diff --git a/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaIndexTask.java b/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaIndexTask.java
index 4eaca8ea6ecd..3639bebd62a5 100644
--- a/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaIndexTask.java
+++ b/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaIndexTask.java
@@ -753,9 +753,12 @@ private boolean isPaused()
 
   private Appenderator newAppenderator(FireDepartmentMetrics metrics, TaskToolbox toolbox)
   {
+    final int maxRowsInMemoryPerPartition = (tuningConfig.getMaxRowsInMemory() /
+                                             ioConfig.getStartPartitions().getPartitionOffsetMap().size());
     return Appenderators.createRealtime(
         dataSchema,
-        tuningConfig.withBasePersistDirectory(new File(toolbox.getTaskWorkDir(), "persist")),
+        tuningConfig.withBasePersistDirectory(new File(toolbox.getTaskWorkDir(), "persist"))
+                    .withMaxRowsInMemory(maxRowsInMemoryPerPartition),
         metrics,
         toolbox.getSegmentPusher(),
         toolbox.getObjectMapper(),
diff --git a/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaTuningConfig.java b/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaTuningConfig.java
index 374b2dec909c..8a60822fa0f7 100644
--- a/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaTuningConfig.java
+++ b/extensions-core/kafka-indexing-service/src/main/java/io/druid/indexing/kafka/KafkaTuningConfig.java
@@ -144,4 +144,19 @@ public KafkaTuningConfig withBasePersistDirectory(File dir)
         handoffConditionTimeout
     );
   }
+
+  public KafkaTuningConfig withMaxRowsInMemory(int rows)
+  {
+    return new KafkaTuningConfig(
+        rows,
+        maxRowsPerSegment,
+        intermediatePersistPeriod,
+        basePersistDirectory,
+        maxPendingPersists,
+        indexSpec,
+        buildV9Directly,
+        reportParseExceptions,
+        handoffConditionTimeout
+    );
+  }
 }
diff --git a/server/src/main/java/io/druid/segment/realtime/appenderator/Appenderator.java b/server/src/main/java/io/druid/segment/realtime/appenderator/Appenderator.java
index 7d8e3d60fa2f..be305c5c13ea 100644
--- a/server/src/main/java/io/druid/segment/realtime/appenderator/Appenderator.java
+++ b/server/src/main/java/io/druid/segment/realtime/appenderator/Appenderator.java
@@ -33,7 +33,7 @@
  * An Appenderator indexes data. It has some in-memory data and some persisted-on-disk data. It can serve queries on
  * both of those. It can also push data to deep storage. But, it does not decide which segments data should go into.
  * It also doesn't publish segments to the metadata store or monitor handoff; you have to do that yourself!
- * <p/>
+ * <p>
  * Any time you call one of the methods that adds, persists, or pushes data, you must provide a Committer, or a
  * Supplier of one, that represents all data you have given to the Appenderator so far. The Committer will be used when
  * that data has been persisted to disk.
@@ -54,13 +54,13 @@ public interface Appenderator extends QuerySegmentWalker, Closeable
 
   /**
    * Add a row. Must not be called concurrently from multiple threads.
-   * <p/>
+   * <p>
    * If no pending segment exists for the provided identifier, a new one will be created.
-   * <p/>
+   * <p>
    * This method may trigger a {@link #persistAll(Committer)} using the supplied Committer. If it does this, the
    * Committer is guaranteed to be *created* synchronously with the call to add, but will actually be used
    * asynchronously.
-   * <p/>
+   * <p>
    * The add, clear, persistAll, and push methods should all be called from the same thread.
    *
    * @param identifier        the segment into which this row should be added
@@ -95,6 +95,8 @@ int add(SegmentIdentifier identifier, InputRow row, Supplier<Committer> committe
    * Drop all in-memory and on-disk data, and forget any previously-remembered commit metadata. This could be useful if,
    * for some reason, rows have been added that we do not actually want to hand off. Blocks until all data has been
    * cleared. This may take some time, since all pending persists must finish first.
+   *
+   * The add, clear, persistAll, and push methods should all be called from the same thread.
    */
   void clear() throws InterruptedException;
 
@@ -102,7 +104,7 @@ int add(SegmentIdentifier identifier, InputRow row, Supplier<Committer> committe
    * Drop all data associated with a particular pending segment. Unlike {@link #clear()}), any on-disk commit
    * metadata will remain unchanged. If there is no pending segment with this identifier, then this method will
    * do nothing.
-   * <p/>
+   * <p>
    * You should not write to the dropped segment after calling "drop". If you need to drop all your data and
    * re-write it, consider {@link #clear()} instead.
    *
@@ -117,7 +119,7 @@ int add(SegmentIdentifier identifier, InputRow row, Supplier<Committer> committe
    * machine's local disk. The Committer will be made synchronously will the call to persistAll, but will actually
    * be used asynchronously. Any metadata returned by the committer will be associated with the data persisted to
    * disk.
-   * <p/>
+   * <p>
    * The add, clear, persistAll, and push methods should all be called from the same thread.
    *
    * @param committer a committer associated with all data that has been added so far
@@ -129,9 +131,9 @@ int add(SegmentIdentifier identifier, InputRow row, Supplier<Committer> committe
   /**
    * Merge and push particular segments to deep storage. This will trigger an implicit {@link #persistAll(Committer)}
    * using the provided Committer.
-   * <p/>
+   * <p>
    * After this method is called, you cannot add new data to any segments that were previously under construction.
-   * <p/>
+   * <p>
    * The add, clear, persistAll, and push methods should all be called from the same thread.
    *
    * @param identifiers list of segments to push
diff --git a/server/src/main/java/io/druid/segment/realtime/appenderator/AppenderatorImpl.java b/server/src/main/java/io/druid/segment/realtime/appenderator/AppenderatorImpl.java
index 9d013ea7c507..3ab523e266c6 100644
--- a/server/src/main/java/io/druid/segment/realtime/appenderator/AppenderatorImpl.java
+++ b/server/src/main/java/io/druid/segment/realtime/appenderator/AppenderatorImpl.java
@@ -18,6 +18,7 @@
 package io.druid.segment.realtime.appenderator;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Function;
 import com.google.common.base.Joiner;
 import com.google.common.base.Preconditions;
@@ -85,6 +86,7 @@
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
 
 /**
  */
@@ -108,10 +110,11 @@ public class AppenderatorImpl implements Appenderator
   private final VersionedIntervalTimeline<String, Sink> sinkTimeline = new VersionedIntervalTimeline<>(
       String.CASE_INSENSITIVE_ORDER
   );
+  private final AtomicInteger rowsCurrentlyInMemory = new AtomicInteger();
   private final QuerySegmentWalker texasRanger;
 
   private volatile ListeningExecutorService persistExecutor = null;
-  private volatile ListeningExecutorService mergeExecutor = null;
+  private volatile ListeningExecutorService pushExecutor = null;
   private volatile long nextFlush;
   private volatile FileLock basePersistDirLock = null;
   private volatile FileChannel basePersistDirLockChannel = null;
@@ -188,26 +191,34 @@ public int add(
     }
 
     final Sink sink = getOrCreateSink(identifier);
-    int sinkRetVal;
+    final int sinkRowsInMemoryBeforeAdd = sink.getNumRowsInMemory();
+    final int sinkRowsInMemoryAfterAdd;
 
     try {
-      sinkRetVal = sink.add(row);
+      sinkRowsInMemoryAfterAdd = sink.add(row);
     }
     catch (IndexSizeExceededException e) {
-      // Try one more time after swapping, then throw the exception out if it happens again.
-      persistAll(committerSupplier.get());
-      sinkRetVal = sink.add(row);
+      // Uh oh, we can't do anything about this! We can't persist (commit metadata would be out of sync) and we
+      // can't add the row (it just failed). This should never actually happen, though, because we check
+      // sink.canAddRow after returning from add.
+      log.error(e, "Sink for segment[%s] was unexpectedly full!", identifier);
+      throw e;
     }
 
-    if (!sink.canAppendRow() || System.currentTimeMillis() > nextFlush) {
-      persistAll(committerSupplier.get());
+    if (sinkRowsInMemoryAfterAdd < 0) {
+      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
     }
 
-    if (sinkRetVal < 0) {
-      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
-    } else {
-      return sink.getNumRows();
+    rowsCurrentlyInMemory.addAndGet(sinkRowsInMemoryAfterAdd - sinkRowsInMemoryBeforeAdd);
+
+    if (!sink.canAppendRow()
+        || System.currentTimeMillis() > nextFlush
+        || rowsCurrentlyInMemory.get() >= tuningConfig.getMaxRowsInMemory()) {
+      // persistAll clears rowsCurrentlyInMemory, no need to update it.
+      persistAll(committerSupplier.get());
     }
+
+    return sink.getNumRows();
   }
 
   @Override
@@ -228,6 +239,12 @@ public int getRowCount(final SegmentIdentifier identifier)
     }
   }
 
+  @VisibleForTesting
+  int getRowsInMemory()
+  {
+    return rowsCurrentlyInMemory.get();
+  }
+
   private Sink getOrCreateSink(final SegmentIdentifier identifier)
   {
     Sink retVal = sinks.get(identifier);
@@ -410,6 +427,9 @@ public String apply(Map.Entry<SegmentIdentifier, Integer> entry)
     runExecStopwatch.stop();
     resetNextFlush();
 
+    // NB: The rows are still in memory until they're done persisting, but we only count rows in active indexes.
+    rowsCurrentlyInMemory.set(0);
+
     return future;
   }
 
@@ -455,7 +475,7 @@ public SegmentsAndMetadata apply(Object commitMetadata)
             return new SegmentsAndMetadata(dataSegments, commitMetadata);
           }
         },
-        mergeExecutor
+        pushExecutor
     );
   }
 
@@ -464,9 +484,9 @@ public SegmentsAndMetadata apply(Object commitMetadata)
    * This is useful if we're going to do something that would otherwise potentially break currently in-progress
    * pushes.
    */
-  private ListenableFuture<?> mergeBarrier()
+  private ListenableFuture<?> pushBarrier()
   {
-    return mergeExecutor.submit(
+    return pushExecutor.submit(
         new Runnable()
         {
           @Override
@@ -480,7 +500,7 @@ public void run()
 
   /**
    * Merge segment, push to deep storage. Should only be used on segments that have been fully persisted. Must only
-   * be run in the single-threaded mergeExecutor.
+   * be run in the single-threaded pushExecutor.
    *
    * @param identifier sink identifier
    * @param sink       sink to push
@@ -589,7 +609,7 @@ public void close()
     try {
       shutdownExecutors();
       Preconditions.checkState(persistExecutor.awaitTermination(365, TimeUnit.DAYS), "persistExecutor not terminated");
-      Preconditions.checkState(mergeExecutor.awaitTermination(365, TimeUnit.DAYS), "mergeExecutor not terminated");
+      Preconditions.checkState(pushExecutor.awaitTermination(365, TimeUnit.DAYS), "pushExecutor not terminated");
     }
     catch (InterruptedException e) {
       Thread.currentThread().interrupt();
@@ -647,9 +667,9 @@ private void initializeExecutors()
           )
       );
     }
-    if (mergeExecutor == null) {
+    if (pushExecutor == null) {
       // use a blocking single threaded executor to throttle the firehose when write to disk is slow
-      mergeExecutor = MoreExecutors.listeningDecorator(
+      pushExecutor = MoreExecutors.listeningDecorator(
           Execs.newBlockingSingleThreaded(
               "appenderator_merge_%d", 1
           )
@@ -660,7 +680,7 @@ private void initializeExecutors()
   private void shutdownExecutors()
   {
     persistExecutor.shutdownNow();
-    mergeExecutor.shutdownNow();
+    pushExecutor.shutdownNow();
   }
 
   private void resetNextFlush()
@@ -830,12 +850,18 @@ private ListenableFuture<?> abandonSegment(
       final boolean removeOnDiskData
   )
   {
-    // Mark this identifier as dropping, so no future merge tasks will pick it up.
+    // Ensure no future writes will be made to this sink.
+    sink.finishWriting();
+
+    // Mark this identifier as dropping, so no future push tasks will pick it up.
     droppingSinks.add(identifier);
 
-    // Wait for any outstanding merges to finish, then abandon the segment inside the persist thread.
+    // Decrement this sink's rows from rowsCurrentlyInMemory (we only count active sinks).
+    rowsCurrentlyInMemory.addAndGet(-sink.getNumRowsInMemory());
+
+    // Wait for any outstanding pushes to finish, then abandon the segment inside the persist thread.
     return Futures.transform(
-        mergeBarrier(),
+        pushBarrier(),
         new Function<Object, Object>()
         {
           @Nullable
diff --git a/server/src/main/java/io/druid/segment/realtime/plumber/Sink.java b/server/src/main/java/io/druid/segment/realtime/plumber/Sink.java
index 91326192fdd9..29e97dbd5464 100644
--- a/server/src/main/java/io/druid/segment/realtime/plumber/Sink.java
+++ b/server/src/main/java/io/druid/segment/realtime/plumber/Sink.java
@@ -228,6 +228,18 @@ public int getNumRows()
     }
   }
 
+  public int getNumRowsInMemory()
+  {
+    synchronized (hydrantLock) {
+      IncrementalIndex index = currHydrant.getIndex();
+      if (index == null) {
+        return 0;
+      }
+
+      return currHydrant.getIndex().size();
+    }
+  }
+
   private FireHydrant makeNewCurrIndex(long minTimestamp, DataSchema schema)
   {
     final IncrementalIndexSchema indexSchema = new IncrementalIndexSchema.Builder()
diff --git a/server/src/test/java/io/druid/segment/realtime/appenderator/AppenderatorTest.java b/server/src/test/java/io/druid/segment/realtime/appenderator/AppenderatorTest.java
index 12e9e364dc42..620abdc84088 100644
--- a/server/src/test/java/io/druid/segment/realtime/appenderator/AppenderatorTest.java
+++ b/server/src/test/java/io/druid/segment/realtime/appenderator/AppenderatorTest.java
@@ -135,6 +135,56 @@ public SegmentIdentifier apply(DataSegment input)
     }
   }
 
+  @Test
+  public void testMaxRowsInMemory() throws Exception
+  {
+    try (final AppenderatorTester tester = new AppenderatorTester(3)) {
+      final Appenderator appenderator = tester.getAppenderator();
+      final AtomicInteger eventCount = new AtomicInteger(0);
+      final Supplier<Committer> committerSupplier = new Supplier<Committer>()
+      {
+        @Override
+        public Committer get()
+        {
+          final Object metadata = ImmutableMap.of("eventCount", eventCount.get());
+
+          return new Committer()
+          {
+            @Override
+            public Object getMetadata()
+            {
+              return metadata;
+            }
+
+            @Override
+            public void run()
+            {
+              // Do nothing
+            }
+          };
+        }
+      };
+
+      Assert.assertEquals(0, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.startJob();
+      Assert.assertEquals(0, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(0), IR("2000", "foo", 1), committerSupplier);
+      Assert.assertEquals(1, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(1), IR("2000", "bar", 1), committerSupplier);
+      Assert.assertEquals(2, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(1), IR("2000", "bar", 1), committerSupplier);
+      Assert.assertEquals(2, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(0), IR("2000", "baz", 1), committerSupplier);
+      Assert.assertEquals(0, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(1), IR("2000", "qux", 1), committerSupplier);
+      Assert.assertEquals(1, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.add(IDENTIFIERS.get(0), IR("2000", "bob", 1), committerSupplier);
+      Assert.assertEquals(2, ((AppenderatorImpl) appenderator).getRowsInMemory());
+      appenderator.close();
+      Assert.assertEquals(0, ((AppenderatorImpl) appenderator).getRowsInMemory());
+    }
+  }
+
   @Test
   public void testRestoreFromDisk() throws Exception
   {