iota_core/execution_cache/

writeback_cache.rs

// Copyright (c) Mysten Labs, Inc.
// Modifications Copyright (c) 2024 IOTA Stiftung
// SPDX-License-Identifier: Apache-2.0

//! MemoryCache is a cache for the transaction execution which delays writes to
//! the database until transaction results are certified (i.e. they appear in a
//! certified checkpoint, or an effects cert is observed by a fullnode). The
//! cache also stores committed data in memory in order to serve future reads
//! without hitting the database.
//!
//! For storing uncommitted transaction outputs, we cannot evict the data at all
//! until it is written to disk. Committed data not only can be evicted, but it
//! is also unbounded (imagine a stream of transactions that keep splitting a
//! coin into smaller coins).
//!
//! We also want to be able to support negative cache hits (i.e. the case where
//! we can determine an object does not exist without hitting the database).
//!
//! To achieve both of these goals, we split the cache data into two pieces, a
//! dirty set and a cached set. The dirty set has no automatic evictions, data
//! is only removed after being committed. The cached set is in a bounded-sized
//! cache with automatic evictions. In order to support negative cache hits, we
//! treat the two halves of the cache as FIFO queue. Newly written (dirty)
//! versions are inserted to one end of the dirty queue. As versions are
//! committed to disk, they are removed from the other end of the dirty queue
//! and inserted into the cache queue. The cache queue is truncated if it
//! exceeds its maximum size, by removing all but the N newest versions.
//!
//! This gives us the property that the sequence of versions in the dirty and
//! cached queues are the most recent versions of the object, i.e. there can be
//! no "gaps". This allows for the following:
//!
//!   - Negative cache hits: If the queried version is not in memory, but is
//!     higher than the smallest version in the cached queue, it does not exist
//!     in the db either.
//!   - Bounded reads: When reading the most recent version that is <= some
//!     version bound, we can correctly satisfy this query from the cache, or
//!     determine that we must go to the db.
//!
//! Note that at any time, either or both the dirty or the cached queue may be
//! non-existent. There may be no dirty versions of the objects, in which case
//! there will be no dirty queue. And, the cached queue may be evicted from the
//! cache, in which case there will be no cached queue. Because only the cached
//! queue can be evicted (the dirty queue can only become empty by moving
//! versions from it to the cached queue), the "highest versions" property still
//! holds in all cases.
//!
//! The above design is used for both objects and markers.

use std::{
    collections::{BTreeMap, BTreeSet},
    hash::Hash,
    sync::Arc,
};

use dashmap::{DashMap, mapref::entry::Entry as DashMapEntry};
use futures::{FutureExt, future::BoxFuture};
use iota_common::sync::notify_read::NotifyRead;
use iota_macros::fail_point_async;
use iota_types::{
    accumulator::Accumulator,
    base_types::{EpochId, ObjectID, ObjectRef, SequenceNumber, VerifiedExecutionData},
    digests::{ObjectDigest, TransactionDigest, TransactionEffectsDigest, TransactionEventsDigest},
    effects::{TransactionEffects, TransactionEvents},
    error::{IotaError, IotaResult, UserInputError},
    iota_system_state::{IotaSystemState, get_iota_system_state},
    message_envelope::Message,
    messages_checkpoint::CheckpointSequenceNumber,
    object::Object,
    storage::{MarkerValue, ObjectKey, ObjectOrTombstone, ObjectStore, PackageObject},
    transaction::{VerifiedSignedTransaction, VerifiedTransaction},
};
use moka::sync::Cache as MokaCache;
use parking_lot::Mutex;
use prometheus::Registry;
use tap::TapOptional;
use tracing::{debug, info, instrument, trace, warn};

use super::{
    CheckpointCache, ExecutionCacheAPI, ExecutionCacheCommit, ExecutionCacheMetrics,
    ExecutionCacheReconfigAPI, ExecutionCacheWrite, ObjectCacheRead, StateSyncAPI, TestingAPI,
    TransactionCacheRead,
    cache_types::{CachedVersionMap, IsNewer, MonotonicCache},
    implement_passthrough_traits,
    object_locks::ObjectLocks,
};
use crate::{
    authority::{
        AuthorityStore,
        authority_per_epoch_store::AuthorityPerEpochStore,
        authority_store::{ExecutionLockWriteGuard, IotaLockResult, ObjectLockStatus},
        authority_store_tables::LiveObject,
        epoch_start_configuration::{EpochFlag, EpochStartConfiguration},
    },
    state_accumulator::AccumulatorStore,
    transaction_outputs::TransactionOutputs,
};

#[cfg(test)]
#[path = "unit_tests/writeback_cache_tests.rs"]
pub mod writeback_cache_tests;

#[derive(Clone, PartialEq, Eq)]
enum ObjectEntry {
    Object(Object),
    Deleted,
    Wrapped,
}

impl ObjectEntry {
    #[cfg(test)]
    fn unwrap_object(&self) -> &Object {
        match self {
            ObjectEntry::Object(o) => o,
            _ => panic!("unwrap_object called on non-Object"),
        }
    }

    fn is_tombstone(&self) -> bool {
        match self {
            ObjectEntry::Deleted | ObjectEntry::Wrapped => true,
            ObjectEntry::Object(_) => false,
        }
    }
}

impl std::fmt::Debug for ObjectEntry {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ObjectEntry::Object(o) => {
                write!(f, "ObjectEntry::Object({:?})", o.compute_object_reference())
            }
            ObjectEntry::Deleted => write!(f, "ObjectEntry::Deleted"),
            ObjectEntry::Wrapped => write!(f, "ObjectEntry::Wrapped"),
        }
    }
}

impl From<Object> for ObjectEntry {
    fn from(object: Object) -> Self {
        ObjectEntry::Object(object)
    }
}

impl From<ObjectOrTombstone> for ObjectEntry {
    fn from(object: ObjectOrTombstone) -> Self {
        match object {
            ObjectOrTombstone::Object(o) => o.into(),
            ObjectOrTombstone::Tombstone(obj_ref) => {
                if obj_ref.2.is_deleted() {
                    ObjectEntry::Deleted
                } else if obj_ref.2.is_wrapped() {
                    ObjectEntry::Wrapped
                } else {
                    panic!("tombstone digest must either be deleted or wrapped");
                }
            }
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq)]
enum LatestObjectCacheEntry {
    Object(SequenceNumber, ObjectEntry),
    NonExistent,
}

impl LatestObjectCacheEntry {
    #[cfg(test)]
    fn version(&self) -> Option<SequenceNumber> {
        match self {
            LatestObjectCacheEntry::Object(version, _) => Some(*version),
            LatestObjectCacheEntry::NonExistent => None,
        }
    }
}

impl IsNewer for LatestObjectCacheEntry {
    fn is_newer_than(&self, other: &LatestObjectCacheEntry) -> bool {
        match (self, other) {
            (LatestObjectCacheEntry::Object(v1, _), LatestObjectCacheEntry::Object(v2, _)) => {
                v1 > v2
            }
            (LatestObjectCacheEntry::Object(_, _), LatestObjectCacheEntry::NonExistent) => true,
            _ => false,
        }
    }
}

type MarkerKey = (EpochId, ObjectID);

enum CacheResult<T> {
    /// Entry is in the cache
    Hit(T),
    /// Entry is not in the cache and is known to not exist
    NegativeHit,
    /// Entry is not in the cache and may or may not exist in the store
    Miss,
}

/// UncommittedData stores execution outputs that are not yet written to the db.
/// Entries in this struct can only be purged after they are committed.
struct UncommittedData {
    /// The object dirty set. All writes go into this table first. After we
    /// flush the data to the db, the data is removed from this table and
    /// inserted into the object_cache.
    ///
    /// This table may contain both live and dead objects, since we flush both
    /// live and dead objects to the db in order to support past object
    /// queries on fullnodes.
    ///
    /// Further, we only remove objects in FIFO order, which ensures that the
    /// cached sequence of objects has no gaps. In other words, if we have
    /// versions 4, 8, 13 of an object, we can deduce that version 9 does
    /// not exist. This also makes child object reads efficient.
    /// `object_cache` cannot contain a more recent version of an object than
    /// `objects`, and neither can have any gaps. Therefore if there is any
    /// object <= the version bound for a child read in objects, it is the
    /// correct object to return.
    objects: DashMap<ObjectID, CachedVersionMap<ObjectEntry>>,

    // Markers for received objects and deleted shared objects. This contains all of the dirty
    // marker state, which is committed to the db at the same time as other transaction data.
    // After markers are committed to the db we remove them from this table and insert them into
    // marker_cache.
    markers: DashMap<MarkerKey, CachedVersionMap<MarkerValue>>,

    transaction_effects: DashMap<TransactionEffectsDigest, TransactionEffects>,

    // Because TransactionEvents are not unique to the transaction that created them, we must
    // reference count them in order to know when we can remove them from the cache. For now
    // we track all referrers explicitly, but we can use a ref count when we are confident in
    // the correctness of the code.
    transaction_events:
        DashMap<TransactionEventsDigest, (BTreeSet<TransactionDigest>, TransactionEvents)>,

    executed_effects_digests: DashMap<TransactionDigest, TransactionEffectsDigest>,

    // Transaction outputs that have not yet been written to the DB. Items are removed from this
    // table as they are flushed to the db.
    pending_transaction_writes: DashMap<TransactionDigest, Arc<TransactionOutputs>>,
}

impl UncommittedData {
    fn new() -> Self {
        Self {
            objects: DashMap::new(),
            markers: DashMap::new(),
            transaction_effects: DashMap::new(),
            executed_effects_digests: DashMap::new(),
            pending_transaction_writes: DashMap::new(),
            transaction_events: DashMap::new(),
        }
    }

    fn clear(&self) {
        self.objects.clear();
        self.markers.clear();
        self.transaction_effects.clear();
        self.executed_effects_digests.clear();
        self.pending_transaction_writes.clear();
        self.transaction_events.clear();
    }

    fn is_empty(&self) -> bool {
        let empty = self.pending_transaction_writes.is_empty();
        if empty && cfg!(debug_assertions) {
            assert!(
                self.objects.is_empty()
                    && self.markers.is_empty()
                    && self.transaction_effects.is_empty()
                    && self.executed_effects_digests.is_empty()
                    && self.transaction_events.is_empty()
            );
        }
        empty
    }
}

// TODO: set this via the config
static MAX_CACHE_SIZE: u64 = 10000;

/// CachedData stores data that has been committed to the db, but is likely to
/// be read soon.
struct CachedCommittedData {
    // See module level comment for an explanation of caching strategy.
    object_cache: MokaCache<ObjectID, Arc<Mutex<CachedVersionMap<ObjectEntry>>>>,

    // We separately cache the latest version of each object. Although this seems
    // redundant, it is the only way to support populating the cache after a read.
    // We cannot simply insert objects that we read off the disk into `object_cache`,
    // since that may violate the no-missing-versions property.
    // `object_by_id_cache` is also written to on writes so that it is always coherent.
    object_by_id_cache: MonotonicCache<ObjectID, LatestObjectCacheEntry>,

    // See module level comment for an explanation of caching strategy.
    marker_cache: MokaCache<MarkerKey, Arc<Mutex<CachedVersionMap<MarkerValue>>>>,

    transactions: MokaCache<TransactionDigest, Arc<VerifiedTransaction>>,

    transaction_effects: MokaCache<TransactionEffectsDigest, Arc<TransactionEffects>>,

    transaction_events: MokaCache<TransactionEventsDigest, Arc<TransactionEvents>>,

    executed_effects_digests: MokaCache<TransactionDigest, TransactionEffectsDigest>,

    // Objects that were read at transaction signing time - allows us to access them again at
    // execution time with a single lock / hash lookup
    _transaction_objects: MokaCache<TransactionDigest, Vec<Object>>,
}

impl CachedCommittedData {
    fn new() -> Self {
        let object_cache = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let marker_cache = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let transactions = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let transaction_effects = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let transaction_events = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let executed_effects_digests = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        let transaction_objects = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();

        Self {
            object_cache,
            object_by_id_cache: MonotonicCache::new(MAX_CACHE_SIZE),
            marker_cache,
            transactions,
            transaction_effects,
            transaction_events,
            executed_effects_digests,
            _transaction_objects: transaction_objects,
        }
    }

    fn clear_and_assert_empty(&self) {
        self.object_cache.invalidate_all();
        self.object_by_id_cache.invalidate_all();
        self.marker_cache.invalidate_all();
        self.transactions.invalidate_all();
        self.transaction_effects.invalidate_all();
        self.transaction_events.invalidate_all();
        self.executed_effects_digests.invalidate_all();
        self._transaction_objects.invalidate_all();

        assert_empty(&self.object_cache);
        assert!(&self.object_by_id_cache.is_empty());
        assert_empty(&self.marker_cache);
        assert_empty(&self.transactions);
        assert_empty(&self.transaction_effects);
        assert_empty(&self.transaction_events);
        assert_empty(&self.executed_effects_digests);
        assert_empty(&self._transaction_objects);
    }
}

fn assert_empty<K, V>(cache: &MokaCache<K, V>)
where
    K: std::hash::Hash + std::cmp::Eq + std::cmp::PartialEq + Send + Sync + 'static,
    V: std::clone::Clone + std::marker::Send + std::marker::Sync + 'static,
{
    if cache.iter().next().is_some() {
        panic!("cache should be empty");
    }
}

pub struct WritebackCache {
    dirty: UncommittedData,
    cached: CachedCommittedData,

    // The packages cache is treated separately from objects, because they are immutable and can be
    // used by any number of transactions. Additionally, many operations require loading large
    // numbers of packages (due to dependencies), so we want to try to keep all packages in memory.
    //
    // Also, this cache can contain packages that are dirty or committed, so it does not live in
    // UncachedData or CachedCommittedData. The cache is populated in two ways:
    // - when packages are written (in which case they will also be present in the dirty set)
    // - after a cache miss. Because package IDs are unique (only one version exists for each ID)
    //   we do not need to worry about the contiguous version property.
    // - note that we removed any unfinalized packages from the cache during revert_state_update().
    packages: MokaCache<ObjectID, PackageObject>,

    object_locks: ObjectLocks,

    executed_effects_digests_notify_read: NotifyRead<TransactionDigest, TransactionEffectsDigest>,
    store: Arc<AuthorityStore>,
    metrics: Arc<ExecutionCacheMetrics>,
}

macro_rules! check_cache_entry_by_version {
    ($self: ident, $table: expr, $level: expr, $cache: expr, $version: expr) => {
        $self.metrics.record_cache_request($table, $level);
        if let Some(cache) = $cache {
            if let Some(entry) = cache.get(&$version) {
                $self.metrics.record_cache_hit($table, $level);
                return CacheResult::Hit(entry.clone());
            }

            if let Some(least_version) = cache.get_least() {
                if least_version.0 < $version {
                    // If the version is greater than the least version in the cache, then we know
                    // that the object does not exist anywhere
                    $self.metrics.record_cache_negative_hit($table, $level);
                    return CacheResult::NegativeHit;
                }
            }
        }
        $self.metrics.record_cache_miss($table, $level);
    };
}

macro_rules! check_cache_entry_by_latest {
    ($self: ident, $table: expr, $level: expr, $cache: expr) => {
        $self.metrics.record_cache_request($table, $level);
        if let Some(cache) = $cache {
            if let Some((version, entry)) = cache.get_highest() {
                $self.metrics.record_cache_hit($table, $level);
                return CacheResult::Hit((*version, entry.clone()));
            } else {
                panic!("empty CachedVersionMap should have been removed");
            }
        }
        $self.metrics.record_cache_miss($table, $level);
    };
}

impl WritebackCache {
    pub fn new(store: Arc<AuthorityStore>, metrics: Arc<ExecutionCacheMetrics>) -> Self {
        let packages = MokaCache::builder()
            .max_capacity(MAX_CACHE_SIZE)
            .max_capacity(MAX_CACHE_SIZE)
            .build();
        Self {
            dirty: UncommittedData::new(),
            cached: CachedCommittedData::new(),
            packages,
            object_locks: ObjectLocks::new(),
            executed_effects_digests_notify_read: NotifyRead::new(),
            store,
            metrics,
        }
    }

    pub fn new_for_tests(store: Arc<AuthorityStore>, registry: &Registry) -> Self {
        Self::new(store, ExecutionCacheMetrics::new(registry).into())
    }

    #[cfg(test)]
    pub fn reset_for_test(&mut self) {
        let mut new = Self::new(self.store.clone(), self.metrics.clone());
        std::mem::swap(self, &mut new);
    }

    async fn write_object_entry(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
        object: ObjectEntry,
    ) {
        trace!(?object_id, ?version, ?object, "inserting object entry");
        fail_point_async!("write_object_entry");
        self.metrics.record_cache_write("object");

        // We must hold the lock for the object entry while inserting to the
        // object_by_id_cache. Otherwise, a surprising bug can occur:
        //
        // 1. A thread executing TX1 can write object (O,1) to the dirty set and then
        //    pause.
        // 2. TX2, which reads (O,1) can begin executing, because TransactionManager
        //    immediately schedules transactions if their inputs are available. It does
        //    not matter that TX1 hasn't finished executing yet.
        // 3. TX2 can write (O,2) to both the dirty set and the object_by_id_cache.
        // 4. The thread executing TX1 can resume and write (O,1) to the
        //    object_by_id_cache.
        //
        // Now, any subsequent attempt to get the latest version of O will return (O,1)
        // instead of (O,2).
        //
        // This seems very unlikely, but it may be possible under the following
        // circumstances:
        // - While a thread is unlikely to pause for so long, moka cache uses optimistic
        //   lock-free algorithms that have retry loops. Possibly, under high
        //   contention, this code might spin for a surprisingly long time.
        // - Additionally, many concurrent re-executions of the same tx could happen due
        //   to the tx finalizer, plus checkpoint executor, consensus, and RPCs from
        //   fullnodes.
        let mut entry = self.dirty.objects.entry(*object_id).or_default();

        self.cached.object_by_id_cache.insert(
            object_id,
            LatestObjectCacheEntry::Object(version, object.clone()),
        );

        entry.insert(version, object);
    }

    async fn write_marker_value(
        &self,
        epoch_id: EpochId,
        object_key: &ObjectKey,
        marker_value: MarkerValue,
    ) {
        tracing::trace!(
            "inserting marker value {:?}: {:?}",
            object_key,
            marker_value
        );
        fail_point_async!("write_marker_entry");
        self.metrics.record_cache_write("marker");
        self.dirty
            .markers
            .entry((epoch_id, object_key.0))
            .or_default()
            .value_mut()
            .insert(object_key.1, marker_value);
    }

    // lock both the dirty and committed sides of the cache, and then pass the
    // entries to the callback. Written with the `with` pattern because any
    // other way of doing this creates lifetime hell.
    fn with_locked_cache_entries<K, V, R>(
        dirty_map: &DashMap<K, CachedVersionMap<V>>,
        cached_map: &MokaCache<K, Arc<Mutex<CachedVersionMap<V>>>>,
        key: &K,
        cb: impl FnOnce(Option<&CachedVersionMap<V>>, Option<&CachedVersionMap<V>>) -> R,
    ) -> R
    where
        K: Copy + Eq + Hash + Send + Sync + 'static,
        V: Send + Sync + 'static,
    {
        let dirty_entry = dirty_map.entry(*key);
        let dirty_entry = match &dirty_entry {
            DashMapEntry::Occupied(occupied) => Some(occupied.get()),
            DashMapEntry::Vacant(_) => None,
        };

        let cached_entry = cached_map.get(key);
        let cached_lock = cached_entry.as_ref().map(|entry| entry.lock());
        let cached_entry = cached_lock.as_deref();

        cb(dirty_entry, cached_entry)
    }

    // Attempt to get an object from the cache. The DB is not consulted.
    // Can return Hit, Miss, or NegativeHit (if the object is known to not exist).
    fn get_object_entry_by_key_cache_only(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
    ) -> CacheResult<ObjectEntry> {
        Self::with_locked_cache_entries(
            &self.dirty.objects,
            &self.cached.object_cache,
            object_id,
            |dirty_entry, cached_entry| {
                check_cache_entry_by_version!(
                    self,
                    "object_by_version",
                    "uncommitted",
                    dirty_entry,
                    version
                );
                check_cache_entry_by_version!(
                    self,
                    "object_by_version",
                    "committed",
                    cached_entry,
                    version
                );
                CacheResult::Miss
            },
        )
    }

    fn get_object_by_key_cache_only(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
    ) -> CacheResult<Object> {
        match self.get_object_entry_by_key_cache_only(object_id, version) {
            CacheResult::Hit(entry) => match entry {
                ObjectEntry::Object(object) => CacheResult::Hit(object),
                ObjectEntry::Deleted | ObjectEntry::Wrapped => CacheResult::NegativeHit,
            },
            CacheResult::Miss => CacheResult::Miss,
            CacheResult::NegativeHit => CacheResult::NegativeHit,
        }
    }

    fn get_object_entry_by_id_cache_only(
        &self,
        request_type: &'static str,
        object_id: &ObjectID,
    ) -> CacheResult<(SequenceNumber, ObjectEntry)> {
        self.metrics
            .record_cache_request(request_type, "object_by_id");
        let entry = self.cached.object_by_id_cache.get(object_id);

        if cfg!(debug_assertions) {
            if let Some(entry) = &entry {
                // check that cache is coherent
                let highest: Option<ObjectEntry> = self
                    .dirty
                    .objects
                    .get(object_id)
                    .and_then(|entry| entry.get_highest().map(|(_, o)| o.clone()))
                    .or_else(|| {
                        let obj: Option<ObjectEntry> = self
                            .store
                            .get_latest_object_or_tombstone(*object_id)
                            .unwrap()
                            .map(|(_, o)| o.into());
                        obj
                    });

                let cache_entry = match &*entry.lock() {
                    LatestObjectCacheEntry::Object(_, entry) => Some(entry.clone()),
                    LatestObjectCacheEntry::NonExistent => None,
                };

                // If the cache entry is a tombstone, the db entry may be missing if it was
                // pruned.
                let tombstone_possibly_pruned = highest.is_none()
                    && cache_entry
                        .as_ref()
                        .map(|e| e.is_tombstone())
                        .unwrap_or(false);

                if highest != cache_entry && !tombstone_possibly_pruned {
                    tracing::error!(
                        ?highest,
                        ?cache_entry,
                        ?tombstone_possibly_pruned,
                        "object_by_id cache is incoherent for {:?}",
                        object_id
                    );
                    panic!("object_by_id cache is incoherent for {object_id:?}");
                }
            }
        }

        if let Some(entry) = entry {
            let entry = entry.lock();
            match &*entry {
                LatestObjectCacheEntry::Object(latest_version, latest_object) => {
                    self.metrics.record_cache_hit(request_type, "object_by_id");
                    return CacheResult::Hit((*latest_version, latest_object.clone()));
                }
                LatestObjectCacheEntry::NonExistent => {
                    self.metrics
                        .record_cache_negative_hit(request_type, "object_by_id");
                    return CacheResult::NegativeHit;
                }
            }
        } else {
            self.metrics.record_cache_miss(request_type, "object_by_id");
        }

        Self::with_locked_cache_entries(
            &self.dirty.objects,
            &self.cached.object_cache,
            object_id,
            |dirty_entry, cached_entry| {
                check_cache_entry_by_latest!(self, request_type, "uncommitted", dirty_entry);
                check_cache_entry_by_latest!(self, request_type, "committed", cached_entry);
                CacheResult::Miss
            },
        )
    }

    fn get_object_by_id_cache_only(
        &self,
        request_type: &'static str,
        object_id: &ObjectID,
    ) -> CacheResult<(SequenceNumber, Object)> {
        match self.get_object_entry_by_id_cache_only(request_type, object_id) {
            CacheResult::Hit((version, entry)) => match entry {
                ObjectEntry::Object(object) => CacheResult::Hit((version, object)),
                ObjectEntry::Deleted | ObjectEntry::Wrapped => CacheResult::NegativeHit,
            },
            CacheResult::NegativeHit => CacheResult::NegativeHit,
            CacheResult::Miss => CacheResult::Miss,
        }
    }

    fn get_marker_value_cache_only(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
        epoch_id: EpochId,
    ) -> CacheResult<MarkerValue> {
        Self::with_locked_cache_entries(
            &self.dirty.markers,
            &self.cached.marker_cache,
            &(epoch_id, *object_id),
            |dirty_entry, cached_entry| {
                check_cache_entry_by_version!(
                    self,
                    "marker_by_version",
                    "uncommitted",
                    dirty_entry,
                    version
                );
                check_cache_entry_by_version!(
                    self,
                    "marker_by_version",
                    "committed",
                    cached_entry,
                    version
                );
                CacheResult::Miss
            },
        )
    }

    fn get_latest_marker_value_cache_only(
        &self,
        object_id: &ObjectID,
        epoch_id: EpochId,
    ) -> CacheResult<(SequenceNumber, MarkerValue)> {
        Self::with_locked_cache_entries(
            &self.dirty.markers,
            &self.cached.marker_cache,
            &(epoch_id, *object_id),
            |dirty_entry, cached_entry| {
                check_cache_entry_by_latest!(self, "marker_latest", "uncommitted", dirty_entry);
                check_cache_entry_by_latest!(self, "marker_latest", "committed", cached_entry);
                CacheResult::Miss
            },
        )
    }

    fn get_object_impl(
        &self,
        request_type: &'static str,
        id: &ObjectID,
    ) -> IotaResult<Option<Object>> {
        match self.get_object_by_id_cache_only(request_type, id) {
            CacheResult::Hit((_, object)) => Ok(Some(object)),
            CacheResult::NegativeHit => Ok(None),
            CacheResult::Miss => {
                let obj = self.store.get_object(id)?;
                if let Some(obj) = &obj {
                    self.cache_latest_object_by_id(
                        id,
                        LatestObjectCacheEntry::Object(obj.version(), obj.clone().into()),
                    );
                } else {
                    self.cache_object_not_found(id);
                }
                Ok(obj)
            }
        }
    }

    fn record_db_get(&self, request_type: &'static str) -> &AuthorityStore {
        self.metrics.record_cache_request(request_type, "db");
        &self.store
    }

    fn record_db_multi_get(&self, request_type: &'static str, count: usize) -> &AuthorityStore {
        self.metrics
            .record_cache_multi_request(request_type, "db", count);
        &self.store
    }

    #[instrument(level = "debug", skip_all)]
    async fn write_transaction_outputs(
        &self,
        epoch_id: EpochId,
        tx_outputs: Arc<TransactionOutputs>,
    ) -> IotaResult {
        trace!(digest = ?tx_outputs.transaction.digest(), "writing transaction outputs to cache");

        let TransactionOutputs {
            transaction,
            effects,
            markers,
            written,
            deleted,
            wrapped,
            events,
            ..
        } = &*tx_outputs;

        // Deletions and wraps must be written first. The reason is that one of the
        // deletes may be a child object, and if we write the parent object
        // first, a reader may or may not see the previous version of the child
        // object, instead of the deleted/wrapped tombstone, which would cause
        // an execution fork
        for ObjectKey(id, version) in deleted.iter() {
            self.write_object_entry(id, *version, ObjectEntry::Deleted)
                .await;
        }

        for ObjectKey(id, version) in wrapped.iter() {
            self.write_object_entry(id, *version, ObjectEntry::Wrapped)
                .await;
        }

        // Update all markers
        for (object_key, marker_value) in markers.iter() {
            self.write_marker_value(epoch_id, object_key, *marker_value)
                .await;
        }

        // Write children before parents to ensure that readers do not observe a parent
        // object before its most recent children are visible.
        for (object_id, object) in written.iter() {
            if object.is_child_object() {
                self.write_object_entry(object_id, object.version(), object.clone().into())
                    .await;
            }
        }
        for (object_id, object) in written.iter() {
            if !object.is_child_object() {
                self.write_object_entry(object_id, object.version(), object.clone().into())
                    .await;
                if object.is_package() {
                    debug!("caching package: {:?}", object.compute_object_reference());
                    self.packages
                        .insert(*object_id, PackageObject::new(object.clone()));
                }
            }
        }

        let tx_digest = *transaction.digest();
        let effects_digest = effects.digest();

        self.metrics.record_cache_write("transaction_block");
        self.dirty
            .pending_transaction_writes
            .insert(tx_digest, tx_outputs.clone());

        // insert transaction effects before executed_effects_digests so that there
        // are never dangling entries in executed_effects_digests
        self.metrics.record_cache_write("transaction_effects");
        self.dirty
            .transaction_effects
            .insert(effects_digest, effects.clone());

        // note: if events.data.is_empty(), then there are no events for this
        // transaction. We store it anyway to avoid special cases in
        // commint_transaction_outputs, and translate an empty events structure
        // to None when reading.
        self.metrics.record_cache_write("transaction_events");
        match self.dirty.transaction_events.entry(events.digest()) {
            DashMapEntry::Occupied(mut occupied) => {
                occupied.get_mut().0.insert(tx_digest);
            }
            DashMapEntry::Vacant(entry) => {
                let mut txns = BTreeSet::new();
                txns.insert(tx_digest);
                entry.insert((txns, events.clone()));
            }
        }

        self.metrics.record_cache_write("executed_effects_digests");
        self.dirty
            .executed_effects_digests
            .insert(tx_digest, effects_digest);

        self.executed_effects_digests_notify_read
            .notify(&tx_digest, &effects_digest);

        self.metrics
            .pending_notify_read
            .set(self.executed_effects_digests_notify_read.num_pending() as i64);

        Ok(())
    }

    // Commits dirty data for the given TransactionDigest to the db.
    #[instrument(level = "debug", skip_all)]
    async fn commit_transaction_outputs(
        &self,
        epoch: EpochId,
        digests: &[TransactionDigest],
    ) -> IotaResult {
        fail_point_async!("writeback-cache-commit");
        trace!(?digests);

        let mut all_outputs = Vec::with_capacity(digests.len());
        for tx in digests {
            let Some(outputs) = self
                .dirty
                .pending_transaction_writes
                .get(tx)
                .map(|o| o.clone())
            else {
                // This can happen in the following rare case:
                // All transactions in the checkpoint are committed to the db (by
                // commit_transaction_outputs,
                // called in CheckpointExecutor::process_executed_transactions), but the process
                // crashes before the checkpoint water mark is bumped. We will
                // then re-commit the checkpoint at startup, despite that all
                // transactions are already executed.
                warn!("Attempt to commit unknown transaction {:?}", tx);
                continue;
            };
            all_outputs.push(outputs);
        }

        // Flush writes to disk before removing anything from dirty set. otherwise,
        // a cache eviction could cause a value to disappear briefly, even if we insert
        // to the cache before removing from the dirty set.
        self.store
            .write_transaction_outputs(epoch, &all_outputs)
            .await?;

        for outputs in all_outputs.iter() {
            let tx_digest = outputs.transaction.digest();
            assert!(
                self.dirty
                    .pending_transaction_writes
                    .remove(tx_digest)
                    .is_some()
            );
            self.flush_transactions_from_dirty_to_cached(epoch, *tx_digest, outputs);
        }

        Ok(())
    }

    fn flush_transactions_from_dirty_to_cached(
        &self,
        epoch: EpochId,
        tx_digest: TransactionDigest,
        outputs: &TransactionOutputs,
    ) {
        // Now, remove each piece of committed data from the dirty state and insert it
        // into the cache. TODO: outputs should have a strong count of 1 so we
        // should be able to move out of it
        let TransactionOutputs {
            transaction,
            effects,
            markers,
            written,
            deleted,
            wrapped,
            events,
            ..
        } = outputs;

        let effects_digest = effects.digest();
        let events_digest = events.digest();

        // Update cache before removing from self.dirty to avoid
        // unnecessary cache misses
        self.cached
            .transactions
            .insert(tx_digest, transaction.clone());
        self.cached
            .transaction_effects
            .insert(effects_digest, effects.clone().into());
        self.cached
            .executed_effects_digests
            .insert(tx_digest, effects_digest);
        self.cached
            .transaction_events
            .insert(events_digest, events.clone().into());

        self.dirty
            .transaction_effects
            .remove(&effects_digest)
            .expect("effects must exist");

        match self.dirty.transaction_events.entry(events.digest()) {
            DashMapEntry::Occupied(mut occupied) => {
                let txns = &mut occupied.get_mut().0;
                assert!(txns.remove(&tx_digest), "transaction must exist");
                if txns.is_empty() {
                    occupied.remove();
                }
            }
            DashMapEntry::Vacant(_) => {
                panic!("events must exist");
            }
        }

        self.dirty
            .executed_effects_digests
            .remove(&tx_digest)
            .expect("executed effects must exist");

        // Move dirty markers to cache
        for (object_key, marker_value) in markers.iter() {
            Self::move_version_from_dirty_to_cache(
                &self.dirty.markers,
                &self.cached.marker_cache,
                (epoch, object_key.0),
                object_key.1,
                marker_value,
            );
        }

        for (object_id, object) in written.iter() {
            Self::move_version_from_dirty_to_cache(
                &self.dirty.objects,
                &self.cached.object_cache,
                *object_id,
                object.version(),
                &ObjectEntry::Object(object.clone()),
            );
        }

        for ObjectKey(object_id, version) in deleted.iter() {
            Self::move_version_from_dirty_to_cache(
                &self.dirty.objects,
                &self.cached.object_cache,
                *object_id,
                *version,
                &ObjectEntry::Deleted,
            );
        }

        for ObjectKey(object_id, version) in wrapped.iter() {
            Self::move_version_from_dirty_to_cache(
                &self.dirty.objects,
                &self.cached.object_cache,
                *object_id,
                *version,
                &ObjectEntry::Wrapped,
            );
        }
    }

    async fn persist_transactions(&self, digests: &[TransactionDigest]) -> IotaResult {
        let mut txns = Vec::with_capacity(digests.len());
        for tx_digest in digests {
            let Some(tx) = self
                .dirty
                .pending_transaction_writes
                .get(tx_digest)
                .map(|o| o.transaction.clone())
            else {
                // tx should exist in the db if it is not in dirty set.
                debug_assert!(
                    self.store
                        .get_transaction_block(tx_digest)
                        .unwrap()
                        .is_some()
                );
                // If the transaction is not in dirty, it does not need to be committed.
                // This situation can happen if we build a checkpoint locally which was just
                // executed via state sync.
                continue;
            };

            txns.push((*tx_digest, (*tx).clone()));
        }

        self.store.commit_transactions(&txns)
    }

    // Move the oldest/least entry from the dirty queue to the cache queue.
    // This is called after the entry is committed to the db.
    fn move_version_from_dirty_to_cache<K, V>(
        dirty: &DashMap<K, CachedVersionMap<V>>,
        cache: &MokaCache<K, Arc<Mutex<CachedVersionMap<V>>>>,
        key: K,
        version: SequenceNumber,
        value: &V,
    ) where
        K: Eq + std::hash::Hash + Clone + Send + Sync + Copy + 'static,
        V: Send + Sync + Clone + Eq + std::fmt::Debug + 'static,
    {
        static MAX_VERSIONS: usize = 3;

        // IMPORTANT: lock both the dirty set entry and the cache entry before modifying
        // either. this ensures that readers cannot see a value temporarily
        // disappear.
        let dirty_entry = dirty.entry(key);
        let cache_entry = cache.entry(key).or_default();
        let mut cache_map = cache_entry.value().lock();

        // insert into cache and drop old versions.
        cache_map.insert(version, value.clone());
        // TODO: make this automatic by giving CachedVersionMap an optional max capacity
        cache_map.truncate_to(MAX_VERSIONS);

        let DashMapEntry::Occupied(mut occupied_dirty_entry) = dirty_entry else {
            panic!("dirty map must exist");
        };

        let removed = occupied_dirty_entry.get_mut().pop_oldest(&version);

        assert_eq!(removed.as_ref(), Some(value), "dirty version must exist");

        // if there are no versions remaining, remove the map entry
        if occupied_dirty_entry.get().is_empty() {
            occupied_dirty_entry.remove();
        }
    }

    // Updates the latest object id cache with an entry that was read from the db.
    fn cache_latest_object_by_id(&self, object_id: &ObjectID, object: LatestObjectCacheEntry) {
        trace!("caching object by id: {:?} {:?}", object_id, object);
        self.metrics.record_cache_write("object_by_id");
        self.cached.object_by_id_cache.insert(object_id, object);
    }

    fn cache_object_not_found(&self, object_id: &ObjectID) {
        self.cache_latest_object_by_id(object_id, LatestObjectCacheEntry::NonExistent);
    }

    fn clear_state_end_of_epoch_impl(&self, _execution_guard: &ExecutionLockWriteGuard<'_>) {
        info!("clearing state at end of epoch");
        assert!(
            self.dirty.pending_transaction_writes.is_empty(),
            "should be empty due to revert_state_update"
        );
        self.dirty.clear();
        info!("clearing old transaction locks");
        self.object_locks.clear();
    }

    fn revert_state_update_impl(&self, tx: &TransactionDigest) -> IotaResult {
        // TODO: remove revert_state_update_impl entirely, and simply drop all dirty
        // state when clear_state_end_of_epoch_impl is called.
        // Further, once we do this, we can delay the insertion of the transaction into
        // pending_consensus_transactions until after the transaction has executed.
        let Some((_, outputs)) = self.dirty.pending_transaction_writes.remove(tx) else {
            assert!(
                !self.is_tx_already_executed(tx).expect("read cannot fail"),
                "attempt to revert committed transaction"
            );

            // A transaction can be inserted into pending_consensus_transactions, but then
            // reconfiguration can happen before the transaction executes.
            info!("Not reverting {:?} as it was not executed", tx);
            return Ok(());
        };

        for (object_id, object) in outputs.written.iter() {
            if object.is_package() {
                info!("removing non-finalized package from cache: {:?}", object_id);
                self.packages.invalidate(object_id);
            }
            self.cached.object_by_id_cache.invalidate(object_id);
            self.cached.object_cache.invalidate(object_id);
        }

        for ObjectKey(object_id, _) in outputs.deleted.iter().chain(outputs.wrapped.iter()) {
            self.cached.object_by_id_cache.invalidate(object_id);
            self.cached.object_cache.invalidate(object_id);
        }

        // Note: individual object entries are removed when
        // clear_state_end_of_epoch_impl is called
        Ok(())
    }

    fn bulk_insert_genesis_objects_impl(&self, objects: &[Object]) -> IotaResult {
        self.store.bulk_insert_genesis_objects(objects)?;
        for obj in objects {
            self.cached.object_cache.invalidate(&obj.id());
            self.cached.object_by_id_cache.invalidate(&obj.id());
        }
        Ok(())
    }

    fn insert_genesis_object_impl(&self, object: Object) -> IotaResult {
        self.cached.object_by_id_cache.invalidate(&object.id());
        self.cached.object_cache.invalidate(&object.id());
        self.store.insert_genesis_object(object)
    }

    pub fn clear_caches_and_assert_empty(&self) {
        info!("clearing caches");
        self.cached.clear_and_assert_empty();
        self.packages.invalidate_all();
        assert_empty(&self.packages);
    }
}

impl ExecutionCacheAPI for WritebackCache {}

impl ExecutionCacheCommit for WritebackCache {
    fn commit_transaction_outputs<'a>(
        &'a self,
        epoch: EpochId,
        digests: &'a [TransactionDigest],
    ) -> BoxFuture<'a, IotaResult> {
        WritebackCache::commit_transaction_outputs(self, epoch, digests).boxed()
    }

    fn persist_transactions<'a>(
        &'a self,
        digests: &'a [TransactionDigest],
    ) -> BoxFuture<'a, IotaResult> {
        WritebackCache::persist_transactions(self, digests).boxed()
    }
}

impl ObjectCacheRead for WritebackCache {
    fn get_package_object(&self, package_id: &ObjectID) -> IotaResult<Option<PackageObject>> {
        self.metrics
            .record_cache_request("package", "package_cache");
        if let Some(p) = self.packages.get(package_id) {
            if cfg!(debug_assertions) {
                if let Some(store_package) = self.store.get_object(package_id).unwrap() {
                    assert_eq!(
                        store_package.digest(),
                        p.object().digest(),
                        "Package object cache is inconsistent for package {package_id:?}"
                    );
                }
            }
            self.metrics.record_cache_hit("package", "package_cache");
            return Ok(Some(p));
        } else {
            self.metrics.record_cache_miss("package", "package_cache");
        }

        // We try the dirty objects cache as well before going to the database. This is
        // necessary because the package could be evicted from the package cache
        // before it is committed to the database.
        if let Some(p) = self.get_object_impl("package", package_id)? {
            if p.is_package() {
                let p = PackageObject::new(p);
                tracing::trace!(
                    "caching package: {:?}",
                    p.object().compute_object_reference()
                );
                self.metrics.record_cache_write("package");
                self.packages.insert(*package_id, p.clone());
                Ok(Some(p))
            } else {
                Err(IotaError::UserInput {
                    error: UserInputError::MoveObjectAsPackage {
                        object_id: *package_id,
                    },
                })
            }
        } else {
            Ok(None)
        }
    }

    fn force_reload_system_packages(&self, _system_package_ids: &[ObjectID]) {
        // This is a no-op because all writes go through the cache, therefore it
        // can never be incoherent
    }

    // get_object and variants.

    fn get_object(&self, id: &ObjectID) -> IotaResult<Option<Object>> {
        self.get_object_impl("object_latest", id)
    }

    fn get_object_by_key(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
    ) -> IotaResult<Option<Object>> {
        match self.get_object_by_key_cache_only(object_id, version) {
            CacheResult::Hit(object) => Ok(Some(object)),
            CacheResult::NegativeHit => Ok(None),
            CacheResult::Miss => Ok(self
                .record_db_get("object_by_version")
                .get_object_by_key(object_id, version)?),
        }
    }

    fn multi_get_objects_by_key(
        &self,
        object_keys: &[ObjectKey],
    ) -> Result<Vec<Option<Object>>, IotaError> {
        do_fallback_lookup(
            object_keys,
            |key| {
                Ok(match self.get_object_by_key_cache_only(&key.0, key.1) {
                    CacheResult::Hit(maybe_object) => CacheResult::Hit(Some(maybe_object)),
                    CacheResult::NegativeHit => CacheResult::NegativeHit,
                    CacheResult::Miss => CacheResult::Miss,
                })
            },
            |remaining| {
                self.record_db_multi_get("object_by_version", remaining.len())
                    .multi_get_objects_by_key(remaining)
            },
        )
    }

    fn object_exists_by_key(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
    ) -> IotaResult<bool> {
        match self.get_object_by_key_cache_only(object_id, version) {
            CacheResult::Hit(_) => Ok(true),
            CacheResult::NegativeHit => Ok(false),
            CacheResult::Miss => self
                .record_db_get("object_by_version")
                .object_exists_by_key(object_id, version),
        }
    }

    fn multi_object_exists_by_key(&self, object_keys: &[ObjectKey]) -> IotaResult<Vec<bool>> {
        do_fallback_lookup(
            object_keys,
            |key| {
                Ok(match self.get_object_by_key_cache_only(&key.0, key.1) {
                    CacheResult::Hit(_) => CacheResult::Hit(true),
                    CacheResult::NegativeHit => CacheResult::Hit(false),
                    CacheResult::Miss => CacheResult::Miss,
                })
            },
            |remaining| {
                self.record_db_multi_get("object_by_version", remaining.len())
                    .multi_object_exists_by_key(remaining)
            },
        )
    }

    fn get_latest_object_ref_or_tombstone(
        &self,
        object_id: ObjectID,
    ) -> IotaResult<Option<ObjectRef>> {
        match self.get_object_entry_by_id_cache_only("latest_objref_or_tombstone", &object_id) {
            CacheResult::Hit((version, entry)) => Ok(Some(match entry {
                ObjectEntry::Object(object) => object.compute_object_reference(),
                ObjectEntry::Deleted => (object_id, version, ObjectDigest::OBJECT_DIGEST_DELETED),
                ObjectEntry::Wrapped => (object_id, version, ObjectDigest::OBJECT_DIGEST_WRAPPED),
            })),
            CacheResult::NegativeHit => Ok(None),
            CacheResult::Miss => self
                .record_db_get("latest_objref_or_tombstone")
                .get_latest_object_ref_or_tombstone(object_id),
        }
    }

    fn get_latest_object_or_tombstone(
        &self,
        object_id: ObjectID,
    ) -> Result<Option<(ObjectKey, ObjectOrTombstone)>, IotaError> {
        match self.get_object_entry_by_id_cache_only("latest_object_or_tombstone", &object_id) {
            CacheResult::Hit((version, entry)) => {
                let key = ObjectKey(object_id, version);
                Ok(Some(match entry {
                    ObjectEntry::Object(object) => (key, object.into()),
                    ObjectEntry::Deleted => (
                        key,
                        ObjectOrTombstone::Tombstone((
                            object_id,
                            version,
                            ObjectDigest::OBJECT_DIGEST_DELETED,
                        )),
                    ),
                    ObjectEntry::Wrapped => (
                        key,
                        ObjectOrTombstone::Tombstone((
                            object_id,
                            version,
                            ObjectDigest::OBJECT_DIGEST_WRAPPED,
                        )),
                    ),
                }))
            }
            CacheResult::NegativeHit => Ok(None),
            CacheResult::Miss => self
                .record_db_get("latest_object_or_tombstone")
                .get_latest_object_or_tombstone(object_id),
        }
    }

    #[instrument(level = "trace", skip_all, fields(object_id, version_bound))]
    fn find_object_lt_or_eq_version(
        &self,
        object_id: ObjectID,
        version_bound: SequenceNumber,
    ) -> IotaResult<Option<Object>> {
        macro_rules! check_cache_entry {
            ($level: expr, $objects: expr) => {
                self.metrics
                    .record_cache_request("object_lt_or_eq_version", $level);
                if let Some(objects) = $objects {
                    if let Some((_, object)) = objects
                        .all_versions_lt_or_eq_descending(&version_bound)
                        .next()
                    {
                        if let ObjectEntry::Object(object) = object {
                            self.metrics
                                .record_cache_hit("object_lt_or_eq_version", $level);
                            return Ok(Some(object.clone()));
                        } else {
                            // if we find a tombstone, the object does not exist
                            self.metrics
                                .record_cache_negative_hit("object_lt_or_eq_version", $level);
                            return Ok(None);
                        }
                    } else {
                        self.metrics
                            .record_cache_miss("object_lt_or_eq_version", $level);
                    }
                }
            };
        }

        // if we have the latest version cached, and it is within the bound, we are done
        self.metrics
            .record_cache_request("object_lt_or_eq_version", "object_by_id");
        if let Some(latest) = self.cached.object_by_id_cache.get(&object_id) {
            let latest = latest.lock();
            match &*latest {
                LatestObjectCacheEntry::Object(latest_version, object) => {
                    if *latest_version <= version_bound {
                        if let ObjectEntry::Object(object) = object {
                            self.metrics
                                .record_cache_hit("object_lt_or_eq_version", "object_by_id");
                            return Ok(Some(object.clone()));
                        } else {
                            // object is a tombstone, but is still within the version bound
                            self.metrics.record_cache_negative_hit(
                                "object_lt_or_eq_version",
                                "object_by_id",
                            );
                            return Ok(None);
                        }
                    }
                    // latest object is not within the version bound. fall
                    // through.
                }
                // No object by this ID exists at all
                LatestObjectCacheEntry::NonExistent => {
                    self.metrics
                        .record_cache_negative_hit("object_lt_or_eq_version", "object_by_id");
                    return Ok(None);
                }
            }
        }
        self.metrics
            .record_cache_miss("object_lt_or_eq_version", "object_by_id");

        Self::with_locked_cache_entries(
            &self.dirty.objects,
            &self.cached.object_cache,
            &object_id,
            |dirty_entry, cached_entry| {
                check_cache_entry!("committed", dirty_entry);
                check_cache_entry!("uncommitted", cached_entry);

                // Much of the time, the query will be for the very latest object version, so
                // try that first. But we have to be careful:
                // 1. We must load the tombstone if it is present, because its version may
                //    exceed the version_bound, in which case we must do a scan.
                // 2. You might think we could just call
                //    `self.store.get_latest_object_or_tombstone` here. But we cannot, because
                //    there may be a more recent version in the dirty set, which we skipped over
                //    in check_cache_entry! because of the version bound. However, if we skipped
                //    it above, we will skip it here as well, again due to the version bound.
                // 3. Despite that, we really want to warm the cache here. Why? Because if the
                //    object is cold (not being written to), then we will very soon be able to
                //    start serving reads of it from the object_by_id cache, IF we can warm the
                //    cache. If we don't warm the cache here, and no writes to the object occur,
                //    then we will always have to go to the db for the object.
                //
                // Lastly, it is important to understand the rationale for all this: If the
                // object is write-hot, we will serve almost all reads to it
                // from the dirty set (or possibly the cached set if it is only
                // written to once every few checkpoints). If the object is
                // write-cold (or non-existent) and read-hot, then we will serve almost all
                // reads to it from the object_by_id cache check above.  Most of
                // the apparently wasteful code here exists only to ensure
                // correctness in all the edge cases.
                let latest: Option<(SequenceNumber, ObjectEntry)> =
                    if let Some(dirty_set) = dirty_entry {
                        dirty_set
                            .get_highest()
                            .cloned()
                            .tap_none(|| panic!("dirty set cannot be empty"))
                    } else {
                        self.record_db_get("object_lt_or_eq_version_latest")
                            .get_latest_object_or_tombstone(object_id)?
                            .map(|(ObjectKey(_, version), obj_or_tombstone)| {
                                (version, ObjectEntry::from(obj_or_tombstone))
                            })
                    };

                if let Some((obj_version, obj_entry)) = latest {
                    // we can always cache the latest object (or tombstone), even if it is not
                    // within the version_bound. This is done in order to warm
                    // the cache in the case where a sequence of transactions
                    // all read the same child object without writing to it.
                    self.cache_latest_object_by_id(
                        &object_id,
                        LatestObjectCacheEntry::Object(obj_version, obj_entry.clone()),
                    );

                    if obj_version <= version_bound {
                        match obj_entry {
                            ObjectEntry::Object(object) => Ok(Some(object)),
                            ObjectEntry::Deleted | ObjectEntry::Wrapped => Ok(None),
                        }
                    } else {
                        // The latest object exceeded the bound, so now we have to do a scan
                        // But we already know there is no dirty entry within the bound,
                        // so we go to the db.
                        self.record_db_get("object_lt_or_eq_version_scan")
                            .find_object_lt_or_eq_version(object_id, version_bound)
                    }
                } else {
                    // no object found in dirty set or db, object does not exist
                    // When this is called from a read api (i.e. not the execution path) it is
                    // possible that the object has been deleted and pruned. In this case,
                    // there would be no entry at all on disk, but we may have a tombstone in the
                    // cache
                    let highest = cached_entry.and_then(|c| c.get_highest());
                    assert!(highest.is_none() || highest.unwrap().1.is_tombstone());
                    self.cache_object_not_found(&object_id);
                    Ok(None)
                }
            },
        )
    }

    fn get_iota_system_state_object_unsafe(&self) -> IotaResult<IotaSystemState> {
        get_iota_system_state(self)
    }

    fn get_marker_value(
        &self,
        object_id: &ObjectID,
        version: SequenceNumber,
        epoch_id: EpochId,
    ) -> IotaResult<Option<MarkerValue>> {
        match self.get_marker_value_cache_only(object_id, version, epoch_id) {
            CacheResult::Hit(marker) => Ok(Some(marker)),
            CacheResult::NegativeHit => Ok(None),
            CacheResult::Miss => self
                .record_db_get("marker_by_version")
                .get_marker_value(object_id, &version, epoch_id),
        }
    }

    fn get_latest_marker(
        &self,
        object_id: &ObjectID,
        epoch_id: EpochId,
    ) -> IotaResult<Option<(SequenceNumber, MarkerValue)>> {
        match self.get_latest_marker_value_cache_only(object_id, epoch_id) {
            CacheResult::Hit((v, marker)) => Ok(Some((v, marker))),
            CacheResult::NegativeHit => {
                panic!("cannot have negative hit when getting latest marker")
            }
            CacheResult::Miss => self
                .record_db_get("marker_latest")
                .get_latest_marker(object_id, epoch_id),
        }
    }

    fn get_lock(&self, obj_ref: ObjectRef, epoch_store: &AuthorityPerEpochStore) -> IotaLockResult {
        match self.get_object_by_id_cache_only("lock", &obj_ref.0) {
            CacheResult::Hit((_, obj)) => {
                let actual_objref = obj.compute_object_reference();
                if obj_ref != actual_objref {
                    Ok(ObjectLockStatus::LockedAtDifferentVersion {
                        locked_ref: actual_objref,
                    })
                } else {
                    // requested object ref is live, check if there is a lock
                    Ok(
                        match self
                            .object_locks
                            .get_transaction_lock(&obj_ref, epoch_store)?
                        {
                            Some(tx_digest) => ObjectLockStatus::LockedToTx {
                                locked_by_tx: tx_digest,
                            },
                            None => ObjectLockStatus::Initialized,
                        },
                    )
                }
            }
            CacheResult::NegativeHit => {
                Err(IotaError::from(UserInputError::ObjectNotFound {
                    object_id: obj_ref.0,
                    // even though we know the requested version, we leave it as None to indicate
                    // that the object does not exist at any version
                    version: None,
                }))
            }
            CacheResult::Miss => self.record_db_get("lock").get_lock(obj_ref, epoch_store),
        }
    }

    fn _get_live_objref(&self, object_id: ObjectID) -> IotaResult<ObjectRef> {
        let obj = self.get_object_impl("live_objref", &object_id)?.ok_or(
            UserInputError::ObjectNotFound {
                object_id,
                version: None,
            },
        )?;
        Ok(obj.compute_object_reference())
    }

    fn check_owned_objects_are_live(&self, owned_object_refs: &[ObjectRef]) -> IotaResult {
        do_fallback_lookup(
            owned_object_refs,
            |obj_ref| match self.get_object_by_id_cache_only("object_is_live", &obj_ref.0) {
                CacheResult::Hit((version, obj)) => {
                    if obj.compute_object_reference() != *obj_ref {
                        Err(UserInputError::ObjectVersionUnavailableForConsumption {
                            provided_obj_ref: *obj_ref,
                            current_version: version,
                        }
                        .into())
                    } else {
                        Ok(CacheResult::Hit(()))
                    }
                }
                CacheResult::NegativeHit => Err(UserInputError::ObjectNotFound {
                    object_id: obj_ref.0,
                    version: None,
                }
                .into()),
                CacheResult::Miss => Ok(CacheResult::Miss),
            },
            |remaining| {
                self.record_db_multi_get("object_is_live", remaining.len())
                    .check_owned_objects_are_live(remaining)?;
                Ok(vec![(); remaining.len()])
            },
        )?;
        Ok(())
    }

    fn get_highest_pruned_checkpoint(&self) -> IotaResult<CheckpointSequenceNumber> {
        self.store.perpetual_tables.get_highest_pruned_checkpoint()
    }
}

impl TransactionCacheRead for WritebackCache {
    fn multi_get_transaction_blocks(
        &self,
        digests: &[TransactionDigest],
    ) -> IotaResult<Vec<Option<Arc<VerifiedTransaction>>>> {
        do_fallback_lookup(
            digests,
            |digest| {
                self.metrics
                    .record_cache_request("transaction_block", "uncommitted");
                if let Some(tx) = self.dirty.pending_transaction_writes.get(digest) {
                    self.metrics
                        .record_cache_hit("transaction_block", "uncommitted");
                    return Ok(CacheResult::Hit(Some(tx.transaction.clone())));
                }
                self.metrics
                    .record_cache_miss("transaction_block", "uncommitted");

                self.metrics
                    .record_cache_request("transaction_block", "committed");
                if let Some(tx) = self.cached.transactions.get(digest) {
                    self.metrics
                        .record_cache_hit("transaction_block", "committed");
                    return Ok(CacheResult::Hit(Some(tx.clone())));
                }
                self.metrics
                    .record_cache_miss("transaction_block", "committed");

                Ok(CacheResult::Miss)
            },
            |remaining| {
                self.record_db_multi_get("transaction_block", remaining.len())
                    .multi_get_transaction_blocks(remaining)
                    .map(|v| v.into_iter().map(|o| o.map(Arc::new)).collect())
            },
        )
    }

    fn multi_get_executed_effects_digests(
        &self,
        digests: &[TransactionDigest],
    ) -> IotaResult<Vec<Option<TransactionEffectsDigest>>> {
        do_fallback_lookup(
            digests,
            |digest| {
                self.metrics
                    .record_cache_request("executed_effects_digests", "uncommitted");
                if let Some(digest) = self.dirty.executed_effects_digests.get(digest) {
                    self.metrics
                        .record_cache_hit("executed_effects_digests", "uncommitted");
                    return Ok(CacheResult::Hit(Some(*digest)));
                }
                self.metrics
                    .record_cache_miss("executed_effects_digests", "uncommitted");

                self.metrics
                    .record_cache_request("executed_effects_digests", "committed");
                if let Some(digest) = self.cached.executed_effects_digests.get(digest) {
                    self.metrics
                        .record_cache_hit("executed_effects_digests", "committed");
                    return Ok(CacheResult::Hit(Some(digest)));
                }
                self.metrics
                    .record_cache_miss("executed_effects_digests", "committed");

                Ok(CacheResult::Miss)
            },
            |remaining| {
                self.record_db_multi_get("executed_effects_digests", remaining.len())
                    .multi_get_executed_effects_digests(remaining)
            },
        )
    }

    fn multi_get_effects(
        &self,
        digests: &[TransactionEffectsDigest],
    ) -> IotaResult<Vec<Option<TransactionEffects>>> {
        do_fallback_lookup(
            digests,
            |digest| {
                self.metrics
                    .record_cache_request("transaction_effects", "uncommitted");
                if let Some(effects) = self.dirty.transaction_effects.get(digest) {
                    self.metrics
                        .record_cache_hit("transaction_effects", "uncommitted");
                    return Ok(CacheResult::Hit(Some(effects.clone())));
                }
                self.metrics
                    .record_cache_miss("transaction_effects", "uncommitted");

                self.metrics
                    .record_cache_request("transaction_effects", "committed");
                if let Some(effects) = self.cached.transaction_effects.get(digest) {
                    self.metrics
                        .record_cache_hit("transaction_effects", "committed");
                    return Ok(CacheResult::Hit(Some((*effects).clone())));
                }
                self.metrics
                    .record_cache_miss("transaction_effects", "committed");

                Ok(CacheResult::Miss)
            },
            |remaining| {
                self.record_db_multi_get("transaction_effects", remaining.len())
                    .multi_get_effects(remaining.iter())
            },
        )
    }

    fn notify_read_executed_effects_digests<'a>(
        &'a self,
        digests: &'a [TransactionDigest],
    ) -> BoxFuture<'a, IotaResult<Vec<TransactionEffectsDigest>>> {
        self.executed_effects_digests_notify_read
            .read(digests, |digests| {
                self.multi_get_executed_effects_digests(digests)
            })
            .boxed()
    }

    fn multi_get_events(
        &self,
        event_digests: &[TransactionEventsDigest],
    ) -> IotaResult<Vec<Option<TransactionEvents>>> {
        fn map_events(events: TransactionEvents) -> Option<TransactionEvents> {
            if events.data.is_empty() {
                None
            } else {
                Some(events)
            }
        }

        do_fallback_lookup(
            event_digests,
            |digest| {
                self.metrics
                    .record_cache_request("transaction_events", "uncommitted");
                if let Some(events) = self
                    .dirty
                    .transaction_events
                    .get(digest)
                    .map(|e| e.1.clone())
                {
                    self.metrics
                        .record_cache_hit("transaction_events", "uncommitted");

                    return Ok(CacheResult::Hit(map_events(events)));
                }
                self.metrics
                    .record_cache_miss("transaction_events", "uncommitted");

                self.metrics
                    .record_cache_request("transaction_events", "committed");
                if let Some(events) = self
                    .cached
                    .transaction_events
                    .get(digest)
                    .map(|e| (*e).clone())
                {
                    self.metrics
                        .record_cache_hit("transaction_events", "committed");
                    return Ok(CacheResult::Hit(map_events(events)));
                }

                self.metrics
                    .record_cache_miss("transaction_events", "committed");

                Ok(CacheResult::Miss)
            },
            |digests| self.store.multi_get_events(digests),
        )
    }
}

impl ExecutionCacheWrite for WritebackCache {
    fn acquire_transaction_locks<'a>(
        &'a self,
        epoch_store: &'a AuthorityPerEpochStore,
        owned_input_objects: &'a [ObjectRef],
        transaction: VerifiedSignedTransaction,
    ) -> BoxFuture<'a, IotaResult> {
        self.object_locks
            .acquire_transaction_locks(self, epoch_store, owned_input_objects, transaction)
            .boxed()
    }

    fn write_transaction_outputs(
        &self,
        epoch_id: EpochId,
        tx_outputs: Arc<TransactionOutputs>,
    ) -> BoxFuture<'_, IotaResult> {
        WritebackCache::write_transaction_outputs(self, epoch_id, tx_outputs).boxed()
    }
}

/// do_fallback_lookup is a helper function for multi-get operations.
/// It takes a list of keys and first attempts to look up each key in the cache.
/// The cache can return a hit, a miss, or a negative hit (if the object is
/// known to not exist). Any keys that result in a miss are then looked up in
/// the store.
///
/// The "get from cache" and "get from store" behavior are implemented by the
/// caller and provided via the get_cached_key and multiget_fallback functions.
fn do_fallback_lookup<K: Copy, V: Default + Clone>(
    keys: &[K],
    get_cached_key: impl Fn(&K) -> IotaResult<CacheResult<V>>,
    multiget_fallback: impl Fn(&[K]) -> IotaResult<Vec<V>>,
) -> IotaResult<Vec<V>> {
    let mut results = vec![V::default(); keys.len()];
    let mut fallback_keys = Vec::with_capacity(keys.len());
    let mut fallback_indices = Vec::with_capacity(keys.len());

    for (i, key) in keys.iter().enumerate() {
        match get_cached_key(key)? {
            CacheResult::Miss => {
                fallback_keys.push(*key);
                fallback_indices.push(i);
            }
            CacheResult::NegativeHit => (),
            CacheResult::Hit(value) => {
                results[i] = value;
            }
        }
    }

    let fallback_results = multiget_fallback(&fallback_keys)?;
    assert_eq!(fallback_results.len(), fallback_indices.len());
    assert_eq!(fallback_results.len(), fallback_keys.len());

    for (i, result) in fallback_indices
        .into_iter()
        .zip(fallback_results.into_iter())
    {
        results[i] = result;
    }
    Ok(results)
}

implement_passthrough_traits!(WritebackCache);

impl AccumulatorStore for WritebackCache {
    fn get_root_state_accumulator_for_epoch(
        &self,
        epoch: EpochId,
    ) -> IotaResult<Option<(CheckpointSequenceNumber, Accumulator)>> {
        self.store.get_root_state_accumulator_for_epoch(epoch)
    }

    fn get_root_state_accumulator_for_highest_epoch(
        &self,
    ) -> IotaResult<Option<(EpochId, (CheckpointSequenceNumber, Accumulator))>> {
        self.store.get_root_state_accumulator_for_highest_epoch()
    }

    fn insert_state_accumulator_for_epoch(
        &self,
        epoch: EpochId,
        checkpoint_seq_num: &CheckpointSequenceNumber,
        acc: &Accumulator,
    ) -> IotaResult {
        self.store
            .insert_state_accumulator_for_epoch(epoch, checkpoint_seq_num, acc)
    }

    fn iter_live_object_set(&self) -> Box<dyn Iterator<Item = LiveObject> + '_> {
        // The only time it is safe to iterate the live object set is at an epoch
        // boundary, at which point the db is consistent and the dirty cache is
        // empty. So this does read the cache
        assert!(
            self.dirty.is_empty(),
            "cannot iterate live object set with dirty data"
        );
        self.store.iter_live_object_set()
    }

    // A version of iter_live_object_set that reads the cache. Only use for testing.
    // If used on a live validator, can cause the server to block for as long as
    // it takes to iterate the entire live object set.
    fn iter_cached_live_object_set_for_testing(&self) -> Box<dyn Iterator<Item = LiveObject> + '_> {
        // hold iter until we are finished to prevent any concurrent inserts/deletes
        let iter = self.dirty.objects.iter();
        let mut dirty_objects = BTreeMap::new();

        // add everything from the store
        for obj in self.store.iter_live_object_set() {
            dirty_objects.insert(obj.object_id(), obj);
        }

        // add everything from the cache, but also remove deletions
        for entry in iter {
            let id = *entry.key();
            let value = entry.value();
            match value.get_highest().unwrap() {
                (_, ObjectEntry::Object(object)) => {
                    dirty_objects.insert(id, LiveObject::Normal(object.clone()));
                }
                (_version, ObjectEntry::Wrapped) => {
                    dirty_objects.remove(&id);
                }
                (_, ObjectEntry::Deleted) => {
                    dirty_objects.remove(&id);
                }
            }
        }

        Box::new(dirty_objects.into_values())
    }
}