raft-io 0.8.0

//! Core value types shared across the protocol.
//!
//! These are deliberately plain: a [`NodeId`], the monotonic [`Term`] and
//! [`Index`] counters, the [`Role`] a node currently plays, a single
//! [`LogEntry`], and the [`HardState`] that Raft requires to survive a restart.
//! They carry no behaviour beyond construction and small accessors, which keeps
//! them cheap to copy and trivial to serialize once framing lands.

/// Identifier for a node in the cluster.
///
/// Identifiers are opaque to the protocol; any scheme is fine as long as each
/// node in a cluster has a distinct, stable value. A plain integer keeps the
/// common case allocation-free and `Copy`.
pub type NodeId = u64;

/// A Raft term: a monotonically increasing logical clock.
///
/// Terms partition time into epochs, each beginning with an election. Every
/// message carries the sender's term; a node that sees a higher term steps down
/// and adopts it. Term `0` is the initial value before any election.
pub type Term = u64;

/// Position of an entry in the replicated log.
///
/// Indices are 1-based: the first appended entry has index `1`, and index `0`
/// is the sentinel meaning "before the first entry" (with term `0`). Using `0`
/// as a sentinel lets the `prev_log_index` consistency check at the head of the
/// log fall out without a special case.
pub type Index = u64;

/// The role a node currently plays in the consensus protocol.
///
/// A node is always in exactly one role. It starts as a [`Follower`], may
/// become a [`Candidate`] when it stops hearing from a leader, and becomes a
/// [`Leader`] if it wins an election.
///
/// [`Follower`]: Role::Follower
/// [`Candidate`]: Role::Candidate
/// [`Leader`]: Role::Leader
///
/// # Examples
///
/// ```
/// use raft_io::{RaftConfig, RaftNode, Role};
///
/// let node = RaftNode::new(RaftConfig::single(1));
/// assert_eq!(node.role(), Role::Follower);
/// ```
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum Role {
    /// Passive replica: serves the leader and votes in elections.
    Follower,
    /// Standing for election in the current term, collecting votes.
    Candidate,
    /// Won the election for the current term; drives replication.
    Leader,
}

/// What a [`LogEntry`] carries.
///
/// Most entries are [`Normal`](EntryKind::Normal) application commands. A
/// [`Config`](EntryKind::Config) entry instead carries a cluster configuration —
/// the voting membership — and drives a membership change; its
/// [`command`](LogEntry::command) bytes encode the new member set rather than an
/// application command, so the protocol interprets them and the application does
/// not apply them.
///
/// # Examples
///
/// ```
/// use raft_io::{EntryKind, LogEntry};
///
/// assert_eq!(LogEntry::new(1, 1, vec![]).kind, EntryKind::Normal);
/// assert_eq!(LogEntry::config(1, 2, &[1, 2, 3]).kind, EntryKind::Config);
/// ```
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
#[cfg_attr(feature = "framing", derive(pack_io::Serialize, pack_io::Deserialize))]
pub enum EntryKind {
    /// An ordinary application command, applied to the state machine.
    #[default]
    Normal,
    /// A cluster-configuration change. The command bytes encode the new voting
    /// membership; the protocol adopts it and never applies it to the state
    /// machine.
    Config,
}

/// A single entry in the replicated log.
///
/// The [`command`](LogEntry::command) is opaque bytes: for a
/// [`Normal`](EntryKind::Normal) entry the protocol replicates and orders it but
/// never interprets it, and the application's state machine decodes it on apply;
/// for a [`Config`](EntryKind::Config) entry the bytes encode the new voting
/// membership. Each entry records the [`term`](LogEntry::term) in which the
/// leader created it and its [`index`](LogEntry::index) in the log, which
/// together identify it uniquely.
///
/// # Examples
///
/// ```
/// use raft_io::LogEntry;
///
/// let entry = LogEntry::new(2, 7, b"put k v".to_vec());
/// assert_eq!(entry.term, 2);
/// assert_eq!(entry.index, 7);
/// assert_eq!(entry.command, b"put k v");
/// ```
#[derive(Clone, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "framing", derive(pack_io::Serialize, pack_io::Deserialize))]
pub struct LogEntry {
    /// Term in which the leader created this entry.
    pub term: Term,
    /// 1-based position of this entry in the log.
    pub index: Index,
    /// Whether this is a normal command or a configuration change.
    pub kind: EntryKind,
    /// Opaque bytes: an application command, or an encoded member set for a
    /// [`Config`](EntryKind::Config) entry.
    pub command: Vec<u8>,
}

impl LogEntry {
    /// Creates a [`Normal`](EntryKind::Normal) log entry at `index` in `term`
    /// carrying `command`.
    ///
    /// # Examples
    ///
    /// ```
    /// use raft_io::LogEntry;
    ///
    /// let e = LogEntry::new(1, 1, vec![0xAB]);
    /// assert_eq!(e.command, vec![0xAB]);
    /// ```
    #[inline]
    #[must_use]
    pub fn new(term: Term, index: Index, command: Vec<u8>) -> Self {
        Self {
            term,
            index,
            kind: EntryKind::Normal,
            command,
        }
    }

    /// Creates a [`Config`](EntryKind::Config) log entry carrying the voting
    /// membership `members`.
    ///
    /// # Examples
    ///
    /// ```
    /// use raft_io::LogEntry;
    ///
    /// let e = LogEntry::config(3, 9, &[1, 2, 3]);
    /// assert_eq!(e.members(), Some(vec![1, 2, 3]));
    /// ```
    #[inline]
    #[must_use]
    pub fn config(term: Term, index: Index, members: &[NodeId]) -> Self {
        Self {
            term,
            index,
            kind: EntryKind::Config,
            command: encode_members(members),
        }
    }

    /// Returns the voting membership a [`Config`](EntryKind::Config) entry
    /// carries, or `None` for a [`Normal`](EntryKind::Normal) entry.
    ///
    /// # Examples
    ///
    /// ```
    /// use raft_io::LogEntry;
    ///
    /// assert_eq!(LogEntry::new(1, 1, vec![]).members(), None);
    /// assert_eq!(LogEntry::config(1, 2, &[7, 8]).members(), Some(vec![7, 8]));
    /// ```
    #[must_use]
    pub fn members(&self) -> Option<Vec<NodeId>> {
        match self.kind {
            EntryKind::Normal => None,
            EntryKind::Config => Some(decode_members(&self.command)),
        }
    }
}

/// Encodes a voting membership as little-endian `u64`s.
#[must_use]
pub(crate) fn encode_members(members: &[NodeId]) -> Vec<u8> {
    let mut buf = Vec::with_capacity(members.len() * 8);
    for &id in members {
        buf.extend_from_slice(&id.to_le_bytes());
    }
    buf
}

/// Decodes a voting membership written by [`encode_members`]. A trailing partial
/// chunk (only possible from corruption) is ignored.
#[must_use]
pub(crate) fn decode_members(bytes: &[u8]) -> Vec<NodeId> {
    bytes
        .chunks_exact(8)
        .map(|c| {
            let mut id = [0u8; 8];
            id.copy_from_slice(c);
            NodeId::from_le_bytes(id)
        })
        .collect()
}

/// The state Raft must persist before responding to any RPC.
///
/// Safety depends on `current_term` and `voted_for` surviving a crash: a node
/// that forgot it had already voted in a term could vote twice and help elect
/// two leaders. The [`RaftLog`](crate::RaftLog) stores this alongside the log
/// entries; the in-memory [`MemoryLog`](crate::MemoryLog) keeps it in a field.
///
/// # Examples
///
/// ```
/// use raft_io::HardState;
///
/// let hs = HardState::default();
/// assert_eq!(hs.term, 0);
/// assert_eq!(hs.voted_for, None);
/// ```
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct HardState {
    /// The latest term the node has seen.
    pub term: Term,
    /// The candidate this node voted for in `term`, if any.
    pub voted_for: Option<NodeId>,
}

/// A point-in-time capture of the application's state machine, with the log
/// position it covers.
///
/// A snapshot lets the log discard the entries it subsumes (compaction) and lets
/// a leader catch up a follower that has fallen too far behind to replicate
/// entry by entry. [`index`](Snapshot::index) and [`term`](Snapshot::term) are
/// the last log entry the snapshot includes — its replacement "sentinel" once
/// the entries up to there are gone — and [`data`](Snapshot::data) is the opaque
/// serialized state the application produces and restores. The protocol moves
/// the bytes but never interprets them.
///
/// # Examples
///
/// ```
/// use raft_io::Snapshot;
///
/// let snap = Snapshot::new(10, 3, b"serialized state".to_vec());
/// assert_eq!(snap.index, 10);
/// assert_eq!(snap.term, 3);
/// ```
#[derive(Clone, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "framing", derive(pack_io::Serialize, pack_io::Deserialize))]
pub struct Snapshot {
    /// Index of the last log entry the snapshot includes.
    pub index: Index,
    /// Term of the last log entry the snapshot includes.
    pub term: Term,
    /// Voting membership in effect at [`index`](Snapshot::index).
    ///
    /// Carried so a node that catches up from this snapshot — its configuration
    /// log entries having been compacted away — still knows who is in the
    /// cluster. The node fills this in when it takes a snapshot; an application
    /// constructing a snapshot directly with [`new`](Snapshot::new) leaves it
    /// empty.
    pub config: Vec<NodeId>,
    /// Opaque serialized state machine state. The protocol never inspects it.
    pub data: Vec<u8>,
}

impl Snapshot {
    /// Creates a snapshot covering the log through `index` (created in `term`),
    /// carrying serialized state `data` and an empty configuration.
    ///
    /// The node fills the configuration in when it takes a snapshot; use this
    /// constructor for snapshots that do not track membership.
    ///
    /// # Examples
    ///
    /// ```
    /// use raft_io::Snapshot;
    ///
    /// let snap = Snapshot::new(5, 2, vec![1, 2, 3]);
    /// assert_eq!(snap.data, vec![1, 2, 3]);
    /// assert!(snap.config.is_empty());
    /// ```
    #[inline]
    #[must_use]
    pub fn new(index: Index, term: Term, data: Vec<u8>) -> Self {
        Self {
            index,
            term,
            config: Vec::new(),
            data,
        }
    }

    /// Creates a snapshot that also records the voting membership `config` in
    /// effect at `index`.
    ///
    /// # Examples
    ///
    /// ```
    /// use raft_io::Snapshot;
    ///
    /// let snap = Snapshot::with_config(5, 2, vec![1, 2, 3], vec![0xAB]);
    /// assert_eq!(snap.config, vec![1, 2, 3]);
    /// ```
    #[inline]
    #[must_use]
    pub fn with_config(index: Index, term: Term, config: Vec<NodeId>, data: Vec<u8>) -> Self {
        Self {
            index,
            term,
            config,
            data,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_log_entry_new_sets_all_fields() {
        let e = LogEntry::new(3, 9, vec![1, 2, 3]);
        assert_eq!(e.term, 3);
        assert_eq!(e.index, 9);
        assert_eq!(e.command, vec![1, 2, 3]);
    }

    #[test]
    fn test_hard_state_default_is_term_zero_no_vote() {
        let hs = HardState::default();
        assert_eq!(
            hs,
            HardState {
                term: 0,
                voted_for: None
            }
        );
    }

    #[test]
    fn test_role_is_copy_and_comparable() {
        let r = Role::Leader;
        let copy = r;
        assert_eq!(r, copy);
        assert_ne!(Role::Follower, Role::Candidate);
    }

    #[test]
    fn test_normal_entry_has_no_members() {
        let e = LogEntry::new(1, 1, b"cmd".to_vec());
        assert_eq!(e.kind, EntryKind::Normal);
        assert_eq!(e.members(), None);
    }

    #[test]
    fn test_config_entry_round_trips_members() {
        let e = LogEntry::config(3, 9, &[1, 2, 3, 99]);
        assert_eq!(e.kind, EntryKind::Config);
        assert_eq!(e.members(), Some(vec![1, 2, 3, 99]));
    }

    #[test]
    fn test_empty_config_entry() {
        assert_eq!(LogEntry::config(1, 1, &[]).members(), Some(vec![]));
    }

    #[test]
    fn test_member_codec_round_trips() {
        for members in [vec![], vec![0], vec![1, 2, 3], vec![u64::MAX, 0, 7]] {
            assert_eq!(decode_members(&encode_members(&members)), members);
        }
    }

    #[test]
    fn test_decode_members_ignores_trailing_partial_chunk() {
        let mut bytes = encode_members(&[5, 6]);
        bytes.push(0xFF); // a stray byte from corruption
        assert_eq!(decode_members(&bytes), vec![5, 6]);
    }

    #[test]
    fn test_snapshot_with_config_carries_membership() {
        let snap = Snapshot::with_config(5, 2, vec![1, 2, 3], vec![0xAB]);
        assert_eq!(snap.config, vec![1, 2, 3]);
        assert!(Snapshot::new(5, 2, vec![]).config.is_empty());
    }
}