pvary commented on code in PR #8553:
URL: https://github.com/apache/iceberg/pull/8553#discussion_r1339688528


##########
flink/v1.17/flink/src/main/java/org/apache/iceberg/flink/source/reader/WatermarkExtractorRecordEmitter.java:
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@@ -0,0 +1,64 @@
+/*
+ * 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.iceberg.flink.source.reader;
+
+import org.apache.flink.api.common.eventtime.Watermark;
+import org.apache.flink.api.connector.source.SourceOutput;
+import org.apache.iceberg.flink.source.split.IcebergSourceSplit;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Emitter which emits the record with event time and updates the split 
position.
+ *
+ * <p>The Emitter also emits watermarks at the beginning of every split, and 
sets the event
+ * timestamp based on the provided {@link IcebergWatermarkExtractor}.
+ */
+class WatermarkExtractorRecordEmitter<T> implements 
SerializableRecordEmitter<T> {
+  private static final Logger LOG = 
LoggerFactory.getLogger(WatermarkExtractorRecordEmitter.class);
+  private final IcebergWatermarkExtractor timeExtractor;
+  private String lastSplit = null;
+  private long watermark;
+
+  WatermarkExtractorRecordEmitter(IcebergWatermarkExtractor timeExtractor) {
+    this.timeExtractor = timeExtractor;
+  }
+
+  @Override
+  public void emitRecord(
+      RecordAndPosition<T> element, SourceOutput<T> output, IcebergSourceSplit 
split) {
+    if (!split.splitId().equals(lastSplit)) {
+      long newWatermark = timeExtractor.extractWatermark(split);
+      if (newWatermark < watermark) {
+        LOG.warn(
+            "Watermark decreased. PreviousWM {}, currentWM {}, previousSplit 
{}, currentSplit {}.",
+            watermark,
+            newWatermark,
+            lastSplit,
+            split.splitId());
+      }

Review Comment:
   Let me put this on `info` level then.
   TBH, I still think that having non-monotonically increasing watermark is an 
issue, but I am not sure how strict Flink in this regard. It is not too hard to 
use the max of the received watermarks, so Flink could do it, or the extractor 
could to it...



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