[HUDI-1632] Supports merge on read write mode for Flink writer (#2593)

Also supports async compaction with pluggable strategies.

hudi-flink/src/main/java/org/apache/hudi/operator/FlinkOptions.java

@@ -51,8 +51,8 @@ public class FlinkOptions {
       .key("path")
       .stringType()
       .noDefaultValue()
-      .withDescription("Base path for the target hoodie table."
-          + "\nThe path would be created if it does not exist,\n"
+      .withDescription("Base path for the target hoodie table.\n"
+          + "The path would be created if it does not exist,\n"
           + "otherwise a Hoodie table expects to be initialized successfully");
 
   // ------------------------------------------------------------------------
@@ -165,6 +165,42 @@ public class FlinkOptions {
       .defaultValue(128D) // 128MB
       .withDescription("Batch buffer size in MB to flush data into the underneath filesystem");
 
+  // ------------------------------------------------------------------------
+  //  Compaction Options
+  // ------------------------------------------------------------------------
+
+  public static final ConfigOption<Boolean> COMPACTION_ASYNC_ENABLED = ConfigOptions
+      .key("compaction.async.enabled")
+      .booleanType()
+      .defaultValue(true) // default true for MOR write
+      .withDescription("Async Compaction, enabled by default for MOR");
+
+  public static final String NUM_COMMITS = "num_commits";
+  public static final String TIME_ELAPSED = "time_elapsed";
+  public static final String NUM_AND_TIME = "num_and_time";
+  public static final String NUM_OR_TIME = "num_or_time";
+  public static final ConfigOption<String> COMPACTION_TRIGGER_STRATEGY = ConfigOptions
+      .key("compaction.trigger.strategy")
+      .stringType()
+      .defaultValue(NUM_COMMITS) // default true for MOR write
+      .withDescription("Strategy to trigger compaction, options are 'num_commits': trigger compaction when reach N delta commits;\n"
+          + "'time_elapsed': trigger compaction when time elapsed > N seconds since last compaction;\n"
+          + "'num_and_time': trigger compaction when both NUM_COMMITS and TIME_ELAPSED are satisfied;\n"
+          + "'num_or_time': trigger compaction when NUM_COMMITS or TIME_ELAPSED is satisfied.\n"
+          + "Default is 'num_commits'");
+
+  public static final ConfigOption<Integer> COMPACTION_DELTA_COMMITS = ConfigOptions
+      .key("compaction.delta_commits")
+      .intType()
+      .defaultValue(5)
+      .withDescription("Max delta commits needed to trigger compaction, default 5 commits");
+
+  public static final ConfigOption<Integer> COMPACTION_DELTA_SECONDS = ConfigOptions
+      .key("compaction.delta_seconds")
+      .intType()
+      .defaultValue(3600) // default 1 hour
+      .withDescription("Max delta seconds time needed to trigger compaction, default 1 hour");
+
   // -------------------------------------------------------------------------
   //  Utilities
   // -------------------------------------------------------------------------

hudi-flink/src/main/java/org/apache/hudi/operator/StreamWriteFunction.java

@@ -40,7 +40,6 @@ import org.apache.flink.runtime.state.FunctionSnapshotContext;
 import org.apache.flink.streaming.api.checkpoint.CheckpointedFunction;
 import org.apache.flink.streaming.api.functions.KeyedProcessFunction;
 import org.apache.flink.util.Collector;
-import org.apache.flink.util.Preconditions;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -333,8 +332,11 @@ public class StreamWriteFunction<K, I, O>
   @SuppressWarnings("unchecked, rawtypes")
   private void flushBuffer(boolean isFinalBatch) {
     this.currentInstant = this.writeClient.getInflightAndRequestedInstant(this.config.get(FlinkOptions.TABLE_TYPE));
-    Preconditions.checkNotNull(this.currentInstant,
-        "No inflight instant when flushing data");
+    if (this.currentInstant == null) {
+      // in case there are empty checkpoints that has no input data
+      LOG.info("No inflight instant when flushing data, cancel.");
+      return;
+    }
     final List<WriteStatus> writeStatus;
     if (buffer.size() > 0) {
       writeStatus = new ArrayList<>();

hudi-flink/src/main/java/org/apache/hudi/operator/StreamWriteOperatorCoordinator.java

@@ -22,6 +22,7 @@ import org.apache.hudi.client.FlinkTaskContextSupplier;
 import org.apache.hudi.client.HoodieFlinkWriteClient;
 import org.apache.hudi.client.WriteStatus;
 import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.model.HoodieTableType;
 import org.apache.hudi.common.util.Option;
 import org.apache.hudi.exception.HoodieException;
 import org.apache.hudi.operator.event.BatchWriteSuccessEvent;
@@ -48,6 +49,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Objects;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
@@ -101,6 +103,11 @@ public class StreamWriteOperatorCoordinator
    */
   private final int parallelism;
 
+  /**
+   * Whether needs to schedule compaction task on finished checkpoints.
+   */
+  private final boolean needsScheduleCompaction;
+
   /**
    * Constructs a StreamingSinkOperatorCoordinator.
    *
@@ -112,6 +119,7 @@ public class StreamWriteOperatorCoordinator
       int parallelism) {
     this.conf = conf;
     this.parallelism = parallelism;
+    this.needsScheduleCompaction = isNeedsScheduleCompaction();
   }
 
   @Override
@@ -152,6 +160,10 @@ public class StreamWriteOperatorCoordinator
   public void checkpointComplete(long checkpointId) {
     // start to commit the instant.
     checkAndCommitWithRetry();
+    // if async compaction is on, schedule the compaction
+    if (needsScheduleCompaction) {
+      writeClient.scheduleCompaction(Option.empty());
+    }
     // start new instant.
     startInstant();
   }
@@ -202,6 +214,13 @@ public class StreamWriteOperatorCoordinator
   //  Utilities
   // -------------------------------------------------------------------------
 
+  private boolean isNeedsScheduleCompaction() {
+    return this.conf.getString(FlinkOptions.TABLE_TYPE)
+        .toUpperCase(Locale.ROOT)
+        .equals(HoodieTableType.MERGE_ON_READ.name())
+        && this.conf.getBoolean(FlinkOptions.COMPACTION_ASYNC_ENABLED);
+  }
+
   @SuppressWarnings("rawtypes")
   private void initWriteClient() {
     writeClient = new HoodieFlinkWriteClient(

hudi-flink/src/main/java/org/apache/hudi/operator/compact/CompactFunction.java

@@ -0,0 +1,94 @@
+/*
+ * 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.hudi.operator.compact;
+
+import org.apache.hudi.client.FlinkTaskContextSupplier;
+import org.apache.hudi.client.HoodieFlinkWriteClient;
+import org.apache.hudi.client.WriteStatus;
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.config.SerializableConfiguration;
+import org.apache.hudi.common.model.CompactionOperation;
+import org.apache.hudi.table.HoodieFlinkCopyOnWriteTable;
+import org.apache.hudi.table.action.compact.HoodieFlinkMergeOnReadTableCompactor;
+import org.apache.hudi.util.StreamerUtil;
+
+import org.apache.flink.configuration.Configuration;
+import org.apache.flink.streaming.api.functions.KeyedProcessFunction;
+import org.apache.flink.util.Collector;
+
+import java.util.List;
+
+/**
+ * Function to execute the actual compaction task assigned by the compaction plan task.
+ * In order to execute scalable, the input should shuffle by the compact event {@link CompactionPlanEvent}.
+ */
+public class CompactFunction extends KeyedProcessFunction<Long, CompactionPlanEvent, CompactionCommitEvent> {
+
+  /**
+   * Config options.
+   */
+  private final Configuration conf;
+
+  /**
+   * Write Client.
+   */
+  private transient HoodieFlinkWriteClient writeClient;
+
+  /**
+   * Id of current subtask.
+   */
+  private int taskID;
+
+  public CompactFunction(Configuration conf) {
+    this.conf = conf;
+  }
+
+  @Override
+  public void open(Configuration parameters) throws Exception {
+    this.taskID = getRuntimeContext().getIndexOfThisSubtask();
+    initWriteClient();
+  }
+
+  @Override
+  public void processElement(CompactionPlanEvent event, Context context, Collector<CompactionCommitEvent> collector) throws Exception {
+    final String instantTime = event.getCompactionInstantTime();
+    final CompactionOperation compactionOperation = event.getOperation();
+
+    HoodieFlinkMergeOnReadTableCompactor compactor = new HoodieFlinkMergeOnReadTableCompactor();
+    List<WriteStatus> writeStatuses = compactor.compact(
+        new HoodieFlinkCopyOnWriteTable<>(
+            this.writeClient.getConfig(),
+            this.writeClient.getEngineContext(),
+            this.writeClient.getHoodieTable().getMetaClient()),
+        this.writeClient.getHoodieTable().getMetaClient(),
+        this.writeClient.getConfig(),
+        compactionOperation,
+        instantTime);
+    collector.collect(new CompactionCommitEvent(instantTime, writeStatuses, taskID));
+  }
+
+  private void initWriteClient() {
+    HoodieFlinkEngineContext context =
+        new HoodieFlinkEngineContext(
+            new SerializableConfiguration(StreamerUtil.getHadoopConf()),
+            new FlinkTaskContextSupplier(getRuntimeContext()));
+
+    writeClient = new HoodieFlinkWriteClient<>(context, StreamerUtil.getHoodieClientConfig(conf));
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/compact/CompactionCommitEvent.java

@@ -0,0 +1,62 @@
+/*
+ * 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.hudi.operator.compact;
+
+import org.apache.hudi.client.WriteStatus;
+
+import java.io.Serializable;
+import java.util.List;
+
+/**
+ * Represents a commit event from the compaction task {@link CompactFunction}.
+ */
+public class CompactionCommitEvent implements Serializable {
+  private static final long serialVersionUID = 1L;
+
+  /**
+   * The compaction commit instant time.
+   */
+  private final String instant;
+  /**
+   * The write statuses.
+   */
+  private final List<WriteStatus> writeStatuses;
+  /**
+   * The compaction task identifier.
+   */
+  private final int taskID;
+
+  public CompactionCommitEvent(String instant, List<WriteStatus> writeStatuses, int taskID) {
+    this.instant = instant;
+    this.writeStatuses = writeStatuses;
+    this.taskID = taskID;
+  }
+
+  public String getInstant() {
+    return instant;
+  }
+
+  public List<WriteStatus> getWriteStatuses() {
+    return writeStatuses;
+  }
+
+  public int getTaskID() {
+    return taskID;
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/compact/CompactionCommitSink.java

@@ -0,0 +1,150 @@
+/*
+ * 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.hudi.operator.compact;
+
+import org.apache.hudi.avro.model.HoodieCompactionPlan;
+import org.apache.hudi.client.FlinkTaskContextSupplier;
+import org.apache.hudi.client.HoodieFlinkWriteClient;
+import org.apache.hudi.client.WriteStatus;
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.config.SerializableConfiguration;
+import org.apache.hudi.common.model.HoodieCommitMetadata;
+import org.apache.hudi.common.model.HoodieWriteStat;
+import org.apache.hudi.common.table.timeline.HoodieInstant;
+import org.apache.hudi.common.table.timeline.HoodieTimeline;
+import org.apache.hudi.common.util.CompactionUtils;
+import org.apache.hudi.common.util.Option;
+import org.apache.hudi.util.StreamerUtil;
+
+import org.apache.flink.configuration.Configuration;
+import org.apache.flink.streaming.api.functions.sink.RichSinkFunction;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+/**
+ * Function to check and commit the compaction action.
+ *
+ * <p> Each time after receiving a compaction commit event {@link CompactionCommitEvent},
+ * it loads and checks the compaction plan {@link HoodieCompactionPlan},
+ * if all the compaction operations {@link org.apache.hudi.common.model.CompactionOperation}
+ * of the plan are finished, tries to commit the compaction action.
+ */
+public class CompactionCommitSink extends RichSinkFunction<CompactionCommitEvent> {
+  private static final Logger LOG = LoggerFactory.getLogger(CompactionCommitSink.class);
+
+  /**
+   * Config options.
+   */
+  private final Configuration conf;
+
+  /**
+   * Write Client.
+   */
+  private transient HoodieFlinkWriteClient writeClient;
+
+  /**
+   * Buffer to collect the event from each compact task {@code CompactFunction}.
+   */
+  private transient List<CompactionCommitEvent> commitBuffer;
+
+  /**
+   * Current on-going compaction instant time.
+   */
+  private String compactionInstantTime;
+
+  public CompactionCommitSink(Configuration conf) {
+    this.conf = conf;
+  }
+
+  @Override
+  public void open(Configuration parameters) throws Exception {
+    super.open(parameters);
+    initWriteClient();
+    this.commitBuffer = new ArrayList<>();
+  }
+
+  @Override
+  public void invoke(CompactionCommitEvent event, Context context) throws Exception {
+    if (compactionInstantTime == null) {
+      compactionInstantTime = event.getInstant();
+    } else if (!event.getInstant().equals(compactionInstantTime)) {
+      // last compaction still not finish, rolls it back
+      HoodieInstant inflightInstant = HoodieTimeline.getCompactionInflightInstant(this.compactionInstantTime);
+      writeClient.rollbackInflightCompaction(inflightInstant);
+      this.compactionInstantTime = event.getInstant();
+    }
+    this.commitBuffer.add(event);
+    commitIfNecessary();
+  }
+
+  /**
+   * Condition to commit: the commit buffer has equal size with the compaction plan operations
+   * and all the compact commit event {@link CompactionCommitEvent} has the same compaction instant time.
+   */
+  private void commitIfNecessary() throws IOException {
+    HoodieCompactionPlan compactionPlan = CompactionUtils.getCompactionPlan(
+        this.writeClient.getHoodieTable().getMetaClient(), compactionInstantTime);
+    boolean isReady = compactionPlan.getOperations().size() == commitBuffer.size()
+        && commitBuffer.stream().allMatch(event -> event != null && Objects.equals(event.getInstant(), compactionInstantTime));
+    if (!isReady) {
+      return;
+    }
+    List<WriteStatus> statuses = this.commitBuffer.stream()
+        .map(CompactionCommitEvent::getWriteStatuses)
+        .flatMap(Collection::stream)
+        .collect(Collectors.toList());
+
+    if (this.writeClient.getConfig().shouldAutoCommit()) {
+      // Prepare the commit metadata.
+      List<HoodieWriteStat> updateStatusMap = statuses.stream().map(WriteStatus::getStat).collect(Collectors.toList());
+      HoodieCommitMetadata metadata = new HoodieCommitMetadata(true);
+      for (HoodieWriteStat stat : updateStatusMap) {
+        metadata.addWriteStat(stat.getPartitionPath(), stat);
+      }
+      metadata.addMetadata(HoodieCommitMetadata.SCHEMA_KEY, writeClient.getConfig().getSchema());
+      this.writeClient.completeCompaction(
+          metadata, statuses, this.writeClient.getHoodieTable(), compactionInstantTime);
+    }
+    // commit the compaction
+    this.writeClient.commitCompaction(compactionInstantTime, statuses, Option.empty());
+    // reset the status
+    reset();
+  }
+
+  private void reset() {
+    this.commitBuffer.clear();
+    this.compactionInstantTime = null;
+  }
+
+  private void initWriteClient() {
+    HoodieFlinkEngineContext context =
+        new HoodieFlinkEngineContext(
+            new SerializableConfiguration(StreamerUtil.getHadoopConf()),
+            new FlinkTaskContextSupplier(getRuntimeContext()));
+
+    writeClient = new HoodieFlinkWriteClient<>(context, StreamerUtil.getHoodieClientConfig(this.conf));
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/compact/CompactionPlanEvent.java

@@ -0,0 +1,47 @@
+/*
+ * 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.hudi.operator.compact;
+
+import org.apache.hudi.common.model.CompactionOperation;
+
+import java.io.Serializable;
+
+/**
+ * Represents a compact command from the compaction plan task {@link CompactionPlanOperator}.
+ */
+public class CompactionPlanEvent implements Serializable {
+  private static final long serialVersionUID = 1L;
+
+  private final String compactionInstantTime;
+
+  private final CompactionOperation operation;
+
+  public CompactionPlanEvent(String instantTime, CompactionOperation operation) {
+    this.compactionInstantTime = instantTime;
+    this.operation = operation;
+  }
+
+  public String getCompactionInstantTime() {
+    return compactionInstantTime;
+  }
+
+  public CompactionOperation getOperation() {
+    return operation;
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/compact/CompactionPlanOperator.java

@@ -0,0 +1,146 @@
+/*
+ * 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.hudi.operator.compact;
+
+import org.apache.hudi.avro.model.HoodieCompactionPlan;
+import org.apache.hudi.client.FlinkTaskContextSupplier;
+import org.apache.hudi.client.HoodieFlinkWriteClient;
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.config.SerializableConfiguration;
+import org.apache.hudi.common.model.CompactionOperation;
+import org.apache.hudi.common.table.timeline.HoodieInstant;
+import org.apache.hudi.common.table.timeline.HoodieTimeline;
+import org.apache.hudi.common.util.CompactionUtils;
+import org.apache.hudi.common.util.Option;
+import org.apache.hudi.table.HoodieFlinkTable;
+import org.apache.hudi.util.StreamerUtil;
+
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.configuration.Configuration;
+import org.apache.flink.streaming.api.operators.AbstractStreamOperator;
+import org.apache.flink.streaming.api.operators.OneInputStreamOperator;
+import org.apache.flink.streaming.api.operators.Output;
+import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Objects;
+
+import static java.util.stream.Collectors.toList;
+
+/**
+ * Operator that generates the compaction plan with pluggable strategies on finished checkpoints.
+ *
+ * <p>It should be singleton to avoid conflicts.
+ */
+public class CompactionPlanOperator extends AbstractStreamOperator<CompactionPlanEvent>
+    implements OneInputStreamOperator<Object, CompactionPlanEvent> {
+
+  /**
+   * Config options.
+   */
+  private final Configuration conf;
+
+  /**
+   * Write Client.
+   */
+  private transient HoodieFlinkWriteClient writeClient;
+
+  /**
+   * Compaction instant time.
+   */
+  private String compactionInstantTime;
+
+  public CompactionPlanOperator(Configuration conf) {
+    this.conf = conf;
+  }
+
+  @Override
+  public void open() throws Exception {
+    super.open();
+    initWriteClient();
+  }
+
+  @Override
+  public void processElement(StreamRecord<Object> streamRecord) {
+    // no operation
+  }
+
+  @Override
+  public void notifyCheckpointComplete(long checkpointId) throws IOException {
+    HoodieFlinkTable<?> table = writeClient.getHoodieTable();
+    // the last instant takes the highest priority.
+    Option<HoodieInstant> compactionInstant = table.getActiveTimeline().filterPendingCompactionTimeline().lastInstant();
+    String compactionInstantTime = compactionInstant.isPresent() ? compactionInstant.get().getTimestamp() : null;
+    if (compactionInstantTime == null) {
+      // do nothing.
+      LOG.info("No compaction plan for checkpoint " + checkpointId);
+      return;
+    }
+    if (this.compactionInstantTime != null
+        && Objects.equals(this.compactionInstantTime, compactionInstantTime)) {
+      // do nothing
+      LOG.info("Duplicate scheduling for compaction instant: " + compactionInstantTime + ", ignore");
+      return;
+    }
+    // generate compaction plan
+    // should support configurable commit metadata
+    HoodieCompactionPlan compactionPlan = CompactionUtils.getCompactionPlan(
+        table.getMetaClient(), compactionInstantTime);
+
+    if (compactionPlan == null || (compactionPlan.getOperations() == null)
+        || (compactionPlan.getOperations().isEmpty())) {
+      // do nothing.
+      LOG.info("No compaction plan for checkpoint " + checkpointId + " and instant " + compactionInstantTime);
+    } else {
+      this.compactionInstantTime = compactionInstantTime;
+      HoodieInstant instant = HoodieTimeline.getCompactionRequestedInstant(compactionInstantTime);
+      HoodieTimeline pendingCompactionTimeline = table.getActiveTimeline().filterPendingCompactionTimeline();
+      if (!pendingCompactionTimeline.containsInstant(instant)) {
+        throw new IllegalStateException(
+            "No Compaction request available at " + compactionInstantTime + " to run compaction");
+      }
+
+      // Mark instant as compaction inflight
+      table.getActiveTimeline().transitionCompactionRequestedToInflight(instant);
+      table.getMetaClient().reloadActiveTimeline();
+
+      List<CompactionOperation> operations = compactionPlan.getOperations().stream()
+          .map(CompactionOperation::convertFromAvroRecordInstance).collect(toList());
+      LOG.info("CompactionPlanFunction compacting " + operations + " files");
+      for (CompactionOperation operation : operations) {
+        output.collect(new StreamRecord<>(new CompactionPlanEvent(compactionInstantTime, operation)));
+      }
+    }
+  }
+
+  @VisibleForTesting
+  public void setOutput(Output<StreamRecord<CompactionPlanEvent>> output) {
+    this.output = output;
+  }
+
+  private void initWriteClient() {
+    HoodieFlinkEngineContext context =
+        new HoodieFlinkEngineContext(
+            new SerializableConfiguration(StreamerUtil.getHadoopConf()),
+            new FlinkTaskContextSupplier(getRuntimeContext()));
+
+    writeClient = new HoodieFlinkWriteClient<>(context, StreamerUtil.getHoodieClientConfig(this.conf));
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/partitioner/BucketAssignFunction.java

@@ -26,6 +26,7 @@ import org.apache.hudi.common.model.HoodieBaseFile;
 import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.common.model.HoodieRecord;
 import org.apache.hudi.common.model.HoodieRecordLocation;
+import org.apache.hudi.common.model.HoodieTableType;
 import org.apache.hudi.common.model.WriteOperationType;
 import org.apache.hudi.common.util.ParquetUtils;
 import org.apache.hudi.config.HoodieWriteConfig;
@@ -136,7 +137,10 @@ public class BucketAssignFunction<K, I, O extends HoodieRecord<?>>
     this.context = new HoodieFlinkEngineContext(
         new SerializableConfiguration(this.hadoopConf),
         new FlinkTaskContextSupplier(getRuntimeContext()));
-    this.bucketAssigner = new BucketAssigner(context, writeConfig);
+    this.bucketAssigner = BucketAssigners.create(
+        HoodieTableType.valueOf(conf.getString(FlinkOptions.TABLE_TYPE)),
+        context,
+        writeConfig);
 
     // initialize and check the partitions load state
     loadInitialPartitions();
@@ -145,7 +149,7 @@ public class BucketAssignFunction<K, I, O extends HoodieRecord<?>>
 
   @Override
   public void snapshotState(FunctionSnapshotContext context) {
-    // no operation
+    this.bucketAssigner.reset();
   }
 
   @Override
@@ -209,7 +213,6 @@ public class BucketAssignFunction<K, I, O extends HoodieRecord<?>>
   @Override
   public void notifyCheckpointComplete(long l) {
     // Refresh the table state when there are new commits.
-    this.bucketAssigner.reset();
     this.bucketAssigner.refreshTable();
     checkPartitionsLoaded();
   }

hudi-flink/src/main/java/org/apache/hudi/operator/partitioner/BucketAssigner.java

@@ -65,7 +65,7 @@ public class BucketAssigner {
    */
   private final HashMap<String, BucketInfo> bucketInfoMap;
 
-  private HoodieTable<?, ?, ?, ?> table;
+  protected HoodieTable<?, ?, ?, ?> table;
 
   /**
    * Fink engine context.
@@ -75,7 +75,7 @@ public class BucketAssigner {
   /**
    * The write config.
    */
-  private final HoodieWriteConfig config;
+  protected final HoodieWriteConfig config;
 
   /**
    * The average record size.

hudi-flink/src/main/java/org/apache/hudi/operator/partitioner/BucketAssigners.java

@@ -0,0 +1,54 @@
+/*
+ * 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.hudi.operator.partitioner;
+
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.model.HoodieTableType;
+import org.apache.hudi.config.HoodieWriteConfig;
+import org.apache.hudi.operator.partitioner.delta.DeltaBucketAssigner;
+
+/**
+ * Utilities for {@code BucketAssigner}.
+ */
+public abstract class BucketAssigners {
+
+  private BucketAssigners() {}
+
+  /**
+   * Creates a {@code BucketAssigner}.
+   *
+   * @param tableType The table type
+   * @param context   The engine context
+   * @param config    The configuration
+   * @return the bucket assigner instance
+   */
+  public static BucketAssigner create(
+      HoodieTableType tableType,
+      HoodieFlinkEngineContext context,
+      HoodieWriteConfig config) {
+    switch (tableType) {
+      case COPY_ON_WRITE:
+        return new BucketAssigner(context, config);
+      case MERGE_ON_READ:
+        return new DeltaBucketAssigner(context, config);
+      default:
+        throw new AssertionError();
+    }
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/operator/partitioner/delta/DeltaBucketAssigner.java

@@ -0,0 +1,114 @@
+/*
+ * 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.hudi.operator.partitioner.delta;
+
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.fs.FSUtils;
+import org.apache.hudi.common.model.FileSlice;
+import org.apache.hudi.common.model.HoodieLogFile;
+import org.apache.hudi.common.model.HoodieRecordLocation;
+import org.apache.hudi.common.table.timeline.HoodieInstant;
+import org.apache.hudi.common.table.timeline.HoodieTimeline;
+import org.apache.hudi.config.HoodieWriteConfig;
+import org.apache.hudi.operator.partitioner.BucketAssigner;
+import org.apache.hudi.table.action.commit.SmallFile;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * BucketAssigner for MERGE_ON_READ table type, this allows auto correction of small parquet files to larger ones
+ * without the need for an index in the logFile.
+ *
+ * <p>Note: assumes the index can always index log files for Flink write.
+ */
+public class DeltaBucketAssigner extends BucketAssigner {
+  public DeltaBucketAssigner(HoodieFlinkEngineContext context, HoodieWriteConfig config) {
+    super(context, config);
+  }
+
+  @Override
+  protected List<SmallFile> getSmallFiles(String partitionPath) {
+    // smallFiles only for partitionPath
+    List<SmallFile> smallFileLocations = new ArrayList<>();
+
+    // Init here since this class (and member variables) might not have been initialized
+    HoodieTimeline commitTimeline = table.getCompletedCommitsTimeline();
+
+    // Find out all eligible small file slices
+    if (!commitTimeline.empty()) {
+      HoodieInstant latestCommitTime = commitTimeline.lastInstant().get();
+      // find smallest file in partition and append to it
+      List<FileSlice> allSmallFileSlices = new ArrayList<>();
+      // If we can index log files, we can add more inserts to log files for fileIds including those under
+      // pending compaction.
+      List<FileSlice> allFileSlices =
+          table.getSliceView().getLatestFileSlicesBeforeOrOn(partitionPath, latestCommitTime.getTimestamp(), true)
+              .collect(Collectors.toList());
+      for (FileSlice fileSlice : allFileSlices) {
+        if (isSmallFile(fileSlice)) {
+          allSmallFileSlices.add(fileSlice);
+        }
+      }
+      // Create SmallFiles from the eligible file slices
+      for (FileSlice smallFileSlice : allSmallFileSlices) {
+        SmallFile sf = new SmallFile();
+        if (smallFileSlice.getBaseFile().isPresent()) {
+          // TODO : Move logic of file name, file id, base commit time handling inside file slice
+          String filename = smallFileSlice.getBaseFile().get().getFileName();
+          sf.location = new HoodieRecordLocation(FSUtils.getCommitTime(filename), FSUtils.getFileId(filename));
+          sf.sizeBytes = getTotalFileSize(smallFileSlice);
+          smallFileLocations.add(sf);
+        } else {
+          HoodieLogFile logFile = smallFileSlice.getLogFiles().findFirst().get();
+          sf.location = new HoodieRecordLocation(FSUtils.getBaseCommitTimeFromLogPath(logFile.getPath()),
+              FSUtils.getFileIdFromLogPath(logFile.getPath()));
+          sf.sizeBytes = getTotalFileSize(smallFileSlice);
+          smallFileLocations.add(sf);
+        }
+      }
+    }
+    return smallFileLocations;
+  }
+
+  private long getTotalFileSize(FileSlice fileSlice) {
+    if (!fileSlice.getBaseFile().isPresent()) {
+      return convertLogFilesSizeToExpectedParquetSize(fileSlice.getLogFiles().collect(Collectors.toList()));
+    } else {
+      return fileSlice.getBaseFile().get().getFileSize()
+          + convertLogFilesSizeToExpectedParquetSize(fileSlice.getLogFiles().collect(Collectors.toList()));
+    }
+  }
+
+  private boolean isSmallFile(FileSlice fileSlice) {
+    long totalSize = getTotalFileSize(fileSlice);
+    return totalSize < config.getParquetMaxFileSize();
+  }
+
+  // TODO (NA) : Make this static part of utility
+  public long convertLogFilesSizeToExpectedParquetSize(List<HoodieLogFile> hoodieLogFiles) {
+    long totalSizeOfLogFiles = hoodieLogFiles.stream().map(HoodieLogFile::getFileSize)
+        .filter(size -> size > 0).reduce(Long::sum).orElse(0L);
+    // Here we assume that if there is no base parquet file, all log files contain only inserts.
+    // We can then just get the parquet equivalent size of these log files, compare that with
+    // {@link config.getParquetMaxFileSize()} and decide if there is scope to insert more rows
+    return (long) (totalSizeOfLogFiles * config.getLogFileToParquetCompressionRatio());
+  }
+}

hudi-flink/src/main/java/org/apache/hudi/util/StreamerUtil.java

@@ -36,6 +36,7 @@ import org.apache.hudi.exception.HoodieIOException;
 import org.apache.hudi.keygen.KeyGenerator;
 import org.apache.hudi.operator.FlinkOptions;
 import org.apache.hudi.schema.FilebasedSchemaProvider;
+import org.apache.hudi.table.action.compact.CompactionTriggerStrategy;
 
 import org.apache.avro.Schema;
 import org.apache.avro.generic.GenericRecord;
@@ -54,6 +55,7 @@ import java.io.File;
 import java.io.IOException;
 import java.io.StringReader;
 import java.util.List;
+import java.util.Locale;
 import java.util.Objects;
 import java.util.Properties;
 
@@ -229,6 +231,10 @@ public class StreamerUtil {
             .withCompactionConfig(
                 HoodieCompactionConfig.newBuilder()
                     .withPayloadClass(conf.getString(FlinkOptions.PAYLOAD_CLASS))
+                    .withInlineCompactionTriggerStrategy(
+                        CompactionTriggerStrategy.valueOf(conf.getString(FlinkOptions.COMPACTION_TRIGGER_STRATEGY).toUpperCase(Locale.ROOT)))
+                    .withMaxNumDeltaCommitsBeforeCompaction(conf.getInteger(FlinkOptions.COMPACTION_DELTA_COMMITS))
+                    .withMaxDeltaSecondsBeforeCompaction(conf.getInteger(FlinkOptions.COMPACTION_DELTA_SECONDS))
                     .build())
             .forTable(conf.getString(FlinkOptions.TABLE_NAME))
             .withAutoCommit(false)

hudi-flink/src/test/java/org/apache/hudi/operator/StreamWriteITCase.java

@@ -20,6 +20,12 @@ package org.apache.hudi.operator;
 
 import org.apache.hudi.client.WriteStatus;
 import org.apache.hudi.common.model.HoodieRecord;
+import org.apache.hudi.common.model.HoodieTableType;
+import org.apache.hudi.operator.compact.CompactFunction;
+import org.apache.hudi.operator.compact.CompactionCommitEvent;
+import org.apache.hudi.operator.compact.CompactionCommitSink;
+import org.apache.hudi.operator.compact.CompactionPlanEvent;
+import org.apache.hudi.operator.compact.CompactionPlanOperator;
 import org.apache.hudi.operator.partitioner.BucketAssignFunction;
 import org.apache.hudi.operator.transform.RowDataToHoodieFunction;
 import org.apache.hudi.operator.utils.TestConfigurations;
@@ -219,4 +225,81 @@ public class StreamWriteITCase extends TestLogger {
 
     TestData.checkWrittenFullData(tempFile, EXPECTED);
   }
+
+  @Test
+  public void testMergeOnReadWriteWithCompaction() throws Exception {
+    Configuration conf = TestConfigurations.getDefaultConf(tempFile.getAbsolutePath());
+    conf.setInteger(FlinkOptions.COMPACTION_DELTA_COMMITS, 1);
+    conf.setString(FlinkOptions.TABLE_TYPE, HoodieTableType.MERGE_ON_READ.name());
+    StreamExecutionEnvironment execEnv = StreamExecutionEnvironment.getExecutionEnvironment();
+    execEnv.getConfig().disableObjectReuse();
+    execEnv.setParallelism(4);
+    // set up checkpoint interval
+    execEnv.enableCheckpointing(4000, CheckpointingMode.EXACTLY_ONCE);
+    execEnv.getCheckpointConfig().setMaxConcurrentCheckpoints(1);
+
+    // Read from file source
+    RowType rowType =
+        (RowType) AvroSchemaConverter.convertToDataType(StreamerUtil.getSourceSchema(conf))
+            .getLogicalType();
+    StreamWriteOperatorFactory<HoodieRecord> operatorFactory =
+        new StreamWriteOperatorFactory<>(conf);
+
+    JsonRowDataDeserializationSchema deserializationSchema = new JsonRowDataDeserializationSchema(
+        rowType,
+        new RowDataTypeInfo(rowType),
+        false,
+        true,
+        TimestampFormat.ISO_8601
+    );
+    String sourcePath = Objects.requireNonNull(Thread.currentThread()
+        .getContextClassLoader().getResource("test_source.data")).toString();
+
+    TextInputFormat format = new TextInputFormat(new Path(sourcePath));
+    format.setFilesFilter(FilePathFilter.createDefaultFilter());
+    TypeInformation<String> typeInfo = BasicTypeInfo.STRING_TYPE_INFO;
+    format.setCharsetName("UTF-8");
+
+    execEnv
+        // use PROCESS_CONTINUOUSLY mode to trigger checkpoint
+        .readFile(format, sourcePath, FileProcessingMode.PROCESS_CONTINUOUSLY, 1000, typeInfo)
+        .map(record -> deserializationSchema.deserialize(record.getBytes(StandardCharsets.UTF_8)))
+        .setParallelism(4)
+        .map(new RowDataToHoodieFunction<>(rowType, conf), TypeInformation.of(HoodieRecord.class))
+        // Key-by partition path, to avoid multiple subtasks write to a partition at the same time
+        .keyBy(HoodieRecord::getPartitionPath)
+        .transform(
+            "bucket_assigner",
+            TypeInformation.of(HoodieRecord.class),
+            new KeyedProcessOperator<>(new BucketAssignFunction<>(conf)))
+        .uid("uid_bucket_assigner")
+        // shuffle by fileId(bucket id)
+        .keyBy(record -> record.getCurrentLocation().getFileId())
+        .transform("hoodie_stream_write", TypeInformation.of(Object.class), operatorFactory)
+        .uid("uid_hoodie_stream_write")
+        .transform("compact_plan_generate",
+            TypeInformation.of(CompactionPlanEvent.class),
+            new CompactionPlanOperator(conf))
+        .uid("uid_compact_plan_generate")
+        .setParallelism(1) // plan generate must be singleton
+        .keyBy(event -> event.getOperation().hashCode())
+        .transform("compact_task",
+            TypeInformation.of(CompactionCommitEvent.class),
+            new KeyedProcessOperator<>(new CompactFunction(conf)))
+        .addSink(new CompactionCommitSink(conf))
+        .name("compact_commit")
+        .setParallelism(1);
+
+    JobClient client = execEnv.executeAsync(execEnv.getStreamGraph(conf.getString(FlinkOptions.TABLE_NAME)));
+    if (client.getJobStatus().get() != JobStatus.FAILED) {
+      try {
+        TimeUnit.SECONDS.sleep(20); // wait long enough for the compaction to finish
+        client.cancel();
+      } catch (Throwable var1) {
+        // ignored
+      }
+    }
+
+    TestData.checkWrittenFullData(tempFile, EXPECTED);
+  }
 }

hudi-flink/src/test/java/org/apache/hudi/operator/TestWriteCopyOnWrite.java

@@ -22,6 +22,7 @@ import org.apache.hudi.client.HoodieFlinkWriteClient;
 import org.apache.hudi.client.WriteStatus;
 import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.common.model.HoodieRecord;
+import org.apache.hudi.common.model.HoodieTableType;
 import org.apache.hudi.common.table.timeline.HoodieInstant;
 import org.apache.hudi.exception.HoodieException;
 import org.apache.hudi.operator.event.BatchWriteSuccessEvent;
@@ -45,7 +46,6 @@ import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
 
-import static org.apache.hudi.operator.utils.TestData.checkWrittenData;
 import static org.hamcrest.CoreMatchers.instanceOf;
 import static org.hamcrest.CoreMatchers.is;
 import static org.hamcrest.MatcherAssert.assertThat;
@@ -58,13 +58,13 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
 /**
  * Test cases for StreamingSinkFunction.
  */
-public class StreamWriteFunctionTest {
+public class TestWriteCopyOnWrite {
 
-  private static final Map<String, String> EXPECTED1 = new HashMap<>();
+  protected static final Map<String, String> EXPECTED1 = new HashMap<>();
 
-  private static final Map<String, String> EXPECTED2 = new HashMap<>();
+  protected static final Map<String, String> EXPECTED2 = new HashMap<>();
 
-  private static final Map<String, String> EXPECTED3 = new HashMap<>();
+  protected static final Map<String, String> EXPECTED3 = new HashMap<>();
 
   static {
     EXPECTED1.put("par1", "[id1,par1,id1,Danny,23,1,par1, id2,par1,id2,Stephen,33,2,par1]");
@@ -82,14 +82,27 @@ public class StreamWriteFunctionTest {
     EXPECTED3.put("par1", "[id1,par1,id1,Danny,23,1,par1]");
   }
 
-  private StreamWriteFunctionWrapper<RowData> funcWrapper;
+  protected Configuration conf;
+
+  protected StreamWriteFunctionWrapper<RowData> funcWrapper;
 
   @TempDir
   File tempFile;
 
   @BeforeEach
   public void before() throws Exception {
-    this.funcWrapper = new StreamWriteFunctionWrapper<>(tempFile.getAbsolutePath());
+    final String basePath = tempFile.getAbsolutePath();
+    conf = TestConfigurations.getDefaultConf(basePath);
+    conf.setString(FlinkOptions.TABLE_TYPE, getTableType());
+    setUp(conf);
+    this.funcWrapper = new StreamWriteFunctionWrapper<>(tempFile.getAbsolutePath(), conf);
+  }
+
+  /**
+   * Override to have custom configuration.
+   */
+  protected void setUp(Configuration conf) {
+    // for sub-class extension
   }
 
   @AfterEach
@@ -114,7 +127,7 @@ public class StreamWriteFunctionTest {
     funcWrapper.checkpointFunction(1);
 
     String instant = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
 
     final OperatorEvent nextEvent = funcWrapper.getNextEvent();
     MatcherAssert.assertThat("The operator expect to send an event", nextEvent, instanceOf(BatchWriteSuccessEvent.class));
@@ -140,7 +153,7 @@ public class StreamWriteFunctionTest {
     funcWrapper.checkpointFunction(2);
 
     String instant2 = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
     assertNotEquals(instant, instant2);
 
     final OperatorEvent nextEvent2 = funcWrapper.getNextEvent();
@@ -169,7 +182,7 @@ public class StreamWriteFunctionTest {
     funcWrapper.checkpointFunction(1);
 
     String instant = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
     assertNotNull(instant);
 
     final OperatorEvent nextEvent = funcWrapper.getNextEvent();
@@ -191,10 +204,8 @@ public class StreamWriteFunctionTest {
       funcWrapper.invoke(rowData);
     }
 
-    // this triggers NPE cause there is no inflight instant
-    assertThrows(NullPointerException.class,
-        () -> funcWrapper.checkpointFunction(2),
-        "No inflight instant when flushing data");
+    // this returns early cause there is no inflight instant
+    funcWrapper.checkpointFunction(2);
     // do not sent the write event and fails the checkpoint,
     // behaves like the last checkpoint is successful.
     funcWrapper.checkpointFails(2);
@@ -213,7 +224,7 @@ public class StreamWriteFunctionTest {
     funcWrapper.checkpointFunction(1);
 
     String instant = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
 
     final OperatorEvent nextEvent = funcWrapper.getNextEvent();
     assertThat("The operator expect to send an event", nextEvent, instanceOf(BatchWriteSuccessEvent.class));
@@ -232,7 +243,6 @@ public class StreamWriteFunctionTest {
   @Test
   public void testInsertDuplicates() throws Exception {
     // reset the config option
-    Configuration conf = TestConfigurations.getDefaultConf(tempFile.getAbsolutePath());
     conf.setBoolean(FlinkOptions.INSERT_DROP_DUPS, true);
     funcWrapper = new StreamWriteFunctionWrapper<>(tempFile.getAbsolutePath(), conf);
 
@@ -300,11 +310,10 @@ public class StreamWriteFunctionTest {
     funcWrapper.checkpointFunction(2);
 
     String instant = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
 
     nextEvent = funcWrapper.getNextEvent();
     assertThat("The operator expect to send an event", nextEvent, instanceOf(BatchWriteSuccessEvent.class));
-    checkWrittenData(tempFile, EXPECTED2);
 
     funcWrapper.getCoordinator().handleEventFromOperator(0, nextEvent);
     assertNotNull(funcWrapper.getEventBuffer()[0], "The coordinator missed the event");
@@ -319,7 +328,6 @@ public class StreamWriteFunctionTest {
   @Test
   public void testInsertWithMiniBatches() throws Exception {
     // reset the config option
-    Configuration conf = TestConfigurations.getDefaultConf(tempFile.getAbsolutePath());
     conf.setDouble(FlinkOptions.WRITE_BATCH_SIZE, 0.001); // 1Kb batch size
     funcWrapper = new StreamWriteFunctionWrapper<>(tempFile.getAbsolutePath(), conf);
 
@@ -349,16 +357,11 @@ public class StreamWriteFunctionTest {
     assertNotNull(funcWrapper.getEventBuffer()[0], "The coordinator missed the event");
 
     String instant = funcWrapper.getWriteClient()
-        .getInflightAndRequestedInstant("COPY_ON_WRITE");
+        .getInflightAndRequestedInstant(getTableType());
 
     funcWrapper.checkpointComplete(1);
 
-    Map<String, String> expected = new HashMap<>();
-    expected.put("par1", "[id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1]");
+    Map<String, String> expected = getMiniBatchExpected();
     checkWrittenData(tempFile, expected, 1);
 
     // started a new instant already
@@ -384,6 +387,16 @@ public class StreamWriteFunctionTest {
     checkWrittenData(tempFile, expected, 1);
   }
 
+  Map<String, String> getMiniBatchExpected() {
+    Map<String, String> expected = new HashMap<>();
+    expected.put("par1", "[id1,par1,id1,Danny,23,1,par1, "
+        + "id1,par1,id1,Danny,23,1,par1, "
+        + "id1,par1,id1,Danny,23,1,par1, "
+        + "id1,par1,id1,Danny,23,1,par1, "
+        + "id1,par1,id1,Danny,23,1,par1]");
+    return expected;
+  }
+
   @Test
   public void testIndexStateBootstrap() throws Exception {
     // open the function and ingest data
@@ -452,7 +465,7 @@ public class StreamWriteFunctionTest {
 
   @SuppressWarnings("rawtypes")
   private void checkInflightInstant(HoodieFlinkWriteClient writeClient) {
-    final String instant = writeClient.getInflightAndRequestedInstant("COPY_ON_WRITE");
+    final String instant = writeClient.getInflightAndRequestedInstant(getTableType());
     assertNotNull(instant);
   }
 
@@ -464,10 +477,10 @@ public class StreamWriteFunctionTest {
     final String instant;
     switch (state) {
       case REQUESTED:
-        instant = writeClient.getInflightAndRequestedInstant("COPY_ON_WRITE");
+        instant = writeClient.getInflightAndRequestedInstant(getTableType());
         break;
       case COMPLETED:
-        instant = writeClient.getLastCompletedInstant("COPY_ON_WRITE");
+        instant = writeClient.getLastCompletedInstant(getTableType());
         break;
       default:
         throw new AssertionError("Unexpected state");
@@ -475,10 +488,22 @@ public class StreamWriteFunctionTest {
     assertThat(instant, is(instantStr));
   }
 
+  protected String getTableType() {
+    return HoodieTableType.COPY_ON_WRITE.name();
+  }
+
+  protected void checkWrittenData(File baseFile, Map<String, String> expected) throws Exception {
+    checkWrittenData(baseFile, expected, 4);
+  }
+
+  protected void checkWrittenData(File baseFile, Map<String, String> expected, int partitions) throws Exception {
+    TestData.checkWrittenData(baseFile, expected, partitions);
+  }
+
   /**
    * Asserts the data files are empty.
    */
-  private void assertEmptyDataFiles() {
+  protected void assertEmptyDataFiles() {
     File[] dataFiles = tempFile.listFiles(file -> !file.getName().startsWith("."));
     assertNotNull(dataFiles);
     assertThat(dataFiles.length, is(0));

hudi-flink/src/test/java/org/apache/hudi/operator/TestWriteMergeOnRead.java

@@ -0,0 +1,96 @@
+/*
+ * 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.hudi.operator;
+
+import org.apache.hudi.client.FlinkTaskContextSupplier;
+import org.apache.hudi.client.common.HoodieFlinkEngineContext;
+import org.apache.hudi.common.config.SerializableConfiguration;
+import org.apache.hudi.common.fs.FSUtils;
+import org.apache.hudi.common.model.HoodieTableType;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.TableSchemaResolver;
+import org.apache.hudi.common.table.timeline.HoodieActiveTimeline;
+import org.apache.hudi.common.table.timeline.HoodieInstant;
+import org.apache.hudi.config.HoodieWriteConfig;
+import org.apache.hudi.operator.utils.TestData;
+import org.apache.hudi.table.HoodieFlinkTable;
+import org.apache.hudi.util.StreamerUtil;
+
+import org.apache.avro.Schema;
+import org.apache.hadoop.fs.FileSystem;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+/**
+ * Test cases for delta stream write.
+ */
+public class TestWriteMergeOnRead extends TestWriteCopyOnWrite {
+  private FileSystem fs;
+  private HoodieWriteConfig writeConfig;
+  private HoodieFlinkEngineContext context;
+
+  @BeforeEach
+  public void before() throws Exception {
+    super.before();
+    fs = FSUtils.getFs(tempFile.getAbsolutePath(), new org.apache.hadoop.conf.Configuration());
+    writeConfig = StreamerUtil.getHoodieClientConfig(conf);
+    context = new HoodieFlinkEngineContext(
+        new SerializableConfiguration(StreamerUtil.getHadoopConf()),
+        new FlinkTaskContextSupplier(null));
+  }
+
+  @Override
+  protected void checkWrittenData(File baseFile, Map<String, String> expected, int partitions) throws Exception {
+    HoodieTableMetaClient metaClient = HoodieFlinkTable.create(writeConfig, context).getMetaClient();
+    Schema schema = new TableSchemaResolver(metaClient).getTableAvroSchema();
+    String latestInstant = metaClient.getCommitsTimeline().filterCompletedInstants()
+        .getInstants()
+        .filter(x -> x.getAction().equals(HoodieActiveTimeline.DELTA_COMMIT_ACTION))
+        .map(HoodieInstant::getTimestamp)
+        .collect(Collectors.toList()).stream()
+        .max(Comparator.naturalOrder())
+        .orElse(null);
+    TestData.checkWrittenDataMOR(fs, latestInstant, baseFile, expected, partitions, schema);
+  }
+
+  @Disabled
+  @Test
+  public void testIndexStateBootstrap() {
+    // Ignore the index bootstrap because we only support parquet load now.
+  }
+
+  Map<String, String> getMiniBatchExpected() {
+    Map<String, String> expected = new HashMap<>();
+    // MOR mode merges the messages with the same key.
+    expected.put("par1", "[id1,par1,id1,Danny,23,1,par1]");
+    return expected;
+  }
+
+  @Override
+  protected String getTableType() {
+    return HoodieTableType.MERGE_ON_READ.name();
+  }
+}

hudi-flink/src/test/java/org/apache/hudi/operator/TestWriteMergeOnReadWithCompact.java

@@ -0,0 +1,58 @@
+/*
+ * 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.hudi.operator;
+
+import org.apache.hudi.common.model.HoodieTableType;
+
+import org.apache.flink.configuration.Configuration;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Test cases for delta stream write with compaction.
+ */
+public class TestWriteMergeOnReadWithCompact extends TestWriteCopyOnWrite {
+
+  @Override
+  protected void setUp(Configuration conf) {
+    // trigger the compaction for every finished checkpoint
+    conf.setInteger(FlinkOptions.COMPACTION_DELTA_COMMITS, 1);
+  }
+
+  @Disabled
+  @Test
+  public void testIndexStateBootstrap() {
+    // Ignore the index bootstrap because we only support parquet load now.
+  }
+
+  Map<String, String> getMiniBatchExpected() {
+    Map<String, String> expected = new HashMap<>();
+    // MOR mode merges the messages with the same key.
+    expected.put("par1", "[id1,par1,id1,Danny,23,1,par1]");
+    return expected;
+  }
+
+  @Override
+  protected String getTableType() {
+    return HoodieTableType.MERGE_ON_READ.name();
+  }
+}

hudi-flink/src/test/java/org/apache/hudi/operator/utils/CompactFunctionWrapper.java

@@ -0,0 +1,142 @@
+/*
+ * 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.hudi.operator.utils;
+
+import org.apache.hudi.avro.model.HoodieCompactionPlan;
+import org.apache.hudi.operator.compact.CompactFunction;
+import org.apache.hudi.operator.compact.CompactionCommitEvent;
+import org.apache.hudi.operator.compact.CompactionCommitSink;
+import org.apache.hudi.operator.compact.CompactionPlanEvent;
+import org.apache.hudi.operator.compact.CompactionPlanOperator;
+
+import org.apache.flink.configuration.Configuration;
+import org.apache.flink.runtime.io.disk.iomanager.IOManager;
+import org.apache.flink.runtime.io.disk.iomanager.IOManagerAsync;
+import org.apache.flink.runtime.memory.MemoryManager;
+import org.apache.flink.runtime.operators.testutils.MockEnvironment;
+import org.apache.flink.runtime.operators.testutils.MockEnvironmentBuilder;
+import org.apache.flink.streaming.api.operators.Output;
+import org.apache.flink.streaming.api.operators.StreamingRuntimeContext;
+import org.apache.flink.streaming.api.watermark.Watermark;
+import org.apache.flink.streaming.runtime.streamrecord.LatencyMarker;
+import org.apache.flink.streaming.runtime.streamrecord.StreamRecord;
+import org.apache.flink.util.Collector;
+import org.apache.flink.util.OutputTag;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * A wrapper class to manipulate the {@link org.apache.hudi.operator.compact.CompactFunction} instance for testing.
+ */
+public class CompactFunctionWrapper {
+  private final Configuration conf;
+
+  private final IOManager ioManager;
+  private final StreamingRuntimeContext runtimeContext;
+  private final MockFunctionInitializationContext functionInitializationContext;
+
+  /** Function that generates the {@link HoodieCompactionPlan}. */
+  private CompactionPlanOperator compactionPlanFunction;
+  /** Function that executes the compaction task. */
+  private CompactFunction compactFunction;
+  /** Stream sink to handle compaction commits. */
+  private CompactionCommitSink commitSink;
+
+  public CompactFunctionWrapper(Configuration conf) throws Exception {
+    this.ioManager = new IOManagerAsync();
+    MockEnvironment environment = new MockEnvironmentBuilder()
+        .setTaskName("mockTask")
+        .setManagedMemorySize(4 * MemoryManager.DEFAULT_PAGE_SIZE)
+        .setIOManager(ioManager)
+        .build();
+    this.runtimeContext = new MockStreamingRuntimeContext(false, 1, 0, environment);
+    this.conf = conf;
+    this.functionInitializationContext = new MockFunctionInitializationContext();
+  }
+
+  public void openFunction() throws Exception {
+    compactionPlanFunction = new CompactionPlanOperator(conf);
+    compactionPlanFunction.open();
+
+    compactFunction = new CompactFunction(conf);
+    compactFunction.setRuntimeContext(runtimeContext);
+    compactFunction.open(conf);
+
+    commitSink = new CompactionCommitSink(conf);
+    commitSink.setRuntimeContext(runtimeContext);
+    commitSink.open(conf);
+  }
+
+  public void compact(long checkpointID) throws Exception {
+    List<CompactionPlanEvent> events = new ArrayList<>();
+    // collect the CompactEvents.
+    Output<StreamRecord<CompactionPlanEvent>> output = new Output<StreamRecord<CompactionPlanEvent>>() {
+      @Override
+      public void emitWatermark(Watermark watermark) {
+
+      }
+
+      @Override
+      public <X> void collect(OutputTag<X> outputTag, StreamRecord<X> streamRecord) {
+
+      }
+
+      @Override
+      public void emitLatencyMarker(LatencyMarker latencyMarker) {
+
+      }
+
+      @Override
+      public void collect(StreamRecord<CompactionPlanEvent> record) {
+        events.add(record.getValue());
+      }
+
+      @Override
+      public void close() {
+
+      }
+    };
+    compactionPlanFunction.setOutput(output);
+    compactionPlanFunction.notifyCheckpointComplete(checkpointID);
+    // collect the CompactCommitEvents
+    List<CompactionCommitEvent> compactCommitEvents = new ArrayList<>();
+    for (CompactionPlanEvent event: events) {
+      compactFunction.processElement(event, null, new Collector<CompactionCommitEvent>() {
+        @Override
+        public void collect(CompactionCommitEvent event) {
+          compactCommitEvents.add(event);
+        }
+
+        @Override
+        public void close() {
+
+        }
+      });
+    }
+    // handle and commit the compaction
+    for (CompactionCommitEvent event : compactCommitEvents) {
+      commitSink.invoke(event, null);
+    }
+  }
+
+  public void close() throws Exception {
+    ioManager.close();
+  }
+}

hudi-flink/src/test/java/org/apache/hudi/operator/utils/StreamWriteFunctionWrapper.java

@@ -21,6 +21,8 @@ package org.apache.hudi.operator.utils;
 import org.apache.hudi.client.HoodieFlinkWriteClient;
 import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.common.model.HoodieRecord;
+import org.apache.hudi.exception.HoodieException;
+import org.apache.hudi.operator.FlinkOptions;
 import org.apache.hudi.operator.StreamWriteFunction;
 import org.apache.hudi.operator.StreamWriteOperatorCoordinator;
 import org.apache.hudi.operator.event.BatchWriteSuccessEvent;
@@ -64,6 +66,8 @@ public class StreamWriteFunctionWrapper<I> {
   /** Stream write function. */
   private StreamWriteFunction<Object, HoodieRecord<?>, Object> writeFunction;
 
+  private CompactFunctionWrapper compactFunctionWrapper;
+
   public StreamWriteFunctionWrapper(String tablePath) throws Exception {
     this(tablePath, TestConfigurations.getDefaultConf(tablePath));
   }
@@ -81,6 +85,7 @@ public class StreamWriteFunctionWrapper<I> {
     // one function
     this.coordinator = new StreamWriteOperatorCoordinator(conf, 1);
     this.functionInitializationContext = new MockFunctionInitializationContext();
+    this.compactFunctionWrapper = new CompactFunctionWrapper(this.conf);
   }
 
   public void openFunction() throws Exception {
@@ -98,6 +103,10 @@ public class StreamWriteFunctionWrapper<I> {
     writeFunction.setRuntimeContext(runtimeContext);
     writeFunction.setOperatorEventGateway(gateway);
     writeFunction.open(conf);
+
+    if (conf.getBoolean(FlinkOptions.COMPACTION_ASYNC_ENABLED)) {
+      compactFunctionWrapper.openFunction();
+    }
   }
 
   public void invoke(I record) throws Exception {
@@ -149,6 +158,13 @@ public class StreamWriteFunctionWrapper<I> {
     coordinator.checkpointComplete(checkpointId);
     this.bucketAssignerFunction.notifyCheckpointComplete(checkpointId);
     this.writeFunction.notifyCheckpointComplete(checkpointId);
+    if (conf.getBoolean(FlinkOptions.COMPACTION_ASYNC_ENABLED)) {
+      try {
+        compactFunctionWrapper.compact(checkpointId);
+      } catch (Exception e) {
+        throw new HoodieException(e);
+      }
+    }
   }
 
   public void checkpointFails(long checkpointId) {

hudi-flink/src/test/java/org/apache/hudi/operator/utils/TestData.java

@@ -22,10 +22,12 @@ import org.apache.hudi.client.FlinkTaskContextSupplier;
 import org.apache.hudi.client.common.HoodieFlinkEngineContext;
 import org.apache.hudi.common.fs.FSUtils;
 import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.log.HoodieMergedLogRecordScanner;
 import org.apache.hudi.common.testutils.HoodieTestUtils;
 import org.apache.hudi.config.HoodieWriteConfig;
 import org.apache.hudi.table.HoodieFlinkTable;
 
+import org.apache.avro.Schema;
 import org.apache.avro.generic.GenericRecord;
 import org.apache.flink.table.data.RowData;
 import org.apache.flink.table.data.StringData;
@@ -36,6 +38,7 @@ import org.apache.flink.table.data.writer.BinaryWriter;
 import org.apache.flink.table.runtime.types.InternalSerializers;
 import org.apache.flink.table.types.logical.LogicalType;
 import org.apache.flink.table.types.logical.RowType;
+import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
 import org.apache.parquet.Strings;
 import org.apache.parquet.avro.AvroParquetReader;
@@ -49,6 +52,8 @@ import java.util.Arrays;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
+import java.util.Properties;
+import java.util.stream.Collectors;
 import java.util.stream.IntStream;
 
 import static junit.framework.TestCase.assertEquals;
@@ -107,6 +112,28 @@ public class TestData {
             TimestampData.fromEpochMillis(1), StringData.fromString("par1"))));
   }
 
+  public static List<RowData> DATA_SET_FOUR = Arrays.asList(
+      // update: advance the age by 1
+      binaryRow(StringData.fromString("id1"), StringData.fromString("Danny"), 24,
+          TimestampData.fromEpochMillis(2), StringData.fromString("par1")),
+      binaryRow(StringData.fromString("id2"), StringData.fromString("Stephen"), 34,
+          TimestampData.fromEpochMillis(3), StringData.fromString("par1")),
+      binaryRow(StringData.fromString("id3"), StringData.fromString("Julian"), 54,
+          TimestampData.fromEpochMillis(4), StringData.fromString("par2")),
+      binaryRow(StringData.fromString("id4"), StringData.fromString("Fabian"), 32,
+          TimestampData.fromEpochMillis(5), StringData.fromString("par2")),
+      // same with before
+      binaryRow(StringData.fromString("id5"), StringData.fromString("Sophia"), 18,
+          TimestampData.fromEpochMillis(6), StringData.fromString("par3")),
+      // new data
+      binaryRow(StringData.fromString("id9"), StringData.fromString("Jane"), 19,
+          TimestampData.fromEpochMillis(6), StringData.fromString("par3")),
+      binaryRow(StringData.fromString("id10"), StringData.fromString("Ella"), 38,
+          TimestampData.fromEpochMillis(7), StringData.fromString("par4")),
+      binaryRow(StringData.fromString("id11"), StringData.fromString("Phoebe"), 52,
+          TimestampData.fromEpochMillis(8), StringData.fromString("par4"))
+  );
+
   /**
    * Checks the source data TestConfigurations.DATA_SET_ONE are written as expected.
    *
@@ -201,6 +228,75 @@ public class TestData {
 
   }
 
+  /**
+   * Checks the MERGE_ON_READ source data are written as expected.
+   *
+   * <p>Note: Replace it with the Flink reader when it is supported.
+   *
+   * @param fs         The file system
+   * @param latestInstant The latest committed instant of current table
+   * @param baseFile   The file base to check, should be a directory
+   * @param expected   The expected results mapping, the key should be the partition path
+   * @param partitions The expected partition number
+   * @param schema     The read schema
+   */
+  public static void checkWrittenDataMOR(
+      FileSystem fs,
+      String latestInstant,
+      File baseFile,
+      Map<String, String> expected,
+      int partitions,
+      Schema schema) {
+    assert baseFile.isDirectory() : "Base path should be a directory";
+    FileFilter partitionFilter = file -> !file.getName().startsWith(".");
+    File[] partitionDirs = baseFile.listFiles(partitionFilter);
+    assertNotNull(partitionDirs);
+    assertThat(partitionDirs.length, is(partitions));
+    for (File partitionDir : partitionDirs) {
+      File[] dataFiles = partitionDir.listFiles(file ->
+          file.getName().contains(".log.") && !file.getName().startsWith(".."));
+      assertNotNull(dataFiles);
+      HoodieMergedLogRecordScanner scanner = getScanner(
+          fs, baseFile.getPath(), Arrays.stream(dataFiles).map(File::getAbsolutePath)
+              .sorted(Comparator.naturalOrder()).collect(Collectors.toList()),
+          schema, latestInstant);
+      List<String> readBuffer = scanner.getRecords().values().stream()
+          .map(hoodieRecord -> {
+            try {
+              return filterOutVariables((GenericRecord) hoodieRecord.getData().getInsertValue(schema, new Properties()).get());
+            } catch (IOException e) {
+              throw new RuntimeException(e);
+            }
+          })
+          .sorted(Comparator.naturalOrder())
+          .collect(Collectors.toList());
+      assertThat(readBuffer.toString(), is(expected.get(partitionDir.getName())));
+    }
+  }
+
+  /**
+   * Returns the scanner to read avro log files.
+   */
+  private static HoodieMergedLogRecordScanner getScanner(
+      FileSystem fs,
+      String basePath,
+      List<String> logPaths,
+      Schema readSchema,
+      String instant) {
+    return HoodieMergedLogRecordScanner.newBuilder()
+        .withFileSystem(fs)
+        .withBasePath(basePath)
+        .withLogFilePaths(logPaths)
+        .withReaderSchema(readSchema)
+        .withLatestInstantTime(instant)
+        .withReadBlocksLazily(false)
+        .withReverseReader(false)
+        .withBufferSize(16 * 1024 * 1024)
+        .withMaxMemorySizeInBytes(1024 * 1024L)
+        .withSpillableMapBasePath("/tmp/")
+        .build();
+  }
+
   /**
    * Filter out the variables like file name.
    */