'Reactor Kafka consume messages synchronous and process them async

I'm quite new into the reactive world and using Spring Webflux + reactor Kafka.

        kafkaReceiver
                .receive()
//                .publishOn(Schedulers.boundedElastic())
                .doOnNext(a -> log.info("Reading message: {}", a.value()))
                .concatMap(kafkaRecord ->
                 //perform DB operation
                 //kafkaRecord.receiverOffset.ackwnowledge         
                )
                .doOnError(e -> log.error("Error", e))
                .retry()
                .subscribe();

I understood that in order to parallelise message consumption, I have to instantiate one KafkaReceiver for each partition but is it possible/recommended for a partition to read messages in a synchronous manner and process them async (including the manual acknowledge)?

So that this is the desired output:

Reading message:1
Reading message:2
Reading message:3
Reading message:4
Stored message 1 in DB + ack
Reading message:5
Stored message 2 in DB + ack
Stored message 5 in DB + ack
Stored message 3 in DB + ack
Stored message 4 in DB + ack

In case of errors, I'm thinking of publishing the record to a DLT.

I've tried with flatMap too, but it seems that the entire processing happens sequentially on a single thread. Also if I'm publishing to a new scheduler, the processing happens on a new single Thread. If what I'm asking is possible, can someone please help me with a code snippet?

Solution 1:^[1]

What's the output of your current code log ?

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Solution 1	Nicolas Belfis

'Reactor Kafka consume messages synchronous and process them async

Solution 1:[1]

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Solution 1:^[1]