[GitHub] [kafka] cadonna commented on a change in pull request #9609: KAFKA-6687: restrict DSL to allow only Streams from the same source topics

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cadonna commented on a change in pull request #9609:
URL: https://github.com/apache/kafka/pull/9609#discussion_r533990330



##########
File path: streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/SourceGraphNode.java
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@@ -0,0 +1,71 @@
+/*
+ * 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.kafka.streams.kstream.internals.graph;
+
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.regex.Pattern;
+import org.apache.kafka.common.serialization.Serde;
+import org.apache.kafka.streams.kstream.internals.ConsumedInternal;
+
+abstract public class SourceGraphNode<K, V> extends StreamsGraphNode {
+
+    private Collection<String> topicNames;
+    private Pattern topicPattern;
+    private final ConsumedInternal<K, V> consumedInternal;
+
+    public SourceGraphNode(final String nodeName,
+                            final Collection<String> topicNames,
+                            final ConsumedInternal<K, V> consumedInternal) {
+        super(nodeName);
+
+        this.topicNames = topicNames;
+        this.consumedInternal = consumedInternal;
+    }
+
+    public SourceGraphNode(final String nodeName,
+                            final Pattern topicPattern,
+                            final ConsumedInternal<K, V> consumedInternal) {
+
+        super(nodeName);
+
+        this.topicPattern = topicPattern;
+        this.consumedInternal = consumedInternal;
+    }
+
+    public Set<String> topicNames() {
+        return new HashSet<>(topicNames);

Review comment:
       Fair enough and I think that this is nothing urgent or absolute necessary. However, I would like to explain my line of thoughts. I think an interface of a class should also describe the constraints on the the object and as far as I see it does not make any sense to pass the same topic name multiple times to a source node. 
   I do not see an issue with making the same conversion in more places and actually this is even not true because the only place we would do a conversion is in `StreamsBuilder#stream()`. All other dependent calls create a singleton collection which can be easily replaced with a singleton set. Actually, I do not understand why `StreamsBuilder#stream()` takes a collection instead of a set.
   
   I am not sure I can follow your other argument
   
   >  the actual callers of the constructor don't care at all whether it's a Set or any other Collection
    
   Do you refer to the creation of the singleton collection in the callers?
   
   As I said, I do not say we need to follow my proposal. I just wanted to argue in favor of a cleaner and more descriptive interface.




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