rdblue commented on a change in pull request #2945:
URL: https://github.com/apache/iceberg/pull/2945#discussion_r711368342



##########
File path: core/src/main/java/org/apache/iceberg/io/ClusteredFileWriter.java
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@@ -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());

Review comment:
       This doesn't need to be a requirement. We could pass (spec1, part1), 
(spec2, part3), (spec1, part2) and nothing would really break other than this. 
Do you think it is likely that clustering by spec is going to be the case most 
of the time? I'm not sure if this is something we should worry about removing.




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