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
Log
Log只是对于LogSegments的封装，包含loadSegments，append（到active segment），read（需要定位到相应的segment）
LogSegment
Segment是个逻辑概念，为了防止log文件过大， 将log分成许多的LogSegments
Segment又分为两部分，MessageSet文件和Index文件，分别命名为[base_offset].log和[base_offset].index
base_offset就是该Segment的起始offset，比前一个segment里面的offset都要大
Segment提供对于MessageSet的读写接口
写，需要间隔的更新index文件，应该为了尽量减小index的size，所以只是当写入数据大于indexIntervalBytes时，才增加一条索引
读，由于user传入的是逻辑offest，需要先转化为物理地址才能从文件中读到数据，如何转化参考下面
同时index文件是可以根据MessageSet文件重新rebuild的
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: @nonthreadsafe
11: class FileMessageSet private[kafka](@volatile var file: File,
12: private[log] val channel: FileChannel,
13: private[log] val start: Int,
14: private[log] 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 or
11: * 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 use
19: * 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 index
59: * @param n The entry number in the index
60: * @return The offset/position pair at that entry
61: */
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.duplicate
67: 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 = offset
83: 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
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
本文章摘自博客园，原文发布日期：2014-02-18