rdblue commented on a change in pull request #2945:
URL: https://github.com/apache/iceberg/pull/2945#discussion_r711367239



##########
File path: core/src/main/java/org/apache/iceberg/io/ClusteredFileWriter.java
##########
@@ -0,0 +1,121 @@
+/*
+ * 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.io;
+
+import java.io.IOException;
+import java.util.Comparator;
+import java.util.Set;
+import org.apache.iceberg.PartitionSpec;
+import org.apache.iceberg.StructLike;
+import org.apache.iceberg.relocated.com.google.common.base.Preconditions;
+import org.apache.iceberg.relocated.com.google.common.collect.Sets;
+import org.apache.iceberg.types.Comparators;
+import org.apache.iceberg.types.Types;
+import org.apache.iceberg.util.StructLikeSet;
+
+/**
+ * A writer capable of writing to multiple specs and partitions that requires 
the incoming records
+ * to be clustered by partition spec and partition.
+ * <p>
+ * As opposed to {@link FanoutFileWriter}, this writer keeps at most one file 
open to reduce
+ * the memory consumption.
+ */
+abstract class ClusteredFileWriter<T, R> implements 
PartitionAwareFileWriter<T, R> {
+
+  private final Set<Integer> completedSpecs = Sets.newHashSet();
+
+  private PartitionSpec currentSpec = null;
+  private Comparator<StructLike> partitionComparator = null;
+  private Set<StructLike> completedPartitions = null;
+  private StructLike currentPartition = null;
+  private FileWriter<T, R> currentWriter = null;
+
+  private boolean closed = false;
+
+  protected abstract FileWriter<T, R> newWriter(PartitionSpec spec, StructLike 
partition);
+
+  protected abstract void addResult(R result);
+
+  protected abstract R aggregatedResult();
+
+  @Override
+  public void write(T row, PartitionSpec spec, StructLike partition) throws 
IOException {
+    if (!spec.equals(currentSpec)) {
+      if (currentSpec != null) {
+        closeCurrent();
+        completedSpecs.add(currentSpec.specId());
+        completedPartitions.clear();
+      }
+
+      if (completedSpecs.contains(spec.specId())) {
+        throw new IllegalStateException("Already closed files for spec: " + 
spec.specId());
+      }
+
+      Types.StructType partitionType = spec.partitionType();
+
+      currentSpec = spec;
+      partitionComparator = Comparators.forType(partitionType);
+      completedPartitions = StructLikeSet.create(partitionType);

Review comment:
       This seems to assume that partitions will be grouped by spec. Do you 
think that's likely?




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