1. 定義
TTL(Time to Live) 用於限定數據的超時時間。
2.原理
以Column Family的TTL為例介紹，
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默認的TTL值是FOREVER，也就是永不過期。
- 修改TTL的值，CF的TTL的值以秒為單位：
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現有的值：
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
- 過30秒后掃描表
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，連續掃描兩次，數據沒有變化
- 過100秒后掃描表
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
發現r4的f1不見了。這就是TTL的工作原理。
TTL=>的更新超時時間是指：該列最后更新的時間，到超時時間的限制，而不是第一次創建，到超時時間；
同時我們也注意到100秒后r4被刪除，但是只刪除掉了r1的f1列，如果r1有其他列，比如cf，則其他列保留，TTL的概念只針對CELL
如果一個Store file僅包括過期的rows， minor comact的時候會將這些文件刪掉（可以參見HBase compact）。將hbase.store.delete.expired.storefile 設置成false或者將minimum number of versions 設置成除0意外的值可以將這個feature diable掉。number of versions的默認值是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：修改表結構之前，需要先disable 表，否則表中的記錄被清空！HBase不disable直接去alter 表是可以的！ 參加如下測試過程：
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； 新版本因為Cell可以支持tag了，所以可以在cell級別設置TTL了。（待考證)
（ 參見http://hbase.apache.org/book.html#ttl 及https://issues.apache.org/jira/browse/HBASE-10560）
Cell的TTL與Column family的TTL區別：
- Column family的TTL以秒為單位，cell的TTL以毫秒為單位
- 如果有有cell級別的TTL，則cell的TTL override CF的TTL； 但是不能超出CF級別的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.
參考文獻：
http://blog.csdn.net/wulantian/article/details/41010947
http://hbase.apache.org/book.html#ttl
https://issues.apache.org/jira/browse/HBASE-10560