HBase Concept，

HBase Concept，

- Data Model, sparse, distributed, persisted multidimensional sorted map

(row:string, column:string, time:int64) -> string //both key and value are uninterpreted bytes

• Row 

• Column Family, should be declared upfront. basic unit of access control. columns under the column family have same type. disk(compress and index) and memory statistics are based on column family
• Timestamp (64-bit int), multiple versions of cell. values are ordered in timestamp decreasing order. values can be garbage collected by specifying last n versions or new-enough versions (e.g., values written in last several days)

- API, Get, Put, Scan, Delete

HBase doesn't modify data in place. delete is handled by putting tomstones on. These tomstones, along with dead values are cleaned up on major compaction

get rows by,

- Building block

• region, regions split by row range comprise table. they can be distributed and load-balanced in different machines (region servers). 
• HFile, pesisted, ordered immutable map from keys to values. Block indices is loaded to memory for looking up. then load blocks to memory. 

• HLog, one region server one Hlog file. it's a sequence file (HLogKey -> KeyValue). 

HLogKey: table+region+sequence nbr+timestamp

KeyValue: from HFile

cons: need to split log and send splits to different region servers when recover from the region server failure.

pros: only append log to one log file avoiding seeking time to multiple files.

- Architecture view

• client, access hbase via API, cache some info, like region location
• zookeeper

• Master

• region server

- workflow

• locate region (b+ tree of three-level structure

• read/write, 

• region assignment, done by Master who knows region servers, region's affliation and unassigned regions.
• region server up/down, 

• master up/down

reference: http://mvplee.iteye.com/blog/2247221, the bigtable paper