enhancement(observability): Add configuration for buffer utilization EWMA alpha (#24467)

bruceg · web-flow · commit 94a2f4013172 · 2026-01-09T17:20:58.000Z
* enhancement(observability): Add configuration for buffer utilization EWMA alpha

* Restore comment
diff --git a/changelog.d/utilization_ewma_alpha_configuration.enhancement.md b/changelog.d/utilization_ewma_alpha_configuration.enhancement.md
@@ -0,0 +1,6 @@
+Added `buffer_utilization_ewma_alpha` configuration option to the global
+options, allowing users to control the alpha value for the exponentially
+weighted moving average (EWMA) used in source and transform buffer utilization
+metrics.
+
+authors: bruceg
diff --git a/lib/vector-buffers/src/topology/builder.rs b/lib/vector-buffers/src/topology/builder.rs
@@ -191,12 +191,13 @@ impl<T: Bufferable> TopologyBuilder<T> {
         when_full: WhenFull,
         receiver_span: &Span,
         metadata: Option<ChannelMetricMetadata>,
+        ewma_alpha: Option<f64>,
     ) -> (BufferSender<T>, BufferReceiver<T>) {
         let usage_handle = BufferUsageHandle::noop();
         usage_handle.set_buffer_limits(None, Some(max_events.get()));
 
         let limit = MemoryBufferSize::MaxEvents(max_events);
-        let (sender, receiver) = limited(limit, metadata);
+        let (sender, receiver) = limited(limit, metadata, ewma_alpha);
 
         let mode = match when_full {
             WhenFull::Overflow => WhenFull::Block,
@@ -232,7 +233,7 @@ impl<T: Bufferable> TopologyBuilder<T> {
         usage_handle.set_buffer_limits(None, Some(max_events.get()));
 
         let limit = MemoryBufferSize::MaxEvents(max_events);
-        let (sender, receiver) = limited(limit, metadata);
+        let (sender, receiver) = limited(limit, metadata, None);
 
         let mode = match when_full {
             WhenFull::Overflow => WhenFull::Block,
diff --git a/lib/vector-buffers/src/topology/channel/limited_queue.rs b/lib/vector-buffers/src/topology/channel/limited_queue.rs
@@ -23,7 +23,7 @@ use crate::{InMemoryBufferable, config::MemoryBufferSize};
 /// The alpha value for the Exponentially Weighted Moving Average (EWMA) calculation. This is a
 /// measure of how much weight to give to the current value versus the previous values. A value of
 /// 0.9 results in a "half life" of 6-7 measurements.
-const EWMA_ALPHA: f64 = 0.9;
+pub const DEFAULT_EWMA_ALPHA: f64 = 0.9;
 
 /// Error returned by `LimitedSender::send` when the receiver has disconnected.
 #[derive(Debug, PartialEq, Eq)]
@@ -127,7 +127,11 @@ struct Metrics {
 
 impl Metrics {
     #[expect(clippy::cast_precision_loss)] // We have to convert buffer sizes for a gauge, it's okay to lose precision here.
-    fn new(limit: MemoryBufferSize, metadata: ChannelMetricMetadata) -> Self {
+    fn new(
+        limit: MemoryBufferSize,
+        metadata: ChannelMetricMetadata,
+        ewma_alpha: Option<f64>,
+    ) -> Self {
         let ChannelMetricMetadata { prefix, output } = metadata;
         let (gauge_suffix, max_value) = match limit {
             MemoryBufferSize::MaxEvents(max_events) => ("_max_event_size", max_events.get() as f64),
@@ -137,7 +141,7 @@ impl Metrics {
         let histogram_name = format!("{prefix}_utilization");
         let gauge_name = format!("{prefix}_utilization_level");
         let mean_name = format!("{prefix}_utilization_mean");
-        let ewma = Arc::new(AtomicEwma::new(EWMA_ALPHA));
+        let ewma = Arc::new(AtomicEwma::new(ewma_alpha.unwrap_or(DEFAULT_EWMA_ALPHA)));
         #[cfg(test)]
         let recorded_values = Arc::new(Mutex::new(Vec::new()));
         if let Some(label_value) = output {
@@ -202,9 +206,13 @@ impl<T> Clone for Inner<T> {
 }
 
 impl<T: InMemoryBufferable> Inner<T> {
-    fn new(limit: MemoryBufferSize, metric_metadata: Option<ChannelMetricMetadata>) -> Self {
+    fn new(
+        limit: MemoryBufferSize,
+        metric_metadata: Option<ChannelMetricMetadata>,
+        ewma_alpha: Option<f64>,
+    ) -> Self {
         let read_waker = Arc::new(Notify::new());
-        let metrics = metric_metadata.map(|metadata| Metrics::new(limit, metadata));
+        let metrics = metric_metadata.map(|metadata| Metrics::new(limit, metadata, ewma_alpha));
         match limit {
             MemoryBufferSize::MaxEvents(max_events) => Inner {
                 data: Arc::new(ArrayQueue::new(max_events.get())),
@@ -397,8 +405,9 @@ impl<T> Drop for LimitedReceiver<T> {
 pub fn limited<T: InMemoryBufferable + fmt::Debug>(
     limit: MemoryBufferSize,
     metric_metadata: Option<ChannelMetricMetadata>,
+    ewma_alpha: Option<f64>,
 ) -> (LimitedSender<T>, LimitedReceiver<T>) {
-    let inner = Inner::new(limit, metric_metadata);
+    let inner = Inner::new(limit, metric_metadata, ewma_alpha);
 
     let sender = LimitedSender {
         inner: inner.clone(),
@@ -426,7 +435,7 @@ mod tests {
     #[tokio::test]
     async fn send_receive() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(2).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(2, tx.available_capacity());
 
@@ -458,6 +467,7 @@ mod tests {
         let (mut tx, mut rx) = limited(
             limit,
             Some(ChannelMetricMetadata::new("test_channel", None)),
+            None,
         );
 
         let metrics = tx.inner.metrics.as_ref().unwrap().recorded_values.clone();
@@ -477,7 +487,7 @@ mod tests {
 
         // With this configuration a maximum of exactly 10 messages can fit in the channel
         let limit = MemoryBufferSize::MaxSize(NonZeroUsize::new(max_allowed_bytes).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(max_allowed_bytes, tx.available_capacity());
 
@@ -511,7 +521,7 @@ mod tests {
     #[test]
     fn sender_waits_for_more_capacity_when_none_available() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(1).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(1, tx.available_capacity());
 
@@ -573,7 +583,7 @@ mod tests {
     #[test]
     fn sender_waits_for_more_capacity_when_partial_available() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(7).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(7, tx.available_capacity());
 
@@ -662,7 +672,7 @@ mod tests {
     #[test]
     fn empty_receiver_returns_none_when_last_sender_drops() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(1).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(1, tx.available_capacity());
 
@@ -705,7 +715,7 @@ mod tests {
     #[test]
     fn receiver_returns_none_once_empty_when_last_sender_drops() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(1).unwrap());
-        let (tx, mut rx) = limited::<Sample>(limit, None);
+        let (tx, mut rx) = limited::<Sample>(limit, None, None);
 
         assert_eq!(1, tx.available_capacity());
 
@@ -735,7 +745,7 @@ mod tests {
     #[test]
     fn oversized_send_allowed_when_empty() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(1).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(1, tx.available_capacity());
 
@@ -768,7 +778,7 @@ mod tests {
     #[test]
     fn oversized_send_allowed_when_partial_capacity() {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(2).unwrap());
-        let (mut tx, mut rx) = limited(limit, None);
+        let (mut tx, mut rx) = limited(limit, None, None);
 
         assert_eq!(2, tx.available_capacity());
 
diff --git a/lib/vector-buffers/src/topology/channel/mod.rs b/lib/vector-buffers/src/topology/channel/mod.rs
@@ -3,7 +3,7 @@ mod receiver;
 mod sender;
 
 pub use limited_queue::{
-    ChannelMetricMetadata, LimitedReceiver, LimitedSender, SendError, limited,
+    ChannelMetricMetadata, DEFAULT_EWMA_ALPHA, LimitedReceiver, LimitedSender, SendError, limited,
 };
 pub use receiver::*;
 pub use sender::*;
diff --git a/lib/vector-buffers/src/variants/in_memory.rs b/lib/vector-buffers/src/variants/in_memory.rs
@@ -45,7 +45,7 @@ where
 
         usage_handle.set_buffer_limits(max_bytes, max_size);
 
-        let (tx, rx) = limited(self.capacity, None);
+        let (tx, rx) = limited(self.capacity, None, None);
         Ok((tx.into(), rx.into()))
     }
 }
diff --git a/lib/vector-core/src/config/global_options.rs b/lib/vector-core/src/config/global_options.rs
@@ -140,6 +140,19 @@ pub struct GlobalOptions {
     #[serde(skip_serializing_if = "crate::serde::is_default")]
     pub expire_metrics_per_metric_set: Option<Vec<PerMetricSetExpiration>>,
 
+    /// The alpha value for the exponential weighted moving average (EWMA) of source and transform
+    /// buffer utilization metrics.
+    ///
+    /// This value specifies how much of the existing value is retained when each update is made.
+    /// Values closer to 1.0 result in the value adjusting slower to changes. The default value of
+    /// 0.9 is equivalent to a "half life" of 6-7 measurements.
+    ///
+    /// Must be between 0 and 1 exclusive (0 < alpha < 1).
+    #[serde(default, skip_serializing_if = "crate::serde::is_default")]
+    #[configurable(validation(range(min = 0.0, max = 1.0)))]
+    #[configurable(metadata(docs::advanced))]
+    pub buffer_utilization_ewma_alpha: Option<f64>,
+
     /// The interval, in seconds, at which the internal metrics cache for VRL is refreshed.
     /// This must be set to be able to access metrics in VRL functions.
     ///
@@ -295,6 +308,9 @@ impl GlobalOptions {
                 expire_metrics: self.expire_metrics.or(with.expire_metrics),
                 expire_metrics_secs: self.expire_metrics_secs.or(with.expire_metrics_secs),
                 expire_metrics_per_metric_set: merged_expire_metrics_per_metric_set,
+                buffer_utilization_ewma_alpha: self
+                    .buffer_utilization_ewma_alpha
+                    .or(with.buffer_utilization_ewma_alpha),
                 metrics_storage_refresh_period: self
                     .metrics_storage_refresh_period
                     .or(with.metrics_storage_refresh_period),
diff --git a/lib/vector-core/src/fanout.rs b/lib/vector-core/src/fanout.rs
@@ -490,6 +490,7 @@ mod tests {
             WhenFull::Block,
             &Span::current(),
             None,
+            None,
         )
     }
 
diff --git a/lib/vector-core/src/source_sender/builder.rs b/lib/vector-core/src/source_sender/builder.rs
@@ -13,6 +13,7 @@ pub struct Builder {
     named_outputs: HashMap<String, Output>,
     lag_time: Option<Histogram>,
     timeout: Option<Duration>,
+    ewma_alpha: Option<f64>,
 }
 
 impl Default for Builder {
@@ -23,6 +24,7 @@ impl Default for Builder {
             named_outputs: Default::default(),
             lag_time: Some(histogram!(LAG_TIME_NAME)),
             timeout: None,
+            ewma_alpha: None,
         }
     }
 }
@@ -40,6 +42,12 @@ impl Builder {
         self
     }
 
+    #[must_use]
+    pub fn with_ewma_alpha(mut self, alpha: Option<f64>) -> Self {
+        self.ewma_alpha = alpha;
+        self
+    }
+
     pub fn add_source_output(
         &mut self,
         output: SourceOutput,
@@ -60,6 +68,7 @@ impl Builder {
                     log_definition,
                     output_id,
                     self.timeout,
+                    self.ewma_alpha,
                 );
                 self.default_output = Some(output);
                 rx
@@ -72,6 +81,7 @@ impl Builder {
                     log_definition,
                     output_id,
                     self.timeout,
+                    self.ewma_alpha,
                 );
                 self.named_outputs.insert(name, output);
                 rx
diff --git a/lib/vector-core/src/source_sender/output.rs b/lib/vector-core/src/source_sender/output.rs
@@ -115,10 +115,11 @@ impl Output {
         log_definition: Option<Arc<Definition>>,
         output_id: OutputId,
         timeout: Option<Duration>,
+        ewma_alpha: Option<f64>,
     ) -> (Self, LimitedReceiver<SourceSenderItem>) {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(n).unwrap());
         let metrics = ChannelMetricMetadata::new(UTILIZATION_METRIC_PREFIX, Some(output.clone()));
-        let (tx, rx) = channel::limited(limit, Some(metrics));
+        let (tx, rx) = channel::limited(limit, Some(metrics), ewma_alpha);
         (
             Self {
                 sender: tx,
diff --git a/lib/vector-core/src/source_sender/sender.rs b/lib/vector-core/src/source_sender/sender.rs
@@ -119,6 +119,7 @@ impl SourceSender {
             None,
             output_id,
             timeout,
+            None,
         );
         (
             Self {
@@ -192,7 +193,7 @@ impl SourceSender {
             port: Some(name.clone()),
         };
         let (output, recv) =
-            Output::new_with_buffer(100, name.clone(), None, None, output_id, None);
+            Output::new_with_buffer(100, name.clone(), None, None, output_id, None, None);
         let recv = recv.into_stream().map(move |mut item| {
             item.events.iter_events_mut().for_each(|mut event| {
                 let metadata = event.metadata_mut();
diff --git a/lib/vector-tap/src/controller.rs b/lib/vector-tap/src/controller.rs
@@ -361,6 +361,7 @@ async fn tap_handler(
                                 WhenFull::DropNewest,
                                 &Span::current(),
                                 None,
+                                None,
                             );
                             let mut tap_transformer = TapTransformer::new(tx.clone(), output.clone());
 
diff --git a/src/config/compiler.rs b/src/config/compiler.rs
@@ -36,6 +36,10 @@ pub fn compile(mut builder: ConfigBuilder) -> Result<(Config, Vec<String>), Vec<
         errors.extend(output_errors);
     }
 
+    if let Err(alpha_errors) = validation::check_buffer_utilization_ewma_alpha(&builder) {
+        errors.extend(alpha_errors);
+    }
+
     let ConfigBuilder {
         global,
         #[cfg(feature = "api")]
diff --git a/src/config/validation.rs b/src/config/validation.rs
@@ -11,6 +11,14 @@ use super::{
 };
 use crate::config::schema;
 
+/// Minimum value (exclusive) for `utilization_ewma_alpha`.
+/// The alpha value must be strictly greater than this value.
+const EWMA_ALPHA_MIN: f64 = 0.0;
+
+/// Maximum value (exclusive) for `utilization_ewma_alpha`.
+/// The alpha value must be strictly less than this value.
+const EWMA_ALPHA_MAX: f64 = 1.0;
+
 /// Check that provide + topology config aren't present in the same builder, which is an error.
 pub fn check_provider(config: &ConfigBuilder) -> Result<(), Vec<String>> {
     if config.provider.is_some()
@@ -147,6 +155,20 @@ pub fn check_resources(config: &ConfigBuilder) -> Result<(), Vec<String>> {
     }
 }
 
+/// Validates that `buffer_utilization_ewma_alpha` value is within the valid range (0 < alpha < 1)
+/// for the global configuration.
+pub fn check_buffer_utilization_ewma_alpha(config: &ConfigBuilder) -> Result<(), Vec<String>> {
+    if let Some(alpha) = config.global.buffer_utilization_ewma_alpha
+        && (alpha <= EWMA_ALPHA_MIN || alpha >= EWMA_ALPHA_MAX)
+    {
+        Err(vec![format!(
+            "Global `buffer_utilization_ewma_alpha` must be between 0 and 1 exclusive (0 < alpha < 1), got {alpha}"
+        )])
+    } else {
+        Ok(())
+    }
+}
+
 /// To avoid collisions between `output` metric tags, check that a component
 /// does not have a named output with the name [`DEFAULT_OUTPUT`]
 pub fn check_outputs(config: &ConfigBuilder) -> Result<(), Vec<String>> {
diff --git a/src/test_util/mock/sources/basic.rs b/src/test_util/mock/sources/basic.rs
@@ -46,7 +46,7 @@ pub struct BasicSourceConfig {
 impl Default for BasicSourceConfig {
     fn default() -> Self {
         let limit = MemoryBufferSize::MaxEvents(NonZeroUsize::new(1000).unwrap());
-        let (_, receiver) = limited(limit, None);
+        let (_, receiver) = limited(limit, None, None);
         Self {
             receiver: Arc::new(Mutex::new(Some(receiver))),
             event_counter: None,
diff --git a/src/topology/builder.rs b/src/topology/builder.rs
@@ -265,7 +265,8 @@ impl<'a> Builder<'a> {
 
             let mut builder = SourceSender::builder()
                 .with_buffer(*SOURCE_SENDER_BUFFER_SIZE)
-                .with_timeout(source.inner.send_timeout());
+                .with_timeout(source.inner.send_timeout())
+                .with_ewma_alpha(self.config.global.buffer_utilization_ewma_alpha);
             let mut pumps = Vec::new();
             let mut controls = HashMap::new();
             let mut schema_definitions = HashMap::with_capacity(source_outputs.len());
@@ -528,6 +529,7 @@ impl<'a> Builder<'a> {
                 WhenFull::Block,
                 &span,
                 Some(metrics),
+                self.config.global.buffer_utilization_ewma_alpha,
             );
 
             self.inputs
diff --git a/website/cue/reference/generated/configuration.cue b/website/cue/reference/generated/configuration.cue

Original file line number	Diff line number	Diff line change
`@@ -45,7 +45,7 @@ where`
`45`	`45`
`46`	`46`	`usage_handle.set_buffer_limits(max_bytes, max_size);`
`47`	`47`
`48`		`- let (tx, rx) = limited(self.capacity, None);`
	`48`	`+ let (tx, rx) = limited(self.capacity, None, None);`
`49`	`49`	`Ok((tx.into(), rx.into()))`
`50`	`50`	`}`
`51`	`51`	`}`
Original file line number	Diff line number	Diff line change
`@@ -490,6 +490,7 @@ mod tests {`
`490`	`490`	`WhenFull::Block,`
`491`	`491`	`&Span::current(),`
`492`	`492`	`None,`
	`493`	`+ None,`
`493`	`494`	`)`
`494`	`495`	`}`
`495`	`496`