Data Model: Raft Core Replication

Entities

Peer
- Fields: id (u64), region_id (u64), state (Leader/Follower/Candidate), term (u64), commit_index (u64), last_applied (u64)
- Relationships: owns RaftStorage; exchanges RaftLogEntry with other peers.
- Constraints: single region scope for this phase; fixed voter set of 3.
RaftLogEntry
- Fields: index (u64), term (u64), command (bytes), context (bytes, optional)
- Relationships: persisted in raft_log CF; applied to state machine when committed.
- Constraints: indices strictly increasing; term monotonic per election; applied in order.
HardState
- Fields: current_term (u64), voted_for (u64), commit_index (u64)
- Relationships: persisted in raft_state CF; loaded at startup before participating.
- Constraints: must be flushed atomically with log appends when advancing commit index.
ConfState
- Fields: voters (Vec<u64>)
- Relationships: persisted in raft_state CF; defines quorum (majority of 3).
- Constraints: static for this phase; changes require future joint consensus.
ReplicationState
- Fields: match_index (u64), next_index (u64), pending (bool)
- Relationships: maintained per follower in memory; not persisted.
- Constraints: drives AppendEntries backoff and progress.

Peer transitions: Follower → Candidate → Leader on election; Leader → Follower on higher term or failed election.
Log application: when commit_index advances, apply entries in order to state machine; last_applied increases monotonically.
Recovery: on restart, load HardState, ConfState, and log; reconcile with leader via AppendEntries (truncate/append) before applying new entries.