title: What are the use cases of an in-memory database?
body: <p>I'm looking into HSQLDB, and it appears that it stores tables in memory unless you specify otherwise. This sounds like a horrible idea to me, but evidently it's not as people use this database quite a bit. </p>
  
  <p>If you store your database data in-memory, what happens if your application process dies? How on earth could you hope to have the same database when you launch your application again?</p>
  
  <p>Databases are for persistent data, so why would you want to store an entire database in volatile memory instead of persisting on the filesystem? Is there a use case for such a database I'm missing here?</p>