About
BigTable is NoSql database where value may be versioned by time. It's then a time serie database and its open source version is called Hbase.
From Google to support actively update.
- OSDI paper in 2006 (Some overlap with the authors of the MapReduce paper)
- Complementary to MapReduce
Articles Related
Data model
BigTable stores data:
- in tables,
- which contain rows (identified by a row key)
- Data in a row is organized into column families, which are groups of columns. A column qualifier identifies a single column within a column family.
- A cell is at the intersection of a row and a column. A cell contains versioned value.
BigTable is a Column-Oriented DB that stores data in a Multidimensional, sparse, distributed, persistent Sorted Map with the following format:
(row:string, column:string, time:int64) -> String
where:
- row and column define the value (a cell)
- and time is a timestamps permitting to store the history of this value.
Timestamps:
- Each cell can be versioned
- Each new version increments the timestamp
- Policies:
- “keep only latest n versions”
- “keep only versions since time t”
Rows
- Data is sorted lexicographically by row key / row number.
- Row key range broken into tablets (Data are contiguous in a tablet)
- A tablet is the unit of distribution and load balancing
Column families
- Column names of the form family:qualifier
- “family” is the basic unit of:
- access control
- memory accounting
- disk accounting (move around on disc)
- Typically all columns in a family the same type (for instance to compress)
Tablet management
- Master assigns tablets to tablet servers
- Tablet server manages reads and writes from its tablets
- Clients communicate directly with tablet server
- Tablet server splits tablets that have grown too large.
Write processing
- When the memtable size reaches a given threshold, either or both minor and major compaction occur to keep read throughput high
- Minor Compaction: Write memtable buffer to a SSTable
- Major Compaction: Rewrites all SSTables into one SSTable and cleans all deletes.
