Enum EmitStrategy 

pub enum EmitStrategy {
    OnWatermark,
    Periodic(Duration),
    OnUpdate,
    OnWindowClose,
    Changelog,
    Final,
}

Strategy for when window results should be emitted.

This controls the trade-off between result freshness and efficiency:

• OnWatermark is most efficient but has highest latency
• Periodic balances freshness and efficiency
• OnUpdate provides lowest latency but highest overhead
• OnWindowClose is for append-only sinks
• Changelog emits Z-set weighted records for CDC
• Final suppresses all intermediate results

Variants

OnWatermark

Emit final results when watermark passes window end (default).

This is the most efficient strategy as it only emits once per window. Results are guaranteed to be complete (within allowed lateness bounds). May emit retractions if late data arrives within lateness bounds.

Periodic(Duration)

Emit intermediate results at fixed intervals.

Useful for dashboards and monitoring where periodic updates are needed before the window closes. The final result is still emitted on watermark.

The duration specifies the interval between periodic emissions.

OnUpdate

Emit updated results after every state change.

This provides the lowest latency for result visibility but has the highest overhead. Each incoming event triggers an emission.

Use with caution for high-volume streams.

OnWindowClose

Emit ONLY when watermark passes window end. No intermediate emissions.

Critical for append-only sinks (Kafka, S3, Delta Lake, Iceberg). Guarantees exactly one emission per window and strictly append-only output — no retractions, no updates.

Window Close Condition

A window closes when watermark >= window_end + allowed_lateness. At that point, the timer fires, the final result is emitted, and window state is purged immediately.

Late Data Policy

Events arriving after window close are classified as late and handled by LateDataConfig:

• Default: dropped (increment late_events_dropped metric)
• With side output: routed to named side output for separate processing (increment late_events_side_output metric)

Late data never re-opens a closed window. The single emission is final and immutable. This is the key contract that makes EOWC safe for append-only sinks.

Requires

• A watermark definition on the source (otherwise timers never fire)
• A windowed aggregation query

SQL: EMIT ON WINDOW CLOSE

Changelog

Emit changelog records with Z-set weights.

Every emission includes operation type and weight:

• Insert (+1 weight)
• Delete (-1 weight)
• Update (retraction pair: -1 old, +1 new)

Required for:

• CDC pipelines
• Cascading materialized views
• Downstream consumers that need to track changes

SQL: EMIT CHANGES

Final

Suppress ALL intermediate results, emit only finalized.

Similar to OnWindowClose but also suppresses:

• Periodic emissions (even if Periodic was set elsewhere)
• Late data retractions (drops late data entirely after window close)

Use for BI reporting where only final, exact results matter.

SQL: EMIT FINAL

Implementations

impl EmitStrategy

pub fn needs_periodic_timer(&self) -> bool

Returns true if this strategy requires periodic timer registration.

pub fn periodic_interval(&self) -> Option<Duration>

Returns the periodic interval if this is a periodic strategy.

pub fn emits_on_update(&self) -> bool

Returns true if results should be emitted on every update.

pub fn emits_intermediate(&self) -> bool

Returns true if this strategy emits intermediate results.

Strategies that emit intermediate results (before window close):

• OnUpdate: emits after every state change
• Periodic: emits at fixed intervals

Strategies that do NOT emit intermediate results:

• OnWatermark: waits for watermark
• OnWindowClose: only emits when window closes
• Changelog: depends on trigger, but typically on watermark
• Final: only emits final result

pub fn requires_changelog(&self) -> bool

Returns true if this strategy requires changelog/Z-set support.

The Changelog strategy requires the operator to track previous values and emit insert/delete/update records with weights.

pub fn is_append_only_compatible(&self) -> bool

Returns true if this strategy is suitable for append-only sinks.

Append-only sinks (Kafka, S3, Delta Lake, Iceberg) cannot handle retractions or updates. Only these strategies are safe:

• OnWindowClose: guarantees single emission per window
• Final: suppresses all intermediate results

pub fn generates_retractions(&self) -> bool

Returns true if late data should generate retractions.

Strategies that generate retractions for late data:

• OnWatermark: may retract previous result
• OnUpdate: immediately emits updated result
• Changelog: emits -old/+new pair

Strategies that do NOT generate retractions:

• OnWindowClose: drops late data (or routes to side output)
• Final: drops late data silently
• Periodic: depends on whether window is still open

pub fn suppresses_intermediate(&self) -> bool

Returns true if this strategy should suppress intermediate emissions.

Used to override periodic timers when a suppressing strategy is active.

pub fn drops_late_data(&self) -> bool

Returns true if late data should be dropped entirely.

The Final strategy drops late data to ensure only exact, finalized results are emitted.

Trait Implementations

impl Clone for EmitStrategy

fn clone(&self) -> EmitStrategy

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for EmitStrategy

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

Formats the value using the given formatter.

impl Default for EmitStrategy

fn default() -> EmitStrategy

Returns the “default value” for a type.

impl PartialEq for EmitStrategy

fn eq(&self, other: &EmitStrategy) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Eq for EmitStrategy

impl StructuralPartialEq for EmitStrategy