Add span based feature flags

.cursor/rules/feature_flags.mdc

@@ -0,0 +1,24 @@
+---
+alwaysApply: false
+description: Feature Flags
+---
+# Java SDK Feature Flags
+
+There is a scope based and a span based API for tracking feature flag evaluations.
+
+## Scope Based API
+
+The `addFeatureFlag` method can be used to track feature flag evaluations. It exists on `Sentry` static API as well as `IScopes` and `IScope`.
+
+The `maxFeatureFlags` option controls how many flags are tracked per scope and also how many are sent to Sentry as part of events.
+Scope based feature flags can also be disabled by setting the value to 0. Defaults to 100 feature flag evaluations.
+
+Order of feature flag evaluations is important as we only keep track of the last {maxFeatureFlag} items.
+
+When a feature flag evluation with the same name is added, the previous one is removed and the new one is stored so that it'll be dropped last.
+
+When sending out an error event, feature flag buffers from all three scope types (global, isolation and current scope) are merged, chosing the newest {maxFeatureFlag} entries across all scope types. Feature flags are sent as part of the `flags` context.
+
+## Span Based API
+
+tbd

.cursor/rules/overview_dev.mdc

@@ -34,6 +34,14 @@ Use the `fetch_rules` tool to include these rules when working on specific areas
   - Rate limiting, cache rotation
   - Android vs JVM caching differences
 
+- **`feature_flags`**: Use when working with:
+  - Feature flag tracking and evaluation
+  - `addFeatureFlag()`, `getFeatureFlags()` methods
+  - `FeatureFlagBuffer`, `FeatureFlag` protocol
+  - `maxFeatureFlags` option and buffer management
+  - Feature flag merging across scope types
+  - Scope-based vs span-based feature flag APIs
+
 ### Integration & Infrastructure
 - **`opentelemetry`**: Use when working with:
   - OpenTelemetry modules (`sentry-opentelemetry-*`)
@@ -63,3 +71,4 @@ Use the `fetch_rules` tool to include these rules when working on specific areas
    - new module/integration/sample → `new_module`
    - Cache/offline/network → `offline`
    - System test/e2e/sample → `e2e_tests`
+   - Feature flag/addFeatureFlag/flag evaluation → `feature_flags`

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 ## Unreleased
 
+### Features
+
+- Add feature flags API ([#4812](https://github.com/getsentry/sentry-java/pull/4812)) and ([#4831](https://github.com/getsentry/sentry-java/pull/4831))
+  - You may now keep track of your feature flag evaluations and have them show up in Sentry.
+  - Top level API (`Sentry.addFeatureFlag("my-feature-flag", true);`) writes to scopes and the current span (if there is one)
+  - It is also possible to use API on `IScope`, `IScopes`, `ISpan` and `ITransaction` directly
+  - Feature flag evaluations tracked on scope(s) will be added to any errors reported to Sentry.
+    - The SDK keeps the latest 100 evaluations from scope(s), replacing old entries as new evaluations are added.
+  - For feature flag evaluations tracked on spans:
+    - Only 10 evaluations are tracked per span, existing flags are updated but new ones exceeding the limit are ignored
+    - Spans do not inherit evaluations from their parent
+
 ### Fixes
 
 - Removed SentryExecutorService limit for delayed scheduled tasks ([#4846](https://github.com/getsentry/sentry-java/pull/4846))
@@ -69,6 +81,9 @@
       <meta-data android:name="io.sentry.session-replay.screenshot-strategy" android:value="canvas" />
     </application>
     ```
+- Add Feature Flags ([#4812](https://github.com/getsentry/sentry-java/pull/4812)) and ([#4831](https://github.com/getsentry/sentry-java/pull/4831))
+  - You may use top level API (`Sentry.addFeatureFlag`) to add feature flag evaluations to both scope and the current active span
+  - Or you may directly add feature flag evaluations to any scope, transaction or span
 
 ### Fixes
 

sentry-opentelemetry/sentry-opentelemetry-bootstrap/api/sentry-opentelemetry-bootstrap.api

@@ -43,6 +43,7 @@ public final class io/sentry/opentelemetry/OtelSpanFactory : io/sentry/ISpanFact
 
 public final class io/sentry/opentelemetry/OtelStrongRefSpanWrapper : io/sentry/opentelemetry/IOtelSpanWrapper {
 	public fun <init> (Lio/opentelemetry/api/trace/Span;Lio/sentry/opentelemetry/IOtelSpanWrapper;)V
+	public fun addFeatureFlag (Ljava/lang/String;Ljava/lang/Boolean;)V
 	public fun finish ()V
 	public fun finish (Lio/sentry/SpanStatus;)V
 	public fun finish (Lio/sentry/SpanStatus;Lio/sentry/SentryDate;)V
@@ -96,6 +97,7 @@ public final class io/sentry/opentelemetry/OtelStrongRefSpanWrapper : io/sentry/
 
 public final class io/sentry/opentelemetry/OtelTransactionSpanForwarder : io/sentry/ITransaction {
 	public fun <init> (Lio/sentry/opentelemetry/IOtelSpanWrapper;)V
+	public fun addFeatureFlag (Ljava/lang/String;Ljava/lang/Boolean;)V
 	public fun finish ()V
 	public fun finish (Lio/sentry/SpanStatus;)V
 	public fun finish (Lio/sentry/SpanStatus;Lio/sentry/SentryDate;)V

sentry-opentelemetry/sentry-opentelemetry-bootstrap/src/main/java/io/sentry/opentelemetry/OtelStrongRefSpanWrapper.java

@@ -310,4 +310,9 @@ public void setContext(@Nullable String key, @Nullable Object context) {
   public @Nullable Attributes getOpenTelemetrySpanAttributes() {
     return delegate.getOpenTelemetrySpanAttributes();
   }
+
+  @Override
+  public void addFeatureFlag(final @Nullable String flag, final @Nullable Boolean result) {
+    delegate.addFeatureFlag(flag, result);
+  }
 }

sentry-opentelemetry/sentry-opentelemetry-bootstrap/src/main/java/io/sentry/opentelemetry/OtelTransactionSpanForwarder.java

@@ -309,4 +309,9 @@ public void setName(@NotNull String name, @NotNull TransactionNameSource nameSou
     }
     return name;
   }
+
+  @Override
+  public void addFeatureFlag(final @Nullable String flag, final @Nullable Boolean result) {
+    rootSpan.addFeatureFlag(flag, result);
+  }
 }

sentry-opentelemetry/sentry-opentelemetry-core/api/sentry-opentelemetry-core.api

@@ -57,6 +57,7 @@ public final class io/sentry/opentelemetry/OtelSpanUtils {
 
 public final class io/sentry/opentelemetry/OtelSpanWrapper : io/sentry/opentelemetry/IOtelSpanWrapper {
 	public fun <init> (Lio/opentelemetry/sdk/trace/ReadWriteSpan;Lio/sentry/IScopes;Lio/sentry/SentryDate;Lio/sentry/TracesSamplingDecision;Lio/sentry/opentelemetry/IOtelSpanWrapper;Lio/sentry/SpanId;Lio/sentry/Baggage;)V
+	public fun addFeatureFlag (Ljava/lang/String;Ljava/lang/Boolean;)V
 	public fun finish ()V
 	public fun finish (Lio/sentry/SpanStatus;)V
 	public fun finish (Lio/sentry/SpanStatus;Lio/sentry/SentryDate;)V

sentry-opentelemetry/sentry-opentelemetry-core/src/main/java/io/sentry/opentelemetry/OtelSpanWrapper.java

@@ -505,6 +505,11 @@ public Map<String, MeasurementValue> getMeasurements() {
     return scopes;
   }
 
+  @Override
+  public void addFeatureFlag(final @Nullable String flag, final @Nullable Boolean result) {
+    context.addFeatureFlag(flag, result);
+  }
+
   @Override
   public @NotNull Context storeInContext(Context context) {
     final @Nullable ReadWriteSpan otelSpan = getSpan();

sentry-opentelemetry/sentry-opentelemetry-core/src/main/java/io/sentry/opentelemetry/SentrySpanExporter.java

@@ -33,7 +33,10 @@
 import io.sentry.SpanStatus;
 import io.sentry.TransactionContext;
 import io.sentry.TransactionOptions;
+import io.sentry.featureflags.IFeatureFlagBuffer;
 import io.sentry.protocol.Contexts;
+import io.sentry.protocol.FeatureFlag;
+import io.sentry.protocol.FeatureFlags;
 import io.sentry.protocol.SentryId;
 import io.sentry.protocol.TransactionNameSource;
 import java.util.Arrays;
@@ -260,6 +263,16 @@ private void transferSpanDetails(
         targetSpan.setData(entry.getKey(), entry.getValue());
       }
 
+      final @NotNull SpanContext spanContext = sourceSpan.getSpanContext();
+      final @NotNull IFeatureFlagBuffer featureFlagBuffer = spanContext.getFeatureFlagBuffer();
+      final @Nullable FeatureFlags featureFlags = featureFlagBuffer.getFeatureFlags();
+      if (featureFlags != null) {
+        for (FeatureFlag featureFlag : featureFlags.getValues()) {
+          targetSpan.setData(
+              FeatureFlag.DATA_PREFIX + featureFlag.getFlag(), featureFlag.getResult());
+        }
+      }
+
       final @NotNull Map<String, String> tags = sourceSpan.getTags();
       for (Map.Entry<String, String> entry : tags.entrySet()) {
         targetSpan.setTag(entry.getKey(), entry.getValue());

sentry-samples/sentry-samples-console-opentelemetry-noagent/src/main/java/io/sentry/samples/console/Main.java

@@ -61,6 +61,8 @@ public static void main(String[] args) throws InterruptedException {
     // Only data added to the scope on `configureScope` above is included.
     Sentry.captureMessage("Some warning!", SentryLevel.WARNING);
 
+    Sentry.addFeatureFlag("my-feature-flag", true);
+
     // Sending exception:
     Exception exception = new RuntimeException("Some error!");
     Sentry.captureException(exception);

sentry-samples/sentry-samples-console-opentelemetry-noagent/src/test/kotlin/sentry/systemtest/ConsoleApplicationSystemTest.kt

@@ -55,7 +55,7 @@ class ConsoleApplicationSystemTest {
     // Verify we received the RuntimeException
     testHelper.ensureErrorReceived { event ->
       event.exceptions?.any { ex -> ex.type == "RuntimeException" && ex.value == "Some error!" } ==
-        true
+        true && testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
     }
 
     // Verify we received the detailed event with fingerprint

sentry-samples/sentry-samples-console/src/main/java/io/sentry/samples/console/Main.java

@@ -126,6 +126,8 @@ public static void main(String[] args) throws InterruptedException {
     // Only data added to the scope on `configureScope` above is included.
     Sentry.captureMessage("Some warning!", SentryLevel.WARNING);
 
+    Sentry.addFeatureFlag("my-feature-flag", true);
+
     // Sending exception:
     Exception exception = new RuntimeException("Some error!");
     Sentry.captureException(exception);

sentry-samples/sentry-samples-console/src/test/kotlin/io/sentry/systemtest/ConsoleApplicationSystemTest.kt

@@ -51,7 +51,7 @@ class ConsoleApplicationSystemTest {
     // Verify we received the RuntimeException
     testHelper.ensureErrorReceived { event ->
       event.exceptions?.any { ex -> ex.type == "RuntimeException" && ex.value == "Some error!" } ==
-        true
+        true && testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
     }
 
     // Verify we received the detailed event with fingerprint

sentry-samples/sentry-samples-jul/src/main/java/io/sentry/samples/jul/Main.java

@@ -1,5 +1,6 @@
 package io.sentry.samples.jul;
 
+import io.sentry.Sentry;
 import java.util.UUID;
 import java.util.logging.Level;
 import java.util.logging.LogManager;
@@ -22,6 +23,10 @@ public static void main(String[] args) throws Exception {
     MDC.put("userId", UUID.randomUUID().toString());
     MDC.put("requestId", UUID.randomUUID().toString());
 
+    Sentry.addFeatureFlag("my-feature-flag", true);
+
+    LOGGER.warning("important warning");
+
     // logging arguments are converted to Sentry Event parameters
     LOGGER.log(Level.INFO, "User has made a purchase of product: %d", 445);
 

sentry-samples/sentry-samples-jul/src/test/kotlin/io/sentry/systemtest/ConsoleApplicationSystemTest.kt

@@ -57,6 +57,11 @@ class ConsoleApplicationSystemTest {
       } != null
     }
 
+    testHelper.ensureErrorReceived { event ->
+      event.message?.message == "important warning" &&
+        testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
+    }
+
     testHelper.ensureLogsReceived { logs, _ ->
       testHelper.doesContainLogWithBody(logs, "User has made a purchase of product: 445") &&
         testHelper.doesContainLogWithBody(logs, "Something went wrong")

sentry-samples/sentry-samples-log4j2/src/main/java/io/sentry/samples/log4j2/Main.java

@@ -1,5 +1,6 @@
 package io.sentry.samples.log4j2;
 
+import io.sentry.Sentry;
 import java.util.UUID;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -19,6 +20,8 @@ public static void main(String[] args) {
     // ThreadContext tag not listed in log4j2.xml
     ThreadContext.put("context-tag", "context-tag-value");
 
+    Sentry.addFeatureFlag("my-feature-flag", true);
+
     // logging arguments are converted to Sentry Event parameters
     LOGGER.info("User has made a purchase of product: {}", 445);
     // because minimumEventLevel is set to WARN this raises an event

sentry-samples/sentry-samples-log4j2/src/test/kotlin/io/sentry/systemtest/ConsoleApplicationSystemTest.kt

@@ -47,6 +47,11 @@ class ConsoleApplicationSystemTest {
         event.level?.name == "ERROR"
     }
 
+    testHelper.ensureErrorReceived { event ->
+      event.message?.message == "Important warning" &&
+        testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
+    }
+
     testHelper.ensureErrorReceived { event ->
       event.breadcrumbs?.firstOrNull {
         it.message == "Hello Sentry!" && it.level == SentryLevel.DEBUG

sentry-samples/sentry-samples-logback/src/main/java/io/sentry/samples/logback/Main.java

@@ -1,5 +1,6 @@
 package io.sentry.samples.logback;
 
+import io.sentry.Sentry;
 import java.util.UUID;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -17,6 +18,9 @@ public static void main(String[] args) {
     // MDC tag not listed in logback.xml
     MDC.put("context-tag", "context-tag-value");
 
+    Sentry.addFeatureFlag("my-feature-flag", true);
+    LOGGER.warn("important warning");
+
     // logging arguments are converted to Sentry Event parameters
     LOGGER.info("User has made a purchase of product: {}", 445);
 

sentry-samples/sentry-samples-logback/src/test/kotlin/io/sentry/systemtest/ConsoleApplicationSystemTest.kt

@@ -53,6 +53,11 @@ class ConsoleApplicationSystemTest {
       } != null
     }
 
+    testHelper.ensureErrorReceived { event ->
+      event.message?.message == "important warning" &&
+        testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
+    }
+
     testHelper.ensureErrorReceived { event ->
       event.breadcrumbs?.firstOrNull {
         it.message == "User has made a purchase of product: 445" && it.level == SentryLevel.INFO

sentry-samples/sentry-samples-spring-7/src/main/java/io/sentry/samples/spring7/web/PersonController.java

@@ -26,6 +26,7 @@ Person person(@PathVariable("id") Long id) {
     Sentry.logger().warn("warn Sentry logging");
     Sentry.logger().error("error Sentry logging");
     Sentry.logger().info("hello %s %s", "there", "world!");
+    Sentry.addFeatureFlag("my-feature-flag", true);
     LOGGER.info("Loading person with id={}", id);
     if (id > 10L) {
       throw new IllegalArgumentException("Something went wrong [id=" + id + "]");

sentry-samples/sentry-samples-spring-7/src/test/kotlin/io/sentry/systemtest/PersonSystemTest.kt

@@ -20,6 +20,11 @@ class PersonSystemTest {
     restClient.getPerson(11L)
     assertEquals(500, restClient.lastKnownStatusCode)
 
+    testHelper.ensureErrorReceived { event ->
+      testHelper.doesEventHaveExceptionMessage(event, "Something went wrong [id=11]") &&
+        testHelper.doesEventHaveFlag(event, "my-feature-flag", true)
+    }
+
     testHelper.ensureTransactionReceived { transaction, envelopeHeader ->
       testHelper.doesTransactionHaveOp(transaction, "http.server")
     }

sentry-samples/sentry-samples-spring-boot-4-opentelemetry-noagent/src/main/java/io/sentry/samples/spring/boot4/PersonController.java

@@ -31,11 +31,13 @@ public PersonController(PersonService personService, Tracer tracer) {
   @GetMapping("{id}")
   @WithSpan("personSpanThroughOtelAnnotation")
   Person person(@PathVariable Long id) {
+    Sentry.addFeatureFlag("outer-feature-flag", true);
     Span span = tracer.spanBuilder("spanCreatedThroughOtelApi").startSpan();
     try (final @NotNull Scope spanScope = span.makeCurrent()) {
       Sentry.logger().warn("warn Sentry logging");
       Sentry.logger().error("error Sentry logging");
       Sentry.logger().info("hello %s %s", "there", "world!");
+      Sentry.addFeatureFlag("inner-feature-flag", true);
       ISpan currentSpan = Sentry.getSpan();
       ISpan sentrySpan = currentSpan.startChild("spanCreatedThroughSentryApi");
       try {

sentry-samples/sentry-samples-spring-boot-4-opentelemetry-noagent/src/test/kotlin/io/sentry/systemtest/PersonSystemTest.kt

@@ -1,5 +1,6 @@
 package io.sentry.systemtest
 
+import io.sentry.protocol.FeatureFlag
 import io.sentry.systemtest.util.TestHelper
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -20,9 +21,33 @@ class PersonSystemTest {
     restClient.getPerson(1L)
     assertEquals(500, restClient.lastKnownStatusCode)
 
+    testHelper.ensureErrorReceived { event ->
+      event.message?.formatted == "Trying person with id=1" &&
+        testHelper.doesEventHaveFlag(event, "inner-feature-flag", true)
+    }
+
+    testHelper.ensureErrorReceived { event ->
+      testHelper.doesEventHaveExceptionMessage(event, "Something went wrong [id=1]") &&
+        testHelper.doesEventHaveFlag(event, "inner-feature-flag", true)
+    }
+
     testHelper.ensureTransactionReceived { transaction, envelopeHeader ->
-      testHelper.doesTransactionContainSpanWithOp(transaction, "spanCreatedThroughOtelApi") &&
-        testHelper.doesTransactionContainSpanWithOp(transaction, "spanCreatedThroughSentryApi")
+      testHelper.doesTransactionHave(transaction, op = "http.server") &&
+        testHelper.doesTransactionHaveSpanWith(
+          transaction,
+          op = "personSpanThroughOtelAnnotation",
+          featureFlag = FeatureFlag("flag.evaluation.outer-feature-flag", true),
+        ) &&
+        testHelper.doesTransactionHaveSpanWith(
+          transaction,
+          op = "spanCreatedThroughOtelApi",
+          featureFlag = FeatureFlag("flag.evaluation.inner-feature-flag", true),
+        ) &&
+        testHelper.doesTransactionHaveSpanWith(
+          transaction,
+          op = "spanCreatedThroughSentryApi",
+          noFeatureFlags = true,
+        )
     }
 
     Thread.sleep(10000)

sentry-samples/sentry-samples-spring-boot-4-opentelemetry/src/main/java/io/sentry/samples/spring/boot4/PersonController.java

@@ -29,11 +29,13 @@ public PersonController(PersonService personService, Tracer tracer) {
 
   @GetMapping("{id}")
   Person person(@PathVariable Long id) {
+    Sentry.addFeatureFlag("transaction-feature-flag", true);
     Span span = tracer.spanBuilder("spanCreatedThroughOtelApi").startSpan();
     try (final @NotNull Scope spanScope = span.makeCurrent()) {
       Sentry.logger().warn("warn Sentry logging");
       Sentry.logger().error("error Sentry logging");
       Sentry.logger().info("hello %s %s", "there", "world!");
+      Sentry.addFeatureFlag("my-feature-flag", true);
       ISpan currentSpan = Sentry.getSpan();
       ISpan sentrySpan = currentSpan.startChild("spanCreatedThroughSentryApi");
       try {