java.lang.Object
- org.apache.lucene.index.MergePolicy

All Implemented Interfaces:

Closeable, AutoCloseable

Direct Known Subclasses:

LogMergePolicy, NoMergePolicy, TieredMergePolicy, UpgradeIndexMergePolicy
```
public abstract class MergePolicy
extends Object
implements Closeable
```
Expert: a MergePolicy determines the sequence of primitive merge operations.

Whenever the segments in an index have been altered by IndexWriter, either the addition of a newly flushed segment, addition of many segments from addIndexes* calls, or a previous merge that may now need to cascade, IndexWriter invokes findMerges(org.apache.lucene.index.SegmentInfos) to give the MergePolicy a chance to pick merges that are now required. This method returns a MergePolicy.MergeSpecification instance describing the set of merges that should be done, or null if no merges are necessary. When IndexWriter.forceMerge is called, it calls findForcedMerges(SegmentInfos,int,Map) and the MergePolicy should then return the necessary merges.

Note that the policy can return more than one merge at a time. In this case, if the writer is using SerialMergeScheduler, the merges will be run sequentially but if it is using ConcurrentMergeScheduler they will be run concurrently.

The default MergePolicy is TieredMergePolicy.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary

Nested Classes
Modifier and Type	Class	Description
`static class`	`MergePolicy.MergeAbortedException`
`static class`	`MergePolicy.MergeException`	Exception thrown if there are any problems while executing a merge.
`static class`	`MergePolicy.MergeSpecification`	A MergeSpecification instance provides the information necessary to perform multiple merges.
`static class`	`MergePolicy.OneMerge`	OneMerge provides the information necessary to perform an individual primitive merge operation, resulting in a single new segment.

Field Summary

Fields
Modifier and Type Field Description

protected SetOnce<IndexWriter> writer

Constructor Summary

Constructors
Constructor Description

MergePolicy()
Creates a new merge policy instance.

Method Summary

All Methods Instance Methods Abstract Methods Concrete Methods
Modifier and Type	Method	Description
`abstract void`	`close()`	Release all resources for the policy.
`abstract MergePolicy.MergeSpecification`	`findForcedDeletesMerges(SegmentInfos segmentInfos)`	Determine what set of merge operations is necessary in order to expunge all deletes from the index.
`abstract MergePolicy.MergeSpecification`	`findForcedMerges(SegmentInfos segmentInfos, int maxSegmentCount, Map<SegmentInfo,Boolean> segmentsToMerge)`	Determine what set of merge operations is necessary in order to merge to <= the specified segment count.
`abstract MergePolicy.MergeSpecification`	`findMerges(SegmentInfos segmentInfos)`	Determine what set of merge operations are now necessary on the index.
`void`	`setIndexWriter(IndexWriter writer)`	Sets the `IndexWriter` to use by this merge policy.
`abstract boolean`	`useCompoundFile(SegmentInfos segments, SegmentInfo newSegment)`	Returns true if a new segment (regardless of its origin) should use the compound file format.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Field Detail
  - writer
```
protected final SetOnce<IndexWriter> writer
```
- Constructor Detail
  - MergePolicy
```
public MergePolicy()
```
    Creates a new merge policy instance. Note that if you intend to use it without passing it to IndexWriter, you should call setIndexWriter(IndexWriter).
- Method Detail
  - setIndexWriter
```
public void setIndexWriter(IndexWriter writer)
```
    Sets the IndexWriter to use by this merge policy. This method is allowed to be called only once, and is usually set by IndexWriter. If it is called more than once, SetOnce.AlreadySetException is thrown.
    
    See Also:
    
    SetOnce
  - findMerges
```
public abstract MergePolicy.MergeSpecification findMerges(SegmentInfos segmentInfos)
                                                   throws CorruptIndexException,
                                                          IOException
```
    Determine what set of merge operations are now necessary on the index. IndexWriter calls this whenever there is a change to the segments. This call is always synchronized on the IndexWriter instance so only one thread at a time will call this method.
    
    Parameters:
    
    segmentInfos - the total set of segments in the index
    
    Throws:
    
    CorruptIndexException
    
    IOException
  - findForcedMerges
```
public abstract MergePolicy.MergeSpecification findForcedMerges(SegmentInfos segmentInfos,
                                                                int maxSegmentCount,
                                                                Map<SegmentInfo,Boolean> segmentsToMerge)
                                                         throws CorruptIndexException,
                                                                IOException
```
    Determine what set of merge operations is necessary in order to merge to <= the specified segment count. IndexWriter calls this when its IndexWriter.forceMerge(int) method is called. This call is always synchronized on the IndexWriter instance so only one thread at a time will call this method.
    
    Parameters:
    
    segmentInfos - the total set of segments in the index
    
    maxSegmentCount - requested maximum number of segments in the index (currently this is always 1)
    
    segmentsToMerge - contains the specific SegmentInfo instances that must be merged away. This may be a subset of all SegmentInfos. If the value is True for a given SegmentInfo, that means this segment was an original segment present in the to-be-merged index; else, it was a segment produced by a cascaded merge.
    
    Throws:
    
    CorruptIndexException
    
    IOException
  - findForcedDeletesMerges
```
public abstract MergePolicy.MergeSpecification findForcedDeletesMerges(SegmentInfos segmentInfos)
                                                                throws CorruptIndexException,
                                                                       IOException
```
    Determine what set of merge operations is necessary in order to expunge all deletes from the index.
    
    Parameters:
    
    segmentInfos - the total set of segments in the index
    
    Throws:
    
    CorruptIndexException
    
    IOException
  - close
```
public abstract void close()
```
    Release all resources for the policy.
    
    Specified by:
    
    close in interface AutoCloseable
    
    Specified by:
    
    close in interface Closeable
  - useCompoundFile
```
public abstract boolean useCompoundFile(SegmentInfos segments,
                                        SegmentInfo newSegment)
                                 throws IOException
```
    Returns true if a new segment (regardless of its origin) should use the compound file format.
    
    Throws:
    
    IOException

Class MergePolicy

Nested Class Summary

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

writer

Constructor Detail

MergePolicy

Method Detail

setIndexWriter

findMerges

findForcedMerges

findForcedDeletesMerges

close

useCompoundFile