leveldb-Impl: Version.java
作者：互联网
Manifest与Current文件是LevelDB保存在稳定存储中的文件版本信息，在LevelDB被打开后，其会先通过Current文件找到当前的Manifest文件，读取并反序列化其中数据，并在内存中维护文件版本信息，以便后续操作。
先来说说Version，Version表示了当前leveldb的版本信息，
版本信息内容包括：1当前每一层的SSTable文件元信息。2记录被Seek太多次需要Compact的文件元信息，以及文件所在的level。3记录所有level层中compaction score 最大的那一层及其level，用于比较判断是否需要对此level层进行compact。
leveldb/Version.java at master · dain/leveldb · GitHub
public class Version
implements SeekingIterable<InternalKey, Slice>
{
private final AtomicInteger retained = new AtomicInteger(1);
private final VersionSet versionSet;
private final Level0 level0;
private final List<Level> levels;
// move these mutable fields somewhere else
private int compactionLevel;
private double compactionScore;
private FileMetaData fileToCompact;
private int fileToCompactLevel;
AtomicInteger是提供原子操作的Integer类,通过线程安全的方式操作加减。
leveldb内部只有一个VersionSet进行管理所有的Version. 当不断有新版本生成的时候，那么就需要不断地 Append 到 versionset 里面。 当旧的 Version 不再服务读请求之后，这个 Version 就会从Versionset中移除。
leveldb的第一层level0 并新建List<Level> levels记录文件所在level信息。
FileMetaData使用来表示sstable的元数据。fileTocompact记录下一个等待compact的file。当fileMataData文件查找到一定次数时就要执行合并操作。
version改变时结合compactionLevel和compactionScore的信息确定下次compaction的level和文件。
fileToCompactLevel记录文件压缩到哪个level。
对 Version 的修改主要发生于 compaction 之后。compaction 分为 minor compaction 和 major compaction，其中 minor compaction 是落 memtable 到 L0，只会新增文件，而 major compaction 会跨 level 做合并，既新增文件也删除文件。每当这时，便会生成一个新的 VersionEdit 产生新的 Version。
public Version(VersionSet versionSet)
{
this.versionSet = versionSet;
checkArgument(NUM_LEVELS > 1, "levels must be at least 2");
this.level0 = new Level0(new ArrayList<FileMetaData>(), getTableCache(), getInternalKeyComparator());
Builder<Level> builder = ImmutableList.builder();
for (int i = 1; i < NUM_LEVELS; i++) {
List<FileMetaData> files = new ArrayList<>();
builder.add(new Level(i, files, getTableCache(), getInternalKeyComparator()));
}
this.levels = builder.build();
}
先判断versionset里的level层数>1 若为Level0则新建对象level0，由FileMetaData TableCache 和 InternalKeyComparartor组成。
ImmutableList.builder方法新建builder对象，更新产生新的version，将新的level对象add到builder中。
public void assertNoOverlappingFiles()
{
for (int level = 1; level < NUM_LEVELS; level++) {
assertNoOverlappingFiles(level);
}
}
判断SStables有没有重复的Key范围
public void assertNoOverlappingFiles(int level)
{
if (level > 0) {
Collection<FileMetaData> files = getFiles().asMap().get(level);
if (files != null) {
long previousFileNumber = 0;
InternalKey previousEnd = null;
for (FileMetaData fileMetaData : files) {
if (previousEnd != null) {
checkArgument(getInternalKeyComparator().compare(
previousEnd,
fileMetaData.getSmallest()
) < 0, "Overlapping files %s and %s in level %s", previousFileNumber, fileMetaData.getNumber(), level);
}
previousFileNumber = fileMetaData.getNumber();
previousEnd = fileMetaData.getLargest();
}
}
}
}
根据PreviousFileNumber（InternalKey类) ,fileMetaData 和所在的 level，检查是否有重叠的Files
private TableCache getTableCache()
{
return versionSet.getTableCache();
}
获取TableCache
public final InternalKeyComparator getInternalKeyComparator()
{
return versionSet.getInternalKeyComparator();
}
获取InternalKey比较器
public synchronized int getCompactionLevel()
{
return compactionLevel;
}
获取compaction的level
public synchronized void setCompactionLevel(int compactionLevel)
{
this.compactionLevel = compactionLevel;
}
设置compaction的level
public synchronized double getCompactionScore()
{
return compactionScore;
}
获取compactionScore,决定是否触发compaction
public synchronized void setCompactionScore(double compactionScore)
{
this.compactionScore = compactionScore;
}
设置CampactionScore
@Override
public MergingIterator iterator()
{
Builder<InternalIterator> builder = ImmutableList.builder();
builder.add(level0.iterator());
builder.addAll(getLevelIterators());
return new MergingIterator(builder.build(), getInternalKeyComparator());
}
如果每个iterator中的key有序，但是所有iterator中的所有key全局无序，此时，需要一种能够“归并”多路有序iterator的结构。这一结构即MergingIterator
传入level0的iterator和所有Level的iterator，以及用来比较InternalKey的Comparator。InternalKey迭代器迭代器组合了MemTable Iterator、Immutable MemTable Iterator、每个Level-0 SSTable的Iterator，和level>1的所有SSTable的Concatenating Iterator。
List<InternalTableIterator> getLevel0Files()
{
Builder<InternalTableIterator> builder = ImmutableList.builder();
for (FileMetaData file : level0.getFiles()) {
builder.add(getTableCache().newIterator(file));
}
return builder.build();
}
新建InternalTableIterator的对象获取TableCache中的level0的file。迭代方法使用时都反复需要获取其中iterator是否为valid或获取其value，不需要每次都访问到最下层的iterator，只需要访问缓存状态即可。
List<LevelIterator> getLevelIterators()
{
Builder<LevelIterator> builder = ImmutableList.builder();
for (Level level : levels) {
if (!level.getFiles().isEmpty()) {
builder.add(level.iterator());
}
}
return builder.build();
}
遍历每一层Level,获取level的Iterator
public LookupResult get(LookupKey key)
{
// We can search level-by-level since entries never hop across
// levels. Therefore we are guaranteed that if we find data
// in an smaller level, later levels are irrelevant.
ReadStats readStats = new ReadStats();
LookupResult lookupResult = level0.get(key, readStats);
if (lookupResult == null) {
for (Level level : levels) {
lookupResult = level.get(key, readStats);
if (lookupResult != null) {
break;
}
}
}
updateStats(readStats.getSeekFileLevel(), readStats.getSeekFile());
return lookupResult;
}
获取level0的LookupKey
int pickLevelForMemTableOutput(Slice smallestUserKey, Slice largestUserKey)
{
int level = 0;
if (!overlapInLevel(0, smallestUserKey, largestUserKey)) {
// Push to next level if there is no overlap in next level,
// and the #bytes overlapping in the level after that are limited.
InternalKey start = new InternalKey(smallestUserKey, MAX_SEQUENCE_NUMBER, ValueType.VALUE);
InternalKey limit = new InternalKey(largestUserKey, 0, ValueType.VALUE);
while (level < MAX_MEM_COMPACT_LEVEL) {
if (overlapInLevel(level + 1, smallestUserKey, largestUserKey)) {
break;
}
long sum = Compaction.totalFileSize(versionSet.getOverlappingInputs(level + 2, start, limit));
if (sum > MAX_GRAND_PARENT_OVERLAP_BYTES) {
break;
}
level++;
}
}
return level;
}
pickLevelForMemTableOutput:从Memtable中dump到level0
leveldb中的db文件本身没有层次概念，所有的db文件都一样，如何确定这个文件是在哪一层由pickLevelForMemTableOutput方法实现。
public boolean overlapInLevel(int level, Slice smallestUserKey, Slice largestUserKey)
{
checkPositionIndex(level, levels.size(), "Invalid level");
requireNonNull(smallestUserKey, "smallestUserKey is null");
requireNonNull(largestUserKey, "largestUserKey is null");
if (level == 0) {
return level0.someFileOverlapsRange(smallestUserKey, largestUserKey);
}
return levels.get(level - 1).someFileOverlapsRange(smallestUserKey, largestUserKey);
}public boolean overlapInLevel(int level, Slice smallestUserKey, Slice largestUserKey)
{
checkPositionIndex(level, levels.size(), "Invalid level");
requireNonNull(smallestUserKey, "smallestUserKey is null");
requireNonNull(largestUserKey, "largestUserKey is null");
if (level == 0) {
return level0.someFileOverlapsRange(smallestUserKey, largestUserKey);
}
return levels.get(level - 1).someFileOverlapsRange(smallestUserKey, largestUserKey);
}
参考：深入浅出LevelDB —— 08 Iterator - 叉鸽 MrCroxx 的博客
标签：largestUserKey,leveldb,java,level,smallestUserKey,builder,Version,return,public 来源： https://www.cnblogs.com/yunqihao/p/16502905.html