Engineering thread safe updates (getAndSet) to a TrieMap with periodic flushing of its contents.

ThreadSafeCache.scala

class SoldUnitsCache {
  type Item = String
  type Units = Long

  val cache = TrieMap[Item, Units]()
  val cacheLock = new ReentrantReadWriteLock()

  def updateSoldUnits(item: Item, soldUnits: Units): Unit = {
    // The readlock here ensures that multiple updates can happen concurrently 
    // while still blocking them when flusher wants to snapshot and clear the map.
    cache.readLock {
      cache.putIfAbsent(item, 0)
      var oldValue = cache(item)
      var newValue = oldValue + soldUnits
      while (!cache.replace(item, oldValue, newValue) {
        oldValue = cache(item)
        newValue = oldValue + soldUnits
      }
    }
  }

  val flusher = new PeriodicTask(flush)
  flusher.start()

  def flush(): Unit = {
    // The write lock here prevents any updates from happening while we snapshot
    // and clear the cache.
    cacheLock.writeLock {
      val snapshot = cache.asMap
      cache.clear
      }
      durablePersist(snapshot)
  }

  def getSoldUnits(item: Item): Long = {
    // No need to acquire any lock
    cache.get(item).getOrElse(0)
  }
}