title: Kafka not deleting key with tombstone
body: <p>I create a kafka topic with below properties</p>
  
  <p><strong>min.cleanable.dirty.ratio=0.01,delete.retention.ms=100,segment.ms=100,cleanup.policy=compact</strong></p>
  
  <p>Let's say I insert k-v pairs in order <strong>1111:1, 1111:2, 1111:null, 2222:1</strong>
  What happens now is except last message, the log compaction runs on rest of the messages and clears first two but retains <strong>1111:null</strong></p>
  
  <p>Acc to the documentation, </p>
  
  <pre><code>Kafka log compaction also allows for deletes. A message with a key and a null payload acts like a tombstone, a delete marker for that key. Tombstones get cleared after a period.
  </code></pre>
  
  <p>So, I am hoping when delete.retention.ms is achieved, the null marker should delete the message with key <strong>1111</strong></p>
  
  <p>I have two questions - Why is the tombstone marker not working? Why is the last message ignored from compaction?</p>
  
  <p>This is what server.properties file has - </p>
  
  <pre><code>log.retention.ms=100
  log.retention.bytes=1073741824
  log.segment.bytes=1073741824
  log.retention.check.interval.ms=100
  log.cleaner.delete.retention.ms=100
  log.cleaner.enable=true
  log.cleaner.min.cleanable.ratio=0.01
  </code></pre>