[GitHub] [pulsar] ben-manes commented on a change in pull request #14283: [metadata-store] Fix race condition by making metadata cache refresh async

[GitBox <gi...@apache.org>]

ben-manes commented on a change in pull request #14283:
URL: https://github.com/apache/pulsar/pull/14283#discussion_r806473435



##########
File path: pulsar-metadata/src/main/java/org/apache/pulsar/metadata/cache/impl/MetadataCacheImpl.java
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@@ -261,12 +261,17 @@ public void invalidate(String path) {
     }
 
     @Override
-    public void refresh(String path) {
+    public CompletableFuture<Void> refresh(String path) {
         // Refresh object of path if only it is cached before.
-        if (objCache.getIfPresent(path) != null) {
-            objCache.synchronous().invalidate(path);
-            objCache.synchronous().refresh(path);
-        }
+        CompletableFuture<Void> promise = new CompletableFuture<>();
+        this.store.getMetadataCacheScheduler().executeOrdered(path, () -> {
+            if (objCache.getIfPresent(path) != null) {
+                objCache.synchronous().invalidate(path);
+                objCache.synchronous().refresh(path);
+            }
+            promise.complete(null);
+        });
+        return promise;

Review comment:
       I'm not sure if it helps, but in [Caffeine 3.x](https://github.com/ben-manes/caffeine/releases/tag/v3.0.0) the refresh behavior was improved to be linearizable. It also now returns te in-flight `CompletableFuture`. You might consider upgrading.
   
   I am also not sure why this is invalidating and refreshing. Since the value is no longer present then the refresh is simply a cache load, and itself delegates to `AsyncCache.get(key)`, which of course also returns that future. You might also consider using `if (objCache.asMap().remove(key) != null) { ... }` instead of the get/invalidate.
   
   Trying to grok the code, I'd probably use `objCache.asMap().computeIfPresent(key, (k, future) -> ...)` to explicitly store a new future and return that upstream. Then it is all an atomic swap with very clear code that localizes and shows your intent.




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