[GitHub] [druid] jihoonson commented on a change in pull request #11294: Bound memory utilization for dynamic partitioning (i.e. memory growth is constant)

[GitBox <gi...@apache.org>]

jihoonson commented on a change in pull request #11294:
URL: https://github.com/apache/druid/pull/11294#discussion_r664768612



##########
File path: server/src/main/java/org/apache/druid/segment/realtime/appenderator/BatchAppenderator.java
##########
@@ -0,0 +1,1149 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.segment.realtime.appenderator;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Stopwatch;
+import com.google.common.base.Supplier;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.common.primitives.Ints;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import org.apache.druid.data.input.Committer;
+import org.apache.druid.data.input.InputRow;
+import org.apache.druid.java.util.common.FileUtils;
+import org.apache.druid.java.util.common.IAE;
+import org.apache.druid.java.util.common.ISE;
+import org.apache.druid.java.util.common.Pair;
+import org.apache.druid.java.util.common.RetryUtils;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.common.io.Closer;
+import org.apache.druid.java.util.emitter.EmittingLogger;
+import org.apache.druid.query.Query;
+import org.apache.druid.query.QueryRunner;
+import org.apache.druid.query.SegmentDescriptor;
+import org.apache.druid.segment.IndexIO;
+import org.apache.druid.segment.IndexMerger;
+import org.apache.druid.segment.QueryableIndex;
+import org.apache.druid.segment.QueryableIndexSegment;
+import org.apache.druid.segment.ReferenceCountingSegment;
+import org.apache.druid.segment.incremental.IncrementalIndexAddResult;
+import org.apache.druid.segment.incremental.IndexSizeExceededException;
+import org.apache.druid.segment.incremental.ParseExceptionHandler;
+import org.apache.druid.segment.incremental.RowIngestionMeters;
+import org.apache.druid.segment.indexing.DataSchema;
+import org.apache.druid.segment.loading.DataSegmentPusher;
+import org.apache.druid.segment.realtime.FireDepartmentMetrics;
+import org.apache.druid.segment.realtime.FireHydrant;
+import org.apache.druid.segment.realtime.plumber.Sink;
+import org.apache.druid.timeline.DataSegment;
+import org.joda.time.Interval;
+
+import javax.annotation.Nullable;
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
+import java.nio.file.StandardOpenOption;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.Collectors;
+
+public class BatchAppenderator implements Appenderator
+{
+  public static final int ROUGH_OVERHEAD_PER_SINK = 5000;
+  // Rough estimate of memory footprint of empty FireHydrant based on actual heap dumps
+  public static final int ROUGH_OVERHEAD_PER_HYDRANT = 1000;
+
+  private static final EmittingLogger log = new EmittingLogger(BatchAppenderator.class);
+  private static final String IDENTIFIER_FILE_NAME = "identifier.json";
+
+  private final String myId;
+  private final DataSchema schema;
+  private final AppenderatorConfig tuningConfig;
+  private final FireDepartmentMetrics metrics;
+  private final DataSegmentPusher dataSegmentPusher;
+  private final ObjectMapper objectMapper;
+  private final IndexIO indexIO;
+  private final IndexMerger indexMerger;
+  /**
+   * This map needs to be concurrent because it's accessed and mutated from multiple threads from where
+   * this Appenderator is used (and methods like {@link #add(SegmentIdWithShardSpec, InputRow, Supplier, boolean)} are
+   * called). It could also be accessed (but not mutated) potentially in the context
+   * of any thread from {@link #drop}.
+   */
+  private final ConcurrentMap<SegmentIdWithShardSpec, Sink> sinks = new ConcurrentHashMap<>();
+  private final long maxBytesTuningConfig;
+  private final boolean skipBytesInMemoryOverheadCheck;
+
+  /**
+   * The following sinks metadata map and associated class are the way to retain metadata now that sinks
+   * are being completely removed from memory after each incremental persist.
+   */
+  private final ConcurrentHashMap<SegmentIdWithShardSpec, SinkMetadata> sinksMetadata = new ConcurrentHashMap<>();
+
+  /**
+   * This class is used for information that needs to be kept related to Sinks as
+   * they are persisted and removed from memory at every incremental persist.
+   * The information is used for sanity checks and as information required
+   * for functionality, depending in the field that is used. More info about the
+   * fields is annotated as comments in the class
+   */
+  private static class SinkMetadata
+  {
+    /** This is used to maintain the rows in the sink accross persists of the sink
+    * used for functionality (i.e. to detect whether an incremental push
+    * is needed {@link AppenderatorDriverAddResult#isPushRequired(Integer, Long)}
+    **/
+    private int numRowsInSegment;
+    /** For sanity check as well as functionality: to make sure that all hydrants for a sink are restored from disk at
+     * push time and also to remember the fire hydrant "count" when persisting it.
+     */
+    private int numHydrants;
+    /* Reference to directory that holds the persisted data */
+    File persistedFileDir;
+
+    public SinkMetadata()
+    {
+      this(0, 0);
+    }
+
+    public SinkMetadata(int numRowsInSegment, int numHydrants)
+    {
+      this.numRowsInSegment = numRowsInSegment;
+      this.numHydrants = numHydrants;
+    }
+
+    public void addRows(int num)
+    {
+      numRowsInSegment += num;
+    }
+
+    public void addHydrants(int num)
+    {
+      numHydrants += num;
+    }
+
+    public int getNumRowsInSegment()
+    {
+      return numRowsInSegment;
+    }
+
+    public int getNumHydrants()
+    {
+      return numHydrants;
+    }
+
+    public void setPersistedFileDir(File persistedFileDir)
+    {
+      this.persistedFileDir = persistedFileDir;
+    }
+
+    public File getPersistedFileDir()
+    {
+      return persistedFileDir;
+    }
+
+  }
+
+  // This variable updated in add(), persist(), and drop()
+  private final AtomicInteger rowsCurrentlyInMemory = new AtomicInteger();
+  private final AtomicInteger totalRows = new AtomicInteger();
+  private final AtomicLong bytesCurrentlyInMemory = new AtomicLong();
+  private final RowIngestionMeters rowIngestionMeters;
+  private final ParseExceptionHandler parseExceptionHandler;
+
+  private final AtomicBoolean closed = new AtomicBoolean(false);
+
+  private volatile FileLock basePersistDirLock = null;
+  private volatile FileChannel basePersistDirLockChannel = null;
+
+  BatchAppenderator(
+      String id,
+      DataSchema schema,
+      AppenderatorConfig tuningConfig,
+      FireDepartmentMetrics metrics,
+      DataSegmentPusher dataSegmentPusher,
+      ObjectMapper objectMapper,
+      IndexIO indexIO,
+      IndexMerger indexMerger,
+      RowIngestionMeters rowIngestionMeters,
+      ParseExceptionHandler parseExceptionHandler
+  )
+  {
+    this.myId = id;
+    this.schema = Preconditions.checkNotNull(schema, "schema");
+    this.tuningConfig = Preconditions.checkNotNull(tuningConfig, "tuningConfig");
+    this.metrics = Preconditions.checkNotNull(metrics, "metrics");
+    this.dataSegmentPusher = Preconditions.checkNotNull(dataSegmentPusher, "dataSegmentPusher");
+    this.objectMapper = Preconditions.checkNotNull(objectMapper, "objectMapper");
+    this.indexIO = Preconditions.checkNotNull(indexIO, "indexIO");
+    this.indexMerger = Preconditions.checkNotNull(indexMerger, "indexMerger");
+    this.rowIngestionMeters = Preconditions.checkNotNull(rowIngestionMeters, "rowIngestionMeters");
+    this.parseExceptionHandler = Preconditions.checkNotNull(parseExceptionHandler, "parseExceptionHandler");
+
+    maxBytesTuningConfig = tuningConfig.getMaxBytesInMemoryOrDefault();
+    skipBytesInMemoryOverheadCheck = tuningConfig.isSkipBytesInMemoryOverheadCheck();
+  }
+
+  @Override
+  public String getId()
+  {
+    return myId;
+  }
+
+  @Override
+  public String getDataSource()
+  {
+    return schema.getDataSource();
+  }
+
+  @Override
+  public Object startJob()
+  {
+    tuningConfig.getBasePersistDirectory().mkdirs();
+    lockBasePersistDirectory();
+    return null;
+  }
+
+  @Override
+  public AppenderatorAddResult add(
+      final SegmentIdWithShardSpec identifier,
+      final InputRow row,
+      @Nullable final Supplier<Committer> committerSupplier,
+      final boolean allowIncrementalPersists
+  ) throws IndexSizeExceededException, SegmentNotWritableException
+  {
+
+    Preconditions.checkArgument(
+        committerSupplier == null,
+        "Batch appenderator does not need a committer!"
+    );
+
+    Preconditions.checkArgument(
+        allowIncrementalPersists,
+        "Batch appenderator should always allow incremental persists!"
+    );
+
+    if (!identifier.getDataSource().equals(schema.getDataSource())) {
+      throw new IAE(
+          "Expected dataSource[%s] but was asked to insert row for dataSource[%s]?!",
+          schema.getDataSource(),
+          identifier.getDataSource()
+      );
+    }
+
+    final Sink sink = getOrCreateSink(identifier);
+    metrics.reportMessageMaxTimestamp(row.getTimestampFromEpoch());
+    final int sinkRowsInMemoryBeforeAdd = sink.getNumRowsInMemory();
+    final int sinkRowsInMemoryAfterAdd;
+    final long bytesInMemoryBeforeAdd = sink.getBytesInMemory();
+    final long bytesInMemoryAfterAdd;
+    final IncrementalIndexAddResult addResult;
+
+    try {
+      addResult = sink.add(row, false); // allow incrememtal persis is always true for batch
+      sinkRowsInMemoryAfterAdd = addResult.getRowCount();
+      bytesInMemoryAfterAdd = addResult.getBytesInMemory();
+    }
+    catch (IndexSizeExceededException e) {
+      // 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 (sinkRowsInMemoryAfterAdd < 0) {
+      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
+    }
+
+    if (addResult.isRowAdded()) {
+      rowIngestionMeters.incrementProcessed();
+    } else if (addResult.hasParseException()) {
+      parseExceptionHandler.handle(addResult.getParseException());
+    }
+
+    final int numAddedRows = sinkRowsInMemoryAfterAdd - sinkRowsInMemoryBeforeAdd;
+    rowsCurrentlyInMemory.addAndGet(numAddedRows);
+    bytesCurrentlyInMemory.addAndGet(bytesInMemoryAfterAdd - bytesInMemoryBeforeAdd);
+    totalRows.addAndGet(numAddedRows);
+    sinksMetadata.computeIfAbsent(identifier, unused -> new SinkMetadata()).addRows(numAddedRows);
+
+    boolean persist = false;
+    List<String> persistReasons = new ArrayList<>();
+
+    if (!sink.canAppendRow()) {
+      persist = true;
+      persistReasons.add("No more rows can be appended to sink");
+    }
+    if (rowsCurrentlyInMemory.get() >= tuningConfig.getMaxRowsInMemory()) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "rowsCurrentlyInMemory[%d] is greater than maxRowsInMemory[%d]",
+          rowsCurrentlyInMemory.get(),
+          tuningConfig.getMaxRowsInMemory()
+      ));
+    }
+    if (bytesCurrentlyInMemory.get() >= maxBytesTuningConfig) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "bytesCurrentlyInMemory[%d] is greater than maxBytesInMemory[%d]",
+          bytesCurrentlyInMemory.get(),
+          maxBytesTuningConfig
+      ));
+    }
+    if (persist) {
+      // persistAll clears rowsCurrentlyInMemory, no need to update it.
+      log.info("Incremental persist to disk because %s.", String.join(",", persistReasons));
+
+      long bytesToBePersisted = 0L;
+      for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+        final Sink sinkEntry = entry.getValue();
+        if (sinkEntry != null) {
+          bytesToBePersisted += sinkEntry.getBytesInMemory();
+          if (sinkEntry.swappable()) {
+            // Code for batch no longer memory maps hydrants but they still take memory...
+            int memoryStillInUse = calculateMemoryUsedByHydrant();
+            bytesCurrentlyInMemory.addAndGet(memoryStillInUse);
+          }
+        }
+      }
+
+      if (!skipBytesInMemoryOverheadCheck
+          && bytesCurrentlyInMemory.get() - bytesToBePersisted > maxBytesTuningConfig) {
+        // We are still over maxBytesTuningConfig even after persisting.
+        // This means that we ran out of all available memory to ingest (due to overheads created as part of ingestion)
+        final String alertMessage = StringUtils.format(
+            "Task has exceeded safe estimated heap usage limits, failing "
+            + "(numSinks: [%d] numHydrantsAcrossAllSinks: [%d] totalRows: [%d])"
+            + "(bytesCurrentlyInMemory: [%d] - bytesToBePersisted: [%d] > maxBytesTuningConfig: [%d])",
+            sinks.size(),
+            sinks.values().stream().mapToInt(Iterables::size).sum(),
+            getTotalRowCount(),
+            bytesCurrentlyInMemory.get(),
+            bytesToBePersisted,
+            maxBytesTuningConfig
+        );
+        final String errorMessage = StringUtils.format(
+            "%s.\nThis can occur when the overhead from too many intermediary segment persists becomes to "
+            + "great to have enough space to process additional input rows. This check, along with metering the overhead "
+            + "of these objects to factor into the 'maxBytesInMemory' computation, can be disabled by setting "
+            + "'skipBytesInMemoryOverheadCheck' to 'true' (note that doing so might allow the task to naturally encounter "
+            + "a 'java.lang.OutOfMemoryError'). Alternatively, 'maxBytesInMemory' can be increased which will cause an "
+            + "increase in heap footprint, but will allow for more intermediary segment persists to occur before "
+            + "reaching this condition.",
+            alertMessage
+        );
+        log.makeAlert(alertMessage)
+           .addData("dataSource", schema.getDataSource())
+           .emit();
+        throw new RuntimeException(errorMessage);
+      }
+
+      persistAllAndRemoveSinks();
+
+    }
+    return new AppenderatorAddResult(identifier, sinksMetadata.get(identifier).numRowsInSegment, false);
+  }
+
+  @Override
+  /**
+   * Returns all active segments regardless whether they are in memory or persisted
+   */
+  public List<SegmentIdWithShardSpec> getSegments()
+  {
+    return ImmutableList.copyOf(sinksMetadata.keySet());
+  }
+
+  @VisibleForTesting
+  public List<SegmentIdWithShardSpec> getInMemorySegments()
+  {
+    return ImmutableList.copyOf(sinks.keySet());
+  }
+
+  @Override
+  public int getRowCount(final SegmentIdWithShardSpec identifier)
+  {
+    return sinksMetadata.get(identifier).getNumRowsInSegment();
+  }
+
+  @Override
+  public int getTotalRowCount()
+  {
+    return totalRows.get();
+  }
+
+  @VisibleForTesting
+  public int getRowsInMemory()
+  {
+    return rowsCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesCurrentlyInMemory()
+  {
+    return bytesCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesInMemory(SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+
+    if (sink == null) {
+      return 0L; // sinks are removed after a persist
+    } else {
+      return sink.getBytesInMemory();
+    }
+  }
+
+  private Sink getOrCreateSink(final SegmentIdWithShardSpec identifier)
+  {
+    Sink retVal = sinks.get(identifier);
+
+    if (retVal == null) {
+      retVal = new Sink(
+          identifier.getInterval(),
+          schema,
+          identifier.getShardSpec(),
+          identifier.getVersion(),
+          tuningConfig.getAppendableIndexSpec(),
+          tuningConfig.getMaxRowsInMemory(),
+          maxBytesTuningConfig,
+          null
+      );
+      bytesCurrentlyInMemory.addAndGet(calculateSinkMemoryInUsed());
+
+      sinks.put(identifier, retVal);
+      metrics.setSinkCount(sinks.size());
+    }
+
+    return retVal;
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForIntervals(final Query<T> query, final Iterable<Interval> intervals)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForSegments(final Query<T> query, final Iterable<SegmentDescriptor> specs)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public void clear()
+  {
+    clear(true);
+  }
+
+  private void clear(boolean removeOnDiskData)
+  {
+    // Drop commit metadata, then abandon all segments.
+    log.info("Clearing all sinks & hydrants, removing data on disk: [%s]", removeOnDiskData);
+    // Drop everything.
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      removeSink(entry.getKey(), entry.getValue(), removeOnDiskData);
+    }
+  }
+
+  @Override
+  public ListenableFuture<?> drop(final SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+    SinkMetadata sm = sinksMetadata.remove(identifier);
+    if (sm != null) {
+      int originalTotalRows = getTotalRowCount();
+      int rowsToDrop = sm.getNumRowsInSegment();
+      int totalRowsAfter = originalTotalRows - rowsToDrop;
+      if (totalRowsAfter < 0) {
+        log.warn("Total rows[%d] after dropping segment[%s] rows [%d]", totalRowsAfter, identifier, rowsToDrop);
+      }
+      totalRows.set(Math.max(totalRowsAfter, 0));
+    }
+    if (sink != null) {
+      removeSink(identifier, sink, true);
+    }
+    return Futures.immediateFuture(null);
+  }
+
+  @Override
+  public ListenableFuture<Object> persistAll(@Nullable final Committer committer)
+  {
+    if (committer != null) {
+      throw new ISE("committer must be null for BatchAppenderator");
+    }
+    persistAllAndRemoveSinks();
+    return Futures.immediateFuture(null);
+  }
+
+  /**
+   * Persist all sinks & their hydrants, keep their metadata, and then remove them completely from
+   * memory (to be resurrected right before merge & push)
+   */
+  private void persistAllAndRemoveSinks()
+  {
+
+    final List<Pair<FireHydrant, SegmentIdWithShardSpec>> indexesToPersist = new ArrayList<>();
+    int numPersistedRows = 0;
+    long bytesPersisted = 0L;
+    int totalHydrantsCount = 0;
+    final long totalSinks = sinks.size();
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      final SegmentIdWithShardSpec identifier = entry.getKey();
+      final Sink sink = entry.getValue();
+      if (sink == null) {
+        throw new ISE("No sink for identifier: %s", identifier);
+      }
+
+      final List<FireHydrant> hydrants = Lists.newArrayList(sink);
+      // Since everytime we persist we also get rid of the in-memory references to sinks & hydrants
+      // the invariant of exactly one, always swappable, sink with exactly one unpersisted hydrant must hold
+      int totalHydrantsForSink = hydrants.size();
+      if (totalHydrantsForSink != 1) {
+        throw new ISE("There should be only onw hydrant for identifier[%s] but there are[%s]",
+                      identifier, totalHydrantsForSink
+        );
+      }
+      totalHydrantsCount += 1;
+      numPersistedRows += sink.getNumRowsInMemory();
+      bytesPersisted += sink.getBytesInMemory();
+
+      if (!sink.swappable()) {
+        throw new ISE("Sink is not swappable![%s]", identifier);
+      }
+      indexesToPersist.add(Pair.of(sink.swap(), identifier));
+
+    }
+
+    if (indexesToPersist.isEmpty()) {
+      log.info("No indexes will be peristed");
+    }
+    final Stopwatch persistStopwatch = Stopwatch.createStarted();
+    try {
+      for (Pair<FireHydrant, SegmentIdWithShardSpec> pair : indexesToPersist) {
+        metrics.incrementRowOutputCount(persistHydrant(pair.lhs, pair.rhs));
+      }
+
+      log.info(
+          "Persisted in-memory data for segments: %s",
+          indexesToPersist.stream()
+                          .filter(itp -> itp.rhs != null)
+                          .map(itp -> itp.rhs.asSegmentId().toString())
+                          .distinct()
+                          .collect(Collectors.joining(", "))
+      );
+      log.info(
+          "Persisted stats: processed rows: [%d], persisted rows[%d], persisted sinks: [%d], persisted fireHydrants (across sinks): [%d]",
+          rowIngestionMeters.getProcessed(),
+          numPersistedRows,
+          totalSinks,
+          totalHydrantsCount
+      );
+
+    }
+    catch (Exception e) {
+      metrics.incrementFailedPersists();
+      throw e;
+    }
+    finally {
+      metrics.incrementNumPersists();
+      metrics.incrementPersistTimeMillis(persistStopwatch.elapsed(TimeUnit.MILLISECONDS));
+      persistStopwatch.stop();
+    }
+
+    // NB: The rows are still in memory until they're done persisting, but we only count rows in active indexes.
+    rowsCurrentlyInMemory.addAndGet(-numPersistedRows);
+    bytesCurrentlyInMemory.addAndGet(-bytesPersisted);
+
+    // remove all sinks after persisting:
+    clear(false);
+
+    log.info("Persisted rows[%,d] and bytes[%,d] and removed all sinks & hydrants from memory",
+             numPersistedRows, bytesPersisted);
+
+  }
+
+  @Override
+  public ListenableFuture<SegmentsAndCommitMetadata> push(
+      final Collection<SegmentIdWithShardSpec> identifiers,
+      @Nullable final Committer committer,
+      final boolean useUniquePath
+  )
+  {
+
+    if (committer != null) {
+      throw new ISE("There should be no committer for batch ingestion");
+    }
+
+    if (useUniquePath) {
+      throw new ISE("Batch ingestion does not require uniquePath");
+    }
+
+
+    // Any sinks not persisted so far need to be persisted before push:
+    persistAllAndRemoveSinks();
+
+    log.info("Preparing to push...");
+
+    // get the dirs for the identfiers:
+    List<File> identifiersDirs = new ArrayList<>();
+    for (SegmentIdWithShardSpec identifier : identifiers) {
+      SinkMetadata sm = sinksMetadata.get(identifier);
+      if (sm == null) {
+        throw new ISE("No sink has been processed for identifier[%s]", identifier);
+      }
+      File persistedDir = sm.getPersistedFileDir();
+      if (persistedDir == null) {
+        throw new ISE("Sink for identifier[%s] not found in local file system", identifier);
+      }
+      identifiersDirs.add(persistedDir);
+    }
+
+    // push all sinks for identifiers:
+    final List<DataSegment> dataSegments = new ArrayList<>();
+    for (File identifier : identifiersDirs) {
+
+      // retrieve sink from disk:
+      Pair<SegmentIdWithShardSpec, Sink> identifiersAndSinks;
+      try {
+        identifiersAndSinks = getIdentifierAndSinkForPersistedFile(identifier);
+      }
+      catch (IOException e) {
+        throw new ISE(e, "Failed to retrieve sinks for identifier[%s]", identifier);
+      }
+
+      // push it:
+      final DataSegment dataSegment = mergeAndPush(
+          identifiersAndSinks.lhs,
+          identifiersAndSinks.rhs,
+          false
+      );
+
+      // record it:
+      if (dataSegment != null) {
+        dataSegments.add(dataSegment);
+      } else {
+        log.warn("mergeAndPush[%s] returned null, skipping.", identifiersAndSinks.lhs);
+      }
+
+    }
+    log.info("Push complete...");
+
+    return Futures.immediateFuture(new SegmentsAndCommitMetadata(dataSegments, null));
+  }
+
+  /**
+   * Merge segment, push to deep storage. Should only be used on segments that have been fully persisted.
+   *
+   * @param identifier    sink identifier
+   * @param sink          sink to push
+   * @param useUniquePath true if the segment should be written to a path with a unique identifier
+   * @return segment descriptor, or null if the sink is no longer valid
+   */
+  @Nullable
+  private DataSegment mergeAndPush(
+      final SegmentIdWithShardSpec identifier,
+      final Sink sink,
+      final boolean useUniquePath
+  )
+  {
+
+
+    // Use a descriptor file to indicate that pushing has completed.
+    final File persistDir = computePersistDir(identifier);
+    final File mergedTarget = new File(persistDir, "merged");
+    final File descriptorFile = computeDescriptorFile(identifier);
+
+    // Sanity checks
+    if (sink.isWritable()) {
+      throw new ISE("Expected sink to be no longer writable before mergeAndPush for segment[%s].", identifier);
+    }
+
+    int numHydrants = 0;
+    for (FireHydrant hydrant : sink) {
+      synchronized (hydrant) {

Review comment:
       No need for synchronization.

##########
File path: server/src/main/java/org/apache/druid/segment/realtime/appenderator/BatchAppenderator.java
##########
@@ -0,0 +1,1149 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.segment.realtime.appenderator;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Stopwatch;
+import com.google.common.base.Supplier;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.common.primitives.Ints;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import org.apache.druid.data.input.Committer;
+import org.apache.druid.data.input.InputRow;
+import org.apache.druid.java.util.common.FileUtils;
+import org.apache.druid.java.util.common.IAE;
+import org.apache.druid.java.util.common.ISE;
+import org.apache.druid.java.util.common.Pair;
+import org.apache.druid.java.util.common.RetryUtils;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.common.io.Closer;
+import org.apache.druid.java.util.emitter.EmittingLogger;
+import org.apache.druid.query.Query;
+import org.apache.druid.query.QueryRunner;
+import org.apache.druid.query.SegmentDescriptor;
+import org.apache.druid.segment.IndexIO;
+import org.apache.druid.segment.IndexMerger;
+import org.apache.druid.segment.QueryableIndex;
+import org.apache.druid.segment.QueryableIndexSegment;
+import org.apache.druid.segment.ReferenceCountingSegment;
+import org.apache.druid.segment.incremental.IncrementalIndexAddResult;
+import org.apache.druid.segment.incremental.IndexSizeExceededException;
+import org.apache.druid.segment.incremental.ParseExceptionHandler;
+import org.apache.druid.segment.incremental.RowIngestionMeters;
+import org.apache.druid.segment.indexing.DataSchema;
+import org.apache.druid.segment.loading.DataSegmentPusher;
+import org.apache.druid.segment.realtime.FireDepartmentMetrics;
+import org.apache.druid.segment.realtime.FireHydrant;
+import org.apache.druid.segment.realtime.plumber.Sink;
+import org.apache.druid.timeline.DataSegment;
+import org.joda.time.Interval;
+
+import javax.annotation.Nullable;
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
+import java.nio.file.StandardOpenOption;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.Collectors;
+
+public class BatchAppenderator implements Appenderator
+{
+  public static final int ROUGH_OVERHEAD_PER_SINK = 5000;
+  // Rough estimate of memory footprint of empty FireHydrant based on actual heap dumps
+  public static final int ROUGH_OVERHEAD_PER_HYDRANT = 1000;
+
+  private static final EmittingLogger log = new EmittingLogger(BatchAppenderator.class);
+  private static final String IDENTIFIER_FILE_NAME = "identifier.json";
+
+  private final String myId;
+  private final DataSchema schema;
+  private final AppenderatorConfig tuningConfig;
+  private final FireDepartmentMetrics metrics;
+  private final DataSegmentPusher dataSegmentPusher;
+  private final ObjectMapper objectMapper;
+  private final IndexIO indexIO;
+  private final IndexMerger indexMerger;
+  /**
+   * This map needs to be concurrent because it's accessed and mutated from multiple threads from where
+   * this Appenderator is used (and methods like {@link #add(SegmentIdWithShardSpec, InputRow, Supplier, boolean)} are
+   * called). It could also be accessed (but not mutated) potentially in the context
+   * of any thread from {@link #drop}.
+   */
+  private final ConcurrentMap<SegmentIdWithShardSpec, Sink> sinks = new ConcurrentHashMap<>();
+  private final long maxBytesTuningConfig;
+  private final boolean skipBytesInMemoryOverheadCheck;
+
+  /**
+   * The following sinks metadata map and associated class are the way to retain metadata now that sinks
+   * are being completely removed from memory after each incremental persist.
+   */
+  private final ConcurrentHashMap<SegmentIdWithShardSpec, SinkMetadata> sinksMetadata = new ConcurrentHashMap<>();
+
+  /**
+   * This class is used for information that needs to be kept related to Sinks as
+   * they are persisted and removed from memory at every incremental persist.
+   * The information is used for sanity checks and as information required
+   * for functionality, depending in the field that is used. More info about the
+   * fields is annotated as comments in the class
+   */
+  private static class SinkMetadata
+  {
+    /** This is used to maintain the rows in the sink accross persists of the sink
+    * used for functionality (i.e. to detect whether an incremental push
+    * is needed {@link AppenderatorDriverAddResult#isPushRequired(Integer, Long)}
+    **/
+    private int numRowsInSegment;
+    /** For sanity check as well as functionality: to make sure that all hydrants for a sink are restored from disk at
+     * push time and also to remember the fire hydrant "count" when persisting it.
+     */
+    private int numHydrants;
+    /* Reference to directory that holds the persisted data */
+    File persistedFileDir;
+
+    public SinkMetadata()
+    {
+      this(0, 0);
+    }
+
+    public SinkMetadata(int numRowsInSegment, int numHydrants)
+    {
+      this.numRowsInSegment = numRowsInSegment;
+      this.numHydrants = numHydrants;
+    }
+
+    public void addRows(int num)
+    {
+      numRowsInSegment += num;
+    }
+
+    public void addHydrants(int num)
+    {
+      numHydrants += num;
+    }
+
+    public int getNumRowsInSegment()
+    {
+      return numRowsInSegment;
+    }
+
+    public int getNumHydrants()
+    {
+      return numHydrants;
+    }
+
+    public void setPersistedFileDir(File persistedFileDir)
+    {
+      this.persistedFileDir = persistedFileDir;
+    }
+
+    public File getPersistedFileDir()
+    {
+      return persistedFileDir;
+    }
+
+  }
+
+  // This variable updated in add(), persist(), and drop()
+  private final AtomicInteger rowsCurrentlyInMemory = new AtomicInteger();
+  private final AtomicInteger totalRows = new AtomicInteger();
+  private final AtomicLong bytesCurrentlyInMemory = new AtomicLong();
+  private final RowIngestionMeters rowIngestionMeters;
+  private final ParseExceptionHandler parseExceptionHandler;
+
+  private final AtomicBoolean closed = new AtomicBoolean(false);
+
+  private volatile FileLock basePersistDirLock = null;
+  private volatile FileChannel basePersistDirLockChannel = null;
+
+  BatchAppenderator(
+      String id,
+      DataSchema schema,
+      AppenderatorConfig tuningConfig,
+      FireDepartmentMetrics metrics,
+      DataSegmentPusher dataSegmentPusher,
+      ObjectMapper objectMapper,
+      IndexIO indexIO,
+      IndexMerger indexMerger,
+      RowIngestionMeters rowIngestionMeters,
+      ParseExceptionHandler parseExceptionHandler
+  )
+  {
+    this.myId = id;
+    this.schema = Preconditions.checkNotNull(schema, "schema");
+    this.tuningConfig = Preconditions.checkNotNull(tuningConfig, "tuningConfig");
+    this.metrics = Preconditions.checkNotNull(metrics, "metrics");
+    this.dataSegmentPusher = Preconditions.checkNotNull(dataSegmentPusher, "dataSegmentPusher");
+    this.objectMapper = Preconditions.checkNotNull(objectMapper, "objectMapper");
+    this.indexIO = Preconditions.checkNotNull(indexIO, "indexIO");
+    this.indexMerger = Preconditions.checkNotNull(indexMerger, "indexMerger");
+    this.rowIngestionMeters = Preconditions.checkNotNull(rowIngestionMeters, "rowIngestionMeters");
+    this.parseExceptionHandler = Preconditions.checkNotNull(parseExceptionHandler, "parseExceptionHandler");
+
+    maxBytesTuningConfig = tuningConfig.getMaxBytesInMemoryOrDefault();
+    skipBytesInMemoryOverheadCheck = tuningConfig.isSkipBytesInMemoryOverheadCheck();
+  }
+
+  @Override
+  public String getId()
+  {
+    return myId;
+  }
+
+  @Override
+  public String getDataSource()
+  {
+    return schema.getDataSource();
+  }
+
+  @Override
+  public Object startJob()
+  {
+    tuningConfig.getBasePersistDirectory().mkdirs();
+    lockBasePersistDirectory();
+    return null;
+  }
+
+  @Override
+  public AppenderatorAddResult add(
+      final SegmentIdWithShardSpec identifier,
+      final InputRow row,
+      @Nullable final Supplier<Committer> committerSupplier,
+      final boolean allowIncrementalPersists
+  ) throws IndexSizeExceededException, SegmentNotWritableException
+  {
+
+    Preconditions.checkArgument(
+        committerSupplier == null,
+        "Batch appenderator does not need a committer!"
+    );
+
+    Preconditions.checkArgument(
+        allowIncrementalPersists,
+        "Batch appenderator should always allow incremental persists!"
+    );
+
+    if (!identifier.getDataSource().equals(schema.getDataSource())) {
+      throw new IAE(
+          "Expected dataSource[%s] but was asked to insert row for dataSource[%s]?!",
+          schema.getDataSource(),
+          identifier.getDataSource()
+      );
+    }
+
+    final Sink sink = getOrCreateSink(identifier);
+    metrics.reportMessageMaxTimestamp(row.getTimestampFromEpoch());
+    final int sinkRowsInMemoryBeforeAdd = sink.getNumRowsInMemory();
+    final int sinkRowsInMemoryAfterAdd;
+    final long bytesInMemoryBeforeAdd = sink.getBytesInMemory();
+    final long bytesInMemoryAfterAdd;
+    final IncrementalIndexAddResult addResult;
+
+    try {
+      addResult = sink.add(row, false); // allow incrememtal persis is always true for batch
+      sinkRowsInMemoryAfterAdd = addResult.getRowCount();
+      bytesInMemoryAfterAdd = addResult.getBytesInMemory();
+    }
+    catch (IndexSizeExceededException e) {
+      // 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 (sinkRowsInMemoryAfterAdd < 0) {
+      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
+    }
+
+    if (addResult.isRowAdded()) {
+      rowIngestionMeters.incrementProcessed();
+    } else if (addResult.hasParseException()) {
+      parseExceptionHandler.handle(addResult.getParseException());
+    }
+
+    final int numAddedRows = sinkRowsInMemoryAfterAdd - sinkRowsInMemoryBeforeAdd;
+    rowsCurrentlyInMemory.addAndGet(numAddedRows);
+    bytesCurrentlyInMemory.addAndGet(bytesInMemoryAfterAdd - bytesInMemoryBeforeAdd);
+    totalRows.addAndGet(numAddedRows);
+    sinksMetadata.computeIfAbsent(identifier, unused -> new SinkMetadata()).addRows(numAddedRows);
+
+    boolean persist = false;
+    List<String> persistReasons = new ArrayList<>();
+
+    if (!sink.canAppendRow()) {
+      persist = true;
+      persistReasons.add("No more rows can be appended to sink");
+    }
+    if (rowsCurrentlyInMemory.get() >= tuningConfig.getMaxRowsInMemory()) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "rowsCurrentlyInMemory[%d] is greater than maxRowsInMemory[%d]",
+          rowsCurrentlyInMemory.get(),
+          tuningConfig.getMaxRowsInMemory()
+      ));
+    }
+    if (bytesCurrentlyInMemory.get() >= maxBytesTuningConfig) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "bytesCurrentlyInMemory[%d] is greater than maxBytesInMemory[%d]",
+          bytesCurrentlyInMemory.get(),
+          maxBytesTuningConfig
+      ));
+    }
+    if (persist) {
+      // persistAll clears rowsCurrentlyInMemory, no need to update it.
+      log.info("Incremental persist to disk because %s.", String.join(",", persistReasons));
+
+      long bytesToBePersisted = 0L;
+      for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+        final Sink sinkEntry = entry.getValue();
+        if (sinkEntry != null) {
+          bytesToBePersisted += sinkEntry.getBytesInMemory();
+          if (sinkEntry.swappable()) {
+            // Code for batch no longer memory maps hydrants but they still take memory...
+            int memoryStillInUse = calculateMemoryUsedByHydrant();
+            bytesCurrentlyInMemory.addAndGet(memoryStillInUse);
+          }
+        }
+      }
+
+      if (!skipBytesInMemoryOverheadCheck
+          && bytesCurrentlyInMemory.get() - bytesToBePersisted > maxBytesTuningConfig) {
+        // We are still over maxBytesTuningConfig even after persisting.
+        // This means that we ran out of all available memory to ingest (due to overheads created as part of ingestion)
+        final String alertMessage = StringUtils.format(
+            "Task has exceeded safe estimated heap usage limits, failing "
+            + "(numSinks: [%d] numHydrantsAcrossAllSinks: [%d] totalRows: [%d])"
+            + "(bytesCurrentlyInMemory: [%d] - bytesToBePersisted: [%d] > maxBytesTuningConfig: [%d])",
+            sinks.size(),
+            sinks.values().stream().mapToInt(Iterables::size).sum(),
+            getTotalRowCount(),
+            bytesCurrentlyInMemory.get(),
+            bytesToBePersisted,
+            maxBytesTuningConfig
+        );
+        final String errorMessage = StringUtils.format(
+            "%s.\nThis can occur when the overhead from too many intermediary segment persists becomes to "
+            + "great to have enough space to process additional input rows. This check, along with metering the overhead "
+            + "of these objects to factor into the 'maxBytesInMemory' computation, can be disabled by setting "
+            + "'skipBytesInMemoryOverheadCheck' to 'true' (note that doing so might allow the task to naturally encounter "
+            + "a 'java.lang.OutOfMemoryError'). Alternatively, 'maxBytesInMemory' can be increased which will cause an "
+            + "increase in heap footprint, but will allow for more intermediary segment persists to occur before "
+            + "reaching this condition.",
+            alertMessage
+        );
+        log.makeAlert(alertMessage)
+           .addData("dataSource", schema.getDataSource())
+           .emit();
+        throw new RuntimeException(errorMessage);
+      }
+
+      persistAllAndRemoveSinks();
+
+    }
+    return new AppenderatorAddResult(identifier, sinksMetadata.get(identifier).numRowsInSegment, false);
+  }
+
+  @Override
+  /**
+   * Returns all active segments regardless whether they are in memory or persisted
+   */
+  public List<SegmentIdWithShardSpec> getSegments()
+  {
+    return ImmutableList.copyOf(sinksMetadata.keySet());
+  }
+
+  @VisibleForTesting
+  public List<SegmentIdWithShardSpec> getInMemorySegments()
+  {
+    return ImmutableList.copyOf(sinks.keySet());
+  }
+
+  @Override
+  public int getRowCount(final SegmentIdWithShardSpec identifier)
+  {
+    return sinksMetadata.get(identifier).getNumRowsInSegment();
+  }
+
+  @Override
+  public int getTotalRowCount()
+  {
+    return totalRows.get();
+  }
+
+  @VisibleForTesting
+  public int getRowsInMemory()
+  {
+    return rowsCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesCurrentlyInMemory()
+  {
+    return bytesCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesInMemory(SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+
+    if (sink == null) {
+      return 0L; // sinks are removed after a persist
+    } else {
+      return sink.getBytesInMemory();
+    }
+  }
+
+  private Sink getOrCreateSink(final SegmentIdWithShardSpec identifier)
+  {
+    Sink retVal = sinks.get(identifier);
+
+    if (retVal == null) {
+      retVal = new Sink(
+          identifier.getInterval(),
+          schema,
+          identifier.getShardSpec(),
+          identifier.getVersion(),
+          tuningConfig.getAppendableIndexSpec(),
+          tuningConfig.getMaxRowsInMemory(),
+          maxBytesTuningConfig,
+          null
+      );
+      bytesCurrentlyInMemory.addAndGet(calculateSinkMemoryInUsed());
+
+      sinks.put(identifier, retVal);
+      metrics.setSinkCount(sinks.size());
+    }
+
+    return retVal;
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForIntervals(final Query<T> query, final Iterable<Interval> intervals)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForSegments(final Query<T> query, final Iterable<SegmentDescriptor> specs)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public void clear()
+  {
+    clear(true);
+  }
+
+  private void clear(boolean removeOnDiskData)
+  {
+    // Drop commit metadata, then abandon all segments.
+    log.info("Clearing all sinks & hydrants, removing data on disk: [%s]", removeOnDiskData);
+    // Drop everything.
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      removeSink(entry.getKey(), entry.getValue(), removeOnDiskData);
+    }
+  }
+
+  @Override
+  public ListenableFuture<?> drop(final SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+    SinkMetadata sm = sinksMetadata.remove(identifier);
+    if (sm != null) {
+      int originalTotalRows = getTotalRowCount();
+      int rowsToDrop = sm.getNumRowsInSegment();
+      int totalRowsAfter = originalTotalRows - rowsToDrop;
+      if (totalRowsAfter < 0) {
+        log.warn("Total rows[%d] after dropping segment[%s] rows [%d]", totalRowsAfter, identifier, rowsToDrop);
+      }
+      totalRows.set(Math.max(totalRowsAfter, 0));
+    }
+    if (sink != null) {
+      removeSink(identifier, sink, true);
+    }
+    return Futures.immediateFuture(null);
+  }
+
+  @Override
+  public ListenableFuture<Object> persistAll(@Nullable final Committer committer)
+  {
+    if (committer != null) {
+      throw new ISE("committer must be null for BatchAppenderator");
+    }
+    persistAllAndRemoveSinks();
+    return Futures.immediateFuture(null);
+  }
+
+  /**
+   * Persist all sinks & their hydrants, keep their metadata, and then remove them completely from
+   * memory (to be resurrected right before merge & push)
+   */
+  private void persistAllAndRemoveSinks()
+  {
+
+    final List<Pair<FireHydrant, SegmentIdWithShardSpec>> indexesToPersist = new ArrayList<>();
+    int numPersistedRows = 0;
+    long bytesPersisted = 0L;
+    int totalHydrantsCount = 0;
+    final long totalSinks = sinks.size();
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      final SegmentIdWithShardSpec identifier = entry.getKey();
+      final Sink sink = entry.getValue();
+      if (sink == null) {
+        throw new ISE("No sink for identifier: %s", identifier);
+      }
+
+      final List<FireHydrant> hydrants = Lists.newArrayList(sink);
+      // Since everytime we persist we also get rid of the in-memory references to sinks & hydrants
+      // the invariant of exactly one, always swappable, sink with exactly one unpersisted hydrant must hold
+      int totalHydrantsForSink = hydrants.size();
+      if (totalHydrantsForSink != 1) {
+        throw new ISE("There should be only onw hydrant for identifier[%s] but there are[%s]",
+                      identifier, totalHydrantsForSink
+        );
+      }
+      totalHydrantsCount += 1;
+      numPersistedRows += sink.getNumRowsInMemory();
+      bytesPersisted += sink.getBytesInMemory();
+
+      if (!sink.swappable()) {
+        throw new ISE("Sink is not swappable![%s]", identifier);
+      }
+      indexesToPersist.add(Pair.of(sink.swap(), identifier));
+
+    }
+
+    if (indexesToPersist.isEmpty()) {
+      log.info("No indexes will be peristed");
+    }
+    final Stopwatch persistStopwatch = Stopwatch.createStarted();
+    try {
+      for (Pair<FireHydrant, SegmentIdWithShardSpec> pair : indexesToPersist) {
+        metrics.incrementRowOutputCount(persistHydrant(pair.lhs, pair.rhs));
+      }
+
+      log.info(
+          "Persisted in-memory data for segments: %s",
+          indexesToPersist.stream()
+                          .filter(itp -> itp.rhs != null)
+                          .map(itp -> itp.rhs.asSegmentId().toString())
+                          .distinct()
+                          .collect(Collectors.joining(", "))
+      );
+      log.info(
+          "Persisted stats: processed rows: [%d], persisted rows[%d], persisted sinks: [%d], persisted fireHydrants (across sinks): [%d]",
+          rowIngestionMeters.getProcessed(),
+          numPersistedRows,
+          totalSinks,
+          totalHydrantsCount
+      );
+
+    }
+    catch (Exception e) {
+      metrics.incrementFailedPersists();
+      throw e;
+    }
+    finally {
+      metrics.incrementNumPersists();
+      metrics.incrementPersistTimeMillis(persistStopwatch.elapsed(TimeUnit.MILLISECONDS));
+      persistStopwatch.stop();
+    }
+
+    // NB: The rows are still in memory until they're done persisting, but we only count rows in active indexes.
+    rowsCurrentlyInMemory.addAndGet(-numPersistedRows);
+    bytesCurrentlyInMemory.addAndGet(-bytesPersisted);
+
+    // remove all sinks after persisting:
+    clear(false);
+
+    log.info("Persisted rows[%,d] and bytes[%,d] and removed all sinks & hydrants from memory",
+             numPersistedRows, bytesPersisted);
+
+  }
+
+  @Override
+  public ListenableFuture<SegmentsAndCommitMetadata> push(
+      final Collection<SegmentIdWithShardSpec> identifiers,
+      @Nullable final Committer committer,
+      final boolean useUniquePath
+  )
+  {
+
+    if (committer != null) {
+      throw new ISE("There should be no committer for batch ingestion");
+    }
+
+    if (useUniquePath) {
+      throw new ISE("Batch ingestion does not require uniquePath");
+    }
+
+
+    // Any sinks not persisted so far need to be persisted before push:
+    persistAllAndRemoveSinks();
+
+    log.info("Preparing to push...");
+
+    // get the dirs for the identfiers:
+    List<File> identifiersDirs = new ArrayList<>();
+    for (SegmentIdWithShardSpec identifier : identifiers) {
+      SinkMetadata sm = sinksMetadata.get(identifier);
+      if (sm == null) {
+        throw new ISE("No sink has been processed for identifier[%s]", identifier);
+      }
+      File persistedDir = sm.getPersistedFileDir();
+      if (persistedDir == null) {
+        throw new ISE("Sink for identifier[%s] not found in local file system", identifier);
+      }
+      identifiersDirs.add(persistedDir);
+    }
+
+    // push all sinks for identifiers:
+    final List<DataSegment> dataSegments = new ArrayList<>();
+    for (File identifier : identifiersDirs) {
+
+      // retrieve sink from disk:
+      Pair<SegmentIdWithShardSpec, Sink> identifiersAndSinks;
+      try {
+        identifiersAndSinks = getIdentifierAndSinkForPersistedFile(identifier);
+      }
+      catch (IOException e) {
+        throw new ISE(e, "Failed to retrieve sinks for identifier[%s]", identifier);
+      }
+
+      // push it:
+      final DataSegment dataSegment = mergeAndPush(
+          identifiersAndSinks.lhs,
+          identifiersAndSinks.rhs,
+          false
+      );
+
+      // record it:
+      if (dataSegment != null) {
+        dataSegments.add(dataSegment);
+      } else {
+        log.warn("mergeAndPush[%s] returned null, skipping.", identifiersAndSinks.lhs);
+      }
+
+    }
+    log.info("Push complete...");
+
+    return Futures.immediateFuture(new SegmentsAndCommitMetadata(dataSegments, null));
+  }
+
+  /**
+   * Merge segment, push to deep storage. Should only be used on segments that have been fully persisted.
+   *
+   * @param identifier    sink identifier
+   * @param sink          sink to push
+   * @param useUniquePath true if the segment should be written to a path with a unique identifier
+   * @return segment descriptor, or null if the sink is no longer valid
+   */
+  @Nullable
+  private DataSegment mergeAndPush(
+      final SegmentIdWithShardSpec identifier,
+      final Sink sink,
+      final boolean useUniquePath

Review comment:
       `useUniquePath` is always false so you can remove it.

##########
File path: server/src/main/java/org/apache/druid/segment/realtime/appenderator/BatchAppenderator.java
##########
@@ -0,0 +1,1149 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.segment.realtime.appenderator;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Stopwatch;
+import com.google.common.base.Supplier;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.common.primitives.Ints;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import org.apache.druid.data.input.Committer;
+import org.apache.druid.data.input.InputRow;
+import org.apache.druid.java.util.common.FileUtils;
+import org.apache.druid.java.util.common.IAE;
+import org.apache.druid.java.util.common.ISE;
+import org.apache.druid.java.util.common.Pair;
+import org.apache.druid.java.util.common.RetryUtils;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.common.io.Closer;
+import org.apache.druid.java.util.emitter.EmittingLogger;
+import org.apache.druid.query.Query;
+import org.apache.druid.query.QueryRunner;
+import org.apache.druid.query.SegmentDescriptor;
+import org.apache.druid.segment.IndexIO;
+import org.apache.druid.segment.IndexMerger;
+import org.apache.druid.segment.QueryableIndex;
+import org.apache.druid.segment.QueryableIndexSegment;
+import org.apache.druid.segment.ReferenceCountingSegment;
+import org.apache.druid.segment.incremental.IncrementalIndexAddResult;
+import org.apache.druid.segment.incremental.IndexSizeExceededException;
+import org.apache.druid.segment.incremental.ParseExceptionHandler;
+import org.apache.druid.segment.incremental.RowIngestionMeters;
+import org.apache.druid.segment.indexing.DataSchema;
+import org.apache.druid.segment.loading.DataSegmentPusher;
+import org.apache.druid.segment.realtime.FireDepartmentMetrics;
+import org.apache.druid.segment.realtime.FireHydrant;
+import org.apache.druid.segment.realtime.plumber.Sink;
+import org.apache.druid.timeline.DataSegment;
+import org.joda.time.Interval;
+
+import javax.annotation.Nullable;
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
+import java.nio.file.StandardOpenOption;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.Collectors;
+
+public class BatchAppenderator implements Appenderator
+{
+  public static final int ROUGH_OVERHEAD_PER_SINK = 5000;
+  // Rough estimate of memory footprint of empty FireHydrant based on actual heap dumps
+  public static final int ROUGH_OVERHEAD_PER_HYDRANT = 1000;
+
+  private static final EmittingLogger log = new EmittingLogger(BatchAppenderator.class);
+  private static final String IDENTIFIER_FILE_NAME = "identifier.json";
+
+  private final String myId;
+  private final DataSchema schema;
+  private final AppenderatorConfig tuningConfig;
+  private final FireDepartmentMetrics metrics;
+  private final DataSegmentPusher dataSegmentPusher;
+  private final ObjectMapper objectMapper;
+  private final IndexIO indexIO;
+  private final IndexMerger indexMerger;
+  /**
+   * This map needs to be concurrent because it's accessed and mutated from multiple threads from where
+   * this Appenderator is used (and methods like {@link #add(SegmentIdWithShardSpec, InputRow, Supplier, boolean)} are
+   * called). It could also be accessed (but not mutated) potentially in the context
+   * of any thread from {@link #drop}.
+   */
+  private final ConcurrentMap<SegmentIdWithShardSpec, Sink> sinks = new ConcurrentHashMap<>();
+  private final long maxBytesTuningConfig;
+  private final boolean skipBytesInMemoryOverheadCheck;
+
+  /**
+   * The following sinks metadata map and associated class are the way to retain metadata now that sinks
+   * are being completely removed from memory after each incremental persist.
+   */
+  private final ConcurrentHashMap<SegmentIdWithShardSpec, SinkMetadata> sinksMetadata = new ConcurrentHashMap<>();
+
+  /**
+   * This class is used for information that needs to be kept related to Sinks as
+   * they are persisted and removed from memory at every incremental persist.
+   * The information is used for sanity checks and as information required
+   * for functionality, depending in the field that is used. More info about the
+   * fields is annotated as comments in the class
+   */
+  private static class SinkMetadata
+  {
+    /** This is used to maintain the rows in the sink accross persists of the sink
+    * used for functionality (i.e. to detect whether an incremental push
+    * is needed {@link AppenderatorDriverAddResult#isPushRequired(Integer, Long)}
+    **/
+    private int numRowsInSegment;
+    /** For sanity check as well as functionality: to make sure that all hydrants for a sink are restored from disk at
+     * push time and also to remember the fire hydrant "count" when persisting it.
+     */
+    private int numHydrants;
+    /* Reference to directory that holds the persisted data */
+    File persistedFileDir;
+
+    public SinkMetadata()
+    {
+      this(0, 0);
+    }
+
+    public SinkMetadata(int numRowsInSegment, int numHydrants)
+    {
+      this.numRowsInSegment = numRowsInSegment;
+      this.numHydrants = numHydrants;
+    }
+
+    public void addRows(int num)
+    {
+      numRowsInSegment += num;
+    }
+
+    public void addHydrants(int num)
+    {
+      numHydrants += num;
+    }
+
+    public int getNumRowsInSegment()
+    {
+      return numRowsInSegment;
+    }
+
+    public int getNumHydrants()
+    {
+      return numHydrants;
+    }
+
+    public void setPersistedFileDir(File persistedFileDir)
+    {
+      this.persistedFileDir = persistedFileDir;
+    }
+
+    public File getPersistedFileDir()
+    {
+      return persistedFileDir;
+    }
+
+  }
+
+  // This variable updated in add(), persist(), and drop()
+  private final AtomicInteger rowsCurrentlyInMemory = new AtomicInteger();
+  private final AtomicInteger totalRows = new AtomicInteger();
+  private final AtomicLong bytesCurrentlyInMemory = new AtomicLong();
+  private final RowIngestionMeters rowIngestionMeters;
+  private final ParseExceptionHandler parseExceptionHandler;
+
+  private final AtomicBoolean closed = new AtomicBoolean(false);
+
+  private volatile FileLock basePersistDirLock = null;
+  private volatile FileChannel basePersistDirLockChannel = null;
+
+  BatchAppenderator(
+      String id,
+      DataSchema schema,
+      AppenderatorConfig tuningConfig,
+      FireDepartmentMetrics metrics,
+      DataSegmentPusher dataSegmentPusher,
+      ObjectMapper objectMapper,
+      IndexIO indexIO,
+      IndexMerger indexMerger,
+      RowIngestionMeters rowIngestionMeters,
+      ParseExceptionHandler parseExceptionHandler
+  )
+  {
+    this.myId = id;
+    this.schema = Preconditions.checkNotNull(schema, "schema");
+    this.tuningConfig = Preconditions.checkNotNull(tuningConfig, "tuningConfig");
+    this.metrics = Preconditions.checkNotNull(metrics, "metrics");
+    this.dataSegmentPusher = Preconditions.checkNotNull(dataSegmentPusher, "dataSegmentPusher");
+    this.objectMapper = Preconditions.checkNotNull(objectMapper, "objectMapper");
+    this.indexIO = Preconditions.checkNotNull(indexIO, "indexIO");
+    this.indexMerger = Preconditions.checkNotNull(indexMerger, "indexMerger");
+    this.rowIngestionMeters = Preconditions.checkNotNull(rowIngestionMeters, "rowIngestionMeters");
+    this.parseExceptionHandler = Preconditions.checkNotNull(parseExceptionHandler, "parseExceptionHandler");
+
+    maxBytesTuningConfig = tuningConfig.getMaxBytesInMemoryOrDefault();
+    skipBytesInMemoryOverheadCheck = tuningConfig.isSkipBytesInMemoryOverheadCheck();
+  }
+
+  @Override
+  public String getId()
+  {
+    return myId;
+  }
+
+  @Override
+  public String getDataSource()
+  {
+    return schema.getDataSource();
+  }
+
+  @Override
+  public Object startJob()
+  {
+    tuningConfig.getBasePersistDirectory().mkdirs();
+    lockBasePersistDirectory();
+    return null;
+  }
+
+  @Override
+  public AppenderatorAddResult add(
+      final SegmentIdWithShardSpec identifier,
+      final InputRow row,
+      @Nullable final Supplier<Committer> committerSupplier,
+      final boolean allowIncrementalPersists
+  ) throws IndexSizeExceededException, SegmentNotWritableException
+  {
+
+    Preconditions.checkArgument(
+        committerSupplier == null,
+        "Batch appenderator does not need a committer!"
+    );
+
+    Preconditions.checkArgument(
+        allowIncrementalPersists,
+        "Batch appenderator should always allow incremental persists!"
+    );
+
+    if (!identifier.getDataSource().equals(schema.getDataSource())) {
+      throw new IAE(
+          "Expected dataSource[%s] but was asked to insert row for dataSource[%s]?!",
+          schema.getDataSource(),
+          identifier.getDataSource()
+      );
+    }
+
+    final Sink sink = getOrCreateSink(identifier);
+    metrics.reportMessageMaxTimestamp(row.getTimestampFromEpoch());
+    final int sinkRowsInMemoryBeforeAdd = sink.getNumRowsInMemory();
+    final int sinkRowsInMemoryAfterAdd;
+    final long bytesInMemoryBeforeAdd = sink.getBytesInMemory();
+    final long bytesInMemoryAfterAdd;
+    final IncrementalIndexAddResult addResult;
+
+    try {
+      addResult = sink.add(row, false); // allow incrememtal persis is always true for batch
+      sinkRowsInMemoryAfterAdd = addResult.getRowCount();
+      bytesInMemoryAfterAdd = addResult.getBytesInMemory();
+    }
+    catch (IndexSizeExceededException e) {
+      // 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 (sinkRowsInMemoryAfterAdd < 0) {
+      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
+    }
+
+    if (addResult.isRowAdded()) {
+      rowIngestionMeters.incrementProcessed();
+    } else if (addResult.hasParseException()) {
+      parseExceptionHandler.handle(addResult.getParseException());
+    }
+
+    final int numAddedRows = sinkRowsInMemoryAfterAdd - sinkRowsInMemoryBeforeAdd;
+    rowsCurrentlyInMemory.addAndGet(numAddedRows);
+    bytesCurrentlyInMemory.addAndGet(bytesInMemoryAfterAdd - bytesInMemoryBeforeAdd);
+    totalRows.addAndGet(numAddedRows);
+    sinksMetadata.computeIfAbsent(identifier, unused -> new SinkMetadata()).addRows(numAddedRows);
+
+    boolean persist = false;
+    List<String> persistReasons = new ArrayList<>();
+
+    if (!sink.canAppendRow()) {
+      persist = true;
+      persistReasons.add("No more rows can be appended to sink");
+    }
+    if (rowsCurrentlyInMemory.get() >= tuningConfig.getMaxRowsInMemory()) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "rowsCurrentlyInMemory[%d] is greater than maxRowsInMemory[%d]",
+          rowsCurrentlyInMemory.get(),
+          tuningConfig.getMaxRowsInMemory()
+      ));
+    }
+    if (bytesCurrentlyInMemory.get() >= maxBytesTuningConfig) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "bytesCurrentlyInMemory[%d] is greater than maxBytesInMemory[%d]",
+          bytesCurrentlyInMemory.get(),
+          maxBytesTuningConfig
+      ));
+    }
+    if (persist) {
+      // persistAll clears rowsCurrentlyInMemory, no need to update it.
+      log.info("Incremental persist to disk because %s.", String.join(",", persistReasons));
+
+      long bytesToBePersisted = 0L;
+      for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+        final Sink sinkEntry = entry.getValue();
+        if (sinkEntry != null) {
+          bytesToBePersisted += sinkEntry.getBytesInMemory();
+          if (sinkEntry.swappable()) {
+            // Code for batch no longer memory maps hydrants but they still take memory...
+            int memoryStillInUse = calculateMemoryUsedByHydrant();
+            bytesCurrentlyInMemory.addAndGet(memoryStillInUse);
+          }
+        }
+      }
+
+      if (!skipBytesInMemoryOverheadCheck
+          && bytesCurrentlyInMemory.get() - bytesToBePersisted > maxBytesTuningConfig) {
+        // We are still over maxBytesTuningConfig even after persisting.
+        // This means that we ran out of all available memory to ingest (due to overheads created as part of ingestion)
+        final String alertMessage = StringUtils.format(
+            "Task has exceeded safe estimated heap usage limits, failing "
+            + "(numSinks: [%d] numHydrantsAcrossAllSinks: [%d] totalRows: [%d])"
+            + "(bytesCurrentlyInMemory: [%d] - bytesToBePersisted: [%d] > maxBytesTuningConfig: [%d])",
+            sinks.size(),
+            sinks.values().stream().mapToInt(Iterables::size).sum(),
+            getTotalRowCount(),
+            bytesCurrentlyInMemory.get(),
+            bytesToBePersisted,
+            maxBytesTuningConfig
+        );
+        final String errorMessage = StringUtils.format(
+            "%s.\nThis can occur when the overhead from too many intermediary segment persists becomes to "
+            + "great to have enough space to process additional input rows. This check, along with metering the overhead "
+            + "of these objects to factor into the 'maxBytesInMemory' computation, can be disabled by setting "
+            + "'skipBytesInMemoryOverheadCheck' to 'true' (note that doing so might allow the task to naturally encounter "
+            + "a 'java.lang.OutOfMemoryError'). Alternatively, 'maxBytesInMemory' can be increased which will cause an "
+            + "increase in heap footprint, but will allow for more intermediary segment persists to occur before "
+            + "reaching this condition.",
+            alertMessage
+        );
+        log.makeAlert(alertMessage)
+           .addData("dataSource", schema.getDataSource())
+           .emit();
+        throw new RuntimeException(errorMessage);
+      }
+
+      persistAllAndRemoveSinks();
+
+    }
+    return new AppenderatorAddResult(identifier, sinksMetadata.get(identifier).numRowsInSegment, false);
+  }
+
+  @Override
+  /**
+   * Returns all active segments regardless whether they are in memory or persisted
+   */
+  public List<SegmentIdWithShardSpec> getSegments()
+  {
+    return ImmutableList.copyOf(sinksMetadata.keySet());
+  }
+
+  @VisibleForTesting
+  public List<SegmentIdWithShardSpec> getInMemorySegments()
+  {
+    return ImmutableList.copyOf(sinks.keySet());
+  }
+
+  @Override
+  public int getRowCount(final SegmentIdWithShardSpec identifier)
+  {
+    return sinksMetadata.get(identifier).getNumRowsInSegment();
+  }
+
+  @Override
+  public int getTotalRowCount()
+  {
+    return totalRows.get();
+  }
+
+  @VisibleForTesting
+  public int getRowsInMemory()
+  {
+    return rowsCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesCurrentlyInMemory()
+  {
+    return bytesCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesInMemory(SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+
+    if (sink == null) {
+      return 0L; // sinks are removed after a persist
+    } else {
+      return sink.getBytesInMemory();
+    }
+  }
+
+  private Sink getOrCreateSink(final SegmentIdWithShardSpec identifier)
+  {
+    Sink retVal = sinks.get(identifier);
+
+    if (retVal == null) {
+      retVal = new Sink(
+          identifier.getInterval(),
+          schema,
+          identifier.getShardSpec(),
+          identifier.getVersion(),
+          tuningConfig.getAppendableIndexSpec(),
+          tuningConfig.getMaxRowsInMemory(),
+          maxBytesTuningConfig,
+          null
+      );
+      bytesCurrentlyInMemory.addAndGet(calculateSinkMemoryInUsed());
+
+      sinks.put(identifier, retVal);
+      metrics.setSinkCount(sinks.size());
+    }
+
+    return retVal;
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForIntervals(final Query<T> query, final Iterable<Interval> intervals)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForSegments(final Query<T> query, final Iterable<SegmentDescriptor> specs)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public void clear()
+  {
+    clear(true);
+  }
+
+  private void clear(boolean removeOnDiskData)
+  {
+    // Drop commit metadata, then abandon all segments.
+    log.info("Clearing all sinks & hydrants, removing data on disk: [%s]", removeOnDiskData);
+    // Drop everything.
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      removeSink(entry.getKey(), entry.getValue(), removeOnDiskData);
+    }
+  }
+
+  @Override
+  public ListenableFuture<?> drop(final SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+    SinkMetadata sm = sinksMetadata.remove(identifier);
+    if (sm != null) {
+      int originalTotalRows = getTotalRowCount();
+      int rowsToDrop = sm.getNumRowsInSegment();
+      int totalRowsAfter = originalTotalRows - rowsToDrop;
+      if (totalRowsAfter < 0) {
+        log.warn("Total rows[%d] after dropping segment[%s] rows [%d]", totalRowsAfter, identifier, rowsToDrop);
+      }
+      totalRows.set(Math.max(totalRowsAfter, 0));
+    }
+    if (sink != null) {
+      removeSink(identifier, sink, true);
+    }
+    return Futures.immediateFuture(null);
+  }
+
+  @Override
+  public ListenableFuture<Object> persistAll(@Nullable final Committer committer)
+  {
+    if (committer != null) {
+      throw new ISE("committer must be null for BatchAppenderator");
+    }
+    persistAllAndRemoveSinks();
+    return Futures.immediateFuture(null);
+  }
+
+  /**
+   * Persist all sinks & their hydrants, keep their metadata, and then remove them completely from
+   * memory (to be resurrected right before merge & push)
+   */
+  private void persistAllAndRemoveSinks()
+  {
+
+    final List<Pair<FireHydrant, SegmentIdWithShardSpec>> indexesToPersist = new ArrayList<>();
+    int numPersistedRows = 0;
+    long bytesPersisted = 0L;
+    int totalHydrantsCount = 0;
+    final long totalSinks = sinks.size();
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      final SegmentIdWithShardSpec identifier = entry.getKey();
+      final Sink sink = entry.getValue();
+      if (sink == null) {
+        throw new ISE("No sink for identifier: %s", identifier);
+      }
+
+      final List<FireHydrant> hydrants = Lists.newArrayList(sink);
+      // Since everytime we persist we also get rid of the in-memory references to sinks & hydrants
+      // the invariant of exactly one, always swappable, sink with exactly one unpersisted hydrant must hold
+      int totalHydrantsForSink = hydrants.size();
+      if (totalHydrantsForSink != 1) {
+        throw new ISE("There should be only onw hydrant for identifier[%s] but there are[%s]",

Review comment:
       ```suggestion
           throw new ISE("There should be only one hydrant for identifier[%s] but there are[%s]",
   ```

##########
File path: server/src/main/java/org/apache/druid/segment/realtime/appenderator/BatchAppenderator.java
##########
@@ -0,0 +1,1149 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.segment.realtime.appenderator;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Stopwatch;
+import com.google.common.base.Supplier;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+import com.google.common.primitives.Ints;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import org.apache.druid.data.input.Committer;
+import org.apache.druid.data.input.InputRow;
+import org.apache.druid.java.util.common.FileUtils;
+import org.apache.druid.java.util.common.IAE;
+import org.apache.druid.java.util.common.ISE;
+import org.apache.druid.java.util.common.Pair;
+import org.apache.druid.java.util.common.RetryUtils;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.java.util.common.io.Closer;
+import org.apache.druid.java.util.emitter.EmittingLogger;
+import org.apache.druid.query.Query;
+import org.apache.druid.query.QueryRunner;
+import org.apache.druid.query.SegmentDescriptor;
+import org.apache.druid.segment.IndexIO;
+import org.apache.druid.segment.IndexMerger;
+import org.apache.druid.segment.QueryableIndex;
+import org.apache.druid.segment.QueryableIndexSegment;
+import org.apache.druid.segment.ReferenceCountingSegment;
+import org.apache.druid.segment.incremental.IncrementalIndexAddResult;
+import org.apache.druid.segment.incremental.IndexSizeExceededException;
+import org.apache.druid.segment.incremental.ParseExceptionHandler;
+import org.apache.druid.segment.incremental.RowIngestionMeters;
+import org.apache.druid.segment.indexing.DataSchema;
+import org.apache.druid.segment.loading.DataSegmentPusher;
+import org.apache.druid.segment.realtime.FireDepartmentMetrics;
+import org.apache.druid.segment.realtime.FireHydrant;
+import org.apache.druid.segment.realtime.plumber.Sink;
+import org.apache.druid.timeline.DataSegment;
+import org.joda.time.Interval;
+
+import javax.annotation.Nullable;
+import java.io.Closeable;
+import java.io.File;
+import java.io.IOException;
+import java.nio.channels.FileChannel;
+import java.nio.channels.FileLock;
+import java.nio.file.StandardOpenOption;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.Collectors;
+
+public class BatchAppenderator implements Appenderator
+{
+  public static final int ROUGH_OVERHEAD_PER_SINK = 5000;
+  // Rough estimate of memory footprint of empty FireHydrant based on actual heap dumps
+  public static final int ROUGH_OVERHEAD_PER_HYDRANT = 1000;
+
+  private static final EmittingLogger log = new EmittingLogger(BatchAppenderator.class);
+  private static final String IDENTIFIER_FILE_NAME = "identifier.json";
+
+  private final String myId;
+  private final DataSchema schema;
+  private final AppenderatorConfig tuningConfig;
+  private final FireDepartmentMetrics metrics;
+  private final DataSegmentPusher dataSegmentPusher;
+  private final ObjectMapper objectMapper;
+  private final IndexIO indexIO;
+  private final IndexMerger indexMerger;
+  /**
+   * This map needs to be concurrent because it's accessed and mutated from multiple threads from where
+   * this Appenderator is used (and methods like {@link #add(SegmentIdWithShardSpec, InputRow, Supplier, boolean)} are
+   * called). It could also be accessed (but not mutated) potentially in the context
+   * of any thread from {@link #drop}.
+   */
+  private final ConcurrentMap<SegmentIdWithShardSpec, Sink> sinks = new ConcurrentHashMap<>();
+  private final long maxBytesTuningConfig;
+  private final boolean skipBytesInMemoryOverheadCheck;
+
+  /**
+   * The following sinks metadata map and associated class are the way to retain metadata now that sinks
+   * are being completely removed from memory after each incremental persist.
+   */
+  private final ConcurrentHashMap<SegmentIdWithShardSpec, SinkMetadata> sinksMetadata = new ConcurrentHashMap<>();
+
+  /**
+   * This class is used for information that needs to be kept related to Sinks as
+   * they are persisted and removed from memory at every incremental persist.
+   * The information is used for sanity checks and as information required
+   * for functionality, depending in the field that is used. More info about the
+   * fields is annotated as comments in the class
+   */
+  private static class SinkMetadata
+  {
+    /** This is used to maintain the rows in the sink accross persists of the sink
+    * used for functionality (i.e. to detect whether an incremental push
+    * is needed {@link AppenderatorDriverAddResult#isPushRequired(Integer, Long)}
+    **/
+    private int numRowsInSegment;
+    /** For sanity check as well as functionality: to make sure that all hydrants for a sink are restored from disk at
+     * push time and also to remember the fire hydrant "count" when persisting it.
+     */
+    private int numHydrants;
+    /* Reference to directory that holds the persisted data */
+    File persistedFileDir;
+
+    public SinkMetadata()
+    {
+      this(0, 0);
+    }
+
+    public SinkMetadata(int numRowsInSegment, int numHydrants)
+    {
+      this.numRowsInSegment = numRowsInSegment;
+      this.numHydrants = numHydrants;
+    }
+
+    public void addRows(int num)
+    {
+      numRowsInSegment += num;
+    }
+
+    public void addHydrants(int num)
+    {
+      numHydrants += num;
+    }
+
+    public int getNumRowsInSegment()
+    {
+      return numRowsInSegment;
+    }
+
+    public int getNumHydrants()
+    {
+      return numHydrants;
+    }
+
+    public void setPersistedFileDir(File persistedFileDir)
+    {
+      this.persistedFileDir = persistedFileDir;
+    }
+
+    public File getPersistedFileDir()
+    {
+      return persistedFileDir;
+    }
+
+  }
+
+  // This variable updated in add(), persist(), and drop()
+  private final AtomicInteger rowsCurrentlyInMemory = new AtomicInteger();
+  private final AtomicInteger totalRows = new AtomicInteger();
+  private final AtomicLong bytesCurrentlyInMemory = new AtomicLong();
+  private final RowIngestionMeters rowIngestionMeters;
+  private final ParseExceptionHandler parseExceptionHandler;
+
+  private final AtomicBoolean closed = new AtomicBoolean(false);
+
+  private volatile FileLock basePersistDirLock = null;
+  private volatile FileChannel basePersistDirLockChannel = null;
+
+  BatchAppenderator(
+      String id,
+      DataSchema schema,
+      AppenderatorConfig tuningConfig,
+      FireDepartmentMetrics metrics,
+      DataSegmentPusher dataSegmentPusher,
+      ObjectMapper objectMapper,
+      IndexIO indexIO,
+      IndexMerger indexMerger,
+      RowIngestionMeters rowIngestionMeters,
+      ParseExceptionHandler parseExceptionHandler
+  )
+  {
+    this.myId = id;
+    this.schema = Preconditions.checkNotNull(schema, "schema");
+    this.tuningConfig = Preconditions.checkNotNull(tuningConfig, "tuningConfig");
+    this.metrics = Preconditions.checkNotNull(metrics, "metrics");
+    this.dataSegmentPusher = Preconditions.checkNotNull(dataSegmentPusher, "dataSegmentPusher");
+    this.objectMapper = Preconditions.checkNotNull(objectMapper, "objectMapper");
+    this.indexIO = Preconditions.checkNotNull(indexIO, "indexIO");
+    this.indexMerger = Preconditions.checkNotNull(indexMerger, "indexMerger");
+    this.rowIngestionMeters = Preconditions.checkNotNull(rowIngestionMeters, "rowIngestionMeters");
+    this.parseExceptionHandler = Preconditions.checkNotNull(parseExceptionHandler, "parseExceptionHandler");
+
+    maxBytesTuningConfig = tuningConfig.getMaxBytesInMemoryOrDefault();
+    skipBytesInMemoryOverheadCheck = tuningConfig.isSkipBytesInMemoryOverheadCheck();
+  }
+
+  @Override
+  public String getId()
+  {
+    return myId;
+  }
+
+  @Override
+  public String getDataSource()
+  {
+    return schema.getDataSource();
+  }
+
+  @Override
+  public Object startJob()
+  {
+    tuningConfig.getBasePersistDirectory().mkdirs();
+    lockBasePersistDirectory();
+    return null;
+  }
+
+  @Override
+  public AppenderatorAddResult add(
+      final SegmentIdWithShardSpec identifier,
+      final InputRow row,
+      @Nullable final Supplier<Committer> committerSupplier,
+      final boolean allowIncrementalPersists
+  ) throws IndexSizeExceededException, SegmentNotWritableException
+  {
+
+    Preconditions.checkArgument(
+        committerSupplier == null,
+        "Batch appenderator does not need a committer!"
+    );
+
+    Preconditions.checkArgument(
+        allowIncrementalPersists,
+        "Batch appenderator should always allow incremental persists!"
+    );
+
+    if (!identifier.getDataSource().equals(schema.getDataSource())) {
+      throw new IAE(
+          "Expected dataSource[%s] but was asked to insert row for dataSource[%s]?!",
+          schema.getDataSource(),
+          identifier.getDataSource()
+      );
+    }
+
+    final Sink sink = getOrCreateSink(identifier);
+    metrics.reportMessageMaxTimestamp(row.getTimestampFromEpoch());
+    final int sinkRowsInMemoryBeforeAdd = sink.getNumRowsInMemory();
+    final int sinkRowsInMemoryAfterAdd;
+    final long bytesInMemoryBeforeAdd = sink.getBytesInMemory();
+    final long bytesInMemoryAfterAdd;
+    final IncrementalIndexAddResult addResult;
+
+    try {
+      addResult = sink.add(row, false); // allow incrememtal persis is always true for batch
+      sinkRowsInMemoryAfterAdd = addResult.getRowCount();
+      bytesInMemoryAfterAdd = addResult.getBytesInMemory();
+    }
+    catch (IndexSizeExceededException e) {
+      // 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 (sinkRowsInMemoryAfterAdd < 0) {
+      throw new SegmentNotWritableException("Attempt to add row to swapped-out sink for segment[%s].", identifier);
+    }
+
+    if (addResult.isRowAdded()) {
+      rowIngestionMeters.incrementProcessed();
+    } else if (addResult.hasParseException()) {
+      parseExceptionHandler.handle(addResult.getParseException());
+    }
+
+    final int numAddedRows = sinkRowsInMemoryAfterAdd - sinkRowsInMemoryBeforeAdd;
+    rowsCurrentlyInMemory.addAndGet(numAddedRows);
+    bytesCurrentlyInMemory.addAndGet(bytesInMemoryAfterAdd - bytesInMemoryBeforeAdd);
+    totalRows.addAndGet(numAddedRows);
+    sinksMetadata.computeIfAbsent(identifier, unused -> new SinkMetadata()).addRows(numAddedRows);
+
+    boolean persist = false;
+    List<String> persistReasons = new ArrayList<>();
+
+    if (!sink.canAppendRow()) {
+      persist = true;
+      persistReasons.add("No more rows can be appended to sink");
+    }
+    if (rowsCurrentlyInMemory.get() >= tuningConfig.getMaxRowsInMemory()) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "rowsCurrentlyInMemory[%d] is greater than maxRowsInMemory[%d]",
+          rowsCurrentlyInMemory.get(),
+          tuningConfig.getMaxRowsInMemory()
+      ));
+    }
+    if (bytesCurrentlyInMemory.get() >= maxBytesTuningConfig) {
+      persist = true;
+      persistReasons.add(StringUtils.format(
+          "bytesCurrentlyInMemory[%d] is greater than maxBytesInMemory[%d]",
+          bytesCurrentlyInMemory.get(),
+          maxBytesTuningConfig
+      ));
+    }
+    if (persist) {
+      // persistAll clears rowsCurrentlyInMemory, no need to update it.
+      log.info("Incremental persist to disk because %s.", String.join(",", persistReasons));
+
+      long bytesToBePersisted = 0L;
+      for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+        final Sink sinkEntry = entry.getValue();
+        if (sinkEntry != null) {
+          bytesToBePersisted += sinkEntry.getBytesInMemory();
+          if (sinkEntry.swappable()) {
+            // Code for batch no longer memory maps hydrants but they still take memory...
+            int memoryStillInUse = calculateMemoryUsedByHydrant();
+            bytesCurrentlyInMemory.addAndGet(memoryStillInUse);
+          }
+        }
+      }
+
+      if (!skipBytesInMemoryOverheadCheck
+          && bytesCurrentlyInMemory.get() - bytesToBePersisted > maxBytesTuningConfig) {
+        // We are still over maxBytesTuningConfig even after persisting.
+        // This means that we ran out of all available memory to ingest (due to overheads created as part of ingestion)
+        final String alertMessage = StringUtils.format(
+            "Task has exceeded safe estimated heap usage limits, failing "
+            + "(numSinks: [%d] numHydrantsAcrossAllSinks: [%d] totalRows: [%d])"
+            + "(bytesCurrentlyInMemory: [%d] - bytesToBePersisted: [%d] > maxBytesTuningConfig: [%d])",
+            sinks.size(),
+            sinks.values().stream().mapToInt(Iterables::size).sum(),
+            getTotalRowCount(),
+            bytesCurrentlyInMemory.get(),
+            bytesToBePersisted,
+            maxBytesTuningConfig
+        );
+        final String errorMessage = StringUtils.format(
+            "%s.\nThis can occur when the overhead from too many intermediary segment persists becomes to "
+            + "great to have enough space to process additional input rows. This check, along with metering the overhead "
+            + "of these objects to factor into the 'maxBytesInMemory' computation, can be disabled by setting "
+            + "'skipBytesInMemoryOverheadCheck' to 'true' (note that doing so might allow the task to naturally encounter "
+            + "a 'java.lang.OutOfMemoryError'). Alternatively, 'maxBytesInMemory' can be increased which will cause an "
+            + "increase in heap footprint, but will allow for more intermediary segment persists to occur before "
+            + "reaching this condition.",
+            alertMessage
+        );
+        log.makeAlert(alertMessage)
+           .addData("dataSource", schema.getDataSource())
+           .emit();
+        throw new RuntimeException(errorMessage);
+      }
+
+      persistAllAndRemoveSinks();
+
+    }
+    return new AppenderatorAddResult(identifier, sinksMetadata.get(identifier).numRowsInSegment, false);
+  }
+
+  @Override
+  /**
+   * Returns all active segments regardless whether they are in memory or persisted
+   */
+  public List<SegmentIdWithShardSpec> getSegments()
+  {
+    return ImmutableList.copyOf(sinksMetadata.keySet());
+  }
+
+  @VisibleForTesting
+  public List<SegmentIdWithShardSpec> getInMemorySegments()
+  {
+    return ImmutableList.copyOf(sinks.keySet());
+  }
+
+  @Override
+  public int getRowCount(final SegmentIdWithShardSpec identifier)
+  {
+    return sinksMetadata.get(identifier).getNumRowsInSegment();
+  }
+
+  @Override
+  public int getTotalRowCount()
+  {
+    return totalRows.get();
+  }
+
+  @VisibleForTesting
+  public int getRowsInMemory()
+  {
+    return rowsCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesCurrentlyInMemory()
+  {
+    return bytesCurrentlyInMemory.get();
+  }
+
+  @VisibleForTesting
+  public long getBytesInMemory(SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+
+    if (sink == null) {
+      return 0L; // sinks are removed after a persist
+    } else {
+      return sink.getBytesInMemory();
+    }
+  }
+
+  private Sink getOrCreateSink(final SegmentIdWithShardSpec identifier)
+  {
+    Sink retVal = sinks.get(identifier);
+
+    if (retVal == null) {
+      retVal = new Sink(
+          identifier.getInterval(),
+          schema,
+          identifier.getShardSpec(),
+          identifier.getVersion(),
+          tuningConfig.getAppendableIndexSpec(),
+          tuningConfig.getMaxRowsInMemory(),
+          maxBytesTuningConfig,
+          null
+      );
+      bytesCurrentlyInMemory.addAndGet(calculateSinkMemoryInUsed());
+
+      sinks.put(identifier, retVal);
+      metrics.setSinkCount(sinks.size());
+    }
+
+    return retVal;
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForIntervals(final Query<T> query, final Iterable<Interval> intervals)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public <T> QueryRunner<T> getQueryRunnerForSegments(final Query<T> query, final Iterable<SegmentDescriptor> specs)
+  {
+    throw new UnsupportedOperationException("No query runner for batch appenderator");
+  }
+
+  @Override
+  public void clear()
+  {
+    clear(true);
+  }
+
+  private void clear(boolean removeOnDiskData)
+  {
+    // Drop commit metadata, then abandon all segments.
+    log.info("Clearing all sinks & hydrants, removing data on disk: [%s]", removeOnDiskData);
+    // Drop everything.
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      removeSink(entry.getKey(), entry.getValue(), removeOnDiskData);
+    }
+  }
+
+  @Override
+  public ListenableFuture<?> drop(final SegmentIdWithShardSpec identifier)
+  {
+    final Sink sink = sinks.get(identifier);
+    SinkMetadata sm = sinksMetadata.remove(identifier);
+    if (sm != null) {
+      int originalTotalRows = getTotalRowCount();
+      int rowsToDrop = sm.getNumRowsInSegment();
+      int totalRowsAfter = originalTotalRows - rowsToDrop;
+      if (totalRowsAfter < 0) {
+        log.warn("Total rows[%d] after dropping segment[%s] rows [%d]", totalRowsAfter, identifier, rowsToDrop);
+      }
+      totalRows.set(Math.max(totalRowsAfter, 0));
+    }
+    if (sink != null) {
+      removeSink(identifier, sink, true);
+    }
+    return Futures.immediateFuture(null);
+  }
+
+  @Override
+  public ListenableFuture<Object> persistAll(@Nullable final Committer committer)
+  {
+    if (committer != null) {
+      throw new ISE("committer must be null for BatchAppenderator");
+    }
+    persistAllAndRemoveSinks();
+    return Futures.immediateFuture(null);
+  }
+
+  /**
+   * Persist all sinks & their hydrants, keep their metadata, and then remove them completely from
+   * memory (to be resurrected right before merge & push)
+   */
+  private void persistAllAndRemoveSinks()
+  {
+
+    final List<Pair<FireHydrant, SegmentIdWithShardSpec>> indexesToPersist = new ArrayList<>();
+    int numPersistedRows = 0;
+    long bytesPersisted = 0L;
+    int totalHydrantsCount = 0;
+    final long totalSinks = sinks.size();
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      final SegmentIdWithShardSpec identifier = entry.getKey();
+      final Sink sink = entry.getValue();
+      if (sink == null) {
+        throw new ISE("No sink for identifier: %s", identifier);
+      }
+
+      final List<FireHydrant> hydrants = Lists.newArrayList(sink);
+      // Since everytime we persist we also get rid of the in-memory references to sinks & hydrants
+      // the invariant of exactly one, always swappable, sink with exactly one unpersisted hydrant must hold
+      int totalHydrantsForSink = hydrants.size();
+      if (totalHydrantsForSink != 1) {
+        throw new ISE("There should be only onw hydrant for identifier[%s] but there are[%s]",
+                      identifier, totalHydrantsForSink
+        );
+      }
+      totalHydrantsCount += 1;
+      numPersistedRows += sink.getNumRowsInMemory();
+      bytesPersisted += sink.getBytesInMemory();
+
+      if (!sink.swappable()) {
+        throw new ISE("Sink is not swappable![%s]", identifier);
+      }
+      indexesToPersist.add(Pair.of(sink.swap(), identifier));
+
+    }
+
+    if (indexesToPersist.isEmpty()) {
+      log.info("No indexes will be peristed");
+    }
+    final Stopwatch persistStopwatch = Stopwatch.createStarted();
+    try {
+      for (Pair<FireHydrant, SegmentIdWithShardSpec> pair : indexesToPersist) {
+        metrics.incrementRowOutputCount(persistHydrant(pair.lhs, pair.rhs));
+      }
+
+      log.info(
+          "Persisted in-memory data for segments: %s",
+          indexesToPersist.stream()
+                          .filter(itp -> itp.rhs != null)
+                          .map(itp -> itp.rhs.asSegmentId().toString())
+                          .distinct()
+                          .collect(Collectors.joining(", "))
+      );
+      log.info(
+          "Persisted stats: processed rows: [%d], persisted rows[%d], persisted sinks: [%d], persisted fireHydrants (across sinks): [%d]",
+          rowIngestionMeters.getProcessed(),
+          numPersistedRows,
+          totalSinks,
+          totalHydrantsCount
+      );
+
+    }
+    catch (Exception e) {
+      metrics.incrementFailedPersists();
+      throw e;
+    }
+    finally {
+      metrics.incrementNumPersists();
+      metrics.incrementPersistTimeMillis(persistStopwatch.elapsed(TimeUnit.MILLISECONDS));
+      persistStopwatch.stop();
+    }
+
+    // NB: The rows are still in memory until they're done persisting, but we only count rows in active indexes.
+    rowsCurrentlyInMemory.addAndGet(-numPersistedRows);
+    bytesCurrentlyInMemory.addAndGet(-bytesPersisted);
+
+    // remove all sinks after persisting:
+    clear(false);
+
+    log.info("Persisted rows[%,d] and bytes[%,d] and removed all sinks & hydrants from memory",
+             numPersistedRows, bytesPersisted);
+
+  }
+
+  @Override
+  public ListenableFuture<SegmentsAndCommitMetadata> push(
+      final Collection<SegmentIdWithShardSpec> identifiers,
+      @Nullable final Committer committer,
+      final boolean useUniquePath
+  )
+  {
+
+    if (committer != null) {
+      throw new ISE("There should be no committer for batch ingestion");
+    }
+
+    if (useUniquePath) {
+      throw new ISE("Batch ingestion does not require uniquePath");
+    }
+
+
+    // Any sinks not persisted so far need to be persisted before push:
+    persistAllAndRemoveSinks();
+
+    log.info("Preparing to push...");
+
+    // get the dirs for the identfiers:
+    List<File> identifiersDirs = new ArrayList<>();
+    for (SegmentIdWithShardSpec identifier : identifiers) {
+      SinkMetadata sm = sinksMetadata.get(identifier);
+      if (sm == null) {
+        throw new ISE("No sink has been processed for identifier[%s]", identifier);
+      }
+      File persistedDir = sm.getPersistedFileDir();
+      if (persistedDir == null) {
+        throw new ISE("Sink for identifier[%s] not found in local file system", identifier);
+      }
+      identifiersDirs.add(persistedDir);
+    }
+
+    // push all sinks for identifiers:
+    final List<DataSegment> dataSegments = new ArrayList<>();
+    for (File identifier : identifiersDirs) {
+
+      // retrieve sink from disk:
+      Pair<SegmentIdWithShardSpec, Sink> identifiersAndSinks;
+      try {
+        identifiersAndSinks = getIdentifierAndSinkForPersistedFile(identifier);
+      }
+      catch (IOException e) {
+        throw new ISE(e, "Failed to retrieve sinks for identifier[%s]", identifier);
+      }
+
+      // push it:
+      final DataSegment dataSegment = mergeAndPush(
+          identifiersAndSinks.lhs,
+          identifiersAndSinks.rhs,
+          false
+      );
+
+      // record it:
+      if (dataSegment != null) {
+        dataSegments.add(dataSegment);
+      } else {
+        log.warn("mergeAndPush[%s] returned null, skipping.", identifiersAndSinks.lhs);
+      }
+
+    }
+    log.info("Push complete...");
+
+    return Futures.immediateFuture(new SegmentsAndCommitMetadata(dataSegments, null));
+  }
+
+  /**
+   * Merge segment, push to deep storage. Should only be used on segments that have been fully persisted.
+   *
+   * @param identifier    sink identifier
+   * @param sink          sink to push
+   * @param useUniquePath true if the segment should be written to a path with a unique identifier
+   * @return segment descriptor, or null if the sink is no longer valid
+   */
+  @Nullable
+  private DataSegment mergeAndPush(
+      final SegmentIdWithShardSpec identifier,
+      final Sink sink,
+      final boolean useUniquePath
+  )
+  {
+
+
+    // Use a descriptor file to indicate that pushing has completed.
+    final File persistDir = computePersistDir(identifier);
+    final File mergedTarget = new File(persistDir, "merged");
+    final File descriptorFile = computeDescriptorFile(identifier);
+
+    // Sanity checks
+    if (sink.isWritable()) {
+      throw new ISE("Expected sink to be no longer writable before mergeAndPush for segment[%s].", identifier);
+    }
+
+    int numHydrants = 0;
+    for (FireHydrant hydrant : sink) {
+      synchronized (hydrant) {
+        if (!hydrant.hasSwapped()) {
+          throw new ISE("Expected sink to be fully persisted before mergeAndPush for segment[%s].", identifier);
+        }
+      }
+      numHydrants++;
+    }
+
+    SinkMetadata sm = sinksMetadata.get(identifier);
+    if (sm == null) {
+      log.warn("Sink metadata not found just before merge for identifier [%s]", identifier);
+    } else if (numHydrants != sm.getNumHydrants()) {
+      throw new ISE("Number of restored hydrants[%d] for identifier[%s] does not match expected value[%d]",
+                    numHydrants, identifier, sm.getNumHydrants());
+    }
+
+    try {
+      if (descriptorFile.exists()) {
+        // Already pushed.
+        if (useUniquePath) {
+          throw new ISE("Merge and push for batch appenderator does not use unique path");
+        } else {
+          log.info("Segment[%s] already pushed, skipping.", identifier);
+          return objectMapper.readValue(descriptorFile, DataSegment.class);
+        }
+      }
+
+      removeDirectory(mergedTarget);
+
+      if (mergedTarget.exists()) {
+        throw new ISE("Merged target[%s] exists after removing?!", mergedTarget);
+      }
+
+      final File mergedFile;
+      final long mergeFinishTime;
+      final long startTime = System.nanoTime();
+      List<QueryableIndex> indexes = new ArrayList<>();
+      Closer closer = Closer.create();
+      try {
+        for (FireHydrant fireHydrant : sink) {
+          Pair<ReferenceCountingSegment, Closeable> segmentAndCloseable = fireHydrant.getAndIncrementSegment();
+          final QueryableIndex queryableIndex = segmentAndCloseable.lhs.asQueryableIndex();
+          log.debug("Segment[%s] adding hydrant[%s]", identifier, fireHydrant);
+          indexes.add(queryableIndex);
+          closer.register(segmentAndCloseable.rhs);
+        }
+
+        mergedFile = indexMerger.mergeQueryableIndex(
+            indexes,
+            schema.getGranularitySpec().isRollup(),
+            schema.getAggregators(),
+            schema.getDimensionsSpec(),
+            mergedTarget,
+            tuningConfig.getIndexSpec(),
+            tuningConfig.getSegmentWriteOutMediumFactory(),
+            tuningConfig.getMaxColumnsToMerge()
+        );
+
+        mergeFinishTime = System.nanoTime();
+
+        log.debug("Segment[%s] built in %,dms.", identifier, (mergeFinishTime - startTime) / 1000000);
+      }
+      catch (Throwable t) {
+        throw closer.rethrow(t);
+      }
+      finally {
+        closer.close();
+      }
+
+      // Retry pushing segments because uploading to deep storage might fail especially for cloud storage types
+      final DataSegment segment = RetryUtils.retry(
+          // This appenderator is used only for the local indexing task so unique paths are not required
+          () -> dataSegmentPusher.push(
+              mergedFile,
+              sink.getSegment()
+                  .withDimensions(IndexMerger.getMergedDimensionsFromQueryableIndexes(
+                      indexes,
+                      schema.getDimensionsSpec()
+                  )),
+              useUniquePath
+          ),
+          exception -> exception instanceof Exception,
+          5
+      );
+
+      // Drop the queryable indexes behind the hydrants... they are not needed anymore and their
+      // mapped file references
+      // can generate OOMs during merge if enough of them are held back...
+      for (FireHydrant fireHydrant : sink) {
+        fireHydrant.swapSegment(null);
+      }
+
+      // cleanup, sink no longer needed
+      removeDirectory(computePersistDir(identifier));
+
+      final long pushFinishTime = System.nanoTime();
+
+      log.info(
+          "Segment[%s] of %,d bytes "
+          + "built from %d incremental persist(s) in %,dms; "
+          + "pushed to deep storage in %,dms. "
+          + "Load spec is: %s",
+          identifier,
+          segment.getSize(),
+          indexes.size(),
+          (mergeFinishTime - startTime) / 1000000,
+          (pushFinishTime - mergeFinishTime) / 1000000,
+          objectMapper.writeValueAsString(segment.getLoadSpec())
+      );
+
+      return segment;
+    }
+    catch (Exception e) {
+      metrics.incrementFailedHandoffs();
+      log.warn(e, "Failed to push merged index for segment[%s].", identifier);
+      throw new RuntimeException(e);
+    }
+  }
+
+  @Override
+  public void close()
+  {
+    if (!closed.compareAndSet(false, true)) {
+      log.debug("Appenderator already closed, skipping close() call.");
+      return;
+    }
+
+    log.debug("Shutting down...");
+
+    for (Map.Entry<SegmentIdWithShardSpec, Sink> entry : sinks.entrySet()) {
+      removeSink(entry.getKey(), entry.getValue(), false);
+    }
+
+    unlockBasePersistDirectory();
+
+    // cleanup:
+    List<File> persistedIdentifiers = getPersistedidentifierPaths();
+    if (persistedIdentifiers != null) {
+      for (File identifier : persistedIdentifiers) {
+        removeDirectory(identifier);
+      }
+    }
+
+    totalRows.set(0);
+    sinksMetadata.clear();
+  }
+
+  /**
+    Nothing to do since there are no executors
+   */
+  @Override
+  public void closeNow()
+  {
+    if (!closed.compareAndSet(false, true)) {
+      log.debug("Appenderator already closed, skipping closeNow() call.");
+      return;
+    }
+
+    log.debug("Shutting down immediately...");
+  }
+
+  private void lockBasePersistDirectory()
+  {
+    if (basePersistDirLock == null) {
+      try {
+        basePersistDirLockChannel = FileChannel.open(
+            computeLockFile().toPath(),
+            StandardOpenOption.CREATE,
+            StandardOpenOption.WRITE
+        );
+
+        basePersistDirLock = basePersistDirLockChannel.tryLock();
+        if (basePersistDirLock == null) {
+          throw new ISE("Cannot acquire lock on basePersistDir: %s", computeLockFile());
+        }
+      }
+      catch (IOException e) {
+        throw new RuntimeException(e);
+      }
+    }
+  }
+
+  private void unlockBasePersistDirectory()
+  {
+    try {
+      if (basePersistDirLock != null) {
+        basePersistDirLock.release();
+        basePersistDirLockChannel.close();
+        basePersistDirLock = null;
+      }
+    }
+    catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+  }
+
+  @VisibleForTesting
+  @Nullable
+  public List<File> getPersistedidentifierPaths()
+  {
+
+    ArrayList<File> retVal = new ArrayList<>();
+
+    final File baseDir = tuningConfig.getBasePersistDirectory();
+    if (!baseDir.exists()) {
+      return null;
+    }
+
+    final File[] files = baseDir.listFiles();
+    if (files == null) {
+      return null;
+    }
+
+    for (File sinkDir : files) {
+      final File identifierFile = new File(sinkDir, IDENTIFIER_FILE_NAME);
+      if (!identifierFile.isFile()) {
+        // No identifier in this sinkDir; it must not actually be a sink directory. Skip it.
+        continue;
+      }
+      retVal.add(sinkDir);
+    }
+
+    return retVal;
+  }
+
+  private Pair<SegmentIdWithShardSpec, Sink> getIdentifierAndSinkForPersistedFile(File identifierPath)
+      throws IOException
+  {
+
+    final SegmentIdWithShardSpec identifier = objectMapper.readValue(
+        new File(identifierPath, IDENTIFIER_FILE_NAME),
+        SegmentIdWithShardSpec.class
+    );
+
+    // To avoid reading and listing of "merged" dir and other special files
+    final File[] sinkFiles = identifierPath.listFiles(
+        (dir, fileName) -> !(Ints.tryParse(fileName) == null)
+    );
+    if (sinkFiles == null) {
+      throw new ISE("Problem reading persisted sinks in path[%s]", identifierPath);
+    }
+
+    Arrays.sort(
+        sinkFiles,
+        (o1, o2) -> Ints.compare(Integer.parseInt(o1.getName()), Integer.parseInt(o2.getName()))
+    );
+
+    List<FireHydrant> hydrants = new ArrayList<>();
+    for (File hydrantDir : sinkFiles) {
+      final int hydrantNumber = Integer.parseInt(hydrantDir.getName());
+
+      log.debug("Loading previously persisted partial segment at [%s]", hydrantDir);
+      if (hydrantNumber != hydrants.size()) {
+        throw new ISE("Missing hydrant [%,d] in identifier [%s].", hydrants.size(), identifier);
+      }
+
+      hydrants.add(
+          new FireHydrant(
+              new QueryableIndexSegment(indexIO.loadIndex(hydrantDir), identifier.asSegmentId()),
+              hydrantNumber
+          )
+      );
+    }
+
+    Sink currSink = new Sink(
+        identifier.getInterval(),
+        schema,
+        identifier.getShardSpec(),
+        identifier.getVersion(),
+        tuningConfig.getAppendableIndexSpec(),
+        tuningConfig.getMaxRowsInMemory(),
+        maxBytesTuningConfig,
+        null,
+        hydrants
+    );
+    currSink.finishWriting(); // this sink is not writable
+    return new Pair<>(identifier, currSink);
+  }
+
+  private void removeSink(
+      final SegmentIdWithShardSpec identifier,
+      final Sink sink,
+      final boolean removeOnDiskData
+  )
+  {
+    // Ensure no future writes will be made to this sink.
+    if (sink.finishWriting()) {
+      // Decrement this sink's rows from the counters. we only count active sinks so that we don't double decrement,
+      // i.e. those that haven't been persisted for *InMemory counters, or pushed to deep storage for the total counter.
+      rowsCurrentlyInMemory.addAndGet(-sink.getNumRowsInMemory());
+      bytesCurrentlyInMemory.addAndGet(-sink.getBytesInMemory());
+      bytesCurrentlyInMemory.addAndGet(-calculateSinkMemoryInUsed());
+      for (FireHydrant hydrant : sink) {
+        // Decrement memory used by all Memory Mapped Hydrant
+        if (!hydrant.equals(sink.getCurrHydrant())) {
+          bytesCurrentlyInMemory.addAndGet(-calculateMemoryUsedByHydrant());
+        }
+      }
+      // totalRows are not decremented when removing the sink from memory, sink was just persisted and it
+      // still "lives" but it is in hibernation. It will be revived later just before push.
+    }
+
+
+    if (!sinks.remove(identifier, sink)) {
+      log.error("Sink for segment[%s] no longer valid, not abandoning.", identifier);
+    }
+
+    metrics.setSinkCount(sinks.size());
+
+    if (removeOnDiskData) {
+      removeDirectory(computePersistDir(identifier));
+    }
+
+    log.info("Removed sink for segment[%s].", identifier);
+
+  }
+
+  private File computeLockFile()
+  {
+    return new File(tuningConfig.getBasePersistDirectory(), ".lock");
+  }
+
+  private File computePersistDir(SegmentIdWithShardSpec identifier)
+  {
+    return new File(tuningConfig.getBasePersistDirectory(), identifier.toString());
+  }
+
+  private File computeIdentifierFile(SegmentIdWithShardSpec identifier)
+  {
+    return new File(computePersistDir(identifier), IDENTIFIER_FILE_NAME);
+  }
+
+  private File computeDescriptorFile(SegmentIdWithShardSpec identifier)
+  {
+    return new File(computePersistDir(identifier), "descriptor.json");
+  }
+
+  private File createPersistDirIfNeeded(SegmentIdWithShardSpec identifier) throws IOException
+  {
+    final File persistDir = computePersistDir(identifier);
+    org.apache.commons.io.FileUtils.forceMkdir(persistDir);
+
+    objectMapper.writeValue(computeIdentifierFile(identifier), identifier);
+
+    return persistDir;
+  }
+
+  /**
+   * Persists the given hydrant and returns the number of rows persisted.
+   *
+   * @param indexToPersist hydrant to persist
+   * @param identifier     the segment this hydrant is going to be part of
+   * @return the number of rows persisted
+   */
+  private int persistHydrant(FireHydrant indexToPersist, SegmentIdWithShardSpec identifier)
+  {
+    synchronized (indexToPersist) {

Review comment:
       No need for synchronization.




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