本文介绍的索引删除策略是在IndexCommit粒度的控制，具体到每个索引文件是怎么控制的，我们下一篇文章介绍。
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