title: kafka multi-partition only one can receive message
body: <p>I am new to kafka.I read the quick start and create a topic named 'test' with 4 partitions,but when send messages to the topic I find all messages are saved in partition-0,the other three partitions are empty why?</p>
  
  <p>there is my java code to send messages</p>
  
  <pre><code>    for (int i = 0; i &lt; 100; i++){
           producer.send(new ProducerRecord&lt;String, String&gt;("test", "message",Integer.toString(i)));
  
      }
  </code></pre>