HBase Coprocessors allow user code to be run on region servers within each region of an HBase table. Coprocessors are loaded dynamically and scale automatically as regions are split or merged. They provide hooks into various HBase operations via observer classes and define an interface for custom endpoint calls between clients and servers. Examples of use cases include secondary indexes, filters, and replacing MapReduce jobs with server-side processing.

1. HBaseCon, May 2012
HBase Coprocessors
Lars George | Solutions Architect

2. Revision History
Version Revised By Description of Revision
Version 1 Lars George Initial version
2 ©2011 Cloudera, Inc. All Rights Reserved. Confidential.
Reproduction or redistribution without written permission is
prohibited.

3. Overview
• Coprocessors were added to Bigtable
– Mentioned during LADIS 2009 talk
• Runs user code within each region of a
table
– Code split and moves with region
• Defines high level call interface for clients
• Calls addressed to rows or ranges of rows
• Implicit automatic scaling, load balancing,
and request routing

4. Examples Use-Cases
• Bigtable uses Coprocessors
– Scalable metadata management
– Distributed language model for machine
translation
– Distributed query processing for full-text index
– Regular expression search in code repository
• MapReduce jobs over HBase are often map-
only jobs
– Row keys are already sorted and distinct
➜ Could be replaced by Coprocessors

5. HBase Coprocessors
• Inspired by Google’s Coprocessors
– Not much information available, but general idea is
understood
• Define various types of server-side code
extensions
– Associated with table using a table property
– Attribute is a path to JAR file
– JAR is loaded when region is opened
– Blends new functionality with existing
• Can be chained with Priorities and Load Order
➜ Allows for dynamic RPC extensions

6. Coprocessor Classes and Interfaces
• The Coprocessor Interface
– All user code must inherit from this class
• The CoprocessorEnvironment Interface
– Retains state across invocations
– Predefined classes
• The CoprocessorHost Interface
– Ties state and user code together
– Predefined classes

7. Coprocessor Priority
• System or User
/** Highest installation priority */
static final int PRIORITY_HIGHEST = 0;
/** High (system) installation priority */
static final int PRIORITY_SYSTEM = Integer.MAX_VALUE / 4;
/** Default installation prio for user coprocessors */
static final int PRIORITY_USER = Integer.MAX_VALUE / 2;
/** Lowest installation priority */
static final int PRIORITY_LOWEST = Integer.MAX_VALUE;

8. Coprocessor Environment
• Available Methods

9. Coprocessor Host
• Maintains all Coprocessor instances and
their environments (state)
• Concrete Classes
– MasterCoprocessorHost
– RegionCoprocessorHost
– WALCoprocessorHost
• Subclasses provide access to specialized
Environment implementations

10. Control Flow

11. Coprocessor Interface
• Base for all other types of Coprocessors
• start() and stop() methods for lifecycle
management
• State as defined in the interface:

12. Observer Classes
• Comparable to database triggers
– Callback functions/hooks for every explicit API
method, but also all important internal calls
• Concrete Implementations
– MasterObserver
• Hooks into HMaster API
– RegionObserver
• Hooks into Region related operations
– WALObserver
• Hooks into write-ahead log operations

13. Region Observers
• Can mediate (veto) actions
– Used by the security policy extensions
– Priority allows mediators to run first
• Hooks into all CRUD+S API calls and more
– get(), put(), delete(), scan(), increment(),…
– checkAndPut(), checkAndDelete(),…
– flush(), compact(), split(),…
• Pre/Post Hooks for every call
• Can be used to build secondary indexes,
filters

14. Endpoint Classes
• Define a dynamic RPC protocol, used
between client and region server
• Executes arbitrary code, loaded in region
server
– Future development will add code weaving/
inspection to deny any malicious code
• Steps to add your own methods
– Define and implement your own protocol
– Implement endpoint coprocessor
– Call HTable’s coprocessorExec() or
coprocessorProxy()

15. Coprocessor Loading
• There are two ways: dynamic or static
– Static: use configuration files and table schema
– Dynamic: not available (yet)
• For static loading from configuration:
– Order is important (defines the execution order)
– Special property key for each host type
– Region related classes are loaded for all regions
and tables
– Priority is always System
– JAR must be on class path

16. Loading from Configuration
• Example:
<property>!
<name>hbase.coprocessor.region.classes</name> !
<value>coprocessor.RegionObserverExample, !
coprocessor.AnotherCoprocessor</value>!
</property>
<property> !
<name>hbase.coprocessor.master.classes</name> !
<value>coprocessor.MasterObserverExample</value>!
</property>
<property> !
<name>hbase.coprocessor.wal.classes</name> !
<value>coprocessor.WALObserverExample, !
bar.foo.MyWALObserver</value> !
</property> !
!

17. Coprocessor Loading (cont.)
• For static loading from table schema:
– Definition per table
– For all regions of the table
– Only region related classes, not WAL or Master
– Added to HTableDescriptor, when table is created
or altered
– Allows to set the priority and JAR path
COPROCESSOR$<num> ➜ !
<path-to-jar>|<classname>|<priority> !

18. Loading from Table Schema
• Example:
'COPROCESSOR$1' => !
'hdfs://localhost:8020/users/leon/test.jar| !
coprocessor.Test|10' !
!
'COPROCESSOR$2' => !
'/Users/laura/test2.jar| !
coprocessor.AnotherTest|1000' !
!

19. Example: Add Coprocessor
public static void main(String[] args) throws IOException { !
Configuration conf = HBaseConfiguration.create(); !
FileSystem fs = FileSystem.get(conf);
Path path = new Path(fs.getUri() + Path.SEPARATOR +!
"test.jar"); !
HTableDescriptor htd = new HTableDescriptor("testtable");!
htd.addFamily(new HColumnDescriptor("colfam1"));!
htd.setValue("COPROCESSOR$1", path.toString() +!
"|" + RegionObserverExample.class.getCanonicalName() +!
"|" + Coprocessor.PRIORITY_USER); !
HBaseAdmin admin = new HBaseAdmin(conf);!
admin.createTable(htd); !
System.out.println(admin.getTableDescriptor(!
Bytes.toBytes("testtable"))); !
} !

20. Example Output
{NAME => 'testtable', COPROCESSOR$1 =>!
'file:/test.jar|coprocessor.RegionObserverExample|
1073741823', FAMILIES => [{NAME => 'colfam1',
BLOOMFILTER => 'NONE', REPLICATION_SCOPE => '0',
COMPRESSION => 'NONE', VERSIONS => '3', TTL =>
'2147483647', BLOCKSIZE => '65536', IN_MEMORY =>
'false', BLOCKCACHE => 'true'}]} !
!

21. Region Observers
• Handles all region related events
• Hooks for two classes of operations:
– Lifecycle changes
– Client API Calls
• All client API calls have a pre/post hook
– Can be used to grant access on preGet()
– Can be used to update secondary indexes on
postPut()

22. Handling Region Lifecycle Events
• Hook into pending open, open, and pending
close state changes
• Called implicitly by the framework
– preOpen(), postOpen(),…
• Used to piggyback or fail the process, e.g.
– Cache warm up after a region opens
– Suppress region splitting, compactions, flushes

23. Region Environment

24. Special Hook Parameter
public interface RegionObserver extends Coprocessor {!
!
/**!
* Called before the region is reported as open to the master.!
* @param c the environment provided by the region server!
*/!
void preOpen(final!
ObserverContext<RegionCoprocessorEnvironment> c);!
!
/**!
* Called after the region is reported as open to the master.!
* @param c the environment provided by the region server!
*/!
void postOpen(final !
ObserverContext<RegionCoprocessorEnvironment> c);!
!

25. ObserverContext

26. Chain of Command
• Especially the complete() and bypass()
methods allow to change the processing
chain
– complete() ends the chain at the current
coprocessor
– bypass() completes the pre/post chain but
uses the last value returned by the
coprocessors, possibly not calling the actual
API method (for pre-hooks)

27. Example: Pre-Hook Complete
@Override !
public void preSplit(ObserverContext!
<RegionCoprocessorEnvironment> e) {!
e.complete(); !
}!

28. Master Observer
• Handles all HMaster related events
– DDL type calls, e.g. create table, add column
– Region management calls, e.g. move, assign
• Pre/post hooks with Context
• Specialized environment provided

29. Master Environment

30. Master Services (cont.)
• Very powerful features
– Access the AssignmentManager to modify
plans
– Access the MasterFileSystem to create or
access resources on HDFS
– Access the ServerManager to get the list of
known servers
– Use the ExecutorService to run system-wide
background processes
• Be careful (for now)!

31. Example: Master Post Hook
public class MasterObserverExample !
extends BaseMasterObserver { !
@Override public void postCreateTable( !
ObserverContext<MasterCoprocessorEnvironment> env, !
HRegionInfo[] regions, boolean sync) !
throws IOException { !
String tableName = !
regions[0].getTableDesc().getNameAsString(); !
MasterServices services =!
env.getEnvironment().getMasterServices();!
MasterFileSystem masterFileSystem =!
services.getMasterFileSystem(); !
FileSystem fileSystem = masterFileSystem.getFileSystem();!
Path blobPath = new Path(tableName + "-blobs");!
fileSystem.mkdirs(blobPath); !
}!
} !
!

32. Example Output
hbase(main):001:0> create
'testtable', 'colfam1‘!
0 row(s) in 0.4300 seconds !
!
$ bin/hadoop dfs -ls
Found 1 items
drwxr-xr-x - larsgeorge
supergroup 0 ... /user/
larsgeorge/testtable-blobs !

33. Endpoints
• Dynamic RPC extends server-side
functionality
– Useful for MapReduce like implementations
– Handles the Map part server-side, Reduce needs
to be done client side
• Based on CoprocessorProtocol interface
• Routing to regions is based on either single
row keys, or row key ranges
– Call is sent, no matter if row exists or not since
region start and end keys are coarse grained

34. Custom Endpoint Implementation
• Involves two steps:
– Extend the CoprocessorProtocol interface
• Defines the actual protocol
– Extend the BaseEndpointCoprocessor
• Provides the server-side code and the dynamic
RPC method

35. Example: Row Count Protocol
public interface RowCountProtocol!
extends CoprocessorProtocol {!
long getRowCount() !
throws IOException; !
long getRowCount(Filter filter)!
throws IOException; !
long getKeyValueCount() !
throws IOException; !
} !
!

36. Example: Endpoint for Row Count
public class RowCountEndpoint !
extends BaseEndpointCoprocessor !
implements RowCountProtocol { !
!
private long getCount(Filter filter, !
boolean countKeyValues) throws IOException {
Scan scan = new Scan();!
scan.setMaxVersions(1); !
if (filter != null) { !
scan.setFilter(filter); !
} !

37. Example: Endpoint for Row Count
RegionCoprocessorEnvironment environment = !
(RegionCoprocessorEnvironment)!
getEnvironment();!
// use an internal scanner to perform!
// scanning.!
InternalScanner scanner =!
environment.getRegion().getScanner(scan); !
int result = 0;!

38. Example: Endpoint for Row Count
try { !
List<KeyValue> curVals = !
new ArrayList<KeyValue>(); !
boolean done = false;!
do { !
curVals.clear(); !
done = scanner.next(curVals); !
result += countKeyValues ? curVals.size() : 1; !
} while (done); !
} finally { !
scanner.close(); !
} !
return result; !
} !
!

39. Example: Endpoint for Row Count
@Override!
public long getRowCount() throws IOException {!
return getRowCount(new FirstKeyOnlyFilter()); !
} !
!
@Override !
public long getRowCount(Filter filter) throws IOException {!
return getCount(filter, false); !
} !
!
@Override!
public long getKeyValueCount() throws IOException {!
return getCount(null, true); !
} !
}
!
!
!

40. Endpoint Invocation
• There are two ways to invoke the call
– By Proxy, using HTable.coprocessorProxy()
• Uses a delayed model, i.e. the call is send when the
proxied method is invoked
– By Exec, using HTable.coprocessorExec()
• The call is send in parallel to all regions and the results
are collected immediately
• The Batch.Call class is used be
coprocessorExec() to wrap the calls per
region
• The optional Batch.Callback can be used to
react upon completion of the remote call

41. Exec vs. Proxy

42. Example: Invocation by Exec
public static void main(String[] args) throws IOException { !
Configuration conf = HBaseConfiguration.create(); !
HTable table = new HTable(conf, "testtable");!
try { !
Map<byte[], Long> results = !
table.coprocessorExec(RowCountProtocol.class, null, null,!
new Batch.Call<RowCountProtocol, Long>() { !
@Override!
public Long call(RowCountProtocol counter) !
throws IOException { !
return counter.getRowCount(); !
} !
}); !
!

43. Example: Invocation by Exec
long total = 0;!
for (Map.Entry<byte[], Long> entry : !
results.entrySet()) { !
total += entry.getValue().longValue();!
System.out.println("Region: " + !
Bytes.toString(entry.getKey()) +!
", Count: " + entry.getValue()); !
} !
System.out.println("Total Count: " + total); !
} catch (Throwable throwable) { !
throwable.printStackTrace(); !
} !
} !

44. Example Output
Region: testtable,,
1303417572005.51f9e2251c...cbcb
0c66858f., Count: 2 !
Region: testtable,row3,
1303417572005.7f3df4dcba...dbc9
9fce5d87., Count: 3 !
Total Count: 5 !
!

45. Batch Convenience
• The Batch.forMethod() helps to quickly
map a protocol function into a Batch.Call
• Useful for single method calls to the
servers
• Uses the Java reflection API to retrieve the
named method
• Saves you from implementing the
anonymous inline class

46. Batch Convenience
Batch.Call call =!
Batch.forMethod(!
RowCountProtocol.class,!
"getKeyValueCount"); !
Map<byte[], Long> results =!
table.coprocessorExec(!
RowCountProtocol.class, !
null, null, call); !
!

47. Call Multiple Endpoints
• Sometimes you need to call more than
one endpoint in a single roundtrip call to
the servers
• This requires an anonymous inline class,
since Batch.forMethod cannot handle this

48. Call Multiple Endpoints
Map<byte[], Pair<Long, Long>> !
results = table.coprocessorExec( !
RowCountProtocol.class, null, null,!
new Batch.Call<RowCountProtocol,!
Pair<Long, Long>>() { !
public Pair<Long, Long> call(!
RowCountProtocol counter) !
throws IOException {
return new Pair(!
counter.getRowCount(), !
counter.getKeyValueCount()); !
}!
}); !

49. Example: Invocation by Proxy
RowCountProtocol protocol =!
table.coprocessorProxy(!
RowCountProtocol.class,!
Bytes.toBytes("row4")); !
long rowsInRegion =!
protocol.getRowCount(); !
System.out.println(!
"Region Row Count: " +!
rowsInRegion); !
!

50. 50 ©2011 Cloudera, Inc. All Rights Reserved. Confidential.
Reproduction or redistribution without written permission is
prohibited.