title: Zookeeper in-memory log
body: <p>Does ZooKeeper have an "in-memory log"? I've some experience with ZooKeeper but I never seen anything like it before (from the client side), and after searching I haven't found anything related to an in-memory log. As far as I know, every operation (create, setData, delete) is made on disk.</p>
  
  <p>However, in the paper <a href="https://www.cs.princeton.edu/~jrex/papers/ravana15.pdf" rel="nofollow">Ravana: Controller Fault-Tolerance in Software-Defined Networking</a> the authors </p>
  
  <blockquote>
    <p>Event logging: The master saves each event in ZooKeeper’s distributed
    in-memory log. Slaves monitor the log by registering a
    trigger for it. When a new event is propagated to a slave’s log, the
    trigger is activated so that the slave can read the newly arrived event
    locally.</p>
  </blockquote>
  
  <p>So, assuming that there is an in-memory log, how would a (java) client app use it? Or is it server side only?</p>