[GitHub] [lucene-solr] atris commented on a change in pull request #916: LUCENE-8213: Asynchronous Caching in LRUQueryCache

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atris commented on a change in pull request #916: LUCENE-8213: Asynchronous Caching in LRUQueryCache
URL: https://github.com/apache/lucene-solr/pull/916#discussion_r330575572
 
 

 ##########
 File path: lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java
 ##########
 @@ -88,13 +93,36 @@
  * @lucene.experimental
  */
 public class LRUQueryCache implements QueryCache, Accountable {
+  /** Act as key for the inflight queries map */
+  private static class MapKey {
+    private final Query query;
+    private final IndexReader.CacheKey cacheKey;
+
+    public MapKey(Query query, IndexReader.CacheKey cacheKey) {
+      this.query = query;
+      this.cacheKey = cacheKey;
+    }
+
+    public Query getQuery() {
+      return query;
+    }
+
+    public IndexReader.CacheKey getCacheKey() {
+      return cacheKey;
+    }
+  }
 
   private final int maxSize;
   private final long maxRamBytesUsed;
   private final Predicate<LeafReaderContext> leavesToCache;
   // maps queries that are contained in the cache to a singleton so that this
   // cache does not store several copies of the same query
   private final Map<Query, Query> uniqueQueries;
+  // Marks the inflight queries that are being asynchronously loaded into the cache
+  // This is used to ensure that multiple threads do not trigger loading
+  // of the same query in the same cache. We use a set because it is an invariant that
+  // the entries of this data structure be unique.
+  private final Map<MapKey, IndexReader.CacheKey> inFlightAsyncLoadQueries = new HashMap<>();
 
 Review comment:
   I originally used a Set, but moved it to a map specifically to enable tests for the double caching case.

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