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_r168059642
 
 

 ##########
 File path: 
pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerImpl.java
 ##########
 @@ -1248,6 +1256,102 @@ 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"));
+        }
+
+        AtomicLong opTimeoutMs = new 
AtomicLong(client.getConfiguration().getOperationTimeoutMs());
+        Backoff backoff = new Backoff(100, TimeUnit.MILLISECONDS,
+            opTimeoutMs.get() * 2, TimeUnit.MILLISECONDS,
+            0 , TimeUnit.MILLISECONDS);
+        CompletableFuture<MessageId> getLastMessageIdFuture = new 
CompletableFuture<>();
+
+        internalGetLastMessageIdAsync(backoff, opTimeoutMs, 
getLastMessageIdFuture);
+        return getLastMessageIdFuture;
+    }
+
+    private void internalGetLastMessageIdAsync(final Backoff backoff,
+                                               final AtomicLong remainingTime,
+                                               CompletableFuture<MessageId> 
future) {
+        if (isConnected()) {
 
 Review comment:
   I missed this before, but we should make sure the connection doesn't change 
while we're executing this method. 
   
   `isConnected()` is checking the current connection, but that might change 
when we ask for `cnx()` few lines below. 
   
   We need to first get a reference on `ClientCnx` and use that throughout the 
method. 
   
   ```java
   ClientCnx cnx = cnx();
   
   if (cnx != null) {
      // check cnx.getRemoteEndpointProtocolVersion();
      cnx.sendGetLastMessageId()... 
   }
   ```

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