diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 24f04b3..6173ece 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -49,6 +49,10 @@ API Changes
 * LUCENE-4399: Deprecated AppendingCodec. Lucene's term dictionaries
   no longer seek when writing.  (Adrien Grand, Robert Muir)
 
+* LUCENE-4472: MergePolicy now accepts a MergeTrigger that provides 
+  information about the trigger of the merge ie. merge triggered due
+  to a segment merge or a full flush etc. (Simon Willnauer)
+
 Bug Fixes
 
 * LUCENE-1822: BaseFragListBuilder hard-coded 6 char margin is too naive.
diff --git a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
index 9b0204d..6db8d30 100644
--- a/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
+++ b/lucene/core/src/java/org/apache/lucene/index/IndexWriter.java
@@ -36,6 +36,7 @@ import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.codecs.Codec;
 import org.apache.lucene.index.FieldInfos.FieldNumbers;
 import org.apache.lucene.index.IndexWriterConfig.OpenMode;
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
 import org.apache.lucene.index.MergeState.CheckAbort;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.store.AlreadyClosedException;
@@ -181,6 +182,10 @@ import org.apache.lucene.util.ThreadInterruptedException;
  * keeps track of the last non commit checkpoint.
  */
 public class IndexWriter implements Closeable, TwoPhaseCommit {
+  
+  private static final int UNBOUNDED_MAX_MERGE_SEGMENTS = -1;
+
+  
   /**
    * Name of the write lock in the index.
    */
@@ -377,7 +382,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
       }
     }
     if (anySegmentFlushed) {
-      maybeMerge();
+      maybeMerge(MergeTrigger.FULL_FLUSH, UNBOUNDED_MAX_MERGE_SEGMENTS);
     }
     if (infoStream.isEnabled("IW")) {
       infoStream.message("IW", "getReader took " + (System.currentTimeMillis() - tStart) + " msec");
@@ -1226,7 +1231,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
         }
       }
       if (anySegmentFlushed) {
-        maybeMerge();
+        maybeMerge(MergeTrigger.SEGMENT_FLUSH, UNBOUNDED_MAX_MERGE_SEGMENTS);
       }
     } catch (OutOfMemoryError oom) {
       handleOOM(oom, "updateDocuments");
@@ -1448,7 +1453,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
       }
 
       if (anySegmentFlushed) {
-        maybeMerge();
+        maybeMerge(MergeTrigger.SEGMENT_FLUSH, UNBOUNDED_MAX_MERGE_SEGMENTS);
       }
     } catch (OutOfMemoryError oom) {
       handleOOM(oom, "updateDocument");
@@ -1621,7 +1626,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
       }
     }
 
-    maybeMerge(maxNumSegments);
+    maybeMerge(MergeTrigger.EXPLICIT, maxNumSegments);
 
     if (doWait) {
       synchronized(this) {
@@ -1796,25 +1801,28 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
    * Explicit calls to maybeMerge() are usually not
    * necessary. The most common case is when merge policy
    * parameters have changed.
+   * 
+   * This method will call the {@link MergePolicy} with
+   * {@link MergeTrigger#EXPLICIT}.
    *
    * <p><b>NOTE</b>: if this method hits an OutOfMemoryError
    * you should immediately close the writer.  See <a
    * href="#OOME">above</a> for details.</p>
    */
   public final void maybeMerge() throws IOException {
-    maybeMerge(-1);
+    maybeMerge(MergeTrigger.EXPLICIT, UNBOUNDED_MAX_MERGE_SEGMENTS);
   }
 
-  private final void maybeMerge(int maxNumSegments) throws IOException {
+  private final void maybeMerge(MergeTrigger trigger, int maxNumSegments) throws IOException {
     ensureOpen(false);
-    updatePendingMerges(maxNumSegments);
+    updatePendingMerges(trigger, maxNumSegments);
     mergeScheduler.merge(this);
   }
 
-  private synchronized void updatePendingMerges(int maxNumSegments)
+  private synchronized void updatePendingMerges(MergeTrigger trigger, int maxNumSegments)
     throws IOException {
     assert maxNumSegments == -1 || maxNumSegments > 0;
-
+    assert trigger != null;
     if (stopMerges) {
       return;
     }
@@ -1825,7 +1833,9 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
     }
 
     final MergePolicy.MergeSpecification spec;
-    if (maxNumSegments != -1) {
+    if (maxNumSegments != UNBOUNDED_MAX_MERGE_SEGMENTS) {
+      assert trigger == MergeTrigger.EXPLICIT || trigger == MergeTrigger.MERGE_FINISHED :
+        "Expected EXPLICT or MERGE_FINISHED as trigger even with maxNumSegments set but was: " + trigger.name();
       spec = mergePolicy.findForcedMerges(segmentInfos, maxNumSegments, Collections.unmodifiableMap(segmentsToMerge));
       if (spec != null) {
         final int numMerges = spec.merges.size();
@@ -1836,7 +1846,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
       }
 
     } else {
-      spec = mergePolicy.findMerges(segmentInfos);
+      spec = mergePolicy.findMerges(trigger, segmentInfos);
     }
 
     if (spec != null) {
@@ -2653,7 +2663,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
       boolean success = false;
       try {
         if (anySegmentsFlushed) {
-          maybeMerge();
+          maybeMerge(MergeTrigger.FULL_FLUSH, UNBOUNDED_MAX_MERGE_SEGMENTS);
         }
         success = true;
       } finally {
@@ -2809,7 +2819,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
     // We can be called during close, when closing==true, so we must pass false to ensureOpen:
     ensureOpen(false);
     if (doFlush(applyAllDeletes) && triggerMerge) {
-      maybeMerge();
+      maybeMerge(MergeTrigger.FULL_FLUSH, UNBOUNDED_MAX_MERGE_SEGMENTS);
     }
   }
 
@@ -3240,7 +3250,7 @@ public class IndexWriter implements Closeable, TwoPhaseCommit {
           // segments) may now enable new merges, so we call
           // merge policy & update pending merges.
           if (success && !merge.isAborted() && (merge.maxNumSegments != -1 || (!closed && !closing))) {
-            updatePendingMerges(merge.maxNumSegments);
+            updatePendingMerges(MergeTrigger.MERGE_FINISHED, merge.maxNumSegments);
           }
         }
       }
diff --git a/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
index 91a47dd..305519e 100644
--- a/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/LogMergePolicy.java
@@ -24,6 +24,8 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
+
 
 /**
  * <p>This class implements a {@link MergePolicy} that tries
@@ -560,7 +562,7 @@ public abstract class LogMergePolicy extends MergePolicy {
    *  will return multiple merges, allowing the {@link
    *  MergeScheduler} to use concurrency. */
   @Override
-  public MergeSpecification findMerges(SegmentInfos infos) throws IOException {
+  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos infos) throws IOException {
 
     final int numSegments = infos.size();
     if (verbose()) {
diff --git a/lucene/core/src/java/org/apache/lucene/index/MergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/MergePolicy.java
index e0a2333..9979bd1 100644
--- a/lucene/core/src/java/org/apache/lucene/index/MergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/MergePolicy.java
@@ -19,6 +19,7 @@ package org.apache.lucene.index;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
@@ -57,7 +58,7 @@ import org.apache.lucene.util.SetOnce;
  */
 
 public abstract class MergePolicy implements java.io.Closeable, Cloneable {
-
+  
   /** OneMerge provides the information necessary to perform
    *  an individual primitive merge operation, resulting in
    *  a single new segment.  The merge spec includes the
@@ -333,11 +334,11 @@ public abstract class MergePolicy implements java.io.Closeable, Cloneable {
    * {@link IndexWriter} calls this whenever there is a change to the segments.
    * This call is always synchronized on the {@link IndexWriter} instance so
    * only one thread at a time will call this method.
-   * 
+   * @param mergeTrigger the event that triggered the merge
    * @param segmentInfos
    *          the total set of segments in the index
    */
-  public abstract MergeSpecification findMerges(SegmentInfos segmentInfos)
+  public abstract MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos)
       throws IOException;
 
   /**
@@ -379,9 +380,36 @@ public abstract class MergePolicy implements java.io.Closeable, Cloneable {
    * Release all resources for the policy.
    */
   public abstract void close();
-
+  
+  
   /**
    * Returns true if a new segment (regardless of its origin) should use the compound file format.
    */
   public abstract boolean useCompoundFile(SegmentInfos segments, SegmentInfoPerCommit newSegment) throws IOException;
+  
+  /**
+   * MergeTrigger is passed to
+   * {@link MergePolicy#findMerges(MergeTrigger, SegmentInfos)} to indicate the
+   * event that triggered the merge.
+   */
+  public static enum MergeTrigger {
+    /**
+     * Merge was triggered by a segment flush.
+     */
+    SEGMENT_FLUSH, 
+    /**
+     * Merge was triggered by a full flush. Full flushes
+     * can be caused by a commit, NRT reader reopen or a close call on the index writer.
+     */
+    FULL_FLUSH,
+    /**
+     * Merge has been triggered explicitly by the user.
+     */
+    EXPLICIT,
+    
+    /**
+     * Merge was triggered by a successfully finished merge.
+     */
+    MERGE_FINISHED,
+  }
 }
diff --git a/lucene/core/src/java/org/apache/lucene/index/NoMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/NoMergePolicy.java
index 9d83e35..bf147bc 100644
--- a/lucene/core/src/java/org/apache/lucene/index/NoMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/NoMergePolicy.java
@@ -17,8 +17,12 @@ package org.apache.lucene.index;
  * limitations under the License.
  */
 
+import java.io.IOException;
 import java.util.Map;
 
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
+import org.apache.lucene.index.MergePolicy.MergeSpecification;
+
 /**
  * A {@link MergePolicy} which never returns merges to execute (hence it's
  * name). It is also a singleton and can be accessed through
@@ -53,7 +57,7 @@ public final class NoMergePolicy extends MergePolicy {
   public void close() {}
 
   @Override
-  public MergeSpecification findMerges(SegmentInfos segmentInfos) { return null; }
+  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos) { return null; }
 
   @Override
   public MergeSpecification findForcedMerges(SegmentInfos segmentInfos,
diff --git a/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
index fd61b33..5d31390 100644
--- a/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/TieredMergePolicy.java
@@ -27,6 +27,8 @@ import java.util.Comparator;
 import java.util.List;
 import java.util.ArrayList;
 
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
+
 
 /**
  *  Merges segments of approximately equal size, subject to
@@ -298,7 +300,7 @@ public class TieredMergePolicy extends MergePolicy {
   }
 
   @Override
-  public MergeSpecification findMerges(SegmentInfos infos) throws IOException {
+  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos infos) throws IOException {
     if (verbose()) {
       message("findMerges: " + infos.size() + " segments");
     }
diff --git a/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java b/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java
index 3d0de7c..0f81b74 100644
--- a/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java
+++ b/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java
@@ -17,6 +17,7 @@ package org.apache.lucene.index;
  * limitations under the License.
  */
 
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
 import org.apache.lucene.util.Constants;
 
 import java.io.IOException;
@@ -75,8 +76,8 @@ public class UpgradeIndexMergePolicy extends MergePolicy {
   }
   
   @Override
-  public MergeSpecification findMerges(SegmentInfos segmentInfos) throws IOException {
-    return base.findMerges(segmentInfos);
+  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos) throws IOException {
+    return base.findMerges(null, segmentInfos);
   }
   
   @Override
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestNoMergePolicy.java b/lucene/core/src/test/org/apache/lucene/index/TestNoMergePolicy.java
index 4014cf3..187409e 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestNoMergePolicy.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestNoMergePolicy.java
@@ -30,7 +30,7 @@ public class TestNoMergePolicy extends LuceneTestCase {
   @Test
   public void testNoMergePolicy() throws Exception {
     MergePolicy mp = NoMergePolicy.NO_COMPOUND_FILES;
-    assertNull(mp.findMerges(null));
+    assertNull(mp.findMerges(null, (SegmentInfos)null));
     assertNull(mp.findForcedMerges(null, 0, null));
     assertNull(mp.findForcedDeletesMerges(null));
     assertFalse(mp.useCompoundFile(null, null));
diff --git a/lucene/core/src/test/org/apache/lucene/index/TestPerSegmentDeletes.java b/lucene/core/src/test/org/apache/lucene/index/TestPerSegmentDeletes.java
index 5117d39..8312cf7 100644
--- a/lucene/core/src/test/org/apache/lucene/index/TestPerSegmentDeletes.java
+++ b/lucene/core/src/test/org/apache/lucene/index/TestPerSegmentDeletes.java
@@ -24,6 +24,7 @@ import java.util.Map;
 import java.util.Random;
 
 import org.apache.lucene.analysis.MockAnalyzer;
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
 import org.apache.lucene.search.DocIdSetIterator;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.MockDirectoryWrapper;
@@ -257,7 +258,7 @@ public class TestPerSegmentDeletes extends LuceneTestCase {
     public void close() {}
 
     @Override
-    public MergeSpecification findMerges(SegmentInfos segmentInfos)
+    public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos)
         throws IOException {
       MergeSpecification ms = new MergeSpecification();
       if (doMerge) {
diff --git a/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java b/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
index fb55f76..4667ae1 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/index/MockRandomMergePolicy.java
@@ -25,6 +25,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Random;
 
+import org.apache.lucene.index.MergePolicy.MergeTrigger;
 import org.apache.lucene.util._TestUtil;
 
 /**
@@ -40,7 +41,7 @@ public class MockRandomMergePolicy extends MergePolicy {
   }
 
   @Override
-  public MergeSpecification findMerges(SegmentInfos segmentInfos) {
+  public MergeSpecification findMerges(MergeTrigger mergeTrigger, SegmentInfos segmentInfos) {
     MergeSpecification mergeSpec = null;
     //System.out.println("MRMP: findMerges sis=" + segmentInfos);
 
@@ -110,7 +111,7 @@ public class MockRandomMergePolicy extends MergePolicy {
 
   @Override
   public MergeSpecification findForcedDeletesMerges(SegmentInfos segmentInfos) throws IOException {
-    return findMerges(segmentInfos);
+    return findMerges(null, segmentInfos);
   }
 
   @Override
diff --git a/lucene/test-framework/src/java/org/apache/lucene/util/LuceneTestCase.java b/lucene/test-framework/src/java/org/apache/lucene/util/LuceneTestCase.java
index eaefa92..64c123a 100644
--- a/lucene/test-framework/src/java/org/apache/lucene/util/LuceneTestCase.java
+++ b/lucene/test-framework/src/java/org/apache/lucene/util/LuceneTestCase.java
@@ -763,7 +763,6 @@ public abstract class LuceneTestCase extends Assert {
     } else {
       c.setMergePolicy(newLogMergePolicy());
     }
-
     c.setReaderPooling(r.nextBoolean());
     c.setReaderTermsIndexDivisor(_TestUtil.nextInt(r, 1, 4));
     return c;