[GitHub] [pulsar] Shoothzj commented on issue #11783: ConcurrentModificationException occurs when failPendingMessages

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

Shoothzj commented on issue #11783:
URL: https://github.com/apache/pulsar/issues/11783#issuecomment-909347986


   I just figure out why, because the client code retry to send messages when exception occurred to avoid network "Jitter"(Translate on google). the sample code are like this
   ```java
    String topic = "test";
           PulsarClient pulsarClient = PulsarClient.builder()
                   .serviceUrl("http://127.0.0.1:8080")
                   .build();
           ProducerBuilder<String> producerBuilder = pulsarClient.newProducer(Schema.STRING).enableBatching(true);
           Producer<String> producer = producerBuilder.topic(topic).create();
   
           final TypedMessageBuilder<String> stringTypedMessageBuilder = producer.newMessage();
           final TypedMessageBuilder<String> value = stringTypedMessageBuilder.key("1").value("2");
           final CompletableFuture<MessageId> completableFuture = value.sendAsync();
           completableFuture.exceptionally(throwable -> {
               producer.sendAsync("xxxx");
               return null;
           });
   ```
   The `exceptionally` is called from `ProducerImpl.failPendingMessages`'s forEachLoop, and `sendAsync` will add messages to `pendingMessages`, then the error ocurred.
   
   I have a question. Should we allow to retry on `exceptionally` callback or prohibit usage like that ?
   
   @315157973 @eolivelli @merlimat , thanks


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