org.apache.lucene.index

## Class MergePolicy

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

- Direct Known Subclasses:
    FilterMergePolicy, LogMergePolicy, 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.MergePolicy.MergeContext)` 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, MergeContext)` 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.MergeAbortedException` Thrown when a merge was explicitly aborted because `IndexWriter.abortMerges()` was called.
    `static interface` | `MergePolicy.MergeContext` This interface represents the current context of the merge selection process.
    `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.
    `static class` | `MergePolicy.OneMergeProgress` Progress and state for an executing merge.

  - ### 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

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected boolean` | `assertDelCount(int delCount, SegmentCommitInfo info)` Asserts that the delCount for this SegmentCommitInfo is valid
    `abstract MergePolicy.MergeSpecification` | `findForcedDeletesMerges(SegmentInfos segmentInfos, MergePolicy.MergeContext mergeContext)` 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, MergePolicy.MergeContext mergeContext)` Determine what set of merge operations is necessary in order to merge to `<=` the specified segment count.
    `MergePolicy.MergeSpecification` | `findFullFlushMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos, MergePolicy.MergeContext mergeContext)` Identifies merges that we want to execute (synchronously) on commit.
    `abstract MergePolicy.MergeSpecification` | `findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos, MergePolicy.MergeContext mergeContext)` 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, MergePolicy.MergeContext mergeContext)` 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
    `boolean` | `keepFullyDeletedSegment(IOSupplier<CodecReader> readerIOSupplier)` Returns true if the segment represented by the given CodecReader should be keep even if it's fully deleted.
    `protected void` | `message(String message, MergePolicy.MergeContext mergeContext)` Print a debug message to `MergePolicy.MergeContext`'s `infoStream`.
    `int` | `numDeletesToMerge(SegmentCommitInfo info, int delCount, IOSupplier<CodecReader> readerSupplier)` Returns the number of deletes that a merge would claim on the given segment.
    `protected String` | `segString(MergePolicy.MergeContext mergeContext, Iterable<SegmentCommitInfo> infos)` Builds a String representation of the given SegmentCommitInfo instances
    `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, MergePolicy.MergeContext mergeContext)` Return the byte size of the provided `SegmentCommitInfo`, pro-rated by percentage of non-deleted documents is set.
    `boolean` | `useCompoundFile(SegmentInfos infos, SegmentCommitInfo mergedInfo, MergePolicy.MergeContext mergeContext)` Returns true if a new segment (regardless of its origin) should use the compound file format.
    `protected boolean` | `verbose(MergePolicy.MergeContext mergeContext)` Returns `true` if the info-stream is in verbose mode

    - ### 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,
                                                                    MergePolicy.MergeContext mergeContext)
                                                             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
        `mergeContext` - the IndexWriter to find the merges on
      Throws:
        `IOException`

    - #### findForcedMerges

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

    - #### findForcedDeletesMerges

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

    - #### findFullFlushMerges

          public MergePolicy.MergeSpecification findFullFlushMerges(MergeTrigger mergeTrigger,
                                                                    SegmentInfos segmentInfos,
                                                                    MergePolicy.MergeContext mergeContext)
                                                             throws IOException

      Identifies merges that we want to execute (synchronously) on commit. By default, this will do no merging on commit. If you implement this method in your `MergePolicy` you must also set a non-zero timeout using `IndexWriterConfig.setMaxFullFlushMergeWaitMillis(long)`. Any merges returned here will make `IndexWriter.commit()`, `IndexWriter.prepareCommit()` or `IndexWriter.getReader(boolean, boolean)` block until the merges complete or until `LiveIndexWriterConfig.getMaxFullFlushMergeWaitMillis()` has elapsed. This may be used to merge small segments that have just been flushed, reducing the number of segments in the point in time snapshot. If a merge does not complete in the allotted time, it will continue to execute, and eventually finish and apply to future point in time snapshot, but will not be reflected in the current one. If a `MergePolicy.OneMerge` in the returned `MergePolicy.MergeSpecification` includes a segment already included in a registered merge, then `IndexWriter.commit()` or `IndexWriter.prepareCommit()` will throw a `IllegalStateException`. Use `MergePolicy.MergeContext.getMergingSegments()` to determine which segments are currently registered to merge.
      Parameters:
        `mergeTrigger` - the event that triggered the merge (COMMIT or GET_READER).
        `segmentInfos` - the total set of segments in the index (while preparing the commit)
        `mergeContext` - the MergeContext to find the merges on, which should be used to determine which segments are already in a registered merge (see `MergePolicy.MergeContext.getMergingSegments()`).
      Throws:
        `IOException`

    - #### useCompoundFile

          public boolean useCompoundFile(SegmentInfos infos,
                                         SegmentCommitInfo mergedInfo,
                                         MergePolicy.MergeContext mergeContext)
                                  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,
                              MergePolicy.MergeContext mergeContext)
                       throws IOException

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

    - #### assertDelCount

          protected final boolean assertDelCount(int delCount,
                                                 SegmentCommitInfo info)

      Asserts that the delCount for this SegmentCommitInfo is valid

    - #### isMerged

          protected final boolean isMerged(SegmentInfos infos,
                                           SegmentCommitInfo info,
                                           MergePolicy.MergeContext mergeContext)
                                    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 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.

    - #### keepFullyDeletedSegment

          public boolean keepFullyDeletedSegment(IOSupplier<CodecReader> readerIOSupplier)
                                          throws IOException

      Returns true if the segment represented by the given CodecReader should be keep even if it's fully deleted. This is useful for testing of for instance if the merge policy implements retention policies for soft deletes.
      Throws:
        `IOException`

    - #### numDeletesToMerge

          public int numDeletesToMerge(SegmentCommitInfo info,
                                       int delCount,
                                       IOSupplier<CodecReader> readerSupplier)
                                throws IOException

      Returns the number of deletes that a merge would claim on the given segment. This method will by default return the sum of the del count on disk and the pending delete count. Yet, subclasses that wrap merge readers might modify this to reflect deletes that are carried over to the target segment in the case of soft deletes. Soft deletes all deletes to survive across merges in order to control when the soft-deleted data is claimed.
      Parameters:
        `info` - the segment info that identifies the segment
        `delCount` - the number deleted documents for this segment
        `readerSupplier` - a supplier that allows to obtain a `CodecReader` for this segment
      Throws:
        `IOException`
      See Also:
        `IndexWriter.softUpdateDocument(Term, Iterable, Field...)`, `IndexWriterConfig.setSoftDeletesField(String)`

    - #### segString

          protected final String segString(MergePolicy.MergeContext mergeContext,
                                           Iterable<SegmentCommitInfo> infos)

      Builds a String representation of the given SegmentCommitInfo instances

    - #### message

          protected final void message(String message,
                                       MergePolicy.MergeContext mergeContext)

      Print a debug message to `MergePolicy.MergeContext`'s `infoStream`.

    - #### verbose

          protected final boolean verbose(MergePolicy.MergeContext mergeContext)

      Returns `true` if the info-stream is in verbose mode
      See Also:
        `message(String, MergeContext)`

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