node.rs

//! This module incapsulates most of the application-specific logics.
//!
//! It's responsible for
//! - handling proposals,
//! - handling configuration changes,
//! - processing raft `Ready` - persisting entries, communicating with other raft nodes.

use crate::cas;
use crate::governor;
use crate::has_grades;
use crate::instance::Instance;
use crate::kvcell::KVCell;
use crate::loop_start;
use crate::r#loop::FlowControl;
use crate::rpc;
use crate::schema::{Distribution, IndexDef, SpaceDef};
use crate::storage::acl;
use crate::storage::ddl_meta_drop_space;
use crate::storage::SnapshotData;
use crate::storage::{ddl_abort_on_master, ddl_meta_space_update_operable};
use crate::storage::{local_schema_version, set_local_schema_version};
use crate::storage::{Clusterwide, ClusterwideSpaceId, PropertyName};
use crate::stringify_cfunc;
use crate::sync;
use crate::tlog;
use crate::traft;
use crate::traft::error::Error;
use crate::traft::event;
use crate::traft::event::Event;
use crate::traft::notify::{notification, Notifier, Notify};
use crate::traft::op::{Acl, Ddl, Dml, Op, OpResult};
use crate::traft::Address;
use crate::traft::ConnectionPool;
use crate::traft::ContextCoercion as _;
use crate::traft::LogicalClock;
use crate::traft::RaftId;
use crate::traft::RaftIndex;
use crate::traft::RaftSpaceAccess;
use crate::traft::RaftTerm;
use crate::traft::Topology;
use crate::util::AnyWithTypeName;
use crate::warn_or_panic;
use ::raft::prelude as raft;
use ::raft::Error as RaftError;
use ::raft::StateRole as RaftStateRole;
use ::raft::StorageError;
use ::raft::INVALID_ID;
use ::tarantool::error::TarantoolError;
use ::tarantool::fiber;
use ::tarantool::fiber::mutex::MutexGuard;
use ::tarantool::fiber::r#async::timeout::IntoTimeout as _;
use ::tarantool::fiber::r#async::{oneshot, watch};
use ::tarantool::fiber::Mutex;
use ::tarantool::index::FieldType as IFT;
use ::tarantool::index::Part;
use ::tarantool::proc;
use ::tarantool::space::FieldType as SFT;
use ::tarantool::space::SpaceId;
use ::tarantool::time::Instant;
use ::tarantool::tlua;
use ::tarantool::transaction::transaction;
use ::tarantool::tuple::Decode;
use ::tarantool::vclock::Vclock;
use protobuf::Message as _;
use std::cell::Cell;
use std::cell::RefCell;
use std::cmp::Ordering;
use std::collections::{HashMap, HashSet};
use std::convert::TryFrom;
use std::rc::Rc;
use std::time::Duration;
use ApplyEntryResult::*;

type RawNode = raft::RawNode<RaftSpaceAccess>;

::tarantool::define_str_enum! {
    pub enum RaftState {
        Follower = "Follower",
        Candidate = "Candidate",
        Leader = "Leader",
        PreCandidate = "PreCandidate",
    }
}

impl RaftState {
    pub fn is_leader(&self) -> bool {
        matches!(self, Self::Leader)
    }
}

impl From<RaftStateRole> for RaftState {
    fn from(role: RaftStateRole) -> Self {
        match role {
            RaftStateRole::Follower => Self::Follower,
            RaftStateRole::Candidate => Self::Candidate,
            RaftStateRole::Leader => Self::Leader,
            RaftStateRole::PreCandidate => Self::PreCandidate,
        }
    }
}

#[derive(Copy, Clone, Debug, tlua::Push, tlua::PushInto)]
pub struct Status {
    /// `raft_id` of the current instance
    pub id: RaftId,
    /// `raft_id` of the leader instance
    pub leader_id: Option<RaftId>,
    /// Current term number
    pub term: RaftTerm,
    /// Current raft state
    pub raft_state: RaftState,
}

impl Status {
    pub fn check_term(&self, requested_term: RaftTerm) -> traft::Result<()> {
        if requested_term != self.term {
            return Err(Error::TermMismatch {
                requested: requested_term,
                current: self.term,
            });
        }
        Ok(())
    }
}

type StorageWatchers = HashMap<SpaceId, watch::Sender<()>>;
type StorageChanges = HashSet<SpaceId>;

/// The heart of `traft` module - the Node.
pub struct Node {
    /// RaftId of the Node.
    //
    // It appears twice in the Node: here and in `status.id`.
    // This is a concious decision.
    // `self.raft_id()` is used in Rust API, and
    // `self.status()` is mostly useful in Lua API.
    pub(crate) raft_id: RaftId,

    node_impl: Rc<Mutex<NodeImpl>>,
    pub(crate) storage: Clusterwide,
    pub(crate) raft_storage: RaftSpaceAccess,
    pub(crate) main_loop: MainLoop,
    pub(crate) governor_loop: governor::Loop,
    status: watch::Receiver<Status>,
    watchers: Rc<Mutex<StorageWatchers>>,
    topology: Rc<RefCell<Topology>>,
}

impl std::fmt::Debug for Node {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Node")
            .field("raft_id", &self.raft_id)
            .finish_non_exhaustive()
    }
}

impl Node {
    /// Initialize the raft node.
    /// **This function yields**
    pub fn new(storage: Clusterwide, raft_storage: RaftSpaceAccess) -> Result<Self, RaftError> {
        let topology = Rc::new(RefCell::new(Topology::from(storage.clone())));
        let node_impl = NodeImpl::new(storage.clone(), raft_storage.clone(), topology.clone())?;

        let raft_id = node_impl.raft_id();
        let status = node_impl.status.subscribe();

        let node_impl = Rc::new(Mutex::new(node_impl));
        let watchers = Rc::new(Mutex::new(HashMap::new()));

        let node = Node {
            raft_id,
            main_loop: MainLoop::start(node_impl.clone(), watchers.clone()), // yields
            governor_loop: governor::Loop::start(
                status.clone(),
                storage.clone(),
                raft_storage.clone(),
            ),
            node_impl,
            storage,
            raft_storage,
            status,
            watchers,
            topology,
        };

        // Wait for the node to enter the main loop
        node.tick_and_yield(0);
        Ok(node)
    }

    pub fn raft_id(&self) -> RaftId {
        self.raft_id
    }

    pub fn status(&self) -> Status {
        self.status.get()
    }

    pub(crate) fn node_impl(&self) -> MutexGuard<NodeImpl> {
        self.node_impl.lock()
    }

    /// Wait for the status to be changed.
    /// **This function yields**
    pub fn wait_status(&self) {
        fiber::block_on(self.status.clone().changed()).unwrap();
    }

    /// Returns current applied [`RaftIndex`].
    pub fn get_index(&self) -> RaftIndex {
        self.raft_storage
            .applied()
            .expect("reading from memtx should never fail")
    }

    /// Performs the quorum read operation.
    ///
    /// If works the following way:
    ///
    /// 1. The instance forwards a request (`MsgReadIndex`) to a raft
    ///    leader. In case there's no leader at the moment, the function
    ///    returns `Err(ProposalDropped)`.
    /// 2. Raft leader tracks its `commit_index` and broadcasts a
    ///    heartbeat to followers to make certain that it's still a
    ///    leader.
    /// 3. As soon as the heartbeat is acknowlenged by the quorum, the
    ///    function returns that index.
    /// 4. The instance awaits when the index is applied. If timeout
    ///    expires beforehand, the function returns `Err(Timeout)`.
    ///
    /// Returns current applied [`RaftIndex`].
    ///
    /// **This function yields**
    pub fn read_index(&self, timeout: Duration) -> traft::Result<RaftIndex> {
        let deadline = fiber::clock().saturating_add(timeout);

        let notify = self.raw_operation(|node_impl| node_impl.read_index_async())?;
        let index: RaftIndex = fiber::block_on(notify.recv_timeout(timeout))?;

        self.wait_index(index, deadline.duration_since(fiber::clock()))
    }

    /// Waits for [`RaftIndex`] to be applied to the storage locally.
    ///
    /// Returns current applied [`RaftIndex`]. It can be equal to or
    /// greater than the target one. If timeout expires beforehand, the
    /// function returns `Err(Timeout)`.
    ///
    /// **This function yields**
    #[inline]
    pub fn wait_index(&self, target: RaftIndex, timeout: Duration) -> traft::Result<RaftIndex> {
        let deadline = fiber::clock().saturating_add(timeout);
        loop {
            let current = self.get_index();
            if current >= target {
                return Ok(current);
            }

            if event::wait_deadline(event::Event::EntryApplied, deadline)?.is_timeout() {
                return Err(Error::Timeout);
            }
        }
    }

    /// Propose an operation and wait for it's result.
    /// **This function yields**
    pub fn propose_and_wait<T: OpResult + Into<Op>>(
        &self,
        op: T,
        timeout: Duration,
    ) -> traft::Result<T::Result> {
        let notify = self.raw_operation(|node_impl| node_impl.propose_async(op))?;
        fiber::block_on(notify.recv_timeout::<T::Result>(timeout))
    }

    /// Become a candidate and wait for a main loop round so that there's a
    /// chance we become the leader.
    /// **This function yields**
    pub fn campaign_and_yield(&self) -> traft::Result<()> {
        self.raw_operation(|node_impl| node_impl.campaign())?;
        // Even though we don't expect a response, we still should let the
        // main_loop do an iteration. Without rescheduling, the Ready state
        // wouldn't be processed, the Status wouldn't be updated, and some
        // assertions may fail (e.g. in `postjoin()` in `main.rs`).
        fiber::reschedule();
        Ok(())
    }

    /// **This function yields**
    pub fn step_and_yield(&self, msg: raft::Message) {
        self.raw_operation(|node_impl| node_impl.step(msg))
            .map_err(|e| tlog!(Error, "{e}"))
            .ok();
        // even though we don't expect a response, we still should let the
        // main_loop do an iteration
        fiber::reschedule();
    }

    /// **This function yields**
    pub fn tick_and_yield(&self, n_times: u32) {
        self.raw_operation(|node_impl| node_impl.tick(n_times));
        // even though we don't expect a response, we still should let the
        // main_loop do an iteration
        fiber::reschedule();
    }

    /// **This function yields**
    pub fn timeout_now(&self) {
        let raft_id = self.raft_id();
        self.step_and_yield(raft::Message {
            to: raft_id,
            from: raft_id,
            msg_type: raft::MessageType::MsgTimeoutNow,
            ..Default::default()
        })
    }

    /// Processes the [`rpc::join::Request`] and appends necessary
    /// entries to the raft log (if successful).
    ///
    /// Returns the resulting [`Instance`] when the entry is committed.
    // TODO: to make this function async and have an outer timeout,
    // wait_* fns also need to be async.
    pub fn handle_join_request_and_wait(
        &self,
        req: rpc::join::Request,
        timeout: Duration,
    ) -> traft::Result<(Box<Instance>, HashSet<Address>)> {
        let deadline = fiber::clock().saturating_add(timeout);

        loop {
            let instance = self
                .topology
                .borrow()
                .build_instance(
                    req.instance_id.as_ref(),
                    req.replicaset_id.as_ref(),
                    &req.failure_domain,
                )
                .map_err(RaftError::ConfChangeError)?;
            let mut replication_addresses = self.storage.peer_addresses.addresses_by_ids(
                self.topology
                    .borrow()
                    .get_replication_ids(&instance.replicaset_id),
            )?;
            replication_addresses.insert(req.advertise_address.clone());
            let peer_address = traft::PeerAddress {
                raft_id: instance.raft_id,
                address: req.advertise_address.clone(),
            };
            let op_addr = Dml::replace(ClusterwideSpaceId::Address, &peer_address)
                .expect("encoding should not fail");
            let op_instance = Dml::replace(ClusterwideSpaceId::Instance, &instance)
                .expect("encoding should not fail");
            let ranges = vec![
                cas::Range::new(ClusterwideSpaceId::Instance),
                cas::Range::new(ClusterwideSpaceId::Address),
                cas::Range::new(ClusterwideSpaceId::Property)
                    .eq((PropertyName::ReplicationFactor,)),
            ];
            macro_rules! handle_result {
                ($res:expr) => {
                    match $res {
                        Ok((index, term)) => {
                            self.wait_index(index, deadline.duration_since(fiber::clock()))?;
                            if term != raft::Storage::term(&self.raft_storage, index)? {
                                // leader switched - retry
                                self.wait_status();
                                continue;
                            }
                        }
                        Err(err) => {
                            if err.is_cas_err() | err.is_term_mismatch_err() {
                                // cas error - retry
                                fiber::sleep(Duration::from_millis(500));
                                continue;
                            } else {
                                return Err(err);
                            }
                        }
                    }
                };
            }
            // Only in this order - so that when instance exists - address will always be there.
            handle_result!(cas::compare_and_swap(
                Op::Dml(op_addr),
                cas::Predicate {
                    index: self.raft_storage.applied()?,
                    term: self.raft_storage.term()?,
                    ranges: ranges.clone(),
                },
                deadline.duration_since(fiber::clock()),
            ));
            handle_result!(cas::compare_and_swap(
                Op::Dml(op_instance),
                cas::Predicate {
                    index: self.raft_storage.applied()?,
                    term: self.raft_storage.term()?,
                    ranges,
                },
                deadline.duration_since(fiber::clock()),
            ));

            self.main_loop.wakeup();
            return Ok((instance.into(), replication_addresses));
        }
    }

    /// Processes the [`rpc::update_instance::Request`] and appends
    /// the corresponding [`Op::Dml`] entry to the raft log (if successful).
    ///
    /// Returns `Ok(())` when the entry is committed.
    ///
    /// **This function yields**
    // TODO: for this function to be async and have an outer timeout wait_* fns need to be async
    pub fn handle_update_instance_request_and_wait(
        &self,
        req: rpc::update_instance::Request,
        timeout: Duration,
    ) -> traft::Result<()> {
        let deadline = fiber::clock().saturating_add(timeout);

        loop {
            let instance = self
                .topology
                .borrow()
                .build_updated_instance(&req)
                .map_err(RaftError::ConfChangeError)?;
            let dml = Dml::replace(ClusterwideSpaceId::Instance, &instance)
                .expect("encoding should not fail");

            let ranges = vec![
                cas::Range::new(ClusterwideSpaceId::Instance),
                cas::Range::new(ClusterwideSpaceId::Address),
                cas::Range::new(ClusterwideSpaceId::Property)
                    .eq((PropertyName::ReplicationFactor,)),
            ];
            let res = cas::compare_and_swap(
                Op::Dml(dml),
                cas::Predicate {
                    index: self.raft_storage.applied()?,
                    term: self.raft_storage.term()?,
                    ranges,
                },
                deadline.duration_since(fiber::clock()),
            );
            match res {
                Ok((index, term)) => {
                    self.wait_index(index, deadline.duration_since(fiber::clock()))?;
                    if term != raft::Storage::term(&self.raft_storage, index)? {
                        // leader switched - retry
                        self.wait_status();
                        continue;
                    }
                }
                Err(err) => {
                    if err.is_cas_err() | err.is_term_mismatch_err() {
                        // cas error - retry
                        fiber::sleep(Duration::from_millis(500));
                        continue;
                    } else {
                        return Err(err);
                    }
                }
            }
            self.main_loop.wakeup();
            return Ok(());
        }
    }

    /// Only the conf_change_loop on a leader is eligible to call this function.
    ///
    /// **This function yields**
    pub(crate) fn propose_conf_change_and_wait(
        &self,
        term: RaftTerm,
        conf_change: raft::ConfChangeV2,
    ) -> traft::Result<()> {
        let notify =
            self.raw_operation(|node_impl| node_impl.propose_conf_change_async(term, conf_change))?;
        fiber::block_on(notify).unwrap()?;
        Ok(())
    }

    /// Attempt to transfer leadership to a given node and yield.
    ///
    /// **This function yields**
    pub fn transfer_leadership_and_yield(&self, new_leader_id: RaftId) {
        self.raw_operation(|node_impl| node_impl.raw_node.transfer_leader(new_leader_id));
        fiber::reschedule();
    }

    /// This function **may yield** if `self.node_impl` mutex is acquired.
    #[inline]
    #[track_caller]
    fn raw_operation<R>(&self, f: impl FnOnce(&mut NodeImpl) -> R) -> R {
        let mut node_impl = self.node_impl.lock();
        let res = f(&mut node_impl);
        drop(node_impl);
        self.main_loop.wakeup();
        res
    }

    #[inline]
    pub fn all_traft_entries(&self) -> ::tarantool::Result<Vec<traft::Entry>> {
        self.raft_storage.all_traft_entries()
    }

    /// Returns a watch which will be notified when a clusterwide space is
    /// modified via the specified `index`.
    ///
    /// You can also pass a [ClusterwideSpace](crate::storage::ClusterwideSpace) in which case the space's
    /// primary index will be used.
    #[inline(always)]
    pub fn storage_watcher(&self, space: impl Into<SpaceId>) -> watch::Receiver<()> {
        use std::collections::hash_map::Entry;
        let mut watchers = self.watchers.lock();
        match watchers.entry(space.into()) {
            Entry::Vacant(entry) => {
                let (tx, rx) = watch::channel(());
                entry.insert(tx);
                rx
            }
            Entry::Occupied(entry) => entry.get().subscribe(),
        }
    }
}

pub(crate) struct NodeImpl {
    pub raw_node: RawNode,
    pub notifications: HashMap<LogicalClock, Notifier>,
    topology: Rc<RefCell<Topology>>,
    joint_state_latch: KVCell<RaftIndex, oneshot::Sender<Result<(), RaftError>>>,
    storage: Clusterwide,
    raft_storage: RaftSpaceAccess,
    pool: ConnectionPool,
    lc: LogicalClock,
    status: watch::Sender<Status>,
}

impl NodeImpl {
    fn new(
        storage: Clusterwide,
        raft_storage: RaftSpaceAccess,
        topology: Rc<RefCell<Topology>>,
    ) -> Result<Self, RaftError> {
        let box_err = |e| StorageError::Other(Box::new(e));

        let raft_id: RaftId = raft_storage
            .raft_id()
            .map_err(box_err)?
            .expect("raft_id should be set by the time the node is being initialized");
        let applied: RaftIndex = raft_storage.applied().map_err(box_err)?;
        let lc = {
            let gen = raft_storage.gen().unwrap() + 1;
            raft_storage.persist_gen(gen).unwrap();
            LogicalClock::new(raft_id, gen)
        };

        let pool = ConnectionPool::builder(storage.clone())
            .handler_name(stringify_cfunc!(proc_raft_interact))
            .call_timeout(MainLoop::TICK.saturating_mul(4))
            .build();

        let cfg = raft::Config {
            id: raft_id,
            applied,
            pre_vote: true,
            ..Default::default()
        };

        let raw_node = RawNode::new(&cfg, raft_storage.clone(), &tlog::root())?;

        let (status, _) = watch::channel(Status {
            id: raft_id,
            leader_id: None,
            term: traft::INIT_RAFT_TERM,
            raft_state: RaftState::Follower,
        });

        Ok(Self {
            raw_node,
            notifications: Default::default(),
            topology,
            joint_state_latch: KVCell::new(),
            storage,
            raft_storage,
            pool,
            lc,
            status,
        })
    }

    fn raft_id(&self) -> RaftId {
        self.raw_node.raft.id
    }

    pub fn read_index_async(&mut self) -> Result<Notify, RaftError> {
        // In some states `raft-rs` ignores the ReadIndex request.
        // Check it preliminary, don't wait for the timeout.
        //
        // See for details:
        // - <https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2058>
        // - <https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2323>
        let leader_doesnt_exist = self.raw_node.raft.leader_id == INVALID_ID;
        let term_just_started = // ...
            self.raw_node.raft.state == RaftStateRole::Leader
            && !self.raw_node.raft.commit_to_current_term();

        if leader_doesnt_exist || term_just_started {
            return Err(RaftError::ProposalDropped);
        }

        let (lc, notify) = self.schedule_notification();
        // read_index puts this context into an Entry,
        // so we've got to compose full EntryContext,
        // despite single LogicalClock would be enough
        let ctx = traft::EntryContextNormal::new(lc, Op::Nop);
        self.raw_node.read_index(ctx.to_bytes());
        Ok(notify)
    }

    /// **Doesn't yield**
    #[inline]
    // TODO: rename and document
    pub fn propose_async<T>(&mut self, op: T) -> Result<Notify, RaftError>
    where
        T: Into<Op>,
    {
        let (lc, notify) = self.schedule_notification();
        let ctx = traft::EntryContextNormal::new(lc, op.into());
        self.raw_node.propose(ctx.to_bytes(), vec![])?;
        Ok(notify)
    }

    /// Proposes a raft entry to be appended to the log and returns raft index
    /// at which it is expected to be committed unless it gets rejected.
    ///
    /// **Doesn't yield**
    pub fn propose(&mut self, op: Op) -> Result<RaftIndex, RaftError> {
        self.lc.inc();
        let ctx = traft::EntryContextNormal::new(self.lc, op);
        self.raw_node.propose(ctx.to_bytes(), vec![])?;
        let index = self.raw_node.raft.raft_log.last_index();
        Ok(index)
    }

    pub fn campaign(&mut self) -> Result<(), RaftError> {
        self.raw_node.campaign()
    }

    pub fn step(&mut self, msg: raft::Message) -> Result<(), RaftError> {
        if msg.to != self.raft_id() {
            return Ok(());
        }

        // TODO check it's not a MsgPropose with op::Dml for updating _pico_instance.
        // TODO check it's not a MsgPropose with ConfChange.
        self.raw_node.step(msg)
    }

    pub fn tick(&mut self, n_times: u32) {
        for _ in 0..n_times {
            self.raw_node.tick();
        }
    }

    fn propose_conf_change_async(
        &mut self,
        term: RaftTerm,
        conf_change: raft::ConfChangeV2,
    ) -> Result<oneshot::Receiver<Result<(), RaftError>>, RaftError> {
        // In some states proposing a ConfChange is impossible.
        // Check if there's a reason to reject it.

        // Checking leadership is only needed for the
        // correct latch management. It doesn't affect
        // raft correctness. Checking the instance is a
        // leader makes sure the proposed `ConfChange`
        // is appended to the raft log immediately
        // instead of sending `MsgPropose` over the
        // network.
        if self.raw_node.raft.state != RaftStateRole::Leader {
            return Err(RaftError::ConfChangeError("not a leader".into()));
        }

        if term != self.raw_node.raft.term {
            return Err(RaftError::ConfChangeError("raft term mismatch".into()));
        }

        // Without this check the node would silently ignore the conf change.
        // See https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2014-L2026
        if self.raw_node.raft.has_pending_conf() {
            return Err(RaftError::ConfChangeError(
                "already has pending confchange".into(),
            ));
        }

        let prev_index = self.raw_node.raft.raft_log.last_index();
        self.raw_node.propose_conf_change(vec![], conf_change)?;

        // Ensure the ConfChange was actually appended to the log.
        // Otherwise it's a problem: current instance isn't actually a
        // leader (which is impossible in theory, but we're not sure in
        // practice) and sent the message to the raft network. It may
        // lead to an inconsistency.
        let last_index = self.raw_node.raft.raft_log.last_index();
        assert_eq!(last_index, prev_index + 1);

        if !self.joint_state_latch.is_empty() {
            warn_or_panic!("joint state latch is locked");
        }

        let (tx, rx) = oneshot::channel();
        self.joint_state_latch.insert(last_index, tx);
        event::broadcast(Event::JointStateEnter);

        Ok(rx)
    }

    /// Is called during a transaction
    fn handle_committed_entries(
        &mut self,
        entries: &[raft::Entry],
        wake_governor: &mut bool,
        expelled: &mut bool,
        storage_changes: &mut StorageChanges,
    ) -> traft::Result<()> {
        let mut entries = entries.iter().peekable();

        while let Some(&entry) = entries.peek() {
            let entry = match traft::Entry::try_from(entry) {
                Ok(v) => v,
                Err(e) => {
                    tlog!(Error, "abnormal entry: {e}"; "entry" => ?entry);
                    continue;
                }
            };

            let mut apply_entry_result = EntryApplied;
            transaction(|| -> tarantool::Result<()> {
                let entry_index = entry.index;
                match entry.entry_type {
                    raft::EntryType::EntryNormal => {
                        apply_entry_result = self.handle_committed_normal_entry(
                            entry,
                            wake_governor,
                            expelled,
                            storage_changes,
                        );
                        if apply_entry_result != EntryApplied {
                            return Ok(());
                        }
                    }
                    raft::EntryType::EntryConfChange | raft::EntryType::EntryConfChangeV2 => {
                        self.handle_committed_conf_change(entry)
                    }
                }

                let res = self.raft_storage.persist_applied(entry_index);
                event::broadcast(Event::EntryApplied);
                if let Err(e) = res {
                    tlog!(
                        Error,
                        "error persisting applied index: {e}";
                        "index" => entry_index
                    );
                }

                Ok(())
            })?;

            match apply_entry_result {
                SleepAndRetry => {
                    let timeout = MainLoop::TICK * 4;
                    fiber::sleep(timeout);
                    continue;
                }
                EntryApplied => {
                    // Actually advance the iterator.
                    let _ = entries.next();
                }
            }
        }

        Ok(())
    }

    /// Is called during a transaction
    fn handle_committed_normal_entry(
        &mut self,
        entry: traft::Entry,
        wake_governor: &mut bool,
        expelled: &mut bool,
        storage_changes: &mut StorageChanges,
    ) -> ApplyEntryResult {
        assert_eq!(entry.entry_type, raft::EntryType::EntryNormal);
        let lc = entry.lc();
        let index = entry.index;
        let op = entry.into_op().unwrap_or(Op::Nop);
        tlog!(Debug, "applying entry: {op}"; "index" => index);

        let mut instance_update = None;
        let mut old_instance = None;
        match &op {
            Op::Dml(op) => {
                let space = op.space();
                if space == ClusterwideSpaceId::Property as SpaceId
                    || space == ClusterwideSpaceId::Replicaset as SpaceId
                {
                    *wake_governor = true;
                } else if space == ClusterwideSpaceId::Instance as SpaceId {
                    *wake_governor = true;
                    let instance = match op {
                        Dml::Insert { tuple, .. } => Some(tuple),
                        Dml::Replace { tuple, .. } => Some(tuple),
                        Dml::Update { .. } => None,
                        Dml::Delete { .. } => None,
                    };
                    if let Some(instance) = instance {
                        let instance: Instance = rmp_serde::from_slice(instance.as_ref())
                            .expect("should be a valid instance tuple");
                        if has_grades!(instance, Expelled -> *)
                            && instance.raft_id == self.raft_id()
                        {
                            // cannot exit during a transaction
                            *expelled = true;
                        }
                        if self
                            .storage
                            .instances
                            .contains(&instance.instance_id)
                            .expect("storage should not fail")
                        {
                            old_instance = Some(
                                self.storage
                                    .instances
                                    .get(&instance.instance_id)
                                    .expect("storage should not fail"),
                            );
                        }
                        instance_update = Some(instance);
                    }
                }
                storage_changes.insert(space);
            }
            Op::DdlPrepare { .. } => {
                *wake_governor = true;
            }
            _ => {}
        }

        let storage_properties = &self.storage.properties;

        // apply the operation
        let mut result = Box::new(()) as Box<dyn AnyWithTypeName>;
        match op {
            Op::Nop => {}
            Op::Dml(op) => {
                let res = match &op {
                    Dml::Insert { space, tuple } => self.storage.insert(*space, tuple).map(Some),
                    Dml::Replace { space, tuple } => self.storage.replace(*space, tuple).map(Some),
                    Dml::Update { space, key, ops } => self.storage.update(*space, key, ops),
                    Dml::Delete { space, key } => self.storage.delete(*space, key),
                };
                result = Box::new(res) as _;
            }
            Op::DdlPrepare {
                ddl,
                schema_version,
            } => {
                self.apply_op_ddl_prepare(ddl, schema_version)
                    .expect("storage error");
            }
            Op::DdlCommit => {
                let v_local = local_schema_version().expect("storage error");
                let v_pending = storage_properties
                    .pending_schema_version()
                    .expect("storage error")
                    .expect("granted we don't mess up log compaction, this should not be None");
                let ddl = storage_properties
                    .pending_schema_change()
                    .expect("storage error")
                    .expect("granted we don't mess up log compaction, this should not be None");

                // This instance is catching up to the cluster.
                if v_local < v_pending {
                    if self.is_readonly() {
                        return SleepAndRetry;
                    } else {
                        // Master applies schema change at this point.
                        let resp = rpc::ddl_apply::apply_schema_change(
                            &self.storage,
                            &ddl,
                            v_pending,
                            true,
                        )
                        .expect("storage error");
                        match resp {
                            rpc::ddl_apply::Response::Abort { reason } => {
                                tlog!(Warning, "failed applying committed ddl operation: {reason}";
                                    "ddl" => ?ddl,
                                );
                                return SleepAndRetry;
                            }
                            rpc::ddl_apply::Response::Ok => {}
                        }
                    }
                }

                // Update pico metadata.
                match ddl {
                    Ddl::CreateSpace { id, .. } => {
                        ddl_meta_space_update_operable(&self.storage, id, true)
                            .expect("storage shouldn't fail");
                    }

                    Ddl::DropSpace { id } => {
                        ddl_meta_drop_space(&self.storage, id).expect("storage shouldn't fail");
                    }

                    _ => {
                        todo!()
                    }
                }

                storage_properties
                    .delete(PropertyName::PendingSchemaChange)
                    .expect("storage error");
                storage_properties
                    .delete(PropertyName::PendingSchemaVersion)
                    .expect("storage error");
                storage_properties
                    .put(PropertyName::GlobalSchemaVersion, &v_pending)
                    .expect("storage error");
            }
            Op::DdlAbort => {
                let v_local = local_schema_version().expect("storage error");
                let v_pending: u64 = storage_properties
                    .pending_schema_version()
                    .expect("storage error")
                    .expect("granted we don't mess up log compaction, this should not be None");
                let ddl = storage_properties
                    .pending_schema_change()
                    .expect("storage error")
                    .expect("granted we don't mess up log compaction, this should not be None");
                // This condition means, schema versions must always increase
                // even after an DdlAbort
                if v_local == v_pending {
                    if self.is_readonly() {
                        return SleepAndRetry;
                    } else {
                        let v_global = storage_properties
                            .global_schema_version()
                            .expect("storage error");
                        ddl_abort_on_master(&ddl, v_global).expect("storage error");
                    }
                }

                // Update pico metadata.
                match ddl {
                    Ddl::CreateSpace { id, .. } => {
                        ddl_meta_drop_space(&self.storage, id).expect("storage shouldn't fail");
                    }

                    Ddl::DropSpace { id } => {
                        ddl_meta_space_update_operable(&self.storage, id, true)
                            .expect("storage shouldn't fail");
                    }

                    _ => {
                        todo!()
                    }
                }

                storage_properties
                    .delete(PropertyName::PendingSchemaChange)
                    .expect("storage error");
                storage_properties
                    .delete(PropertyName::PendingSchemaVersion)
                    .expect("storage error");
            }

            Op::Acl(acl) => {
                let v_local = local_schema_version().expect("storage error");
                let v_pending = acl.schema_version();
                if v_local < v_pending {
                    if self.is_readonly() {
                        // Wait for tarantool replication with master to progress.
                        return SleepAndRetry;
                    } else {
                        match &acl {
                            Acl::CreateUser { user_def } => {
                                acl::on_master_create_user(user_def)
                                    .expect("creating user shouldn't fail");
                            }
                            Acl::ChangeAuth { user_id, auth, .. } => {
                                acl::on_master_change_user_auth(*user_id, auth)
                                    .expect("changing user auth shouldn't fail");
                            }