Log Data Layout

There are 5 significant log ids reflecting raft log state:

old                           new
øøøøøøLLLLLLLLLLLLLLLLLLLL
+----+----+----+----+----+--->
     |    |    |    |    |
     |    |    |    |    '--- last_log_id
     |    |    |    '-------- committed
     |    |    '------------- applied
     |    '------------------ snapshot_last_log_id
     '----------------------- last_purged_log_id

These log ids follow a strict order: last_purged_log_id ≤ snapshot_last_log_id ≤ applied ≤ committed ≤ last_log_id

• last_log_id: is the last known log entry.

• committed: the last log entry that is committed, i.e., accepted and persisted by a quorum of voters, and will always be seen by all future leaders.

• applied: the committed log entry that is applied to the state-machine. In raft only committed log entry can be applied.

• snapshot_last_log_id: the greatest log id that is included in the snapshot. Openraft takes a snapshot of the state-machine periodically. Therefore, a snapshot is a compacted format of a continuous range of log entries.

• last_purged_log_id: the id of the last purged log entries. Log entries that are included in a snapshot will be purged. openraft purges logs from the start up to a configured position, when a fresh snapshot is built, last_purged_log_id is the greatest id that openraft does not store.