BigTable & Hbase
A Distributed Storage System for Structured Data
Edward J. Yoon
edwardyoon@apache.org

Three Major Component
• Master Server
- Responsible for assigning tablets to tablet servers, detecting the addition and
expiration of tablet servers, balancing tablet-server load, and garbage collection of
files in HDFS
- Handles schema changed such as table and CF creations
• Tablet Server
- Manages a set tablets(10~1000 per tablet server)
- Handles read write requests to the tablets
- Splits tablets that have grown too large (100-200 MB)
• Client Library
- Communicate directly with tablet servers for reads and writes

Architecture
- Assigning tablets
- Detecting the addition and expiration of tablet
Client
- Balancing tablet-server load
- Handle schema changed
Master Server
Chubby
GFS master
server, CMS
- Handle master election
client
- Store the bootstrap location of Hbase data
- Discover region server
- Store access control lists
CMS server
Tablet Server Tablet Server Tablet Server
- Scheduling jobs
- Managing resources on the cluster
GFS chunk GFS chunk GFS chunk - dealing with machine failures
server, CMS server, CMS server, CMS
client client client

Data Model
• Doesn’t support a full relational data model
• Multi-dimensional sorted map
• Indexed by a row, column, timestamp
(row: string, column : string, time : int 64)  string
• Column-oriented storage
- Most queries only involve a few columns out of many, so greatly reduces I/O.

Tablet Location
• Use three-level hierarchy analogous to that of a B+
tree
- Location is ip : port of relevant server
- 1st level: Bootstrapped from lock server, points to location of root tablet
- 2nd level: Uses META 0 data to find owner of appropriate META 1 tablet
- 3rd level: META1 table holds locations of tablets of all other tables

Tablet Assignment
Tablet servers
Master keeps track of the set of live tablet
servers the current assignment of tablets to
Cluster region servers, including which tablets are
manager unassigned.
Chubby
1) Start
a server
2) Create a lock
8) Acquire and
9) Reassign unassigned
Delete the lock
tablets
3) Acquire the lock
4) Monitor
5) Assign tablets
Region Server Master Server
6) Check lock status

Tablet Serving
• To recover a tablet
- reads its metadata from the METADATA table
- metadata contains
- the list of SS-Tables that comprise a tablet
- a set of a redo points, which are pointers into any commit logs that may contain data for
the tablet.
- reads the indices of the SSTables into memory
- reconstructs the memtable by applying all of the updates that have committed
since the redo points

Compaction
V5.0
Create new
memtable
Read op
memtable Deleted data are
removed
Frozen Storage can be re-
memtable used
Memory Major compaction
DFS Memtable + all
Tablet log SSTables
-> to one SSTable
Write op V4.0 V3.0 V2.0 V1.0
Merging compaction
Memtable + a few SSTables
-> A new SSTable
SSTable files
Periodically done.
Minor compaction
Deleted data are still alive.
Memtable -> a new
V6.0
SSTable

Compression
• Clients can control whether or not SSTables for a
locality group are compressed
• Tow-pass custom compression scheme
- First-pass: long common strings across a large window (BMDiff)
- Second-pass: looks for repetitions in a small 16KB window (zippy)
- Both compression passes are very fast
- Space reduction
• Allow to identify large amounts of shared boilerplate in
pages from same host
- Choose their row names so that similar data ends up clustered and therefore
achieve very good performance

Caching for read performance
• Use two level of caching to improve read performance
• Scan cache
- Higher-level cache
- Most useful for applications that tend to read the same data
• Block cache
- Lower-level cache
- Useful for applications or random read of different columns in same locality
group within a hot row

Hbase : BigTable clone project
• http://hadoop.apache.org/hbase/
• Written in java
• we do not have chubby or a CMS server, we have Job-
Tracker and zookeeper coming soon.
• Since Hadoop (GFS) doesn't provide file-append
function, Current Hbase have a problem of data loss
when Hbase crashed.
- Hadoop 0.19.x provides file append function