HBase的TTL介绍

1. 定义

TTL(Time to Live) 用于限定数据的超时时间。

2.原理

以Column Family的TTL为例介绍，

hbase(main):001:0> desc 'wxy:test'
Table wxy:test is ENABLED
wxy:test
COLUMN FAMILIES DESCRIPTION
{NAME => 'cf', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', VERSIONS =
> '2', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => 'FOREVER', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
{NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', COMPRESSIO
N => 'NONE', VERSIONS => '5', TTL => 'FOREVER', MIN_VERSIONS => '0', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
2 row(s) in 0.9730 seconds

CF默认的TTL值是FOREVER，也就是永不过期。

• 修改TTL的值，CF的TTL的值以秒为单位：

hbase(main):003:0> disable 'wxy:test'
0 row(s) in 1.3500 seconds

hbase(main):004:0> alter 'wxy:test', {NAME=>'f1', TTL => '100'}
Updating all regions with the new schema...
1/1 regions updated.
Done.
0 row(s) in 1.1780 seconds

hbase(main):002:0> desc 'wxy:test'
Table wxy:test is DISABLED
wxy:test
COLUMN FAMILIES DESCRIPTION
{NAME => 'cf', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', VERSIONS =
> '2', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => 'FOREVER', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
{NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', COMPRESSIO
N => 'NONE', VERSIONS => '5', TTL => '100 SECONDS (1 MINUTE 40 SECOND)', MIN_VERSIONS => '0', KEEP_DELET
ED_CELLS => 'FALSE', BLOCKSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
2 row(s) in 0.0680 seconds

hbase(main):003:0> enable 'wxy:test'
0 row(s) in 0.2460 seconds

• scan现有的值：

hbase(main):007:0> scan 'wxy:test'
ROW                         COLUMN+CELL
 r1                         column=cf:name, timestamp=1503047499079, value=lisi4
 r1                         column=cf:sex, timestamp=1502788726648, value=male
 r2                         column=cf:age, timestamp=1503041691183, value=20
 r3                         column=cf:age, timestamp=1503041723715, value=23
 r4                         column=cf:name, timestamp=1503041738224, value=Alex
4 row(s) in 0.1140 seconds

• 更新表

hbase(main):007:0> put 'wxy:test' ,'r4','f1:address','shandi'
0 row(s) in 0.2590 seconds

hbase(main):008:0> scan 'wxy:test'
ROW                         COLUMN+CELL
 r1                         column=cf:name, timestamp=1503047499079, value=lisi4
 r1                         column=cf:sex, timestamp=1502788726648, value=male
 r2                         column=cf:age, timestamp=1503041691183, value=20
 r3                         column=cf:age, timestamp=1503041723715, value=23
 r4                         column=cf:name, timestamp=1503041738224, value=Alex
 r4                         column=f1:address, timestamp=1505976958276, value=shandi
4 row(s) in 0.0680 seconds

• 过30秒后扫描表 　　

hbase(main):012:0> scan 'wxy:test'
ROW                         COLUMN+CELL
 r1                         column=cf:name, timestamp=1503047499079, value=lisi4
 r1                         column=cf:sex, timestamp=1502788726648, value=male
 r2                         column=cf:age, timestamp=1503041691183, value=20
 r3                         column=cf:age, timestamp=1503041723715, value=23
 r4                         column=cf:name, timestamp=1503041738224, value=Alex
 r4                         column=f1:address, timestamp=1505976958276, value=shandi
4 row(s) in 0.0460 seconds

hbase(main):013:0> scan 'wxy:test'
ROW                         COLUMN+CELL
 r1                         column=cf:name, timestamp=1503047499079, value=lisi4
 r1                         column=cf:sex, timestamp=1502788726648, value=male
 r2                         column=cf:age, timestamp=1503041691183, value=20
 r3                         column=cf:age, timestamp=1503041723715, value=23
 r4                         column=cf:name, timestamp=1503041738224, value=Alex
 r4                         column=f1:address, timestamp=1505976958276, value=shandi
4 row(s) in 0.0390 seconds

如上，连续扫描两次，数据没有变化

• 过100秒后扫描表

hbase(main):019:0> scan 'wxy:test'
ROW                         COLUMN+CELL
 r1                         column=cf:name, timestamp=1503047499079, value=lisi4
 r1                         column=cf:sex, timestamp=1502788726648, value=male
 r2                         column=cf:age, timestamp=1503041691183, value=20
 r3                         column=cf:age, timestamp=1503041723715, value=23
 r4                         column=cf:name, timestamp=1503041738224, value=Alex
4 row(s) in 0.0280 seconds

　发现r4的f1不见了。这就是TTL的工作原理。

TTL=>的更新超时时间是指：该列最后更新的时间，到超时时间的限制，而不是第一次创建，到超时时间；

     同时我们也注意到100秒后r4被删除，但是只删除掉了r1的f1列，如果r1有其他列，比如cf，则其他列保留，TTL的概念只针对CELL

如果一个Store file仅包括过期的rows， minor comact的时候会将这些文件删掉（可以参见HBase compact）。将hbase.store.delete.expired.storefile 设置成false或者将minimum number of versions 设置成除0意外的值可以将这个feature diable掉。number of versions的默认值是0：

hbase(main):001:0> desc 'wxy:test'
Table wxy:test is ENABLED
wxy:test
COLUMN FAMILIES DESCRIPTION
{NAME => 'cf', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', VERSIONS =
> '2', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => 'FOREVER', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
{NAME => 'f1', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', COMPRESSIO
N => 'NONE', VERSIONS => '5', TTL => 'FOREVER', MIN_VERSIONS => '0', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
2 row(s) in 0.9730 seconds

     注意：修改表结构之前，需要先disable 表，否则表中的记录被清空！HBase不disable直接去alter 表是可以的！ 参加如下测试过程：

hbase(main):004:0> scan 'test'
ROW                         COLUMN+CELL
 row1                       column=cf:a, timestamp=1500967679327, value=value1
 row2                       column=cf:b, timestamp=1500967692945, value=value2
 row3                       column=cf:c, timestamp=1500967715743, value=value3
3 row(s) in 0.2490 seconds

hbase(main):005:0> desc 'test'
Table test is ENABLED
test
COLUMN FAMILIES DESCRIPTION
{NAME => 'cf', DATA_BLOCK_ENCODING => 'NONE', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', VERSIONS =
> '1', COMPRESSION => 'NONE', MIN_VERSIONS => '0', TTL => 'FOREVER', KEEP_DELETED_CELLS => 'FALSE', BLOC
KSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}
1 row(s) in 0.0880 seconds

hbase(main):006:0> alter 'test',{NAME => 'cf',TTL => '100'}
Updating all regions with the new schema...
0/1 regions updated.
1/1 regions updated.
Done.
0 row(s) in 2.2200 seconds

hbase(main):007:0> scan 'test'
ROW                         COLUMN+CELL
0 row(s) in 0.0190 seconds

3. 粒度

早期版本控制粒度是column family； 新版本因为Cell可以支持tag了，所以可以在cell级别设置TTL了。（待考证)

（ 参见http://hbase.apache.org/book.html#ttl 及https://issues.apache.org/jira/browse/HBASE-10560）

Cell的TTL与Column family的TTL区别：

• Column family的TTL以秒为单位，cell的TTL以毫秒为单位
• 如果有有cell级别的TTL，则cell的TTL override CF的TTL； 但是不能超出CF级别的TTL

以下引自：http://hbase.apache.org/book.html#ttl

• Cell TTLs are expressed in units of milliseconds instead of seconds.

• A cell TTLs cannot extend the effective lifetime of a cell beyond a ColumnFamily level TTL setting.

以下引自：https://issues.apache.org/jira/browse/HBASE-10560 作者的comments：

We can keep the existing column level definition and enforcement mechanism and extend it to look for a TTL cell tag during compaction. If one is found, it can override the CF setting. TTL overrides can be passed up to the server in an operation attribute.

参考文献：

http://blog.csdn.net/wulantian/article/details/41010947

http://hbase.apache.org/book.html#ttl

https://issues.apache.org/jira/browse/HBASE-10560