Application State

Inside Raft Process, RaftApp and Log are most important data structures for application state.

RaftApp

RaftApp is an abstraction that represents the application state and the snapshot repository. The snapshot repository isn’t separated because the contents of the snapshot is strongly coupled with the application state.

The application state can be updated by applying the log entries. In this figure, the lastly applied entry is of index 55.

RaftApp can arbitrarily generate a snapshot to compact the log. When RaftApp makes a snapshot and stores it in the snapshot repository, The newest snapshot is immediately picked up by an internal thread to update the log (right in the figure) by replacing the snapshot entry. In this figure, the snapshot index is 51.

Old snapshots will be garbage collected.

Snapshots might be fetched from other nodes. This happens when the Raft process is far behind the leader and the leader doesn’t have the log entries as they are previously garbage collected.

Log

In the figure, log entries from 45 to 50 are scheduled for garbage collection. Snapshot entry is of index 51 and it is guaranteed that the corresponding snapshot exists in the snapshot repository. 52 to 55 are applied. 56 or later are not applied yet: They are either uncommitted or committed.