Hi, using the Pulsar Java Client, is there a way, when batching is enabled, to be called when messages are effectively send to pulsar ? As send is asynchronous, how can we be sure that message are effectively sent to pulsar (for example, in case of OOM) ?

The

CompletableFuture

of each message will get completed when the send completes. If it never completes or fails, the sender will have to assume that the message hasn't been sent. There's at-least-once semantics by default. If duplicates are a concern, you should use message deduplication https://pulsar.apache.org/docs/3.2.x/cookbooks-deduplication/ or transactions. The limitations of idempotent producer are explained here: https://pulsar.apache.org/docs/3.2.x/txn-why/#limitation-of-idempotent-producer .

Thank you. Do we have a guarantee that the returns (CompletableFuture) will arrive in order ? (I mean, if 2 batches are sent async, do CompletableFuture will be called in the same order ?)

Yes. If you have complex message sending pipelines, you might find the Reactive Pulsar Client more convenient. Unfortunately there isn't good docs and examples for the Reactive Pulsar client (contributions are welcome!) https://github.com/apache/pulsar-client-reactive/blob/ea3a2e4eb7b40e78a6a0f4bbfa7c[…]rg/apache/pulsar/reactive/client/api/ReactiveMessageSender.java