1. Relaxed Transactions for HBase
Francis Liu, Software Engineer
5/22/12

2. Mutable Data
 Writes are effective immediately
Read_Job
Table1
Map1
C1=1
C2=2
Map2
C3=3
Map3

3. Mutable Data
 Writes are effective immediately
Job1
Read_Job
Table1
Map1
Map1
C1=1
Map2 C2=2
Write_Job Map2
C3=4
Map3
Map3

4. Mutable Data
 Partial writes in the midst of failures
Write_Job
Table1
Map1
C1=1
Map2
C2=2
C3=?
Map3

5. Mutable Data
 Partial writes in the midst of failures
Write_Job
Table1
Map1
C1=1
Map2 Read_Job
C2=2
C3=?
Map3

6. Revision Manager
 Optimized for batch processing
› Large number of writes (ie Data Ingestion, Batch
updates)
 Cross row write transactions within a table
 Coprocessor Endpoint
› Leverage HBase Security
 Zookeeper for persistence
› table revision information
 Experimental feature in Hcatalog 0.4

7. Architecture
Revision Mgr
Revision Mgr
Service
Client
(Coprocessor)
InputFormat/
RegionServer Zookeeper
OutputFormat

8. API
 For reads
› RevisionManager.createSnapshot(tableName)
› SnapshotFilter.filter(result)
 For writes
› RevisionManager.beginWriteTransaction(table, families)
› RevisionManager.commitWriteTransaction(transaction)
› RevisionManager.abortWriteTransaction(transaction)

9. Concepts
 Revision
› Monotonically increasing number
› All “Puts” of a job are written with the same revision number as the
cell version
 TableSnapshot
› Point-in-time consistent view of a table
› Used for reading
› Latest committed revision
› List of aborted revisions
› Upper bound on visible revision per CF
 Transaction
› Write transaction
› Revision Number
› List of column Families being written to

10. Relaxed Transaction Properties
 Immutable Input
 Change After Commit
 Precedence Preservation

11. Immutable Input
Consistent Read
Write
CellA=1
Read
Snapshot1 CellA=1 CellA=1 CellA=1
Read_Job1
Begin t1 CellA=2 Commit
Write_Job1

12. Change After Commit
 Revisions are only viewable after commit
› A job cannot see it‟s own writes
 Aborted revisions are added to a table‟s aborted
list
 Timed out revisions are aborted

13. Change After Commit
Write
CellA=1
Read
Snapshot1 CellA=1
Read_Job1
Begin t1 CellA=2 Commit
Write_Job1
t1 change read
Snapshot2 CellA=2
Read_Job2

14. Precedence Preservation
 Snapshot Isolation
› Transaction is aborted when a write conflict is detected
 Conflicts
› Concurrent transactions to the same Column Family
› Inefficient to abort
 Resolved during read time
• For every CF
– find: min_rev = min(active_revision)
– Only return closest revision to min_rev
• min_rev is what‟s stored in a snapshot

15. Precedence Preservation
CellA=1 Write
CellB=1 Read
Begin t1 CellA=2 CellB=2 Commit
Write_Job1
Changes are not visible due to t1
Begin t2 CellA=3 Commit
Write_Job2
Snapshot1 CellA=1 CellB=1
Read_Job1
* CellA and CellB are members of the same column family

16. Snapshot Filter
 Consumes TableSnapshot
 Read time filtering
› Aborted revisions
› Revisions written after snapshot was taken
› Conflicting/Blocked revisions

17. Flow - Read
 User/Client
› RevisionManager.createSnapshot()
• TableSnapshot instance is serialized into JobConf
 RecordReader
› Using SnapshotFilter.filter(result)

18. Flow - Read
SnapshotRecordReader SnapshotFilter ScannerIterator
next(key,value)
Loop
result != null
and
filtered == null
next()
next result
filter(result)
filtered result
next record

19. Flow - Write
 User/Client
› HBaseOutputFormat.checkOutputSpecs(FileSystem, JobConf)
• Write transaction is started by calling beginWriteTransaction(Transaction)
• Transaction instance is serialized into JobConf
 RecordWriter
› Puts make use of the revision number as the version
 OutputCommitter
› OutputCommitter.commitJob(JobContext)
• RevisionManager.commitWriteTransaction(Transaction)
› OutputCommitter.abortJob(JobContext)
• RevisionManager.abortWriteTransaction(Transaction)

20. Usage
 Using HCatalog Revision Manager usage is done
under the covers.
 Work is being done to decouple HCatalog from
HBaseInputFormat/HBaseOutputFormat
 Other frameworks can make use of the
RevisionManager API

21. Usage: HCatalog
Create Table
hcat –e “create table my_table(key string, gpa string) STORED BY
'org.apache.hcatalog.hbase.HBaseHCatStorageHandler'
TBLPROPERTIES ('hbase.columns.mapping'=':key,info:gpa');”
Using Pig
A = LOAD „table1‟ USING org.apache.hcatalog.pig.HCatLoader();
STORE A INTO „table1‟ USING org.apache.hcatalog.pig.HCatStorer();
Using MapReduce
HCatInputFormat.setInput(job,…)
HCatOutputFormat.setOutput(job,…)

22. Future Work
 Compaction of aborted transactions
 Server-side filtering using HBase Filters
 Compatibility with Hive

23. Further Info
 hcatalog-user@incubator.apache.org
 http://incubator.apache.org/hcatalog/
 toffer@apache.org

24. Questions?