org.apache.lucene.index

## Class LogMergePolicy

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

- Direct Known Subclasses:
    LogByteSizeMergePolicy, LogDocMergePolicy

      public abstract class LogMergePolicy
      extends MergePolicy

  This class implements a `MergePolicy` that tries to merge segments into levels of exponentially increasing size, where each level has fewer segments than the value of the merge factor. Whenever extra segments (beyond the merge factor upper bound) are encountered, all segments within the level are merged. You can get or set the merge factor using `getMergeFactor()` and `setMergeFactor(int)` respectively.

  This class is abstract and requires a subclass to define the `MergePolicy.size(org.apache.lucene.index.SegmentCommitInfo, org.apache.lucene.index.MergePolicy.MergeContext)` method which specifies how a segment's size is determined. `LogDocMergePolicy` is one subclass that measures size by document count in the segment. `LogByteSizeMergePolicy` is another subclass that measures size as the total byte size of the file(s) for the segment.

- - ### Nested Class Summary

    - ### Nested classes/interfaces inherited from class org.apache.lucene.index.MergePolicy

      `MergePolicy.MergeAbortedException, MergePolicy.MergeContext, MergePolicy.MergeException, MergePolicy.MergeSpecification, MergePolicy.OneMerge, MergePolicy.OneMergeProgress`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected boolean` | `calibrateSizeByDeletes` If true, we pro-rate a segment's size by the percentage of non-deleted documents.
    `static int` | `DEFAULT_MAX_MERGE_DOCS` Default maximum segment size.
    `static int` | `DEFAULT_MERGE_FACTOR` Default merge factor, which is how many segments are merged at a time
    `static double` | `DEFAULT_NO_CFS_RATIO` Default noCFSRatio.
    `static double` | `LEVEL_LOG_SPAN` Defines the allowed range of log(size) for each level.
    `protected int` | `maxMergeDocs` If a segment has more than this many documents then it will never be merged.
    `protected long` | `maxMergeSize` If the size of a segment exceeds this value then it will never be merged.
    `protected long` | `maxMergeSizeForForcedMerge` If the size of a segment exceeds this value then it will never be merged during `IndexWriter.forceMerge(int)`.
    `protected int` | `mergeFactor` How many segments to merge at a time.
    `protected long` | `minMergeSize` Any segments whose size is smaller than this value will be rounded up to this value.

    - ### Fields inherited from class org.apache.lucene.index.MergePolicy

      `DEFAULT_MAX_CFS_SEGMENT_SIZE, maxCFSSegmentSize, noCFSRatio`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `LogMergePolicy()` Sole constructor.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `MergePolicy.MergeSpecification` | `findForcedDeletesMerges(SegmentInfos segmentInfos, MergePolicy.MergeContext mergeContext)` Finds merges necessary to force-merge all deletes from the index.
    `MergePolicy.MergeSpecification` | `findForcedMerges(SegmentInfos infos, int maxNumSegments, Map<SegmentCommitInfo,Boolean> segmentsToMerge, MergePolicy.MergeContext mergeContext)` Returns the merges necessary to merge the index down to a specified number of segments.
    `MergePolicy.MergeSpecification` | `findMerges(MergeTrigger mergeTrigger, SegmentInfos infos, MergePolicy.MergeContext mergeContext)` Checks if any merges are now necessary and returns a `MergePolicy.MergeSpecification` if so.
    `boolean` | `getCalibrateSizeByDeletes()` Returns true if the segment size should be calibrated by the number of deletes when choosing segments for merge.
    `int` | `getMaxMergeDocs()` Returns the largest segment (measured by document count) that may be merged with other segments.
    `int` | `getMergeFactor()` Returns the number of segments that are merged at once and also controls the total number of segments allowed to accumulate in the index.
    `protected boolean` | `isMerged(SegmentInfos infos, int maxNumSegments, Map<SegmentCommitInfo,Boolean> segmentsToMerge, MergePolicy.MergeContext mergeContext)` Returns true if the number of segments eligible for merging is less than or equal to the specified `maxNumSegments`.
    `void` | `setCalibrateSizeByDeletes(boolean calibrateSizeByDeletes)` Sets whether the segment size should be calibrated by the number of deletes when choosing segments for merge.
    `void` | `setMaxMergeDocs(int maxMergeDocs)` Determines the largest segment (measured by document count) that may be merged with other segments.
    `void` | `setMergeFactor(int mergeFactor)` Determines how often segment indices are merged by addDocument().
    `protected long` | `sizeBytes(SegmentCommitInfo info, MergePolicy.MergeContext mergeContext)` Return the byte size of the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents if `setCalibrateSizeByDeletes(boolean)` is set.
    `protected long` | `sizeDocs(SegmentCommitInfo info, MergePolicy.MergeContext mergeContext)` Return the number of documents in the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents if `setCalibrateSizeByDeletes(boolean)` is set.
    `String` | `toString()`

    - ### Methods inherited from class org.apache.lucene.index.MergePolicy

      `assertDelCount, getMaxCFSSegmentSizeMB, getNoCFSRatio, isMerged, keepFullyDeletedSegment, message, numDeletesToMerge, segString, setMaxCFSSegmentSizeMB, setNoCFSRatio, size, useCompoundFile, verbose`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### LEVEL_LOG_SPAN

          public static final double LEVEL_LOG_SPAN

      Defines the allowed range of log(size) for each level. A level is computed by taking the max segment log size, minus LEVEL_LOG_SPAN, and finding all segments falling within that range.
      See Also:
        Constant Field Values

    - #### DEFAULT_MERGE_FACTOR

          public static final int DEFAULT_MERGE_FACTOR

      Default merge factor, which is how many segments are merged at a time
      See Also:
        Constant Field Values

    - #### DEFAULT_MAX_MERGE_DOCS

          public static final int DEFAULT_MAX_MERGE_DOCS

      Default maximum segment size. A segment of this size or larger will never be merged. @see setMaxMergeDocs
      See Also:
        Constant Field Values

    - #### DEFAULT_NO_CFS_RATIO

          public static final double DEFAULT_NO_CFS_RATIO

      Default noCFSRatio. If a merge's size is `>= 10%` of the index, then we disable compound file for it.
      See Also:
        `MergePolicy.setNoCFSRatio(double)`, Constant Field Values

    - #### mergeFactor

          protected int mergeFactor

      How many segments to merge at a time.

    - #### minMergeSize

          protected long minMergeSize

      Any segments whose size is smaller than this value will be rounded up to this value. This ensures that tiny segments are aggressively merged.

    - #### maxMergeSize

          protected long maxMergeSize

      If the size of a segment exceeds this value then it will never be merged.

    - #### maxMergeSizeForForcedMerge

          protected long maxMergeSizeForForcedMerge

      If the size of a segment exceeds this value then it will never be merged during `IndexWriter.forceMerge(int)`.

    - #### maxMergeDocs

          protected int maxMergeDocs

      If a segment has more than this many documents then it will never be merged.

    - #### calibrateSizeByDeletes

          protected boolean calibrateSizeByDeletes

      If true, we pro-rate a segment's size by the percentage of non-deleted documents.

  - ### Constructor Detail

    - #### LogMergePolicy

          public LogMergePolicy()

      Sole constructor. (For invocation by subclass constructors, typically implicit.)

  - ### Method Detail

    - #### getMergeFactor

          public int getMergeFactor()

      Returns the number of segments that are merged at once and also controls the total number of segments allowed to accumulate in the index.

    - #### setMergeFactor

          public void setMergeFactor(int mergeFactor)

      Determines how often segment indices are merged by addDocument(). With smaller values, less RAM is used while indexing, and searches are faster, but indexing speed is slower. With larger values, more RAM is used during indexing, and while searches is slower, indexing is faster. Thus larger values (`> 10`) are best for batch index creation, and smaller values (`< 10`) for indices that are interactively maintained.

    - #### setCalibrateSizeByDeletes

          public void setCalibrateSizeByDeletes(boolean calibrateSizeByDeletes)

      Sets whether the segment size should be calibrated by the number of deletes when choosing segments for merge.

    - #### getCalibrateSizeByDeletes

          public boolean getCalibrateSizeByDeletes()

      Returns true if the segment size should be calibrated by the number of deletes when choosing segments for merge.

    - #### sizeDocs

          protected long sizeDocs(SegmentCommitInfo info,
                                  MergePolicy.MergeContext mergeContext)
                           throws IOException

      Return the number of documents in the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents if `setCalibrateSizeByDeletes(boolean)` is set.
      Throws:
        `IOException`

    - #### sizeBytes

          protected long sizeBytes(SegmentCommitInfo info,
                                   MergePolicy.MergeContext mergeContext)
                            throws IOException

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

    - #### isMerged

          protected boolean isMerged(SegmentInfos infos,
                                     int maxNumSegments,
                                     Map<SegmentCommitInfo,Boolean> segmentsToMerge,
                                     MergePolicy.MergeContext mergeContext)
                              throws IOException

      Returns true if the number of segments eligible for merging is less than or equal to the specified `maxNumSegments`.
      Throws:
        `IOException`

    - #### findForcedMerges

          public MergePolicy.MergeSpecification findForcedMerges(SegmentInfos infos,
                                                                 int maxNumSegments,
                                                                 Map<SegmentCommitInfo,Boolean> segmentsToMerge,
                                                                 MergePolicy.MergeContext mergeContext)
                                                          throws IOException

      Returns the merges necessary to merge the index down to a specified number of segments. This respects the `maxMergeSizeForForcedMerge` setting. By default, and assuming `maxNumSegments=1`, only one segment will be left in the index, where that segment has no deletions pending nor separate norms, and it is in compound file format if the current useCompoundFile setting is true. This method returns multiple merges (mergeFactor at a time) so the `MergeScheduler` in use may make use of concurrency.
      Specified by:
        `findForcedMerges` in class `MergePolicy`
      Parameters:
        `infos` - the total set of segments in the index
        `maxNumSegments` - 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.
        `mergeContext` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### findForcedDeletesMerges

          public MergePolicy.MergeSpecification findForcedDeletesMerges(SegmentInfos segmentInfos,
                                                                        MergePolicy.MergeContext mergeContext)
                                                                 throws IOException

      Finds merges necessary to force-merge all deletes from the index. We simply merge adjacent segments that have deletes, up to mergeFactor at a time.
      Specified by:
        `findForcedDeletesMerges` in class `MergePolicy`
      Parameters:
        `segmentInfos` - the total set of segments in the index
        `mergeContext` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### findMerges

          public MergePolicy.MergeSpecification findMerges(MergeTrigger mergeTrigger,
                                                           SegmentInfos infos,
                                                           MergePolicy.MergeContext mergeContext)
                                                    throws IOException

      Checks if any merges are now necessary and returns a `MergePolicy.MergeSpecification` if so. A merge is necessary when there are more than `setMergeFactor(int)` segments at a given level. When multiple levels have too many segments, this method will return multiple merges, allowing the `MergeScheduler` to use concurrency.
      Specified by:
        `findMerges` in class `MergePolicy`
      Parameters:
        `mergeTrigger` - the event that triggered the merge
        `infos` - the total set of segments in the index
        `mergeContext` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### setMaxMergeDocs

          public void setMaxMergeDocs(int maxMergeDocs)

      Determines the largest segment (measured by document count) that may be merged with other segments. Small values (e.g., less than 10,000) are best for interactive indexing, as this limits the length of pauses while indexing to a few seconds. Larger values are best for batched indexing and speedier searches.

      The default value is `Integer.MAX_VALUE`.

      The default merge policy (`LogByteSizeMergePolicy`) also allows you to set this limit by net size (in MB) of the segment, using `LogByteSizeMergePolicy.setMaxMergeMB(double)`.

    - #### getMaxMergeDocs

          public int getMaxMergeDocs()

      Returns the largest segment (measured by document count) that may be merged with other segments.
      See Also:
        `setMaxMergeDocs(int)`

    - #### toString

          public String toString()

      Overrides:
        `toString` in class `Object`

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