lianetm commented on code in PR #17199:
URL: https://github.com/apache/kafka/pull/17199#discussion_r1872002514


##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java:
##########
@@ -1893,25 +1901,30 @@ private void subscribeInternal(Collection<String> 
topics, Optional<ConsumerRebal
      * It is possible that {@link ErrorEvent an error}
      * could occur when processing the events. In such cases, the processor 
will take a reference to the first
      * error, continue to process the remaining events, and then throw the 
first error that occurred.
+     *
+     * Visible for testing.
      */
-    private boolean processBackgroundEvents() {
+    boolean processBackgroundEvents() {
         AtomicReference<KafkaException> firstError = new AtomicReference<>();
 
-        LinkedList<BackgroundEvent> events = new LinkedList<>();
-        backgroundEventQueue.drainTo(events);
-
-        for (BackgroundEvent event : events) {
-            try {
-                if (event instanceof CompletableEvent)
-                    backgroundEventReaper.add((CompletableEvent<?>) event);
-
-                backgroundEventProcessor.process(event);
-            } catch (Throwable t) {
-                KafkaException e = ConsumerUtils.maybeWrapAsKafkaException(t);
-
-                if (!firstError.compareAndSet(null, e))
-                    log.warn("An error occurred when processing the background 
event: {}", e.getMessage(), e);
+        List<BackgroundEvent> events = backgroundEventHandler.drainEvents();
+        if (!events.isEmpty()) {
+            long startMs = time.milliseconds();
+            for (BackgroundEvent event : events) {
+                
kafkaConsumerMetrics.recordBackgroundEventQueueTime(time.milliseconds() - 
event.enqueuedMs());
+                try {
+                    if (event instanceof CompletableEvent)
+                        backgroundEventReaper.add((CompletableEvent<?>) event);
+
+                    backgroundEventProcessor.process(event);
+                } catch (Throwable t) {
+                    KafkaException e = 
ConsumerUtils.maybeWrapAsKafkaException(t);
+
+                    if (!firstError.compareAndSet(null, e))
+                        log.warn("An error occurred when processing the 
background event: {}", e.getMessage(), e);
+                }
             }
+            
kafkaConsumerMetrics.recordBackgroundEventQueueProcessingTime(time.milliseconds()
 - startMs);
         }
 
         backgroundEventReaper.reap(time.milliseconds());

Review Comment:
   Interesting, and if we agree on what we want we could just send an update in 
the KIP email thread to add it to the KIP and here.
   
   To align internally first, I guess we would be interested in the num/avg of 
expired events, but we need to consider how that metric would go crazy and be a 
false alarm in cases like poll(0) right? Should we consider the expiration 
relevant only if there was a non-zero timeout? Thoughts?



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