org.apache.lucene.index

## Class MergePolicy

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

- Direct Known Subclasses:
    LogMergePolicy, MergePolicyWrapper, NoMergePolicy, TieredMergePolicy

      public abstract class MergePolicy
      extends Object

  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.MergeTrigger, org.apache.lucene.index.SegmentInfos, org.apache.lucene.index.IndexWriter)` 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, IndexWriter)` 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 and Description
    --- | ---
    `static class` | `MergePolicy.DocMap` A map of doc IDs.
    `static class` | `MergePolicy.MergeAbortedException` Thrown when a merge was explicity aborted because `IndexWriter.abortMerges()` was called.
    `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 and Description
    --- | ---
    `protected static long` | `DEFAULT_MAX_CFS_SEGMENT_SIZE` Default max segment size in order to use compound file system.
    `protected static double` | `DEFAULT_NO_CFS_RATIO` Default ratio for compound file system usage.
    `protected long` | `maxCFSSegmentSize` If the size of the merged segment exceeds this value then it will not use compound file format.
    `protected double` | `noCFSRatio` If the size of the merge segment exceeds this ratio of the total index size then it will remain in non-compound format

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `` | `MergePolicy()` Creates a new merge policy instance.
    `protected` | `MergePolicy(double defaultNoCFSRatio, long defaultMaxCFSSegmentSize)` Creates a new merge policy instance with default settings for noCFSRatio and maxCFSSegmentSize.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `abstract MergePolicy.MergeSpecification` | `findForcedDeletesMerges(SegmentInfos segmentInfos, IndexWriter writer)` 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<SegmentCommitInfo,Boolean> segmentsToMerge, IndexWriter writer)` Determine what set of merge operations is necessary in order to merge to `<=` the specified segment count.
    `abstract MergePolicy.MergeSpecification` | `findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos, IndexWriter writer)` Determine what set of merge operations are now necessary on the index.
    `double` | `getMaxCFSSegmentSizeMB()` Returns the largest size allowed for a compound file segment
    `double` | `getNoCFSRatio()` Returns current `noCFSRatio`.
    `protected boolean` | `isMerged(SegmentInfos infos, SegmentCommitInfo info, IndexWriter writer)` Returns true if this single info is already fully merged (has no pending deletes, is in the same dir as the writer, and matches the current compound file setting
    `void` | `setMaxCFSSegmentSizeMB(double v)` If a merged segment will be more than this value, leave the segment as non-compound file even if compound file is enabled.
    `void` | `setNoCFSRatio(double noCFSRatio)` If a merged segment will be more than this percentage of the total size of the index, leave the segment as non-compound file even if compound file is enabled.
    `protected long` | `size(SegmentCommitInfo info, IndexWriter writer)` Return the byte size of the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents is set.
    `boolean` | `useCompoundFile(SegmentInfos infos, SegmentCommitInfo mergedInfo, IndexWriter writer)` 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

    - #### DEFAULT_NO_CFS_RATIO

          protected static final double DEFAULT_NO_CFS_RATIO

      Default ratio for compound file system usage. Set to `1.0`, always use compound file system.
      See Also:
        Constant Field Values

    - #### DEFAULT_MAX_CFS_SEGMENT_SIZE

          protected static final long DEFAULT_MAX_CFS_SEGMENT_SIZE

      Default max segment size in order to use compound file system. Set to `Long.MAX_VALUE`.
      See Also:
        Constant Field Values

    - #### noCFSRatio

          protected double noCFSRatio

      If the size of the merge segment exceeds this ratio of the total index size then it will remain in non-compound format

    - #### maxCFSSegmentSize

          protected long maxCFSSegmentSize

      If the size of the merged segment exceeds this value then it will not use compound file format.

  - ### Constructor Detail

    - #### MergePolicy

          public MergePolicy()

      Creates a new merge policy instance.

    - #### MergePolicy

          protected MergePolicy(double defaultNoCFSRatio,
                     long defaultMaxCFSSegmentSize)

      Creates a new merge policy instance with default settings for noCFSRatio and maxCFSSegmentSize. This ctor should be used by subclasses using different defaults than the `MergePolicy`

  - ### Method Detail

    - #### findMerges

          public abstract MergePolicy.MergeSpecification findMerges(MergeTrigger mergeTrigger,
                                                  SegmentInfos segmentInfos,
                                                  IndexWriter writer)
                                                             throws 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:
        `mergeTrigger` - the event that triggered the merge
        `segmentInfos` - the total set of segments in the index
        `writer` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### findForcedMerges

          public abstract MergePolicy.MergeSpecification findForcedMerges(SegmentInfos segmentInfos,
                                                        int maxSegmentCount,
                                                        Map<SegmentCommitInfo,Boolean> segmentsToMerge,
                                                        IndexWriter writer)
                                                                   throws 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.
        `writer` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### findForcedDeletesMerges

          public abstract MergePolicy.MergeSpecification findForcedDeletesMerges(SegmentInfos segmentInfos,
                                                               IndexWriter writer)
                                                                          throws 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
        `writer` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### useCompoundFile

          public boolean useCompoundFile(SegmentInfos infos,
                                SegmentCommitInfo mergedInfo,
                                IndexWriter writer)
                                  throws IOException

      Returns true if a new segment (regardless of its origin) should use the compound file format. The default implementation returns `true` iff the size of the given mergedInfo is less or equal to `getMaxCFSSegmentSizeMB()` and the size is less or equal to the TotalIndexSize * `getNoCFSRatio()` otherwise `false`.
      Throws:
        `IOException`

    - #### size

          protected long size(SegmentCommitInfo info,
                  IndexWriter writer)
                       throws IOException

      Return the byte size of the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents is set.
      Throws:
        `IOException`

    - #### isMerged

          protected final boolean isMerged(SegmentInfos infos,
                         SegmentCommitInfo info,
                         IndexWriter writer)
                                    throws IOException

      Returns true if this single info is already fully merged (has no pending deletes, is in the same dir as the writer, and matches the current compound file setting
      Throws:
        `IOException`

    - #### getNoCFSRatio

          public double getNoCFSRatio()

      Returns current `noCFSRatio`.
      See Also:
        `setNoCFSRatio(double)`

    - #### setNoCFSRatio

          public void setNoCFSRatio(double noCFSRatio)

      If a merged segment will be more than this percentage of the total size of the index, leave the segment as non-compound file even if compound file is enabled. Set to 1.0 to always use CFS regardless of merge size.

    - #### getMaxCFSSegmentSizeMB

          public final double getMaxCFSSegmentSizeMB()

      Returns the largest size allowed for a compound file segment

    - #### setMaxCFSSegmentSizeMB

          public void setMaxCFSSegmentSizeMB(double v)

      If a merged segment will be more than this value, leave the segment as non-compound file even if compound file is enabled. Set this to Double.POSITIVE_INFINITY (default) and noCFSRatio to 1.0 to always use CFS regardless of merge size.

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