iqdb_cache/

cached.rs

//! The [`CachedIndex`] wrapper.

use core::time::Duration;
use std::sync::Arc;

use clock_lib::{Clock, Monotonic, SystemClock};
use iqdb_index::{IndexCore, IndexStats};
use iqdb_types::{DistanceMetric, Hit, Metadata, Result, SearchParams, VectorId};

use crate::config::{CacheConfig, EvictionPolicy};
use crate::key::ResultKey;
use crate::policy::PolicyCache;
use crate::stats::CacheStats;
use crate::sync::{AtomicU64, Mutex, MutexGuard, Ordering};

/// A cached search result plus the moment it was stored.
///
/// `stamp` is recorded only when a TTL is configured; with no TTL it is `None`
/// and the entry never expires on time.
struct CacheEntry {
    /// The memoized hits, ready to clone out on a hit.
    hits: Box<[Hit]>,
    /// When the entry was written, for TTL expiry; `None` when no TTL applies.
    stamp: Option<Monotonic>,
}

/// A drop-in [`IndexCore`] wrapper that memoizes search results.
///
/// `CachedIndex` holds any `I: IndexCore` and forwards every call to it, with
/// one addition: identical [`search`](IndexCore::search) calls — same query
/// and same [`SearchParams`] — are served from an in-memory LRU cache instead
/// of re-running the search. Because it *is* an [`IndexCore`], it slots in
/// anywhere the wrapped index does, including behind `Box<dyn IndexCore>`.
///
/// ## Correctness
///
/// The cache never returns a stale result. Every mutation that can change the
/// search space — [`insert`](IndexCore::insert),
/// [`insert_batch`](IndexCore::insert_batch), and
/// [`delete`](IndexCore::delete) — invalidates the cache, so a search after a
/// write always recomputes against the current index. Operations that do not
/// change the result set ([`flush`](IndexCore::flush) and the read-only
/// accessors) leave the cache intact.
///
/// ## Opt-in
///
/// Caching is an optimization a caller chooses by wrapping an index; the
/// database leaves indexes unwrapped by default. Construct a cache that holds
/// a fixed number of recent searches with [`new`](CachedIndex::new) or
/// [`with_capacity`](CachedIndex::with_capacity), or tune it through a
/// [`CacheConfig`] with [`with_config`](CachedIndex::with_config). A capacity of
/// `0` disables caching entirely: every search passes straight through, which is
/// useful for A/B measuring the cache's effect without changing call sites.
///
/// ## Time-to-live
///
/// A [`CacheConfig::ttl`] gives entries an expiry: a cached result older than
/// the TTL is treated as a miss and recomputed. Mutations through this wrapper
/// already invalidate exactly, so the TTL exists to bound staleness from changes
/// the wrapper *cannot* see — for example, the wrapped index mutated through
/// another handle. With no TTL (the default) the clock is never consulted.
///
/// ## Concurrency
///
/// `CachedIndex<I>` is `Send + Sync` whenever `I` is (which every `IndexCore`
/// is). Reads share the cache behind a [`Mutex`] held only for the lookup and
/// the insert — never across the wrapped search — so concurrent misses run the
/// underlying search in parallel rather than serializing on the lock.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// use iqdb_cache::CachedIndex;
/// use iqdb_index::{Index, IndexCore, IndexStats};
/// use iqdb_types::{DistanceMetric, Hit, IqdbError, Metadata, Result, SearchParams, VectorId};
///
/// // A minimal index that returns one hit per search; enough to show the wrap.
/// struct Stub {
///     dim: usize,
///     metric: DistanceMetric,
///     ids: Vec<VectorId>,
/// }
///
/// impl IndexCore for Stub {
///     fn insert(&mut self, id: VectorId, _v: Arc<[f32]>, _m: Option<Metadata>) -> Result<()> {
///         self.ids.push(id);
///         Ok(())
///     }
///     fn delete(&mut self, id: &VectorId) -> Result<()> {
///         match self.ids.iter().position(|x| x == id) {
///             Some(pos) => { let _ = self.ids.remove(pos); Ok(()) }
///             None => Err(IqdbError::NotFound),
///         }
///     }
///     fn search(&self, _q: &[f32], params: &SearchParams) -> Result<Vec<Hit>> {
///         Ok(self.ids.iter().take(params.k).map(|id| Hit::new(id.clone(), 0.0)).collect())
///     }
///     fn len(&self) -> usize { self.ids.len() }
///     fn dim(&self) -> usize { self.dim }
///     fn metric(&self) -> DistanceMetric { self.metric }
///     fn flush(&mut self) -> Result<()> { Ok(()) }
///     fn stats(&self) -> IndexStats {
///         IndexStats { n_vectors: self.ids.len(), index_type: "stub", ..IndexStats::default() }
///     }
/// }
///
/// # fn main() -> Result<()> {
/// let stub = Stub { dim: 3, metric: DistanceMetric::Cosine, ids: vec![VectorId::from(1u64)] };
/// let mut cached = CachedIndex::new(stub);
///
/// let params = SearchParams::new(1, DistanceMetric::Cosine);
/// let first = cached.search(&[1.0, 0.0, 0.0], &params)?;  // miss: runs the search
/// let again = cached.search(&[1.0, 0.0, 0.0], &params)?;  // hit: served from cache
/// assert_eq!(first, again);
///
/// let stats = cached.cache_stats();
/// assert_eq!(stats.hits, 1);
/// assert_eq!(stats.misses, 1);
/// # Ok(())
/// # }
/// ```
pub struct CachedIndex<I> {
    /// The wrapped index every call forwards to.
    inner: I,
    /// The result cache, guarded for `&self` search access.
    cache: Mutex<PolicyCache<ResultKey, CacheEntry>>,
    /// Configured capacity, mirrored here for `0`-means-disabled fast paths.
    capacity: usize,
    /// Configured eviction policy, mirrored here for introspection.
    policy: EvictionPolicy,
    /// Optional per-entry time-to-live; `None` means entries expire only on
    /// mutation.
    ttl: Option<Duration>,
    /// Time source for TTL expiry. `SystemClock` in production; a mock clock is
    /// injected in tests. Only read when `ttl` is `Some`.
    clock: Arc<dyn Clock>,
    /// Lifetime count of cache hits.
    hits: AtomicU64,
    /// Lifetime count of cache misses.
    misses: AtomicU64,
    /// Lifetime count of entries discarded by the eviction policy.
    evictions: AtomicU64,
}

impl<I: IndexCore> CachedIndex<I> {
    /// Wraps `inner` with a result cache of the default capacity (1024 recent
    /// searches) and no TTL.
    ///
    /// # Examples
    ///
    /// ```
    /// # use iqdb_cache::CachedIndex;
    /// # use iqdb_cache::doc_stub::stub_index;
    /// let cached = CachedIndex::new(stub_index());
    /// assert!(cached.is_enabled());
    /// ```
    #[must_use]
    pub fn new(inner: I) -> Self {
        Self::with_config(inner, CacheConfig::new())
    }

    /// Wraps `inner` with a result cache that holds at most `capacity` recent
    /// searches and no TTL.
    ///
    /// A `capacity` of `0` disables caching: searches pass straight through and
    /// nothing is stored.
    ///
    /// # Examples
    ///
    /// ```
    /// # use iqdb_cache::CachedIndex;
    /// # use iqdb_cache::doc_stub::stub_index;
    /// let cached = CachedIndex::with_capacity(stub_index(), 256);
    /// assert_eq!(cached.capacity(), 256);
    ///
    /// let bypass = CachedIndex::with_capacity(stub_index(), 0);
    /// assert!(!bypass.is_enabled());
    /// ```
    #[must_use]
    pub fn with_capacity(inner: I, capacity: usize) -> Self {
        Self::with_config(inner, CacheConfig::new().capacity(capacity))
    }

    /// Wraps `inner` with a result cache built from `config` (the Tier-2 path).
    ///
    /// Use [`CacheConfig`] to set the capacity and an optional TTL together.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::time::Duration;
    ///
    /// use iqdb_cache::{CacheConfig, CachedIndex};
    /// # use iqdb_cache::doc_stub::stub_index;
    /// let config = CacheConfig::new().capacity(512).ttl(Duration::from_secs(30));
    /// let cached = CachedIndex::with_config(stub_index(), config);
    /// assert_eq!(cached.capacity(), 512);
    /// assert_eq!(cached.ttl(), Some(Duration::from_secs(30)));
    /// ```
    #[must_use]
    pub fn with_config(inner: I, config: CacheConfig) -> Self {
        Self::with_config_in(inner, config, Arc::new(SystemClock::new()))
    }

    /// Construction core shared by every public constructor, with an injectable
    /// clock for deterministic TTL tests.
    pub(crate) fn with_config_in(inner: I, config: CacheConfig, clock: Arc<dyn Clock>) -> Self {
        Self {
            inner,
            cache: Mutex::new(PolicyCache::new(config.policy, config.capacity)),
            capacity: config.capacity,
            policy: config.policy,
            ttl: config.ttl,
            clock,
            hits: AtomicU64::new(0),
            misses: AtomicU64::new(0),
            evictions: AtomicU64::new(0),
        }
    }

    /// The configured cache capacity. `0` means caching is disabled.
    #[inline]
    #[must_use]
    pub fn capacity(&self) -> usize {
        self.capacity
    }

    /// The configured per-entry time-to-live, or `None` if results expire only
    /// on mutation.
    #[inline]
    #[must_use]
    pub fn ttl(&self) -> Option<Duration> {
        self.ttl
    }

    /// The configured eviction policy.
    #[inline]
    #[must_use]
    pub fn policy(&self) -> EvictionPolicy {
        self.policy
    }

    /// Whether caching is active (`capacity > 0`).
    #[inline]
    #[must_use]
    pub fn is_enabled(&self) -> bool {
        self.capacity > 0
    }

    /// Borrows the wrapped index.
    #[inline]
    #[must_use]
    pub fn get_ref(&self) -> &I {
        &self.inner
    }

    /// Unwraps the cache, returning the index it held.
    ///
    /// # Examples
    ///
    /// ```
    /// # use iqdb_cache::CachedIndex;
    /// # use iqdb_cache::doc_stub::stub_index;
    /// # use iqdb_index::IndexCore;
    /// let cached = CachedIndex::new(stub_index());
    /// let inner = cached.into_inner();
    /// assert_eq!(inner.dim(), 3);
    /// ```
    #[must_use]
    pub fn into_inner(self) -> I {
        self.inner
    }

    /// Drops every cached result, keeping the wrapped index untouched.
    ///
    /// Mutations invalidate automatically; call this only to force a cold cache
    /// (for example, after the wrapped index was changed through a handle other
    /// than this wrapper).
    pub fn clear_cache(&mut self) {
        self.lock_cache().clear();
    }

    /// A snapshot of the cache's hit/miss counters and occupancy.
    #[must_use]
    pub fn cache_stats(&self) -> CacheStats {
        let len = self.lock_cache().len();
        CacheStats {
            hits: self.hits.load(Ordering::Relaxed),
            misses: self.misses.load(Ordering::Relaxed),
            evictions: self.evictions.load(Ordering::Relaxed),
            len,
            capacity: self.capacity,
        }
    }

    /// Locks the cache, recovering the guard if a previous holder panicked.
    ///
    /// A poisoned result cache is safe to keep using: a half-finished insert
    /// can at worst drop or duplicate a memoized entry, never corrupt a result,
    /// so recovery is preferable to propagating the panic.
    fn lock_cache(&self) -> MutexGuard<'_, PolicyCache<ResultKey, CacheEntry>> {
        self.cache
            .lock()
            .unwrap_or_else(|poisoned| poisoned.into_inner())
    }

    /// Whether a cached entry is still live under the configured TTL.
    ///
    /// Always `true` when no TTL is set, so the clock is never read on the
    /// non-TTL hot path.
    #[inline]
    fn is_live(&self, entry: &CacheEntry) -> bool {
        match (self.ttl, entry.stamp) {
            (Some(ttl), Some(stamp)) => self.clock.now().saturating_duration_since(stamp) < ttl,
            _ => true,
        }
    }

    /// Empties the cache after a mutation. Takes `&mut self`, so the lock is
    /// always uncontended here.
    fn invalidate(&mut self) {
        self.lock_cache().clear();
    }
}

impl<I: IndexCore> IndexCore for CachedIndex<I> {
    fn insert(
        &mut self,
        id: VectorId,
        vector: std::sync::Arc<[f32]>,
        metadata: Option<Metadata>,
    ) -> Result<()> {
        let result = self.inner.insert(id, vector, metadata);
        if result.is_ok() {
            self.invalidate();
        }
        result
    }

    fn insert_batch(
        &mut self,
        items: Vec<(VectorId, std::sync::Arc<[f32]>, Option<Metadata>)>,
    ) -> Result<()> {
        // `insert_batch` is fail-fast and may apply partially, so any outcome
        // can have changed the search space: always invalidate.
        let result = self.inner.insert_batch(items);
        self.invalidate();
        result
    }

    fn delete(&mut self, id: &VectorId) -> Result<()> {
        let result = self.inner.delete(id);
        if result.is_ok() {
            self.invalidate();
        }
        result
    }

    fn search(&self, query: &[f32], params: &SearchParams) -> Result<Vec<Hit>> {
        if self.capacity == 0 {
            let _ = self.misses.fetch_add(1, Ordering::Relaxed);
            return self.inner.search(query, params);
        }

        let key = ResultKey::new(query, params);
        {
            let mut cache = self.lock_cache();
            if let Some(entry) = cache.get(&key) {
                if self.is_live(entry) {
                    let _ = self.hits.fetch_add(1, Ordering::Relaxed);
                    return Ok(entry.hits.to_vec());
                }
                // Expired: fall through to recompute. The stale entry stays
                // until the `put` below overwrites it with a fresh result.
            }
        }

        // Miss (or expired): run the search without holding the lock so
        // concurrent misses do not serialize on it.
        let hits = self.inner.search(query, params)?;
        let _ = self.misses.fetch_add(1, Ordering::Relaxed);
        let stamp = self.ttl.map(|_| self.clock.now());
        let evicted = {
            let mut cache = self.lock_cache();
            cache.put(
                key,
                CacheEntry {
                    hits: hits.clone().into_boxed_slice(),
                    stamp,
                },
            )
        };
        if evicted {
            let _ = self.evictions.fetch_add(1, Ordering::Relaxed);
        }
        Ok(hits)
    }

    fn len(&self) -> usize {
        self.inner.len()
    }

    fn is_empty(&self) -> bool {
        self.inner.is_empty()
    }

    fn dim(&self) -> usize {
        self.inner.dim()
    }

    fn metric(&self) -> DistanceMetric {
        self.inner.metric()
    }

    fn flush(&mut self) -> Result<()> {
        // Flush commits durable state without changing the searchable set, so
        // the cache stays valid.
        self.inner.flush()
    }

    fn stats(&self) -> IndexStats {
        self.inner.stats()
    }
}

#[cfg(test)]
mod tests {
    #![allow(clippy::unwrap_used)]

    use clock_lib::ManualClock;

    use super::*;
    use crate::doc_stub::stub_index;

    fn params() -> SearchParams {
        SearchParams::new(1, DistanceMetric::Cosine)
    }

    #[test]
    fn ttl_entry_is_recomputed_after_expiry() {
        let clock = Arc::new(ManualClock::new());
        let config = CacheConfig::new().capacity(8).ttl(Duration::from_secs(60));
        let cached = CachedIndex::with_config_in(stub_index(), config, clock.clone());

        let _miss = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        let _hit = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 1);

        // Just inside the TTL: still a hit.
        clock.advance(Duration::from_secs(59));
        let _hit2 = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 2);

        // Past the TTL: the entry expires and the search recomputes (a miss).
        clock.advance(Duration::from_secs(2));
        let _expired = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 2);
        assert_eq!(cached.cache_stats().misses, 2);

        // The recompute refreshed the entry, so the next search hits again.
        let _hit3 = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 3);
    }

    #[test]
    fn ttl_boundary_is_exclusive() {
        let clock = Arc::new(ManualClock::new());
        let config = CacheConfig::new().capacity(8).ttl(Duration::from_secs(10));
        let cached = CachedIndex::with_config_in(stub_index(), config, clock.clone());

        let _miss = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        // Exactly at the TTL counts as expired (`elapsed >= ttl`).
        clock.advance(Duration::from_secs(10));
        let _again = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 0);
        assert_eq!(cached.cache_stats().misses, 2);
    }

    #[test]
    fn no_ttl_never_expires_even_as_time_passes() {
        let clock = Arc::new(ManualClock::new());
        let config = CacheConfig::new().capacity(8); // no TTL
        let cached = CachedIndex::with_config_in(stub_index(), config, clock.clone());

        let _miss = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        let _hit = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        // Advance far beyond any plausible TTL: still a hit, because none is set.
        clock.advance(Duration::from_secs(60 * 60 * 24 * 365));
        let _hit2 = cached.search(&[1.0, 0.0, 0.0], &params()).unwrap();
        assert_eq!(cached.cache_stats().hits, 2);
        assert_eq!(cached.cache_stats().misses, 1);
    }
}