[logback-ecs-encoder] Emit event.sequence when SequenceNumberGenerator is configured

ecs-logging-core/src/main/java/co/elastic/logging/EcsJsonSerializer.java

@@ -53,6 +53,7 @@ public class EcsJsonSerializer {
             "event.dataset",
             "process.thread.name",
             "process.thread.id",
+            "event.sequence",
             "ecs.version"));
 
     public static CharSequence toNullSafeString(final CharSequence s) {
@@ -144,6 +145,12 @@ public static void serializeEventDataset(StringBuilder builder, String eventData
         }
     }
 
+    public static void serializeEventSequence(StringBuilder builder, long sequenceNumber) {
+        builder.append("\"event.sequence\":");
+        builder.append(sequenceNumber);
+        builder.append(",");
+    }
+
     public static void serializeLogLevel(StringBuilder builder, String level) {
         builder.append("\"log.level\":");
         // add padding so that all levels line up

ecs-logging-core/src/test/java/co/elastic/logging/EcsJsonSerializerTest.java

@@ -226,6 +226,26 @@ void getMessageStringBuilderReuseNormallySizedBuffer()  {
         assertThat(sb2.length()).isZero();
     }
 
+    @Test
+    void serializeEventSequence() throws JsonProcessingException {
+        StringBuilder jsonBuilder = new StringBuilder();
+        jsonBuilder.append('{');
+        EcsJsonSerializer.serializeEventSequence(jsonBuilder, 42);
+        EcsJsonSerializer.serializeObjectEnd(jsonBuilder);
+        JsonNode jsonNode = objectMapper.readTree(jsonBuilder.toString());
+        assertThat(jsonNode.get("event.sequence").longValue()).isEqualTo(42);
+    }
+
+    @Test
+    void serializeEventSequenceZero() throws JsonProcessingException {
+        StringBuilder jsonBuilder = new StringBuilder();
+        jsonBuilder.append('{');
+        EcsJsonSerializer.serializeEventSequence(jsonBuilder, 0);
+        EcsJsonSerializer.serializeObjectEnd(jsonBuilder);
+        JsonNode jsonNode = objectMapper.readTree(jsonBuilder.toString());
+        assertThat(jsonNode.get("event.sequence").longValue()).isEqualTo(0);
+    }
+
     private void assertRemoveIfEndsWith(String builder, String ending, String expected) {
         StringBuilder sb = new StringBuilder(builder);
         EcsJsonSerializer.removeIfEndsWith(sb, ending);

logback-ecs-encoder/src/main/java/co/elastic/logging/logback/EcsEncoder.java

@@ -37,6 +37,7 @@
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.lang.reflect.Method;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Iterator;
@@ -45,6 +46,16 @@
 public class EcsEncoder extends EncoderBase<ILoggingEvent> {
 
     private static final Charset UTF_8 = Charset.forName("UTF-8");
+    private static final Method GET_SEQUENCE_NUMBER;
+    static {
+        Method m = null;
+        try {
+            m = ILoggingEvent.class.getMethod("getSequenceNumber");
+        } catch (NoSuchMethodException ignored) {
+        }
+        GET_SEQUENCE_NUMBER = m;
+    }
+    private boolean emitEventSequence;
     private boolean stackTraceAsArray = false;
     private String serviceName;
     private String serviceVersion;
@@ -72,6 +83,13 @@ public void start() {
             throwableConverter.start();
         }
         eventDataset = EcsJsonSerializer.computeEventDataset(eventDataset, serviceName);
+        if (GET_SEQUENCE_NUMBER != null && getContext() != null) {
+            try {
+                Method getGenerator = getContext().getClass().getMethod("getSequenceNumberGenerator");
+                emitEventSequence = getGenerator.invoke(getContext()) != null;
+            } catch (Exception ignored) {
+            }
+        }
     }
 
     /**
@@ -115,6 +133,14 @@ public byte[] encode(ILoggingEvent event) {
         EcsJsonSerializer.serializeServiceEnvironment(builder, serviceEnvironment);
         EcsJsonSerializer.serializeServiceNodeName(builder, serviceNodeName);
         EcsJsonSerializer.serializeEventDataset(builder, eventDataset);
+        if (emitEventSequence) {
+            try {
+                EcsJsonSerializer.serializeEventSequence(builder, (Long) GET_SEQUENCE_NUMBER.invoke(event));
+            } catch (Exception ignored) {
+                // Reflection call failed at runtime; disable for subsequent events
+                emitEventSequence = false;
+            }
+        }
         EcsJsonSerializer.serializeThreadName(builder, event.getThreadName());
         EcsJsonSerializer.serializeLoggerName(builder, event.getLoggerName());
         EcsJsonSerializer.serializeAdditionalFields(builder, additionalFields);

logback-ecs-encoder/src/test/java/co/elastic/logging/logback/EcsEncoderTest.java

@@ -31,6 +31,9 @@
 
 import java.io.IOException;
 
+import org.junit.jupiter.api.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+
 public class EcsEncoderTest extends AbstractEcsEncoderTest {
 
     private OutputStreamAppender appender;
@@ -61,4 +64,13 @@ void setUp() {
     public JsonNode getLastLogLine() throws IOException {
         return objectMapper.readTree(appender.getBytes());
     }
+
+    @Test
+    void testEventSequenceAbsentWithoutGenerator() throws Exception {
+        logger.debug("test");
+        JsonNode logLine = getLastLogLine();
+        // No SequenceNumberGenerator on the context (and logback 1.2.x lacks
+        // the API entirely), so event.sequence must not appear.
+        assertThat(logLine.has("event.sequence")).isFalse();
+    }
 }