Event emitter provider

api/events/build.gradle.kts

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+plugins {
+  id("otel.java-conventions")
+  id("otel.publish-conventions")
+
+  id("otel.animalsniffer-conventions")
+}
+
+description = "OpenTelemetry Events API"
+otelJava.moduleName.set("io.opentelemetry.api.events")
+
+dependencies {
+  api(project(":api:all"))
+}

api/events/gradle.properties

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+otel.release=alpha

api/events/src/main/java/io/opentelemetry/api/events/DefaultEventEmitter.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import io.opentelemetry.api.common.Attributes;
+
+class DefaultEventEmitter implements EventEmitter {
+
+  private static final EventEmitter INSTANCE = new DefaultEventEmitter();
+
+  private DefaultEventEmitter() {}
+
+  static EventEmitter getInstance() {
+    return INSTANCE;
+  }
+
+  @Override
+  public void emit(String eventName, Attributes attributes) {}
+}

api/events/src/main/java/io/opentelemetry/api/events/DefaultEventEmitterProvider.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+class DefaultEventEmitterProvider implements EventEmitterProvider {
+
+  private static final EventEmitterProvider INSTANCE = new DefaultEventEmitterProvider();
+  private static final EventEmitterBuilder NOOP_EVENT_EMITTER_BUILDER =
+      new NoopEventEmitterBuilder();
+
+  private DefaultEventEmitterProvider() {}
+
+  static EventEmitterProvider getInstance() {
+    return INSTANCE;
+  }
+
+  @Override
+  public EventEmitterBuilder eventEmitterBuilder(String instrumentationScopeName) {
+    return NOOP_EVENT_EMITTER_BUILDER;
+  }
+
+  private static class NoopEventEmitterBuilder implements EventEmitterBuilder {
+
+    @Override
+    public EventEmitterBuilder setSchemaUrl(String schemaUrl) {
+      return this;
+    }
+
+    @Override
+    public EventEmitterBuilder setInstrumentationVersion(String instrumentationVersion) {
+      return this;
+    }
+
+    @Override
+    public EventEmitterBuilder setEventDomain(String eventDomain) {
+      return this;
+    }
+
+    @Override
+    public EventEmitter build() {
+      return DefaultEventEmitter.getInstance();
+    }
+  }
+}

api/events/src/main/java/io/opentelemetry/api/events/EventEmitter.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import io.opentelemetry.api.common.Attributes;
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A {@link EventEmitter} is the entry point into an event pipeline.
+ *
+ * <p>Example usage emitting events:
+ *
+ * <pre>{@code
+ * class MyClass {
+ *   private final EventEmitter eventEmitter = openTelemetryEventEmitterProvider.eventEmitterBuilder("scope-name")
+ *         .setEventDomain("acme.observability")
+ *         .build();
+ *
+ *   void doWork() {
+ *     eventEmitter.emit("my-event", Attributes.builder()
+ *         .put("key1", "value1")
+ *         .put("key2", "value2")
+ *         .build())
+ *     // do work
+ *   }
+ * }
+ * }</pre>
+ */
+@ThreadSafe
+public interface EventEmitter {
+
+  /**
+   * Emit an event.
+   *
+   * @param eventName the event name, which acts as a classifier for events. Within a particular
+   *     event domain, event name defines a particular class or type of event.
+   * @param attributes attributes associated with the event
+   */
+  void emit(String eventName, Attributes attributes);
+}

api/events/src/main/java/io/opentelemetry/api/events/EventEmitterBuilder.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+/**
+ * Builder class for creating {@link EventEmitter} instances.
+ *
+ * <p>{@link EventEmitter}s are identified by their scope name, version, and schema URL. These
+ * identifying fields, along with attributes, combine to form the instrumentation scope, which is
+ * attached to all events produced by the {@link EventEmitter}.
+ */
+public interface EventEmitterBuilder {
+
+  /**
+   * Sets the event domain. Event domain is not part of {@link EventEmitter} identity.
+   *
+   * @param eventDomain The event domain, which acts as a namespace for event names. Within a
+   *     particular event domain, event name defines a particular class or type of event.
+   * @return this
+   */
+  EventEmitterBuilder setEventDomain(String eventDomain);
+
+  /**
+   * Set the scope schema URL of the resulting {@link EventEmitter}. Schema URL is part of {@link
+   * EventEmitter} identity.
+   *
+   * @param schemaUrl The schema URL.
+   * @return this
+   */
+  EventEmitterBuilder setSchemaUrl(String schemaUrl);
+
+  /**
+   * Sets the instrumentation scope version of the resulting {@link EventEmitter}. Version is part
+   * of {@link EventEmitter} identity.
+   *
+   * @param instrumentationScopeVersion The instrumentation scope version.
+   * @return this
+   */
+  EventEmitterBuilder setInstrumentationVersion(String instrumentationScopeVersion);
+
+  /**
+   * Gets or creates a {@link EventEmitter} instance.
+   *
+   * @return a {@link EventEmitter} instance configured with the provided options.
+   */
+  EventEmitter build();
+}

api/events/src/main/java/io/opentelemetry/api/events/EventEmitterProvider.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A registry for creating scoped {@link EventEmitter}s. The name <i>Provider</i> is for consistency
+ * with other languages and it is <b>NOT</b> loaded using reflection.
+ *
+ * @see EventEmitter
+ */
+@ThreadSafe
+public interface EventEmitterProvider {
+
+  /**
+   * Gets or creates a named EventEmitter instance which emits events to the {@code eventDomain}.
+   *
+   * @param instrumentationScopeName A name uniquely identifying the instrumentation scope, such as
+   *     the instrumentation library, package, or fully qualified class name. Must not be null.
+   * @return a Logger instance.
+   */
+  default EventEmitter get(String instrumentationScopeName) {
+    return eventEmitterBuilder(instrumentationScopeName).build();
+  }
+
+  /**
+   * Creates a LoggerBuilder for a named EventEmitter instance.
+   *
+   * @param instrumentationScopeName A name uniquely identifying the instrumentation scope, such as
+   *     the instrumentation library, package, or fully qualified class name. Must not be null.
+   * @return a LoggerBuilder instance.
+   */
+  EventEmitterBuilder eventEmitterBuilder(String instrumentationScopeName);
+
+  /**
+   * Returns a no-op {@link EventEmitterProvider} which provides Loggers which do not record or
+   * emit.
+   */
+  static EventEmitterProvider noop() {
+    return DefaultEventEmitterProvider.getInstance();
+  }
+}

api/events/src/main/java/io/opentelemetry/api/events/GlobalEventEmitterProvider.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import io.opentelemetry.api.GlobalOpenTelemetry;
+import java.util.concurrent.atomic.AtomicReference;
+import javax.annotation.Nullable;
+
+/**
+ * This class provides a temporary global accessor for {@link EventEmitterProvider} until the event
+ * API is marked stable. It will eventually be merged into {@link GlobalOpenTelemetry}.
+ */
+// We intentionally assign to be used for error reporting.
+@SuppressWarnings("StaticAssignmentOfThrowable")
+public final class GlobalEventEmitterProvider {
+
+  private static final AtomicReference<EventEmitterProvider> instance =
+      new AtomicReference<>(EventEmitterProvider.noop());
+
+  @Nullable private static volatile Throwable setInstanceCaller;
+
+  private GlobalEventEmitterProvider() {}
+
+  /** Returns the globally registered {@link EventEmitterProvider}. */
+  // instance cannot be set to null
+  @SuppressWarnings("NullAway")
+  public static EventEmitterProvider get() {
+    return instance.get();
+  }
+
+  /**
+   * Sets the global {@link EventEmitterProvider}. Future calls to {@link #get()} will return the
+   * provided {@link EventEmitterProvider} instance. This should be called once as early as possible
+   * in your application initialization logic.
+   */
+  public static void set(EventEmitterProvider eventEmitterProvider) {
+    boolean changed = instance.compareAndSet(EventEmitterProvider.noop(), eventEmitterProvider);
+    if (!changed && (eventEmitterProvider != EventEmitterProvider.noop())) {
+      throw new IllegalStateException(
+          "GlobalEventEmitterProvider.set has already been called. GlobalEventEmitterProvider.set "
+              + "must be called only once before any calls to GlobalEventEmitterProvider.get. "
+              + "Previous invocation set to cause of this exception.",
+          setInstanceCaller);
+    }
+    setInstanceCaller = new Throwable();
+  }
+
+  /**
+   * Unsets the global {@link EventEmitterProvider}. This is only meant to be used from tests which
+   * need to reconfigure {@link EventEmitterProvider}.
+   */
+  public static void resetForTest() {
+    instance.set(EventEmitterProvider.noop());
+  }
+}

api/events/src/test/java/io/opentelemetry/api/events/DefaultEventEmitterProviderTest.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+import io.opentelemetry.api.common.Attributes;
+import org.junit.jupiter.api.Test;
+
+class DefaultEventEmitterProviderTest {
+
+  @Test
+  void noopEventEmitterProvider_doesNotThrow() {
+    EventEmitterProvider provider = EventEmitterProvider.noop();
+
+    assertThat(provider).isSameAs(DefaultEventEmitterProvider.getInstance());
+    assertThatCode(() -> provider.get("scope-name")).doesNotThrowAnyException();
+    assertThatCode(
+            () ->
+                provider
+                    .eventEmitterBuilder("scope-name")
+                    .setEventDomain("event-domain")
+                    .setInstrumentationVersion("1.0")
+                    .setSchemaUrl("http://schema.com")
+                    .build())
+        .doesNotThrowAnyException();
+
+    assertThatCode(
+            () ->
+                provider
+                    .eventEmitterBuilder("scope-name")
+                    .build()
+                    .emit("event-name", Attributes.empty()))
+        .doesNotThrowAnyException();
+  }
+}

api/events/src/test/java/io/opentelemetry/api/events/DefaultEventEmitterTest.java

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+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.events;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+import io.opentelemetry.api.common.Attributes;
+import org.junit.jupiter.api.Test;
+
+class DefaultEventEmitterTest {
+
+  @Test
+  void emit() {
+    assertThatCode(() -> DefaultEventEmitter.getInstance().emit("event-name", Attributes.empty()))
+        .doesNotThrowAnyException();
+    assertThatCode(
+            () ->
+                DefaultEventEmitter.getInstance()
+                    .emit("event-name", Attributes.builder().put("key1", "value1").build()))
+        .doesNotThrowAnyException();
+  }
+}