[GitHub] [kafka] cmccabe commented on a change in pull request #11311: KAFKA-13280: Avoid O(N) behavior in KRaftMetadataCache#topicNamesToIds

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cmccabe commented on a change in pull request #11311:
URL: https://github.com/apache/kafka/pull/11311#discussion_r712456110



##########
File path: metadata/src/main/java/org/apache/kafka/image/TopicsImage.java
##########
@@ -92,6 +98,148 @@ public int hashCode() {
         return Objects.hash(topicsById, topicsByName);
     }
 
+    /**
+     * Expose a view of this TopicsImage as a map from topic names to IDs.
+     *
+     * Like TopicsImage itself, this map is immutable.
+     */
+    public Map<String, Uuid> topicNameToIdView() {
+        return new TopicNameToIdMap();
+    }
+
+    class TopicNameToIdMap extends AbstractMap<String, Uuid> {
+        private final TopicNameToIdMapEntrySet set = new TopicNameToIdMapEntrySet();
+
+        @Override
+        public boolean containsKey(Object key) {
+            return topicsByName.containsKey(key);
+        }
+
+        @Override
+        public Uuid get(Object key) {
+            TopicImage image = topicsByName.get(key);
+            if (image == null) return null;
+            return image.id();
+        }
+
+        @Override
+        public Set<Entry<String, Uuid>> entrySet() {
+            return set;
+        }
+    }
+
+    class TopicNameToIdMapEntrySet extends AbstractSet<Entry<String, Uuid>> {
+        @Override
+        public Iterator<Entry<String, Uuid>> iterator() {
+            return new TopicNameToIdMapEntrySetIterator(topicsByName.entrySet().iterator());
+        }
+
+        @SuppressWarnings("rawtypes")
+        @Override
+        public boolean contains(Object o) {
+            if (!(o instanceof Entry)) return false;
+            Entry other = (Entry) o;
+            TopicImage image = topicsByName.get(other.getKey());
+            if (image == null) return false;
+            return image.id().equals(other.getValue());
+        }
+
+        @Override
+        public int size() {
+            return topicsByName.size();
+        }
+    }
+
+    static class TopicNameToIdMapEntrySetIterator implements Iterator<Entry<String, Uuid>> {
+        private final Iterator<Entry<String, TopicImage>> iterator;
+
+        TopicNameToIdMapEntrySetIterator(Iterator<Entry<String, TopicImage>> iterator) {
+            this.iterator = iterator;
+        }
+
+        @Override
+        public boolean hasNext() {
+            return this.iterator.hasNext();
+        }
+
+        @Override
+        public Entry<String, Uuid> next() {
+            Entry<String, TopicImage> entry = iterator.next();
+            return new SimpleImmutableEntry<>(entry.getKey(), entry.getValue().id());

Review comment:
       Allocating a short-lived temporary object is typically almost free, unless the number of objects you allocate overwhelms the eden space in the GC. And people are not supposed to be doing O(num_topics) iterations anyway (if they are, that is a bug)




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