Struct LastValueCache 

pub struct LastValueCache { /* private fields */ }

Last Value Cache — stores the most recent point per series.

Thread Safety

Uses DashMap for lock-free concurrent reads and sharded writes. Multiple threads can read and update the cache simultaneously.

Memory

Memory usage scales linearly with the number of unique series. Each entry stores one Point (typically 100-500 bytes).

LRU Eviction

An optional max_entries cap prevents unbounded memory growth in high-cardinality workloads. When full, the least recently used entries are evicted in O(1) time using a HashMap-backed doubly-linked list (replaces an O(N) VecDeque::retain approach).

Implementations

impl LastValueCache

pub fn new() -> Self

Create a new, empty LVC with no entry limit.

pub fn with_max_entries(max: usize) -> Self

Create an LVC with a maximum number of entries.

When full, the least recently used (LRU) entry is evicted to make room for the new one. Eviction is O(1).

pub fn with_capacity(n: usize) -> Self

Create an LVC with pre-allocated capacity for n series.

pub fn update(&self, point: &Point)

Update the cache with a new point. Only overwrites if the new point has a timestamp ≥ the existing cached point.

When max_entries > 0, the key is moved to the most-recently-used position in O(1) time. If the cache is full and the key is new, the least-recently-used entry is evicted — also O(1).

pub fn get( &self, measurement: &str, tags: &BTreeMap<String, String>, ) -> Option<Point>

Look up the most recent point for a series.

Returns None if the series has never been written to, or has been evicted.

pub fn get_by_key(&self, key: &SeriesKey) -> Option<Point>

Look up by SeriesKey.

pub fn evict(&self, measurement: &str, tags: &BTreeMap<String, String>)

Remove a series from the cache.

pub fn evict_by_key(&self, key: &SeriesKey)

Remove a series by its SeriesKey.

pub fn retain_series(&self, keep: impl Fn(&str) -> bool)

Keep only the series keep accepts, by canonical form.

The LVC is a copy of the newest row of every series, and a copy is not an answer: once retention has dropped a series’ data the raw query returns nothing while the cache kept returning its last reading — a device offline for longer than the retention window went on reporting a “current” value for ever. It is also how the cache stays bounded on a long-lived deployment: the live series set is bounded by retention, and without this the cache was not.

Called by the cardinality repair, against the budget it just rebuilt — as a predicate rather than a materialised set, so pruning the cache does not cost a copy of every live series.

pub fn evict_measurement(&self, measurement: &str)

Remove all entries for a measurement.

pub fn clear(&self)

Clear all entries.

pub fn len(&self) -> usize

Returns the number of cached series.

pub fn is_empty(&self) -> bool

Returns true if the cache is empty.

pub fn update_count(&self) -> usize

Returns the total number of updates applied.

Trait Implementations

impl Debug for LastValueCache

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for LastValueCache

fn default() -> Self

Returns the “default value” for a type.