for example,

whenever we send a data, this callback function will execute.

새 메타데이터
Topic: first-topic
Partition: 2
Offset: 13
Timestamp: 1606555474565

Send "Hello World!" string to topic named "first-topic".

Producer sends a data to Consumer asynchronous, so we have to flush to see results right away.

Execute below command to see "Hello World!"

>> kafka-console-consumer --bootstrap-server localhost:9092 --topic first-topic --group group-one