## org.apache.lucene.index
Class ConcurrentMergeScheduler

    java.lang.Object
      org.apache.lucene.index.MergeScheduler
          org.apache.lucene.index.ConcurrentMergeScheduler

All Implemented Interfaces:
  Closeable, Cloneable

    public class ConcurrentMergeScheduler

extends MergeScheduler

A `MergeScheduler` that runs each merge using a separate thread.

Specify the max number of threads that may run at once, and the maximum number of simultaneous merges with `setMaxMergesAndThreads(int, int)`.

If the number of merges exceeds the max number of threads then the largest merges are paused until one of the smaller merges completes.

If more than `getMaxMergeCount()` merges are requested then this class will forcefully throttle the incoming threads by pausing until one more more merges complete.

Nested Class Summary
`protected class` `ConcurrentMergeScheduler.MergeThread` Runs a merge thread, which may run one or more merges in sequence.

Field Summary
`protected static Comparator<ConcurrentMergeScheduler.MergeThread>` `compareByMergeDocCount` Sorts `ConcurrentMergeScheduler.MergeThread`s; larger merges come first.
`static int` `DEFAULT_MAX_MERGE_COUNT` Default `maxMergeCount`.
`static int` `DEFAULT_MAX_THREAD_COUNT` Default `maxThreadCount`.
`protected Directory` `dir` `Directory` that holds the index.
`protected int` `mergeThreadCount` How many `ConcurrentMergeScheduler.MergeThread`s have kicked off (this is use to name them).
`protected List<ConcurrentMergeScheduler.MergeThread>` `mergeThreads` List of currently active `ConcurrentMergeScheduler.MergeThread`s.
`protected IndexWriter` `writer` `IndexWriter` that owns this instance.

Constructor Summary
`ConcurrentMergeScheduler()` Sole constructor, with all settings set to default values.

Method Summary
`MergeScheduler` `clone()`
`void` `close()` Close this MergeScheduler.
`protected void` `doMerge(MergePolicy.OneMerge merge)` Does the actual merge, by calling `IndexWriter.merge(org.apache.lucene.index.MergePolicy.OneMerge)`
`int` `getMaxMergeCount()` See `setMaxMergesAndThreads(int, int)`.
`int` `getMaxThreadCount()` Returns `maxThreadCount`.
`protected ConcurrentMergeScheduler.MergeThread` `getMergeThread(IndexWriter writer, MergePolicy.OneMerge merge)` Create and return a new MergeThread
`int` `getMergeThreadPriority()` Return the priority that merge threads run at.
`protected void` `handleMergeException(Throwable exc)` Called when an exception is hit in a background merge thread
`void` `merge(IndexWriter writer)` Run the merges provided by `IndexWriter.getNextMerge()`.
`protected int` `mergeThreadCount()` Returns the number of merge threads that are alive.
`protected void` `message(String message)` Outputs the given message - this method assumes `verbose()` was called and returned true.
`void` `setMaxMergesAndThreads(int maxMergeCount, int maxThreadCount)` Sets the maximum number of merge threads and simultaneous merges allowed.
`void` `setMergeThreadPriority(int pri)` Set the base priority that merge threads run at.
`void` `sync()` Wait for any running merge threads to finish.
`String` `toString()`
`protected void` `updateMergeThreads()` Called whenever the running merges have changed, to pause & unpause threads.
`protected boolean` `verbose()` Returns true if verbosing is enabled.

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

Field Detail

### mergeThreads

    protected List<ConcurrentMergeScheduler.MergeThread> mergeThreads

  List of currently active `ConcurrentMergeScheduler.MergeThread`s.

### DEFAULT_MAX_THREAD_COUNT

    public static final int DEFAULT_MAX_THREAD_COUNT

  Default `maxThreadCount`. We default to 1: tests on spinning-magnet drives showed slower indexing performance if more than one merge thread runs at once (though on an SSD it was faster)

  See Also:
    Constant Field Values

### DEFAULT_MAX_MERGE_COUNT

    public static final int DEFAULT_MAX_MERGE_COUNT

  Default `maxMergeCount`.

  See Also:
    Constant Field Values

### dir

    protected Directory dir

  `Directory` that holds the index.

### writer

    protected IndexWriter writer

  `IndexWriter` that owns this instance.

### mergeThreadCount

    protected int mergeThreadCount

  How many `ConcurrentMergeScheduler.MergeThread`s have kicked off (this is use to name them).

### compareByMergeDocCount

    protected static final Comparator<ConcurrentMergeScheduler.MergeThread> compareByMergeDocCount

  Sorts `ConcurrentMergeScheduler.MergeThread`s; larger merges come first.

Constructor Detail

### ConcurrentMergeScheduler

    public ConcurrentMergeScheduler()

  Sole constructor, with all settings set to default values.

Method Detail

### setMaxMergesAndThreads

    public void setMaxMergesAndThreads(int maxMergeCount,
                                       int maxThreadCount)

  Sets the maximum number of merge threads and simultaneous merges allowed.

  Parameters:
    `maxMergeCount` - the max # simultaneous merges that are allowed. If a merge is necessary yet we already have this many threads running, the incoming thread (that is calling add/updateDocument) will block until a merge thread has completed. Note that we will only run the smallest `maxThreadCount` merges at a time.
    `maxThreadCount` - the max # simultaneous merge threads that should be running at once. This must be <= `maxMergeCount`

### getMaxThreadCount

    public int getMaxThreadCount()

  Returns `maxThreadCount`.

  See Also:
    `setMaxMergesAndThreads(int, int)`

### getMaxMergeCount

    public int getMaxMergeCount()

  See `setMaxMergesAndThreads(int, int)`.

### getMergeThreadPriority

    public int getMergeThreadPriority()

  Return the priority that merge threads run at. By default the priority is 1 plus the priority of (ie, slightly higher priority than) the first thread that calls merge.

### setMergeThreadPriority

    public void setMergeThreadPriority(int pri)

  Set the base priority that merge threads run at. Note that CMS may increase priority of some merge threads beyond this base priority. It's best not to set this any higher than Thread.MAX_PRIORITY-maxThreadCount, so that CMS has room to set relative priority among threads.

### updateMergeThreads

    protected void updateMergeThreads()

  Called whenever the running merges have changed, to pause & unpause threads. This method sorts the merge threads by their merge size in descending order and then pauses/unpauses threads from first to last -- that way, smaller merges are guaranteed to run before larger ones.

### verbose

    protected boolean verbose()

  Returns true if verbosing is enabled. This method is usually used in conjunction with `message(String)`, like that:

       if (verbose()) {
         message("your message");
       }

### message

    protected void message(String message)

  Outputs the given message - this method assumes `verbose()` was called and returned true.

### close

    public void close()

  Description copied from class: `MergeScheduler`
  Close this MergeScheduler.

  Specified by:
    `close` in interface `Closeable`
  Specified by:
    `close` in class `MergeScheduler`

### sync

    public void sync()

  Wait for any running merge threads to finish. This call is not interruptible as used by `close()`.

### mergeThreadCount

    protected int mergeThreadCount()

  Returns the number of merge threads that are alive. Note that this number is ≤ `mergeThreads` size.

### merge

    public void merge(IndexWriter writer)
               throws IOException

  Description copied from class: `MergeScheduler`
  Run the merges provided by `IndexWriter.getNextMerge()`.

  Specified by:
    `merge` in class `MergeScheduler`
  Throws:
    `IOException`

### doMerge

    protected void doMerge(MergePolicy.OneMerge merge)
                    throws IOException

  Does the actual merge, by calling `IndexWriter.merge(org.apache.lucene.index.MergePolicy.OneMerge)`

  Throws:
    `IOException`

### getMergeThread

    protected ConcurrentMergeScheduler.MergeThread getMergeThread(IndexWriter writer,
                                                                  MergePolicy.OneMerge merge)
                                                           throws IOException

  Create and return a new MergeThread

  Throws:
    `IOException`

### handleMergeException

    protected void handleMergeException(Throwable exc)

  Called when an exception is hit in a background merge thread

### toString

    public String toString()

  Overrides:
    `toString` in class `Object`

### clone

    public MergeScheduler clone()

  Overrides:
    `clone` in class `MergeScheduler`

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