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fix: safely initialize exemplar reservoir storage #8524

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Merged
jack-berg merged 2 commits into from
Jun 24, 2026
Merged

fix: safely initialize exemplar reservoir storage #8524

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990f3dd
fix: safely initialize exemplar reservoir storage
seongjun-rpls Jun 23, 2026
0f19b96
fix: read exemplar storage once when collecting
seongjun-rpls Jun 24, 2026
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32 changes: 22 additions & 10 deletions .../main/java/io/opentelemetry/sdk/metrics/internal/exemplar/FixedSizeExemplarReservoir.java
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Original file line number Diff line number Diff line change
Expand Up @@ -20,10 +20,11 @@
/** Base for fixed-size reservoir sampling of Exemplars. */
class FixedSizeExemplarReservoir implements DoubleExemplarReservoir, LongExemplarReservoir {

@Nullable private ReservoirCell[] storage;
@Nullable private volatile ReservoirCell[] storage;
private final ReservoirCellSelector reservoirCellSelector;
private final int size;
private final Clock clock;
private final Object storageLock = new Object();
private volatile boolean hasMeasurements = false;

/** Instantiates an exemplar reservoir of fixed size. */
Expand All @@ -36,12 +37,10 @@ class FixedSizeExemplarReservoir implements DoubleExemplarReservoir, LongExempla

@Override
public void offerLongMeasurement(long value, Attributes attributes, Context context) {
if (storage == null) {
storage = initStorage();
}
ReservoirCell[] storage = getOrInitStorage();
int bucket = reservoirCellSelector.reservoirCellIndexFor(storage, value, attributes, context);
if (bucket != -1) {
this.storage[bucket].recordLongMeasurement(value, attributes, context);
storage[bucket].recordLongMeasurement(value, attributes, context);
this.hasMeasurements = true;
}
}
Expand All @@ -58,16 +57,28 @@ public List<LongExemplarData> collectAndResetLongs(Attributes pointAttributes) {

@Override
public void offerDoubleMeasurement(double value, Attributes attributes, Context context) {
if (storage == null) {
storage = initStorage();
}
ReservoirCell[] storage = getOrInitStorage();
int bucket = reservoirCellSelector.reservoirCellIndexFor(storage, value, attributes, context);
if (bucket != -1) {
this.storage[bucket].recordDoubleMeasurement(value, attributes, context);
storage[bucket].recordDoubleMeasurement(value, attributes, context);
this.hasMeasurements = true;
}
}

private ReservoirCell[] getOrInitStorage() {
ReservoirCell[] storage = this.storage;
if (storage == null) {
synchronized (storageLock) {
storage = this.storage;
if (storage == null) {
storage = initStorage();
this.storage = storage;
}
}
}
return storage;
}

private ReservoirCell[] initStorage() {
ReservoirCell[] storage = new ReservoirCell[this.size];
for (int i = 0; i < size; ++i) {
Expand All @@ -78,13 +89,14 @@ private ReservoirCell[] initStorage() {

public <T extends ExemplarData> List<T> doCollectAndReset(
Attributes pointAttributes, BiFunction<ReservoirCell, Attributes, T> mapAndResetCell) {
ReservoirCell[] storage = this.storage;
if (!hasMeasurements || storage == null) {
return Collections.emptyList();
}
// Note: we are collecting exemplars from buckets piecemeal, but we
// could still be sampling exemplars during this process.
List<T> results = new ArrayList<>();
for (ReservoirCell reservoirCell : this.storage) {
for (ReservoirCell reservoirCell : storage) {
T result = mapAndResetCell.apply(reservoirCell, pointAttributes);
if (result != null) {
results.add(result);
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