merlimat commented on a change in pull request #1066: Issue 937: add 
CommandGetLastMessageId to make reader know the end of topic
URL: https://github.com/apache/incubator-pulsar/pull/1066#discussion_r168005589
 
 

 ##########
 File path: 
pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerImpl.java
 ##########
 @@ -1248,6 +1254,103 @@ public void seek(MessageId messageId) throws 
PulsarClientException {
         return seekFuture;
     }
 
+    public boolean hasMessageAvailable() throws PulsarClientException {
+        try {
+            if (lastMessageIdInBroker.compareTo(lastDequeuedMessage) > 0 &&
+                ((MessageIdImpl)lastMessageIdInBroker).getEntryId() != -1) {
+                return true;
+            }
+
+            return hasMessageAvailableAsync().get();
+        } catch (ExecutionException | InterruptedException e) {
+            throw new PulsarClientException(e);
+        }
+    }
+
+    public CompletableFuture<Boolean> hasMessageAvailableAsync() {
+        final CompletableFuture<Boolean> booleanFuture = new 
CompletableFuture<>();
+
+        if (lastMessageIdInBroker.compareTo(lastDequeuedMessage) > 0 &&
+            ((MessageIdImpl)lastMessageIdInBroker).getEntryId() != -1) {
+            booleanFuture.complete(true);
+        } else {
+            getLastMessageIdAsync().thenAccept(messageId -> {
+                lastMessageIdInBroker = messageId;
+                if (lastMessageIdInBroker.compareTo(lastDequeuedMessage) > 0 &&
+                    ((MessageIdImpl)lastMessageIdInBroker).getEntryId() != -1) 
{
+                    booleanFuture.complete(true);
+                } else {
+                    booleanFuture.complete(false);
+                }
+            }).exceptionally(e -> {
+                log.error("[{}][{}] Failed getLastMessageId command", topic, 
subscription);
+                booleanFuture.completeExceptionally(e.getCause());
+                return null;
+            });
+        }
+        return booleanFuture;
+    }
+
+    private CompletableFuture<MessageId> getLastMessageIdAsync() {
+        if (getState() == State.Closing || getState() == State.Closed) {
+            return FutureUtil
+                .failedFuture(new 
PulsarClientException.AlreadyClosedException("Consumer was already closed"));
+        }
+
+        if (!isConnected()) {
+            long opTimeoutMs = 
client.getConfiguration().getOperationTimeoutMs();
 
 Review comment:
   This implementation will block the caller of an asynchrounous method, which 
might be unexpected. One way to do the retries would be to have an internal 
method that gets called asynchrounously in recursion. For example, something 
like : 
   
   ```java
   private CompletableFuture<MessageId> internalGetLastMessageIdAsync(Backoff 
backoff, long remainingTime) {
       if (connected) { 
          // write on socket and return future
       } else {
          // if time is not elapsed yet... 
          long nextDelay = backoff.next();
          executor.schedule(() -> {
                     remainingTime -= (timeSpentSinceLastCall);
                      internalGetLastMessageIdAsync(backoff, remainingTime);
          }, nextDelay, TimeUnit.MILLISECONDS);
       }
   }
   ```
   
   (Don't read too much in the previous example, I'm just trying to illustrate 
the basic idea)

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