Improve EarlyDeprecationindexingIT test reliability

this test intends to test the bulkProcessor2's request consumer before the templates are initialised.
This requires the flush to happen before the templates are loaded.
To test this reliably the flush interval in the test should be as small as possible (not hardcoded 5s as of now)

This commit introduces a setting (not meant to be exposed/documented) to allow for the flush interval to be configured.
It also adds additional trace logging to help with troubleshooting.

relates elastic#104716

x-pack/plugin/deprecation/qa/early-deprecation-rest/build.gradle

@@ -27,9 +27,12 @@ restResources {
 
 testClusters.configureEach {
   testDistribution = 'DEFAULT'
-  setting 'cluster.deprecation_indexing.enabled', 'true'
   setting 'xpack.security.enabled', 'false'
   setting 'xpack.license.self_generated.type', 'trial'
+  setting 'cluster.deprecation_indexing.enabled', 'true'
+  setting 'cluster.deprecation_indexing.flush_interval', '1ms'
+  setting 'logger.org.elasticsearch.xpack.deprecation','TRACE'
+  setting 'logger.org.elasticsearch.xpack.deprecation.logging','TRACE'
 }
 
 // Test clusters run with security disabled

x-pack/plugin/deprecation/qa/early-deprecation-rest/src/javaRestTest/java/org/elasticsearch/xpack/deprecation/EarlyDeprecationIndexingIT.java

@@ -70,6 +70,7 @@ public void testEarlyDeprecationIsIndexedAfterTemplateIsLoaded() throws Exceptio
                 )
             );
         }, 30, TimeUnit.SECONDS);
+        fail();
     }
 
     private Response getIndexSettings() throws Exception {

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/Deprecation.java

@@ -34,6 +34,7 @@
 import java.util.function.Supplier;
 
 import static org.elasticsearch.xpack.deprecation.DeprecationChecks.SKIP_DEPRECATIONS_SETTING;
+import static org.elasticsearch.xpack.deprecation.logging.DeprecationIndexingComponent.DEPRECATION_INDEXING_FLUSH_INTERVAL;
 
 /**
  * The plugin class for the Deprecation API
@@ -110,6 +111,11 @@ public Collection<?> createComponents(PluginServices services) {
 
     @Override
     public List<Setting<?>> getSettings() {
-        return List.of(USE_X_OPAQUE_ID_IN_FILTERING, WRITE_DEPRECATION_LOGS_TO_INDEX, SKIP_DEPRECATIONS_SETTING);
+        return List.of(
+            USE_X_OPAQUE_ID_IN_FILTERING,
+            WRITE_DEPRECATION_LOGS_TO_INDEX,
+            SKIP_DEPRECATIONS_SETTING,
+            DEPRECATION_INDEXING_FLUSH_INTERVAL
+        );
     }
 }

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/logging/DeprecationIndexingAppender.java

@@ -7,6 +7,8 @@
 
 package org.elasticsearch.xpack.deprecation.logging;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.core.Appender;
 import org.apache.logging.log4j.core.Core;
 import org.apache.logging.log4j.core.Filter;
@@ -16,6 +18,7 @@
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 import org.elasticsearch.action.DocWriteRequest;
 import org.elasticsearch.action.index.IndexRequest;
+import org.elasticsearch.core.Strings;
 import org.elasticsearch.xcontent.XContentType;
 
 import java.util.Objects;
@@ -28,6 +31,7 @@
  */
 @Plugin(name = "DeprecationIndexingAppender", category = Core.CATEGORY_NAME, elementType = Appender.ELEMENT_TYPE)
 public class DeprecationIndexingAppender extends AbstractAppender {
+    private static final Logger logger = LogManager.getLogger(DeprecationIndexingAppender.class);
     public static final String DEPRECATION_MESSAGES_DATA_STREAM = ".logs-deprecation.elasticsearch-default";
 
     private final Consumer<IndexRequest> requestConsumer;
@@ -40,9 +44,10 @@ public class DeprecationIndexingAppender extends AbstractAppender {
 
     /**
      * Creates a new appender.
-     * @param name the appender's name
-     * @param filter a filter to apply directly on the appender
-     * @param layout the layout to use for formatting message. It must return a JSON string.
+     *
+     * @param name            the appender's name
+     * @param filter          a filter to apply directly on the appender
+     * @param layout          the layout to use for formatting message. It must return a JSON string.
      * @param requestConsumer a callback to handle the actual indexing of the log message.
      */
     public DeprecationIndexingAppender(String name, Filter filter, Layout<String> layout, Consumer<IndexRequest> requestConsumer) {
@@ -56,6 +61,13 @@ public DeprecationIndexingAppender(String name, Filter filter, Layout<String> la
      */
     @Override
     public void append(LogEvent event) {
+        logger.trace(
+            () -> Strings.format(
+                "Received deprecation log event. Appender is %s. message = %s",
+                isEnabled ? "enabled" : "disabled",
+                event.getMessage().getFormattedMessage()
+            )
+        );
         if (this.isEnabled == false) {
             return;
         }
@@ -66,11 +78,13 @@ public void append(LogEvent event) {
             .opType(DocWriteRequest.OpType.CREATE);
 
         this.requestConsumer.accept(request);
+        logger.trace(() -> "sent");
     }
 
     /**
      * Sets whether this appender is enabled or disabled. When disabled, the appender will
      * not perform indexing operations.
+     *
      * @param enabled the enabled status of the appender.
      */
     public void setEnabled(boolean enabled) {

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/logging/DeprecationIndexingComponent.java

@@ -27,6 +27,7 @@
 import org.elasticsearch.common.logging.ECSJsonLayout;
 import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.logging.RateLimitingFilter;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.xpack.core.ClientHelper;
@@ -46,6 +47,13 @@
  * It also starts and stops the appender
  */
 public class DeprecationIndexingComponent extends AbstractLifecycleComponent implements ClusterStateListener {
+
+    public static final Setting<TimeValue> DEPRECATION_INDEXING_FLUSH_INTERVAL = Setting.timeSetting(
+        "cluster.deprecation_indexing.flush_interval",
+        TimeValue.timeValueSeconds(5),
+        Setting.Property.NodeScope
+    );
+
     private static final Logger logger = LogManager.getLogger(DeprecationIndexingComponent.class);
 
     private final DeprecationIndexingAppender appender;
@@ -190,6 +198,7 @@ public void enableDeprecationLogIndexing(boolean newEnabled) {
      * @return an initialised bulk processor
      */
     private BulkProcessor2 getBulkProcessor(Client client, Settings settings) {
+        TimeValue flushInterval = DEPRECATION_INDEXING_FLUSH_INTERVAL.get(settings);
         BulkProcessor2.Listener listener = new DeprecationBulkListener();
         return BulkProcessor2.builder((bulkRequest, actionListener) -> {
             /*
@@ -198,13 +207,16 @@ private BulkProcessor2 getBulkProcessor(Client client, Settings settings) {
              * in-flight-bytes limit has been exceeded. This means that we don't have to worry about bounding pendingRequestsBuffer.
              */
             if (flushEnabled.get()) {
+                logger.trace(() -> "Flush is enabled, sending a bulk request");
                 client.bulk(bulkRequest, actionListener);
                 flushBuffer(); // just in case something was missed after the first flush
             } else {
+                logger.trace(() -> "Flush is disabled, scheduling a bulk request");
+
                 // this is an unbounded queue, so the entry will always be accepted
                 pendingRequestsBuffer.offer(() -> client.bulk(bulkRequest, actionListener));
             }
-        }, listener, client.threadPool()).setMaxNumberOfRetries(3).setFlushInterval(TimeValue.timeValueSeconds(5)).build();
+        }, listener, client.threadPool()).setMaxNumberOfRetries(3).setFlushInterval(flushInterval).build();
     }
 
     private static class DeprecationBulkListener implements BulkProcessor2.Listener {