HBase-HLog分析

HLog的全部实现在包:

org.apache.hadoop.hbase.regionserver.wal 中

相关的配置为

hbase.regionserver.hlog.lowreplication.rolllimit

其中在Hlog.syncer方法中调用checkLowReplication方法用来判断是否hlog在hdfs上的副本数低于配置项，

若低于则requestLogRoll，最终调用logRollRequested方法，但是调用次数不超过默认5次

核心类HLog，负责创建读，写接口的实现，完成最终的写入数据，读数据等。

HLog.Reader  HLog读接口，实现类:SequenceFileLogReader

HLog.Writer    HLog写接口，实现类:SequenceFileLogWriter

HLog中包含了若干 HLog.Entry，Entry是一个K/V键值对

键为:HLogKey

值为:WALEdit

工具类:

    HLogSplitter(./hbase hlog命令实现类)

    Compressor(负责将input内容压缩到output中，或者input内容解压到output中)

WALActionsListener  日志的观察者监听类，当日志发生变化了可以触发观察者，比如replication功能就是实现了这个接口。 

HLog是一个二进制格式的文件，文件开头二进制信息:

SEQ0org.apache.hadoop.hbase.regionserver.wal.HLogKey0org.apache.hadoop.hbase.regionserver.wal.WALEdit

一个HLog文件如下:

这里注明了key和value使用的分别是HLogKey和WALEdit两个类

HLog中包含若干Entry，HLog是一个顺序的文件结构，没有索引，所有的Entry都是顺序挨个读取的。

HLog格式如下图：

HLog$Entry包含一个key，value
key:    HLogKey
value: WALEdit
Entry二进制内容如下:
1.总数据长度(hlogkey+waledit)
2.key长度
3.数据

HLogKey
1.变长int
2.编码后的region名字(若启用压缩则写入压缩后的内容)
3.表名(若启用压缩则写入压缩后的内容)
4.long长度的序列号
5.long长度的时间戳
6.1字节UUID标识(是否用默认的UUID标识)
7.UUID前8个字节
8.UUID后8个字节

WALEdit
1.int长度版本号
2.int长度KeyValue个数
3.遍历写入KeyValue(若启用压缩则写入压缩后的内容)3-1.KeyValue长度(int)3-2.KeyValue的二进制数据(4字节的key长，4字节的value长,2字节的rowkey长,key,1字节的family长，family,qualify，8字节的timestampe，1字节的keytype,变成long的memstoreMVC
4.socpe个数(若scope为null则写入0)
5.遍历写入每个scope(int长度)

HLog读取，写入的列子

	/*** 读取HLog中的内容* @throws IOException*/public void read()throws IOException {FileSystem fs = FileSystem.get(cfg);Path path = new Path("/test/hbase/hlog.1234567890");HLog.Reader reader = HLog.getReader(fs, path, cfg);HLog.Entry entry = reader.next();HLogKey key = entry.getKey();WALEdit edit = entry.getEdit();System.out.println( Bytes.toString(key.getEncodedRegionName()) );System.out.println( Bytes.toString(key.getTablename()) );System.out.println( key.getLogSeqNum() );System.out.println( key.getWriteTime() );System.out.println( key.getClusterId().toString() );List<KeyValue> list = edit.getKeyValues();for(KeyValue kv:list) {System.out.println(kv.toString());}while( (entry=reader.next()) !=null ) {System.out.println(entry);}}

	/*** 将数据写入到HLog，用HLog.Writer直接写入数据* @throws IOException*/public void write() throws IOException {FileSystem fs = FileSystem.get(cfg);Path path = new Path("/test/hbase/hlog.1234567890");HLog.Writer writer =  HLog.createWriter(fs, path, cfg);byte[] encodedRegionName = Bytes.toBytes("myregion");byte[] tablename = Bytes.toBytes("test");long logSeqNum = 100;long now = System.currentTimeMillis();UUID clusterId = UUID.randomUUID();HLogKey key1 = new HLogKey(encodedRegionName, tablename, logSeqNum, now, clusterId);WALEdit edit1 = new WALEdit();edit1.add(generator("key111111111111111111111111", "column", "a",System.currentTimeMillis(), new byte[] { '2' }));		HLog.Entry entry1 = new Entry(key1, edit1);HLogKey key2 = new HLogKey(encodedRegionName, tablename, 101L, now, UUID.randomUUID());WALEdit edit2 = new WALEdit();edit2.add(generator("key333333333333333333333333", "column", "b",System.currentTimeMillis(), VALUES));HLog.Entry entry2 = new Entry(key2, edit2);writer.append(entry2);writer.append(entry1);writer.sync();writer.close();}

	/*** 使用HLog写入数据* @throws IOException*/public void hlogWriter() throws IOException {FileSystem fs = FileSystem.get(cfg);String regionDir = "/test/hbase/hlog";HLog log = new HLog(fs, new Path(regionDir, "logs"), new Path(regionDir, "oldlogs"), cfg);byte[] tableName = Bytes.toBytes("test");byte[] startKey = Bytes.toBytes("aaaaa1111111111");byte[] endKey = Bytes.toBytes("zzzzz9999999999");HRegionInfo info = new HRegionInfo(tableName, startKey, endKey);WALEdit edit = new WALEdit();edit.add(generator("key333333333333333333333333", "column", "a",System.currentTimeMillis(), new byte[] { '2' }));edit.add(generator("key333333333333333333333333", "column", "b",System.currentTimeMillis(), VALUES));UUID uuid = UUID.randomUUID();long now = System.currentTimeMillis();HColumnDescriptor family = new HColumnDescriptor("column");HTableDescriptor descriptor = new HTableDescriptor();descriptor.addFamily(family);log.append(info, tableName, edit, uuid, now, descriptor);log.sync();log.hflush();log.hsync();log.close();}

	/*** 生成KeyValue* @param key* @param column* @param qualifier* @param timestamp* @param value* @return*/public KeyValue generator(String key, String column, String qualifier,long timestamp, byte[] value) {byte[] keyBytes = Bytes.toBytes(key);byte[] familyBytes = Bytes.toBytes(column);byte[] qualifierBytes = Bytes.toBytes(qualifier);Type type = Type.Put;byte[] valueBytes = value;KeyValue kv = new KeyValue(keyBytes, 0, keyBytes.length, familyBytes,0, familyBytes.length, qualifierBytes, 0,qualifierBytes.length, timestamp, type, valueBytes, 0,valueBytes.length);return kv;}