Apache Kafka源码分析 – Log Management
LogManager
LogManager会管理broker上所有的logs（在一个log目录下），一个topic的一个partition对应于一个log（一个log子目录）
首先loadLogs会加载每个partition所对应的log对象， 然后提供createLog，getLog，deleteLog之类的管理接口
并且会创建些后台线程来进行，cleanup，flush，checkpoint生成之类的工作
/**
* The entry point to the kafka log management subsystem. The log manager is responsible for log creation, retrieval, and cleaning.
* All read and write operations are delegated to the individual log instances.
*
* The log manager maintains logs in one or more directories. New logs are created in the data directory
* with the fewest logs. No attempt is made to move partitions after the fact or balance based on
* size or I/O rate.
*
* A background thread handles log retention by periodically truncating excess log segments.
*/
@threadsafe
class LogManager(val logDirs: Array,
val topicConfigs: Map,
val defaultConfig: LogConfig,
val cleanerConfig: CleanerConfig,
val flushCheckMs: Long,
val flushCheckpointMs: Long,
val retentionCheckMs: Long,
scheduler: Scheduler,
private val time: Time) extends Logging {
//kafka.utils.Pool，对ConcurrentHashMap的封装
private val logs = new Pool() //一个topic的partition对应于一个log
/**
* Recover and load all logs in the given data directories
*/
private def loadLogs(dirs: Seq) {
for(dir0) {
// append an entry to the index (if needed)
if(bytesSinceLastIndexEntry > indexIntervalBytes) { //仅index部分message
index.append(offset, log.sizeInBytes())//写index文件
this.bytesSinceLastIndexEntry = 0
}
// append the messages
log.append(messages) //写messageset文件
this.bytesSinceLastIndexEntry += messages.sizeInBytes
}
}
/**
* Read a message set from this segment beginning with the first offset >= startOffset. The message set will include
* no more than maxSize bytes and will end before maxOffset if a maxOffset is specified.
*
* @param startOffset A lower bound on the first offset to include in the message set we read
* @param maxSize The maximum number of bytes to include in the message set we read
* @param maxOffset An optional maximum offset for the message set we read
*
* @return The message set read or null if the startOffset is larger than the largest offset in this log.
*/
@threadsafe
def read(startOffset: Long, maxOffset: Option, maxSize: Int): MessageSet = {
val logSize = log.sizeInBytes // this may change, need to save a consistent copy
val startPosition = translateOffset(startOffset)
// calculate the length of the message set to read based on whether or not they gave us a maxOffset
val length =
maxOffset match {
case None =>
// no max offset, just use the max size they gave unmolested
maxSize
case Some(offset) => {
// there is a max offset, translate it to a file position and use that to calculate the max read size
if(offset < startOffset)
throw new IllegalArgumentException("Attempt to read with a maximum offset (%d) less than the start offset (%d).".format(offset, startOffset))
val mapping = translateOffset(offset, startPosition.position)
val endPosition =
if(mapping == null)
logSize // the max offset is off the end of the log, use the end of the file
else
mapping.position
min(endPosition - startPosition.position, maxSize)
}
}
log.read(startPosition.position, length) //读出messageset
}
/**
* Run recovery on the given segment. This will rebuild the index from the log file and lop off any invalid bytes from the end of the log and index.
*
* @param maxMessageSize A bound the memory allocation in the case of a corrupt message size--we will assume any message larger than this
* is corrupt.
*
* @return The number of bytes truncated from the log
*/
@nonthreadsafe
def recover(maxMessageSize: Int): Int = {...}
}
FileMessageSet
Segment中实际存放log message的文件，通过FileChannel可以读写文件
1: /** 2:* An on-disk message set. An optional start and end position can be applied to the message set 3:* which will allow slicing a subset of the file. 4:* @param file The file name for the underlying log data 5:* @param channel the underlying file channel used 6:* @param start A lower bound on the absolute position in the file from which the message set begins 7:* @param end The upper bound on the absolute position in the file at which the message set ends 8:* @param isSlice Should the start and end parameters be used for slicing? 9:*/10: @nonthreadsafe11: class FileMessageSet private(@volatile var file: File,12: private val channel: FileChannel,13: private val start: Int,14: private val end: Int,15: isSlice: Boolean) extends MessageSet with Logging {...} OffsetIndex
Segment的index文件， 这是0.8后加上的，之前message直接使用物理offset标识
新版本中还是改成了使用逻辑offset，让物理地址对用户透明， 这样就需要一个index来匹配逻辑offset和物理地址
index考虑到效率，最好放在内存中，但是考虑到size问题， 所以使用MappedByteBuffer（参考，Java RandomAccessFile用法 ）
注释里面说，
Index是sparse的，不保证每个message在index都有索引的entry
Index由entry组成，每个entry为8-byte，逻辑offset4-byte，物理地址4-byte
并且逻辑offset是基于base offset的相对offset，否则无法保证只使用4-byte
1: /** 2:* An index that maps offsets to physical file locations for a particular log segment. This index may be sparse: 3:* that is it may not hold an entry for all messages in the log. 4:* 5:* The index is stored in a file that is pre-allocated to hold a fixed maximum number of 8-byte entries. 6:* 7:* The index supports lookups against a memory-map of this file. These lookups are done using a simple binary search variant 8:* to locate the offset/location pair for the greatest offset less than or equal to the target offset. 9:* 10:* Index files can be opened in two ways: either as an empty, mutable index that allows appends or11:* an immutable read-only index file that has previously been populated. The makeReadOnly method will turn a mutable file into an 12:* immutable one and truncate off any extra bytes. This is done when the index file is rolled over.13:* 14:* No attempt is made to checksum the contents of this file, in the event of a crash it is rebuilt.15:* 16:* The file format is a series of entries. The physical format is a 4 byte "relative" offset and a 4 byte file location for the 17:* message with that offset. The offset stored is relative to the base offset of the index file. So, for example,18:* if the base offset was 50, then the offset 55 would be stored as 5. Using relative offsets in this way let's us use19:* only 4 bytes for the offset.20:* 21:* The frequency of entries is up to the user of this class.22:* 23:* All external APIs translate from relative offsets to full offsets, so users of this class do not interact with the internal 24:* storage format.25:*/26: class OffsetIndex(@volatile var file: File, val baseOffset: Long, val maxIndexSize: Int = -1) extends Logging {27: private val lock = new ReentrantLock//操作index文件需要加锁28: 29: /* initialize the memory mapping for this index */30: private var mmap: MappedByteBuffer =//使用MappedByteBuffer来操作index文件以应对大文件31: {32: val newlyCreated = file.createNewFile()33: val raf = new RandomAccessFile(file, "rw")34: val len = raf.length()35: val idx = raf.getChannel.map(FileChannel.MapMode.READ_WRITE, 0, len) 36: }37: 38: //通过byte偏移从buffer中读出某个entry的内容，offset和physical地址39: /* return the nth offset relative to the base offset */40: private def relativeOffset(buffer: ByteBuffer, n: Int): Int = buffer.getInt(n * 8)41: /* return the nth physical position */42: private def physical(buffer: ByteBuffer, n: Int): Int = buffer.getInt(n * 8 + 4)43: 44: //通过二分查找找到targetOffset或最接近的offset（less than）45: /**46: * Find the largest offset less than or equal to the given targetOffset 47: * and return a pair holding this offset and it's corresponding physical file position.48: * 49: * @param targetOffset The offset to look up.50: * 51: * @return The offset found and the corresponding file position for this offset. 52: * If the target offset is smaller than the least entry in the index (or the index is empty),53: * the pair (baseOffset, 0) is returned.54: */55: def lookup(targetOffset: Long): OffsetPosition = {...}56: 57: /**58:* Get the nth offset mapping from the index59:* @param n The entry number in the index60:* @return The offset/position pair at that entry61:*/62: def entry(n: Int): OffsetPosition = {63: maybeLock(lock) {64: if(n >= entries)65: throw new IllegalArgumentException("Attempt to fetch the %dth entry from an index of size %d.".format(n, entries))66: val idx = mmap.duplicate67: OffsetPosition(relativeOffset(idx, n), physical(idx, n))68: }69: }70: 71: /**72:* Append an entry for the given offset/location pair to the index. This entry must have a larger offset than all subsequent entries.73:*/74: def append(offset: Long, position: Int) {75: inLock(lock) {76: require(!isFull, "Attempt to append to a full index (size = " + size + ").")77: if (size.get == 0 || offset > lastOffset) {78: debug("Adding index entry %d => %d to %s.".format(offset, position, file.getName))79: this.mmap.putInt((offset - baseOffset).toInt)80: this.mmap.putInt(position)81: this.size.incrementAndGet()82: this.lastOffset = offset83: require(entries * 8 == mmap.position, entries + " entries but file position in index is " + mmap.position + ".")84: } else {85: throw new InvalidOffsetException("Attempt to append an offset (%d) to position %d no larger than the last offset appended (%d) to %s."86: .format(offset, entries, lastOffset, file.getAbsolutePath))87: }88: }89: }
具体看看如何从逻辑offset转化为物理地址的？
0.8中增加了逻辑offset，那么就需要做逻辑offset和物理地址间的转化
简单的方法，直接用hashmap，cache所有offset，问题就是这样空间耗费比较大
所以kafka的方式，是分段索引，用offset通过二分查找中index中找出段的起始地址，然后再去file里面遍历找出精确的地址， 时间换空间的设计
1. LogSegment.translateOffset
首先是从index文件中找到近似的物理地址
前面说了，index中从效率考虑并不会为每个offset建立索引entry，只会分段建立offset索引， 所以从index中直接可以找到精确物理地址的概率不大，但是可以找到最接近的那个物理地址
如果你觉得index的粒度比较粗，可以直接给出开始查找的startingFilePosition
所以精确的物理地址需要到MessageSet文件里面去继续找
/**
* Find the physical file position for the first message with offset >= the requested offset.
*
* The lowerBound argument is an optimization that can be used if we already know a valid starting position
* in the file higher than the greast-lower-bound from the index.
*
* @param offset The offset we want to translate
* @param startingFilePosition A lower bound on the file position from which to begin the search. This is purely an optimization and
* when omitted, the search will begin at the position in the offset index.
*
* @return The position in the log storing the message with the least offset >= the requested offset or null if no message meets this criteria.
*/
@threadsafe
private def translateOffset(offset: Long, startingFilePosition: Int = 0): OffsetPosition = {
val mapping = index.lookup(offset) //从index查出近似物理地址
log.searchFor(offset, max(mapping.position, startingFilePosition))
}
2. FileMessageSet.searchFor
在messageSet中，message的构成是，overhead（MessageSize+Offset）和message
而searchFor的逻辑是从startingPosition开始， 逐条遍历各个message，并从overhead中取出offset进行比较，直到找到target offset为止
/**
* Search forward for the file position of the last offset that is greater than or equal to the target offset
* and return its physical position. If no such offsets are found, return null.
* @param targetOffset The offset to search for.
* @param startingPosition The starting position in the file to begin searching from.
*/
def searchFor(targetOffset: Long, startingPosition: Int): OffsetPosition = {
var position = startingPosition
val buffer = ByteBuffer.allocate(MessageSet.LogOverhead) // LogOverhead = MessageSizeLength + OffsetLength
val size = sizeInBytes()
while(position + MessageSet.LogOverhead < size) { //从postion开始逐条遍历
buffer.rewind()
channel.read(buffer, position)
buffer.rewind()
val offset = buffer.getLong()
if(offset >= targetOffset)//判断是否找到offset
return OffsetPosition(offset, position)
val messageSize = buffer.getInt()
position += MessageSet.LogOverhead + messageSize //递进到下个message
}
null
}
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