Relaxed Transactions for HBaseFrancis Liu, Software Engineer

5/22/12

Mutable Data

Writes are effective immediately

Mutable Data

Writes are effective immediately

Mutable Data

Partial writes in the midst of failures

Mutable Data

Partial writes in the midst of failures

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

Architecture

API

For reads› RevisionManager.createSnapshot(tableName)› SnapshotFilter.filter(result)

For writes› RevisionManager.beginWriteTransaction(table, families)› RevisionManager.commitWriteTransaction(transaction)› RevisionManager.abortWriteTransaction(transaction)

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

Relaxed Transaction Properties

Immutable Input Change After Commit Precedence Preservation

Immutable Input

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

Change After Commit

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

Precedence Preservation

Snapshot Filter

Consumes TableSnapshot Read time filtering› Aborted revisions› Revisions written after snapshot was taken› Conflicting/Blocked revisions

Flow - Read

User/Client› RevisionManager.createSnapshot()

• TableSnapshot instance is serialized into JobConf

RecordReader› Using SnapshotFilter.filter(result)

Flow - Read

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)

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

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,…)

Future Work

Compaction of aborted transactions Server-side filtering using HBase Filters Compatibility with Hive

Further Info

hcatalog-user@incubator.apache.org http://incubator.apache.org/hcatalog/ toffer@apache.org

Questions?