org.apache.accumulo.examples.wikisearch.iterator
Class ReadAheadIterator

java.lang.Object
  extended by org.apache.accumulo.examples.wikisearch.iterator.ReadAheadIterator
All Implemented Interfaces:
OptionDescriber, SortedKeyValueIterator<Key,Value>

public class ReadAheadIterator
extends Object
implements SortedKeyValueIterator<Key,Value>, OptionDescriber

This iterator takes the source iterator (the one below it in the iterator stack) and puts it in a background thread. The background thread continues processing and fills a queue with the Keys and Values from the source iterator. When seek() is called on this iterator, it pauses the background thread, clears the queue, calls seek() on the source iterator, then resumes the thread filling the queue. Users of this iterator can set the queue size, default is five elements. Users must be aware of the potential for OutOfMemory errors when using this iterator with large queue sizes or large objects. This iterator copies the Key and Value from the source iterator and puts them into the queue. This iterator introduces some parallelism into the server side iterator stack. One use case for this would be when an iterator takes a relatively long time to process each K,V pair and causes the iterators above it to wait. By putting the longer running iterator in a background thread we should be able to achieve greater throughput. NOTE: Experimental!


Nested Class Summary
 
Nested classes/interfaces inherited from interface org.apache.accumulo.core.iterators.OptionDescriber
OptionDescriber.IteratorOptions
 
Field Summary
static String QUEUE_SIZE
           
static String TIMEOUT
           
 
Constructor Summary
  ReadAheadIterator()
           
protected ReadAheadIterator(ReadAheadIterator other, IteratorEnvironment env)
           
 
Method Summary
 SortedKeyValueIterator<Key,Value> deepCopy(IteratorEnvironment env)
           
 OptionDescriber.IteratorOptions describeOptions()
           
 Key getTopKey()
           
 Value getTopValue()
           
 boolean hasTop()
           
 void init(SortedKeyValueIterator<Key,Value> source, Map<String,String> options, IteratorEnvironment env)
           
 void next()
          Populate the key and value
 void seek(Range range, Collection<ByteSequence> columnFamilies, boolean inclusive)
          Seek to the next matching cell and call next to populate the key and value.
 boolean validateOptions(Map<String,String> options)
           
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

QUEUE_SIZE

public static final String QUEUE_SIZE
See Also:
Constant Field Values

TIMEOUT

public static final String TIMEOUT
See Also:
Constant Field Values
Constructor Detail

ReadAheadIterator

protected ReadAheadIterator(ReadAheadIterator other,
                            IteratorEnvironment env)

ReadAheadIterator

public ReadAheadIterator()
Method Detail

deepCopy

public SortedKeyValueIterator<Key,Value> deepCopy(IteratorEnvironment env)
Specified by:
deepCopy in interface SortedKeyValueIterator<Key,Value>

getTopKey

public Key getTopKey()
Specified by:
getTopKey in interface SortedKeyValueIterator<Key,Value>

getTopValue

public Value getTopValue()
Specified by:
getTopValue in interface SortedKeyValueIterator<Key,Value>

hasTop

public boolean hasTop()
Specified by:
hasTop in interface SortedKeyValueIterator<Key,Value>

init

public void init(SortedKeyValueIterator<Key,Value> source,
                 Map<String,String> options,
                 IteratorEnvironment env)
          throws IOException
Specified by:
init in interface SortedKeyValueIterator<Key,Value>
Throws:
IOException

next

public void next()
          throws IOException
Populate the key and value

Specified by:
next in interface SortedKeyValueIterator<Key,Value>
Throws:
IOException

seek

public void seek(Range range,
                 Collection<ByteSequence> columnFamilies,
                 boolean inclusive)
          throws IOException
Seek to the next matching cell and call next to populate the key and value.

Specified by:
seek in interface SortedKeyValueIterator<Key,Value>
Throws:
IOException

describeOptions

public OptionDescriber.IteratorOptions describeOptions()
Specified by:
describeOptions in interface OptionDescriber

validateOptions

public boolean validateOptions(Map<String,String> options)
Specified by:
validateOptions in interface OptionDescriber


Copyright © 2012 The Apache Software Foundation. All Rights Reserved.