KafkaProducer: Difference between `callback` and returned `Future`?
The KafkaProducer send method both returns a Future and accepts a Callback.
Is there any fundamental difference between using one mechanism over the other to execute an action upon completion of the sending?
Answer
Looking at the documentation you linked to it looks like the main difference between the Future and the Callback lies in who initiates the "request is finished, what now?" question.
Let's say we have a customer C and a baker B. And C is asking B to make him a nice cookie. Now there are 2 possible ways the baker can return the delicious cookie to the customer.
Future
The baker accepts the request and tells the customer: Ok, when I'm finished I'll place your cookie here on the counter. (This agreement is the Future.)
In this scenario, the customer is responsible for checking the counter (Future) to see if the baker has finished his cookie or not.
blocking The customer stays near the counter and looks at it until the cookie is put there (Future.get()) or the baker puts an apology there instead (Error : Out of cookie dough).
non-blocking The customer does some other work, and once in a while checks if the cookie is waiting for him on the counter (Future.isDone()). If the cookie is ready, the customer takes it (Future.get()).
Callback
In this scenario the customer, after ordering his cookie, tells the baker: When my cookie is ready please give it to my pet robot dog here, he'll know what to do with it (This robot is the Callback).
Now the baker when the cookie is ready gives the cookie to the dog and tells him to run back to it's owner. The baker can continue baking the next cookie for another customer.
The dog runs back to the customer and starts wagging it's artificial tail to make the customer aware that his cookie is ready.
Notice how the customer didn't have any idea when the cookie would be given to him, nor was he actively polling the baker to see if it was ready.
That's the main difference between the 2 scenario's. Who is responsible for initiating the "your cookie is ready, what do you want to do with it?" question. With the Future, the customer is responsible for checking when it's ready, either by actively waiting, or by polling every now and then. In case of the callback, the baker will call back to the provided function.
I hope this answer gives you a better insight in what a Future and Calback actually are. Once you got the general idea, you could try to find out on which thread each specific thing is handled. When a thread is blocked, or in what order everything completes. Writing some simple programs that print statements like: "main client thread: cookie recieved" could be a fun way to experiment with this.