title: confused about transaction logs in zookeeper
body: <p>I am studying zookeeper, so I read the paper "A simple totally ordered broadcast protocol". And I dont really understand the following sentence:</p>
  
  <p>"ZooKeeper uses an in-memory database and stores transaction logs and periodic snapshots on disk. Zab’s transaction log doubles as the database write-ahead transaction log so that a transaction will only be written to the disk once."</p>
  
  <p>Is there anyone can explain it to me?</p>