Re: [PR] Prevent race in SingleDirectoryDbLedgerStorage metrics collection [bookkeeper]

["dlg99 (via GitHub)" <gi...@apache.org>]

dlg99 commented on code in PR #4307:
URL: https://github.com/apache/bookkeeper/pull/4307#discussion_r1619621006


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bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java:
##########
@@ -207,10 +209,17 @@ public SingleDirectoryDbLedgerStorage(ServerConfiguration conf, LedgerManager le
         gcThread = new GarbageCollectorThread(conf,
                 ledgerManager, ledgerDirsManager, this, entryLogger, ledgerIndexDirStatsLogger);
 
+        // Because the writeCache and the writeCacheBeingFlushed are swapped during flushes, it is
+        // safer to acquire the underlying cache size and count references and then use those for metrics.
+        final AtomicLong finalWriteCacheSize = writeCache.cacheSize;
+        final AtomicLong finalWriteCacheBeingFlushedSize = writeCacheBeingFlushed.cacheSize;
+        final LongAdder finalWriteCacheCounter = writeCache.cacheCount;
+        final LongAdder finalWriteCacheBeingFlushedCounter = writeCacheBeingFlushed.cacheCount;
+

Review Comment:
   I don't think this is right.
   Lambdas in the DbLedgerStorageStats() constructor used to provide a way to access to metrics of current writeCache/.. instance.
   With this change it will provide metrics for the first used instance, one before the very first swap, so the stats will stop changing after the cache swaps couple of times. 



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