atris commented on a change in pull request #815: LUCENE-8213: Introduce Asynchronous Caching in LRUQueryCache URL: https://github.com/apache/lucene-solr/pull/815#discussion_r324497573
########## File path: lucene/core/src/java/org/apache/lucene/search/LRUQueryCache.java ########## @@ -89,12 +92,30 @@ */ public class LRUQueryCache implements QueryCache, Accountable { + /** Throw this error when a query is asynchronously being loaded in the cache + * and a read for the same is requested + */ + private static final class AsyncQueryLoadInProgressException extends RuntimeException { + + /** + * Sole constructor. + */ + public AsyncQueryLoadInProgressException(String query) { + super(query); + } + } + 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 primarily 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 Set<Query> inFlightAsyncLoadQueries = Collections.newSetFromMap(new ConcurrentHashMap<Query,Boolean>()); Review comment: We add to this map without a lock when triggering a caching operation, hence the need for a concurrent safe set. The idea is that it is a better idea to let two threads request an asynchronous cache load for the same query concurrently than syncing the entire operation under a lock. However, this means that two threads might want to add the same key to `inFlightAsyncLoadQueries` in the same duration, so we need that operation to be idempotent and thread safe. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@lucene.apache.org For additional commands, e-mail: issues-h...@lucene.apache.org