Enforce last-value-wins semantics in AttributesMap without performance regression

sdk/common/src/jmh/java/io/opentelemetry/sdk/common/internal/AttributesMapBenchmark.java

@@ -0,0 +1,139 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.sdk.common.internal;
+
+import static io.opentelemetry.api.common.AttributeKey.booleanKey;
+import static io.opentelemetry.api.common.AttributeKey.stringKey;
+
+import io.opentelemetry.api.common.AttributeKey;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Fork;
+import org.openjdk.jmh.annotations.Measurement;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.Warmup;
+import org.openjdk.jmh.infra.Blackhole;
+
+/**
+ * Microbenchmark for {@link AttributesMap}. Parametrized by number of attributes.
+ *
+ * <p>Write scenarios:
+ *
+ * <ul>
+ *   <li>{@code uniqueKeys} — normal production case: each key name is distinct
+ *   <li>{@code putThenForEach} — combined write + export cycle: N unique puts followed by one
+ *       {@code forEach}, modeling the dominant span lifecycle
+ * </ul>
+ *
+ * <p>Read scenarios (run on a pre-filled map of {@code numAttributes} unique string entries):
+ *
+ * <ul>
+ *   <li>{@code getHit} — lookup with the exact stored key type (hit)
+ *   <li>{@code getTypeMiss} — lookup with a different type for the same key name (type miss)
+ *   <li>{@code forEachAll} — full iteration over all entries
+ * </ul>
+ */
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@Warmup(iterations = 5, time = 200, timeUnit = TimeUnit.MILLISECONDS)
+@Measurement(iterations = 10, time = 200, timeUnit = TimeUnit.MILLISECONDS)
+@Fork(2)
+@State(Scope.Thread)
+public class AttributesMapBenchmark {
+
+  // Default SpanLimits attribute count limit.
+  private static final int CAPACITY = 128;
+
+  @Param({"4", "16", "20", "32", "128"})
+  int numAttributes;
+
+  private List<AttributeKey<String>> stringKeys;
+  private List<AttributeKey<Boolean>> boolKeys;
+  private List<String> values;
+
+  // Pre-filled map used by read benchmarks — populated once in @Setup.
+  private AttributesMap filledMap;
+
+  @Setup
+  public void setup() {
+    stringKeys = new ArrayList<>(numAttributes);
+    boolKeys = new ArrayList<>(numAttributes);
+    values = new ArrayList<>(numAttributes);
+    for (int i = 0; i < numAttributes; i++) {
+      stringKeys.add(stringKey("key" + i));
+      boolKeys.add(booleanKey("key" + i));
+      values.add("value" + i);
+    }
+    filledMap = AttributesMap.create(CAPACITY, Integer.MAX_VALUE);
+    for (int i = 0; i < numAttributes; i++) {
+      filledMap.put(stringKeys.get(i), values.get(i));
+    }
+  }
+
+  /** Each key name is unique — the common production case. */
+  @Benchmark
+  public AttributesMap uniqueKeys() {
+    AttributesMap map = AttributesMap.create(CAPACITY, Integer.MAX_VALUE);
+    for (int i = 0; i < numAttributes; i++) {
+      map.put(stringKeys.get(i), values.get(i));
+    }
+    return map;
+  }
+
+  // ---- Read benchmarks (operate on pre-filled map) ----
+
+  /**
+   * Lookup with the exact stored key type — always a hit. Measures the cost of a successful {@code
+   * get()} for each entry in the map.
+   */
+  @Benchmark
+  public void getHit(Blackhole bh) {
+    for (int i = 0; i < numAttributes; i++) {
+      bh.consume(filledMap.get(stringKeys.get(i)));
+    }
+  }
+
+  /**
+   * Lookup with a different type for the same key name — always returns null.
+   *
+   * <p>The map holds N string-typed entries; boolean keys for the same names locate each entry by
+   * name but fail the type check. Isolates the cost of a name-hit / type-miss lookup.
+   */
+  @Benchmark
+  public void getTypeMiss(Blackhole bh) {
+    for (int i = 0; i < numAttributes; i++) {
+      bh.consume(filledMap.get(boolKeys.get(i)));
+    }
+  }
+
+  /** Full iteration over all entries via {@code forEach}. */
+  @Benchmark
+  public void forEachAll(Blackhole bh) {
+    filledMap.forEach((k, v) -> bh.consume(v));
+  }
+
+  /**
+   * Combined write + read cycle: fill a fresh map with N unique string keys, then iterate all
+   * entries once. Models the dominant production path: N puts during span building, followed by one
+   * forEach at export time.
+   */
+  @Benchmark
+  public void putThenForEach(Blackhole bh) {
+    AttributesMap map = AttributesMap.create(CAPACITY, Integer.MAX_VALUE);
+    for (int i = 0; i < numAttributes; i++) {
+      map.put(stringKeys.get(i), values.get(i));
+    }
+    map.forEach((k, v) -> bh.consume(v));
+  }
+}