Creating Kafka Producer with "acks=all" config.
Is their any significance of calling flush with above config ?
Will it wait for flush to be invoked before being sent to broker.
AS
acks=all This means the leader will wait for the full set of in-sync replicas to acknowledge the record. This guarantees that the record will not be lost as long as at least one in-sync replica remains alive. This is the strongest available guarantee. This is equivalent to the acks=-1 setting.
Let me first try and call out the distinction between flush() and acks before I get to the 2 questions.
flush() - This is a method to be invoked in the producer to push the messages to the brokers from the buffer (configurable) maintained on the producer side. You would either invoke this method or close() to send the messages through to the brokers from the producer buffer. This gets invoked automatically if the buffer memory available to the producer gets full (as described by Manoj in his answer).
acks=ALL is however a responsibility of the broker i.e. to send an acknowledgement back to the producer after the messages have synchronously replicated to other brokers as per the setting requested in the producer. You would use this setting to tune your message delivery semantics. In this case, as soon as the messages are replicated to the designated in-sync replicas, the broker will send the acknowledgement to the producer saying - "I got your messages".
Now, on your questions i.e. If there is any significance of calling flush with the acks setting and whether or not the producer will wait for flush to be invoked before being sent to the broker.
Well, the asynchronous nature of the producer will ensure that the producer does not wait. If however, you invoke flush() explicitly or if it gets invoked on its own then any further sends will be blocked until the producer gets the acknowledgement from the broker. So, the relationship between these 2 is very subtle.
I hope this helps!