Hazelcast's Hot Restart Persistence uses the log-structured storage approach. The following is a top-level design description:

  • The only kind of update operation on persistent data is appending.
  • What is appended are facts about events that happened to the data model represented by the store; either a new value was assigned to a key or a key was removed.
  • Each record associated with a key makes stale the previous record that was associated with that key.
  • Stale records contribute to the amount of garbage present in the persistent storage.
  • Measures are taken to remove garbage from the storage.

This kind of design focuses almost all of the system's complexity into the garbage collection (GC) process, stripping down the client's operation to the bare necessity of guaranteeing persistent behavior: a simple file append operation. Consequently, the latency of operations is close to the theoretical minimum in almost all cases. Complications arise only during prolonged periods of maximum load; this is where the details of the GC process begin to matter.